socat(1) socat(1)

NAME

socat - Multipurpose relay (SOcket CAT)

SYNOPSIS

socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan

DESCRIPTION

Socat is a command line based utility that establishes two
bidirectional byte streams and transfers data between them. Because
the streams can be constructed from a large set of different types of
data sinks and sources (see address types), and because lots of
address options may be applied to the streams, socat can be used for
many different purposes.

Filan is a utility that prints information about its active file
descriptors to stdout. It has been written for debugging socat, but
might be useful for other purposes too. Use the -h option to find
more infos.

Procan is a utility that prints information about process parameters
to stdout. It has been written to better understand some UNIX process
properties and for debugging socat, but might be useful for other
purposes too.

The life cycle of a socat instance typically consists of four phases.

In the init phase, the command line options are parsed and logging is
initialized.

During the open phase, socat opens the first address and afterwards
the second address. These steps are usually blocking; thus,
especially for complex address types like socks, connection requests
or authentication dialogs must be completed before the next step is
started.

In the transfer phase, socat watches both streams' read and write
file descriptors via select() , and, when data is available on one
side and can be written to the other side, socat reads it, performs
newline character conversions if required, and writes the data to the
write file descriptor of the other stream, then continues waiting for
more data in both directions.

When one of the streams effectively reaches EOF, the closing phase
begins. Socat transfers the EOF condition to the other stream, i.e.
tries to shutdown only its write stream, giving it a chance to
terminate gracefully. For a defined time socat continues to transfer
data in the other direction, but then closes all remaining channels
and terminates.

OPTIONS

Socat provides some command line options that modify the behaviour of
the program. They have nothing to do with so called address options
that are used as parts of address specifications.

-V Print version and available feature information to stdout, and
exit.

-h | -?
Print a help text to stdout describing command line options
and available address types, and exit.

-hh | -??
Like -h, plus a list of the short names of all available
address options. Some options are platform dependent, so this
output is helpful for checking the particular implementation.

-hhh | -???
Like -hh, plus a list of all available address option names.

-d Without this option, only fatal, error, and warning messages
are printed; applying this option also prints notice messages.
See DIAGNOSTICS for more information.

-d0 With this option, only fatal and error messages are printed;
this restores the behaviour of socat up to version 1.7.4.

-d -d | -dd | -d2
Prints fatal, error, warning, and notice messages.

-d -d -d | -ddd | -d3
Prints fatal, error, warning, notice, and info messages.

-d -d -d -d | -dddd | -d4
Prints fatal, error, warning, notice, info, and debug
messages.

-D Logs information about file descriptors before starting the
transfer phase.

--experimental
New features that are not well tested or are subject to change
in the future must be explicitly enabled using this option.

-ly[<facility>]
Writes messages to syslog instead of stderr; severity as
defined with -d option. With optional <facility>, the syslog
type can be selected, default is "daemon". Third party
libraries might not obey this option.

-lf <logfile>
Writes messages to <logfile> [filename] instead of stderr.
Some third party libraries, in particular libwrap, might not
obey this option.

-ls Writes messages to stderr (this is the default). Some third
party libraries might not obey this option, in particular
libwrap appears to only log to syslog.

-lp<progname>
Overrides the program name printed in error messages and used
for constructing environment variable names.

-lu Extends the timestamp of error messages to microsecond
resolution. Does not work when logging to syslog.

-lm[<facility>]
Mixed log mode. During startup messages are printed to stderr;
when socat starts the transfer phase loop or daemon mode (i.e.
after opening all streams and before starting data transfer,
or, with listening sockets with fork option, before the first
accept call), it switches logging to syslog. With optional
<facility>, the syslog type can be selected, default is
"daemon".

-lh Adds hostname to log messages. Uses the value from environment
variable HOSTNAME or the value retrieved with uname() if
HOSTNAME is not set.

-v Writes the transferred data not only to their target streams,
but also to stderr. The output format is text with some
conversions for readability, and prefixed with "> " or "< "
indicating flow directions.

-x Writes the transferred data not only to their target streams,
but also to stderr. The output format is hexadecimal, prefixed
with "> " or "< " indicating flow directions. Can be combined
with -v .

-r <file>
Dumps the raw (binary) data flowing from left to right address
to the given file. The file name may contain references to
environment variables and $$ (pid), $PROGNAME (see option
option -lp), $TIMESTAMP (uses format %Y%m%dT%H%M%S), and
MICROS (microseconds of daytime). These references have to be
protected from shell expansion of course.

-R <file>
Dumps the raw (binary) data flowing from right to left address
to the given file. See option -r for customization of file
name.

-b<size>
Sets the data transfer block <size> [size_t]. At most <size>
bytes are transferred per step. Default is 8192 bytes.

-s By default, socat terminates when an error occurred to prevent
the process from running when some option could not be
applied. With this option, socat is sloppy with errors and
tries to continue. Even with this option, socat will exit on
fatals, and will abort connection attempts when security
checks failed.

-S<signals-bitmap>
Changes the set of signals that are caught by socat just for
printing an log message. This catching is useful to get the
information about the signal into socats log, but prevents
core dump or other standard actions. The default set of these
signals is SIGHUP, SIGINT, SIGQUIT, SIGILL, SIGABRT, SIGBUS,
SIGFPE, SIGSEGV, and SIGTERM; replace this set (0x89de on
Linux) with a bitmap (e.g., SIGFPE has value 8 and its bit is
0x0080).
Note: Signals SIGHUP, SIGINT, SIGQUIT, SIGUSR1, SIGPIPE,
SIGALRM, SIGTERM, and SIGCHLD may be handled specially anyway.

-t<timeout>
When one channel has reached EOF, the write part of the other
channel is shut down. Then, socat waits <timeout> [timeval]
seconds before terminating. Default is 0.5 seconds. This
timeout only applies to addresses where write and read part
can be closed independently. When during the timeout interval
the read part gives EOF, socat terminates without awaiting the
timeout.

-T<timeout>
Total inactivity timeout: when socat is already in the
transfer loop and nothing has happened for <timeout> [timeval]
seconds (no data arrived, no interrupt occurred...) then it
terminates. Up to version 1.8.0.0 "0" meant infinite"; since
version 1.8.0.1 "0" means 0 and values <0 mean infinite.
Useful with protocols like UDP that cannot transfer EOF.

-u Uses unidirectional mode. The first address is only used for
reading, and the second address is only used for writing
(example).

-U Uses unidirectional mode in reverse direction. The first
address is only used for writing, and the second address is
only used for reading.

-g During address option parsing, don't check if the option is
considered useful in the given address environment. Use it if
you want to force, e.g., appliance of a socket option to a
serial device.

-L<lockfile>
If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.

-W<lockfile>
If lockfile exists, waits until it disappears. When lockfile
does not exist, creates it and continues, unlinks lockfile on
exit.

-4 Use IP version 4 in case the addresses do not implicitly or
explicitly specify a version. Since version 1.8.0.1 this is
the default.

-6 Use IP version 6 in case the addresses do not implicitly or
explicitly specify a version.

-0 Do not prefer a particular IP version; this lets passive
addresses (LISTEN, RECV, ...) serve both versions on some
platforms (not BSD).

--statistics

-S Logs transfer statistics (bytes and blocks counters for both
directions) before terminating socat.
See also signal USR1.
This feature is experimental and might change in future
versions.

ADDRESS SPECIFICATIONS

With the address command line arguments, the user gives socat
instructions and the necessary information for establishing the byte
streams.

An address specification usually consists of an address type keyword,
zero or more required address parameters separated by ':' from the
keyword and from each other, and zero or more address options
separated by ','.

The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For
some keywords there exist synonyms ('-' for STDIO, TCP for TCP4).
Keywords are case insensitive. For a few special address types, the
keyword may be omitted: Address specifications starting with a number
are assumed to be FD (raw file descriptor) addresses; if a '/' is
found before the first ':' or ',', GOPEN (generic file open) is
assumed.

The required number and type of address parameters depend on the
address type. E.g., TCP4 requires a server specification (name or
address), and a port specification (number or service name).

Zero or more address options may be given with each address. They
influence the address in some ways. Options consist of an option
keyword or an option keyword and a value, separated by '='. Option
keywords are case insensitive. For filtering the options that are
useful with an address type, each option is member of one option
group. For each address type there is a set of option groups allowed.
Only options belonging to one of these address groups may be used
(except with option -g).

Address specifications following the above schema are also called
single address specifications. Two single addresses can be combined
with "!!" to form a dual type address for one channel. Here, the
first address is used by socat for reading data, and the second
address for writing data. There is no way to specify an option only
once for being applied to both single addresses.

Usually, addresses are opened in read/write mode. When an address is
part of a dual address specification, or when option -u or -U is
used, an address might be used only for reading or for writing.
Considering this is important with some address types.

With socat version 1.5.0 and higher, the lexical analysis tries to
handle quotes and parenthesis meaningfully and allows escaping of
special characters. If one of the characters ( { [ ' is found, the
corresponding closing character - ) } ] ' - is looked for; they may
also be nested. Within these constructs, socats special characters
and strings : , !! are not handled specially. All those characters
and strings can be escaped with \ or within ""

ADDRESS TYPES

This section describes the available address types with their
keywords, parameters, and semantics.

CREATE:<filename>
Opens <filename> with creat() and uses the file descriptor for
writing. This is a write-only address because a file opened
with creat cannot be read from. See options -u and -U, and
dual addresses.
Flags like O_LARGEFILE cannot be applied. If you need them use
OPEN with options create,create.
<filename> must be a valid existing or not existing path. If
<filename> is a named pipe, creat() might block; if <filename>
refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options: mode, user, group, unlink-early, unlink-late,
append
See also: OPEN, GOPEN

DCCP-CONNECT:<host>:<port> (DCCP:<host>:<port>)
Establishes a DCCP connect to the specified <host> [IP
address] and <port> [DCCP service] using IP version 4 or 6
depending on address specification, name resolution, or option
pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, connect-timeout, tos, dccp-set-ccid,
nonblock, sourceport, retry, readbytes
See also: DCCP4-CONNECT, DCCP6-CONNECT, DCCP-LISTEN,
TCP-CONNECT SCTP-CONNECT

DCCP4-CONNECT:<host>:<port> (DCCP4:<host>:<port>)
Like DCCP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,DCCP,CHILD,RETRY

DCCP6-CONNECT:<host>:<port> (DCCP6:<host>:<port>)
Like DCCP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,DCCP,CHILD,RETRY


DCCP-LISTEN:<port> (DCCP-L:<port>)
Listens on <port> [DCCP service] and accepts an DCCP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,DCCP,RETRY
Useful options: fork, bind, range, max-children, backlog,
accept-timeout, dccp-set-sid, su, reuseaddr, retry
See also: DCCP4-LISTEN, DCCP6-LISTEN, TCP-LISTEN, SCTP-LISTEN,
DCCP-CONNECT

DCCP4-LISTEN:<port> (DCCP4-L:<port>)
Like DCCP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,DCCP,RETRY

DCCP6-LISTEN:<port> (DCCP6-L:<port>)
Like DCCP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,DCCP,RETRY


EXEC:<command-line>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with execvp()
. <command-line> is a simple command with arguments separated
by single spaces. If the program name contains a '/', the part
after the last '/' is taken as ARGV[0]. If the program name is
a relative path, the execvp() semantics for finding the
program via $PATH apply. After successful program start, socat
writes data to stdin of the process and reads from its stdout
using a UNIX domain socket generated by socketpair() per
default. (example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork,
socktype, pty, stderr, ctty, setsid, pipes, umask, login,
sigint, sigquit, netns
See also: SYSTEM,SHELL

FD:<fdnum>
Uses the file descriptor <fdnum>. It must already exist as
valid UN*X file descriptor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR

GOPEN:<filename>
(Generic open) This address type tries to handle any file
system entry except directories usefully. <filename> may be a
relative or absolute path. If it already exists, its type is
checked. In case of a UNIX domain socket, socat connects; if
connecting fails, socat assumes a datagram socket and uses
sendto() calls. If the entry is not a socket, socat opens it
applying the O_APPEND flag. If it does not exist, it is
opened with flag O_CREAT as a regular file (example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT

IP-SENDTO:<host>:<protocol>
Opens a raw IP socket. Depending on host specification or
option pf, IP protocol version 4 or 6 is used. It uses
<protocol> to send packets to <host> [IP address] and receives
packets from host, ignores packets from other hosts. Protocol
255 uses the raw socket with the IP header being part of the
data.
Option groups: FD,SOCKET,IP4,IP6
Useful options: pf, ttl
See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV,
UDP-SENDTO, UNIX-SENDTO

INTERFACE:<interface>
Communicates with a network connected on an interface using
raw packets including link level data. <interface> is the name
of the network interface. Currently only available on Linux.
Option groups: FD,SOCKET
Useful options: pf, type
See also: ip-recv

IP4-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4

IP6-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6


IP-DATAGRAM:<address>:<protocol>
Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets
arriving on the local socket are checked if their source
addresses match RANGE or TCPWRAP options. This address type
can for example be used for implementing symmetric or
asymmetric broadcast or multicast communications.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast,
ip-multicast-loop, ip-multicast-ttl, ip-multicast-if,
ip-add-membership, ip-add-source-membership, ipv6-join-group,
ipv6-join-source-group, ttl, tos, pf
See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO, IP-RECVFROM,
IP-RECV, UDP-DATAGRAM

IP4-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE

IP6-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv6. Please note that IPv6
does not know broadcasts.
Option groups: FD,SOCKET,IP6,RANGE


IP-RECVFROM:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf,
IP protocol version 4 or 6 is used. It receives one packet
from an unspecified peer and may send one or more answer
packets to that peer. This mode is particularly useful with
fork option where each arriving packet - from arbitrary peers
- is handled by its own sub process. This allows a behaviour
similar to typical UDP based servers like ntpd or named.
Please note that the reply packets might be fetched as
incoming traffic when sender and receiver IP address are
identical because there is no port number to distinguish the
sockets.
This address works well with IP-SENDTO address peers (see
above). Protocol 255 uses the raw socket with the IP header
being part of the data.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: pf, fork, range, ttl, broadcast
See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV,
UDP-RECVFROM, UNIX-RECVFROM

IP4-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE

IP6-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE


IP-RECV:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf,
IP protocol version 4 or 6 is used. It receives packets from
multiple unspecified peers and merges the data. No replies
are possible, this is a read-only address, see options -u and
-U, and dual addresses. It can be, e.g., addressed by socat
IP-SENDTO address peers. Protocol 255 uses the raw socket
with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, range
See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM,
UDP-RECV, UNIX-RECV

IP4-RECV:<protocol>
Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE

IP6-RECV:<protocol>
Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE


OPEN:<filename>
Opens <filename> using the open() system call (example). This
operation fails on UNIX domain sockets.
Note: This address type is rarely useful in bidirectional
mode.
Option groups: FD,REG,NAMED,OPEN
Useful options: creat, excl, noatime, nofollow, append,
rdonly, wronly, lock, readbytes, ignoreeof
See also: CREATE, GOPEN, UNIX-CONNECT

OPENSSL:<host>:<port>
Tries to establish a SSL connection to <port> [TCP service] on
<host> [IP address] using TCP/IP version 4 or 6 depending on
address specification, name resolution, or option pf.
NOTE: Up to version 1.7.2.4 the server certificate was only
checked for validity against the system certificate store or
cafile or capath, but not for match with the server's name or
its IP address. Since version 1.7.3.0 socat checks the peer
certificate for match with the <host> parameter or the value
of the openssl-commonname option. Socat tries to match it
against the certificates subject commonName, and the
certificates extension subjectAltName DNS names. Wildcards in
the certificate are supported.
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify, commonname,
cafile, capath, certificate, key, compress, bind, pf,
connect-timeout, sourceport, retry
See also: OPENSSL-LISTEN, TCP

OPENSSL-LISTEN:<port>
Listens on tcp <port> [TCP service]. The IP version is 4 or
the one specified with pf. When a connection is accepted, this
address behaves as SSL server.
Note: You probably want to use the certificate option with
this address.
NOTE: The client certificate is only checked for validity
against cafile or capath, but not for match with the client's
name or its IP address!
Option groups:
FD,SOCKET,IP4,IP6,TCP,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress, fork,
bind, range, tcpwrap, su, reuseaddr, retry
See also: OPENSSL, TCP-LISTEN

OPENSSL-DTLS-CLIENT:<host>:<port>
Tries to establish a DTLS connection to <port> [UDP service]
on <host> [IP address] using UDP/IP version 4 or 6 depending
on address specification, name resolution, or option pf.
Socat checks the peer certificates subjectAltName or
commonName against the addresses option openssl-commonname or
the host name. Wildcards in the certificate are supported.
Use socat option -b to make datagrams small enough to fit with
overhead on the network. Use option -T to prevent indefinite
hanging when peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY
Useful options: min-proto-version, cipher, verify, commonname,
cafile, capath, certificate, key, compress, bind, pf,
sourceport, retry, rcvtimeo
See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT

OPENSSL-DTLS-SERVER:<port>
Listens on UDP <port> [UDP service]. The IP version is 4 or
the one specified with pf. When a connection is accepted, this
address behaves as DTLS server.
Note: You probably want to use the certificate option with
this address.
NOTE: The client certificate is only checked for validity
against cafile or capath, but not for match with the client's
name or its IP address! Use socat option -b to make datagrams
small enough to fit with overhead on the network. Use option
-T to prevent indefinite hanging when peer went down quietly.
Option groups:
FD,SOCKET,IP4,IP6,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, min-proto-version, cipher, verify,
commonname, cafile, capath, certificate, key, compress, fork,
bind, range, tcpwrap, su, reuseaddr, retry
rcvtimeo
See also: OPENSSL-DTLS-CLIENT, OPENSSL-LISTEN, UDP-LISTEN

PIPE:<filename>
If <filename> already exists, it is opened. If it does not
exist, a named pipe is created and opened. Beginning with
socat version 1.4.3, the named pipe is removed when the
address is closed (but see option unlink-close
Note: When a pipe is used for both reading and writing, it
works as echo service.
Note: When a pipe is used for both reading and writing, and
socat tries to write more bytes than the pipe can buffer
(Linux 2.4: 2048 bytes), socat might block. Consider using
socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode,
unlink-early
See also: unnamed pipe

PIPE Creates an unnamed pipe and uses it for reading and writing.
It works as an echo, because everything written to it appears
immediately as read data.
Note: When socat tries to write more bytes than the pipe can
queue (Linux 2.4: 2048 bytes), socat might block. Consider,
e.g., using option -b 2048
Option groups: FD
See also: named pipe, SOCKETPAIR

SOCKETPAIR
Creates a socketpair and uses it for reading and writing. It
works as an echo, because everything written to it appears
immediately as read data. The default socket type is datagram,
so it keeps packet boundaries.
Option groups: FD
Useful options: socktype
See also: unnamed pipe

POSIXMQ-READ:/<mqueue>
Opens the specified POSIX message queue and reads messages
(packets). It keeps the boundaries.
This is a read-only address, see options -u and -U and dual
addresses.
Socat provides this address type only on Linux because POSIX
MQ is based on UNIX filedescriptors there.
This feature is new in version 1.8.0.0 and might change in the
future, therefore it is experimental.
Useful options: posixmq-priority, unlink-early, unlink-close

POSIXMQ-RECEIVE:/<mqueue>

POSIXMQ-RECV:/<mqueue>
Opens the specified POSIX message queue and reads one message
(packet).
This is a read-only address. See POSIXMQ-READ for more info.
Example: POSIX MQ recv with fork
This feature is experimental.
Useful options: posixmq-priority, fork, max-children,
unlink-early, unlink-close

POSIXMQ-SEND:/<mqueue>
Opens the specified POSIX message queue and writes messages
(packets).
This is a write-only address. See POSIXMQ-READ for more info.
(Example)
This feature is experimental.
Useful options: posixmq-priority, fork, max-children,
unlink-early, unlink-close

POSIXMQ-BIDIRECTIONAL:/mqueue
Opens the specified POSIX message queue and writes and reads
messages (packet). This is probably rarely useful but has been
implemented for functional completeness.

PROXY:<proxy>:<hostname>:<port>
Connects to an HTTP proxy server on port 8080 using TCP/IP
version 4 or 6 depending on address specification, name
resolution, or option pf, and sends a CONNECT request for
hostname:port. If the proxy grants access and succeeds to
connect to the target, data transfer between socat and the
target can start (example). Note that the traffic need not be
HTTP but can be an arbitrary protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options: proxyport, ignorecr, proxyauth, resolve, crnl,
bind, connect-timeout, mss, sourceport, retry
See also: SOCKS, TCP

PTY Generates a pseudo terminal (pty) and uses its master side.
Another process may open the pty's slave side using it like a
serial line or terminal. (example). If both the ptmx and the
openpty mechanisms are available, ptmx is used (POSIX).
Option groups: FD,NAMED,PTY,TERMIOS
Useful options: link, openpty, wait-slave, mode, user, group
See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM, SHELL

READLINE
Uses GNU readline and history on stdio to allow editing and
reusing input lines (example). This requires the GNU readline
and history libraries. Note that stdio should be a (pseudo)
terminal device, otherwise readline does not seem to work.
Option groups: FD,READLINE,TERMIOS
Useful options: history, noecho
See also: STDIO

SCTP-CONNECT:<host>:<port>
Establishes an SCTP stream connection to the specified <host>
[IP address] and <port> [TCP service] using IP version 4 or 6
depending on address specification, name resolution, or option
pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, pf, connect-timeout, tos, mtudiscover,
sctp-maxseg, sctp-nodelay, nonblock, sourceport, retry,
readbytes
See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN,
TCP-CONNECT

SCTP4-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY

SCTP6-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY

SCTP-LISTEN:<port>
Listens on <port> [TCP service] and accepts an SCTP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf,
max-children, backlog, accept-timeout, sctp-maxseg,
sctp-nodelay, su, reuseaddr, retry
See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN, SCTP-CONNECT

SCTP4-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY

SCTP6-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY


SOCKET-CONNECT:<domain>:<protocol>:<remote-address>
Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and
connects to the remote-address. The two socket parameters
have to be specified by int numbers. Consult your OS
documentation and include files to find the appropriate
values. The remote-address must be the data representation of
a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the specified
groups - also use options of higher level protocols when you
apply socat option -g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN,
SOCKET-SENDTO

SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>
Creates a datagram socket using the first three given socket
parameters (see man socket(2)) and sends outgoing data to the
remote-address. The three socket parameters have to be
specified by int numbers. Consult your OS documentation and
include files to find the appropriate values. The
remote-address must be the data representation of a sockaddr
structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified
groups - also use options of higher level protocols when you
apply socat option -g.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt,
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO,
SOCKET-RECV, SOCKET-RECVFROM

SOCKET-LISTEN:<domain>:<protocol>:<local-address>
Creates a stream socket using the first and second given
socket parameters and SOCK_STREAM (see man socket(2)) and
waits for incoming connections on local-address. The two
socket parameters have to be specified by int numbers. Consult
your OS documentation and include files to find the
appropriate values. The local-address must be the data
representation of a sockaddr structure without sa_family and
(BSD) sa_len components.
Please note that you can - beyond the options of the specified
groups - also use options of higher level protocols when you
apply socat option -g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options: setsockopt, setsockopt-listen,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN,
SOCKET-SENDTO, SOCKET-SENDTO

SOCKET-RECV:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters (see
man socket(2)) and binds it to <local-address>. Receives
arriving data. The three parameters have to be specified by
int numbers. Consult your OS documentation and include files
to find the appropriate values. The local-address must be the
data representation of a sockaddr structure without sa_family
and (BSD) sa_len components.
Option groups: FD,SOCKET,RANGE
Useful options: range, setsockopt, setsockopt-listen
See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM,
SOCKET-SENDTO, SOCKET-RECVFROM

SOCKET-RECVFROM:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters (see
man socket(2)) and binds it to <local-address>. Receives
arriving data and sends replies back to the sender. The first
three parameters have to be specified as int numbers. Consult
your OS documentation and include files to find the
appropriate values. The local-address must be the data
representation of a sockaddr structure without sa_family and
(BSD) sa_len components.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt, setsockopt-listen
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM,
SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV

SOCKET-SENDTO:<domain>:<type>:<protocol>:<remote-address>
Creates a socket using the three given socket parameters (see
man socket(2)). Sends outgoing data to the given address and
receives replies. The three parameters have to be specified
as int numbers. Consult your OS documentation and include
files to find the appropriate values. The remote-address must
be the data representation of a sockaddr structure without
sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET
Useful options: bind, setsockopt, setsockopt-listen
See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO, SOCKET-DATAGRAM,
SOCKET-RECV SOCKET-RECVFROM

ACCEPT-FD:<fdnum>
Expects a listening socket in <fdnum> and accepts one or (with
option fork) more connections. This address type is useful
under systemd control with "inetd mode".
Example: (example)
Option groups: FD, SOCKET, TCP, CHILD, RETRY
Useful options: fork, range, sourceport, lowport, tcpwrap

SOCKS4:<socks-server>:<host>:<port>
Connects via <socks-server> [IP address] to <host> [IPv4
address] on <port> [TCP service], using socks version 4
protocol over IP version 4 or 6 depending on address
specification, name resolution, or option pf (example).
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Useful options: socksuser, socksport, sourceport, pf, retry
See also: SOCKS5, SOCKS4A, PROXY, TCP

SOCKS4A:<socks-server>:<host>:<port>
like SOCKS4, but uses socks protocol version 4a, thus leaving
host name resolution to the socks server.
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY


SOCKS5-CONNECT:<socks-server>:<socks-port>:<target-host>:<target-port>
Connects via <socks-server> [IP address] to <target-host>
[IPv4 address] on <target-port> [TCP service], using socks
version 5 protocol over TCP. Currently no authentication
mechanism is provided.
This address type is experimental.
Option groups: FD, SOCKET, IP4, IP6, TCP, CHILD, RETRY
Useful options: socksport, sourceport, pf, retry
See also: SOCKS5-LISTEN, SOCKS4, SOCKS4A, PROXY, TCP

SOCKS5-LISTEN:<socks-server>:<socks-port>:<listen-host>:<listen-port>
Connects to <socks-server> [IP address] using socks version 5
protocol over TCP and makes it listen for incoming connections
on <listen-port> [TCP service], binding to <-listen-host>
[IPv4 address] Currently not authentication mechanism is
provided. This address type is experimental. Option groups:
FD, SOCKET, IP4, IP6, TCP, CHILD, RETRY
Useful options: sourceport, pf, retry
See also: SOCKS5-CONNECT,

STDERR Uses file descriptor 2.
This is a write-only address, see options -u and -U, and dual
addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD

STDIN Uses file descriptor 0.
This is a read-only address, see options -u and -U, and dual
addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD

STDIO Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD

STDOUT Uses file descriptor 1.
This is a write-only address, see options -u and -U, and dual
addresses.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD

SHELL:<shell-command>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with the
configured shell ($SHELL). Note that <shell-command> [string]
must not contain ',' or "!!", and that shell meta characters
may have to be protected. After successful program start,
socat writes data to stdin of the process and reads from its
stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork,
socktype, pty, stderr, ctty, setsid, pipes, umask, sigint,
sigquit
See also: EXEC, SYSTEM

SYSTEM:<shell-command>
Forks a sub process that establishes communication with its
parent process and invokes the specified program with system()
. Please note that <shell-command> [string] must not contain
',' or "!!", and that shell meta characters may have to be
protected. After successful program start, socat writes data
to stdin of the process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork,
socktype, pty, stderr, ctty, setsid, pipes, umask, sigint,
sigquit, netns
See also: EXEC, SHELL

TCP:<host>:<port>
Connects to <port> [TCP service] on <host> [IP address] using
TCP/IP version 4 or 6 depending on address specification, name
resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY
Useful options: connect-timeout, retry, sourceport, netns,
crnl, bind, pf, tos, mtudiscover, mss, nodelay, nonblock,
readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT,
UNIX-CONNECT

TCP4:<host>:<port>
Like TCP, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,IP4,TCP,RETRY

TCP6:<host>:<port>
Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY

TCP-LISTEN:<port>
Listens on <port> [TCP service] and accepts a TCP/IP
connection. The IP version is 4 or the one specified with
address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this
address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf,
max-children, backlog, accept-timeout, mss, su, reuseaddr,
retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, SCTP-LISTEN,
UNIX-LISTEN, OPENSSL-LISTEN, TCP-CONNECT

TCP4-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY

TCP6-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv6 protocol.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY

TUN[:<if-addr>/<bits>]
Creates a Linux TUN/TAP device and optionally assignes it the
address and netmask given by the parameters. The resulting
network interface is almost ready for use by other processes;
socat serves its "wire side". This address requires read and
write access to the tunnel cloning device, usually
/dev/net/tun , as well as permission to set some ioctl()s.
Option iff-up is required to immediately activate the
interface!
Note: If you intend to transfer packets between two Socat
"wire sides" you need a protocol that keeps packet boundaries,
e.g.UDP; TCP might work with option nodelay.
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type,
iff-no-pi, netns
See also: ip-recv

UDP:<host>:<port>
Connects to <port> [UDP service] on <host> [IP address] using
UDP/IP version 4 or 6 depending on address specification, name
resolution, or option pf.
Please note that, due to UDP protocol properties, no real
connection is established; data has to be sent for
`connecting' to the server, and no end-of-file condition can
be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP

UDP4:<host>:<port>
Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4

UDP6:<host>:<port>
Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6

UDP-DATAGRAM:<address>:<port>
Sends outgoing data to the specified address which may in
particular be a broadcast or multicast address. Packets
arriving on the local socket are checked for the correct
remote port only when option sourceport is used (this is a
change with Socat version 1.7.4.0) and if their source
addresses match RANGE or TCPWRAP options. This address type
can for example be used for implementing symmetric or
asymmetric broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast,
ip-multicast-loop, ip-multicast-ttl, ip-multicast-if,
ip-add-membership, ip-add-source-membership, ipv6-join-group,
ipv6-join-source-group, ttl, tos, sourceport, pf
See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO,
UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-DATAGRAM

UDP4-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv4 protocol (example1,
example2).
Option groups: FD,SOCKET,IP4, RANGE

UDP6-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE

UDP-LISTEN:<port>
Waits for a UDP/IP packet arriving on <port> [UDP service] and
`connects' back to sender. The accepted IP version is 4 or
the one specified with option pf. Please note that, due to
UDP protocol properties, no real connection is established;
data has to arrive from the peer first, and no end-of-file
condition can be transported. Note that opening this address
usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options: fork, bind, range, pf
See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN

UDP4-LISTEN:<port>
Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4

UDP6-LISTEN:<port>
Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6

UDP-SENDTO:<host>:<port>
Communicates with the specified peer socket, defined by <port>
[UDP service] on <host> [IP address], using UDP/IP version 4
or 6 depending on address specification, name resolution, or
option pf. It sends packets to and receives packets from that
peer socket only. This address effectively implements a
datagram client. It works well with socat UDP-RECVFROM and
UDP-RECV address peers.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV,
UDP-CONNECT, UDP-LISTEN, IP-SENDTO

UDP4-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4

UDP6-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6

UDP-RECVFROM:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP
version 4 or 6 depending on option pf. It receives one packet
from an unspecified peer and may send one or more answer
packets to that peer. This mode is particularly useful with
fork option where each arriving packet - from arbitrary peers
- is handled by its own sub process. This allows a behaviour
similar to typical UDP based servers like ntpd or named. This
address works well with socat UDP-SENDTO address peers.
Note: When the second address fails before entering the
transfer loop the packet is dropped. Use option retry or
forever on the second address to avoid data loss.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: fork, ttl, tos, bind, sourceport, pf
See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO, UDP-RECV,
UDP-CONNECT, UDP-LISTEN, IP-RECVFROM, UNIX-RECVFROM

UDP4-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE

UDP6-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE

UDP-RECV:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP
version 4 or 6 depending on option pf. It receives packets
from multiple unspecified peers and merges the data. No
replies are possible. It works well with, e.g., socat
UDP-SENDTO address peers; it behaves similar to a syslog
server.
This is a read-only address, see options -u and -U, and dual
addresses.
Note: if you need the fork option, use UDP-RECVFROM in
unidirectional mode (with option -u) instead.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, bind, sourceport, ttl, tos
See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM,
UDP-CONNECT, UDP-LISTEN, IP-RECV, UNIX-RECV

UDP4-RECV:<port>
Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE

UDP6-RECV:<port>
Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE

UDPLITE-CONNECT:<host>:<port>

UDPLITE4-CONNECT:<host>:<port>

UDPLITE6-CONNECT:<host>:<port>

UDPLITE-DATAGRAM:<address>:<port>

UDPLITE4-DATAGRAM:<address>:<port>

UDPLITE6-DATAGRAM:<address>:<port>

UDPLITE-LISTEN:<port>

UDPLITE4-LISTEN:<port>

UDPLITE6-LISTEN:<port>

UDPLITE-SENDTO:<host>:<port>

UDPLITE4-SENDTO:<host>:<port>

UDPLITE6-SENDTO:<host>:<port>

UDPLITE-RECVFROM:<port>

UDPLITE4-RECVFROM:<port>

UDPLITE6-RECVFROM:<port>

UDPLITE-RECV:<port>

UDPLITE4-RECV:<port>

UDPLITE6-RECV:<port>
The UDPLITE addresses are almost identical to the related UDP
addresses but they use UDP-Lite protocol and have the
additional UDPLITE option group.


UNIX-CONNECT:<filename>
Connects to <filename> assuming it is a UNIX domain socket.
If <filename> does not exist, this is an error; if <filename>
is not a UNIX domain socket, this is an error; if <filename>
is a UNIX domain socket, but no process is listening, this is
an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
) Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP

UNIX-LISTEN:<filename>
Listens on <filename> using a UNIX domain stream socket and
accepts a connection. If <filename> exists and is not a
socket, this is an error. If <filename> exists and is a UNIX
domain socket, binding to the address fails (use option
unlink-early!). Note that opening this address usually blocks
until a client connects. Beginning with socat version 1.4.3,
the file system entry is removed when this address is closed
(but see option unlink-close) (example).
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX
Useful options: fork, umask, mode, user, group, unlink-early
See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV, TCP-LISTEN

UNIX-SENDTO:<filename>
Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain datagram socket. It
sends packets to and receives packets from that peer socket
only. Please note that it might be necessary to bind the
local socket to an address (e.g. /tmp/sock1, which must not
exist before). This address type works well with socat
UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT, UDP-SENDTO,
IP-SENDTO

UNIX-RECVFROM:<filename>
Creates a UNIX domain datagram socket [<filename>]. Receives
one packet and may send one or more answer packets to that
peer. This mode is particularly useful with fork option where
each arriving packet - from arbitrary peers - is handled by
its own sub process. This address works well with socat
UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
See the note about RECVFROM addresses.
Useful options: fork
umask
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM,
IP-RECVFROM

UNIX-RECV:<filename>
Creates a UNIX domain datagram socket [<filename>]. Receives
packets from multiple unspecified peers and merges the data.
No replies are possible, this is a read-only address, see
options -u and -U, and dual addresses. It can be, e.g.,
addressed by socat UNIX-SENDTO address peers. It behaves
similar to a syslog server.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: umask
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV,
IP-RECV

UNIX-CLIENT:<filename>
Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain socket. It first
tries to connect and, if that fails, assumes it is a datagram
socket, thus supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN

VSOCK-CONNECT:<cid>:<port>
Establishes a VSOCK stream connection to the specified <cid>
[VSOCK cid] and <port> [VSOCK port].
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, connect-timeout, retry, readbytes
See also: VSOCK-LISTEN,

VSOCK-LISTEN:<port>
Listens on <port> [VSOCK port] and accepts a VSOCK connection.
Note that opening this address usually blocks until a client
connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RETRY
Useful options: fork, bind, max-children, backlog, su,
reuseaddr, retry
See also: VSOCK-CONNECT

ABSTRACT-CONNECT:<string>

ABSTRACT-LISTEN:<string>

ABSTRACT-SENDTO:<string>

ABSTRACT-RECVFROM:<string>

ABSTRACT-RECV:<string>

ABSTRACT-CLIENT:<string>
The ABSTRACT addresses are almost identical to the related
UNIX addresses except that they do not address file system
based sockets but an alternate UNIX domain address space. To
achieve this the socket address strings are prefixed with "\0"
internally. This feature is available (only?) on Linux.
Option groups are the same as with the related UNIX addresses,
except that the ABSTRACT addresses are not member of the NAMED
group.
Useful options: netns

ADDRESS OPTIONS

Address options can be applied to address specifications to influence
the process of opening the addresses and the properties of the
resulting data channels.

For technical reasons not every option can be applied to every
address type; e.g., applying a socket option to a regular file will
fail. To catch most useless combinations as early as in the open
phase, the concept of option groups was introduced. Each option
belongs to one or more option groups. Options can be used only with
address types that support at least one of their option groups (but
see option -g).

Address options have data types that their values must conform to.
Every address option consists of just a keyword or a keyword followed
by "=value", where value must conform to the options type. Some
address options manipulate parameters of system calls; e.g., option
sync sets the O_SYNC flag with the open() call. Other options cause
a system or library call; e.g., with option `ttl=value' the
setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int)) call is applied.
Other options set internal socat variables that are used during data
transfer; e.g., `crnl' causes explicit character conversions. A few
options have more complex implementations; e.g., su-d
(substuser-delayed) inquires some user and group infos, stores them,
and applies them later after a possible chroot() call.

If multiple options are given to an address, their sequence in the
address specification has (almost) no effect on the sequence of their
execution/application. Instead, socat has built in an option phase
model that tries to bring the options in a useful order. Some options
exist in different forms (e.g., unlink, unlink-early, unlink-late) to
control the time of their execution.

If the same option is specified more than once within one address
specification, with equal or different values, the effect depends on
the kind of option. Options resulting in function calls like
setsockopt() cause multiple invocations. With options that set
parameters for a required call like open() or set internal flags, the
value of the last option occurrence is effective.

The existence or semantics of many options are system dependent.
Socat usually does NOT try to emulate missing libc or kernel
features, it just provides an interface to the underlying system. So,
if an operating system lacks a feature, the related option is simply
not available on this platform.

The following paragraphs introduce just the more common address
options. For a more comprehensive reference and to find information
about canonical option names, alias names, option phases, and
platforms see file xio.help.


FD option group

This option group contains options that are applied to a UN*X style
file descriptor, no matter how it was generated. Because all current
socat address types are file descriptor based, these options may be
applied to any address.
Note: Some of these options are also member of another option group,
that provides another, non-fd based mechanism. For these options, it
depends on the actual address type and its option groups which
mechanism is used. The second, non-fd based mechanism is prioritized.

cloexec[=<bool>]
Sets the FD_CLOEXEC flag with the fcntl() system call to value
<bool>. If set, the file descriptor is closed on exec() family
function calls. Socat internally handles this flag for the fds
it controls, so in most cases there will be no need to apply
this option.

setlk[=<bool>]
Tries to set a discretionary write lock to the whole file
using the fcntl(fd, F_SETLK, ...) system call. If the file is
already locked, this call results in an error. On Linux, when
the file permissions for group are "S" (g-x,g+s), and the file
system is locally mounted with the "mand" option, the lock is
mandatory, i.e. prevents other processes from opening the
file.

setlkw[=<bool>]
Tries to set a discretionary waiting write lock to the whole
file using the fcntl(fd, F_SETLKW, ...) system call. If the
file is already locked, this call blocks. See option setlk
for information about making this lock mandatory.

setlk-rd[=<bool>]
Tries to set a discretionary read lock to the whole file using
the fcntl(fd, F_SETLK, ...) system call. If the file is
already write locked, this call results in an error. See
option setlk for information about making this lock mandatory.

setlkw-rd[=<bool>]
Tries to set a discretionary waiting read lock to the whole
file using the fcntl(fd, F_SETLKW, ...) system call. If the
file is already write locked, this call blocks. See option
setlk for information about making this lock mandatory.

flock-ex[=<bool>]
Tries to set a blocking exclusive advisory lock to the file
using the flock(fd, LOCK_EX) system call. Socat hangs in this
call if the file is locked by another process.

flock-ex-nb[=<bool>]
Tries to set a nonblocking exclusive advisory lock to the file
using the flock(fd, LOCK_EX|LOCK_NB) system call. If the file
is already locked, this option results in an error.

flock-sh[=<bool>]
Tries to set a blocking shared advisory lock to the file using
the flock(fd, LOCK_SH) system call. Socat hangs in this call
if the file is locked by another process.

flock-sh-nb[=<bool>]
Tries to set a nonblocking shared advisory lock to the file
using the flock(fd, LOCK_SH|LOCK_NB) system call. If the file
is already locked, this option results in an error.

lock[=<bool>]
Sets a blocking lock on the file. Uses the setlk or flock
mechanism depending on availability on the particular
platform. If both are available, the POSIX variant (setlkw) is
used.

user=<user>
Sets the <user> (owner) of the stream. If the address is
member of the NAMED option group, socat uses the chown()
system call after opening the file or binding to the UNIX
domain socket (race condition!). Without filesystem entry,
socat sets the user of the stream using the fchown() system
call. These calls might require root privilege.

user-late=<user>
Sets the owner of the fd to <user> with the fchown() system
call after opening or connecting the channel. This is useful
only on file system entries.

group=<group>
Sets the <group> of the stream. If the address is member of
the NAMED option group, socat uses the chown() system call
after opening the file or binding to the UNIX domain socket
(race condition!). Without filesystem entry, socat sets the
group of the stream with the fchown() system call. These
calls might require group membership or root privilege.

group-late=<group>
Sets the group of the fd to <group> with the fchown() system
call after opening or connecting the channel. This is useful
only on file system entries.

mode=<mode>
Sets the <mode> [mode_t] (permissions) of the stream. If the
address is member of the NAMED option group and uses the
open() or creat() call, the mode is applied with these. If
the address is member of the NAMED option group without using
these system calls, socat uses the chmod() system call after
opening the filesystem entry or binding to the UNIX domain
socket (race condition!). Otherwise, socat sets the mode of
the stream using fchmod() . These calls might require
ownership or root privilege.

perm-late=<mode>
Sets the permissions of the fd to value <mode> [mode_t] using
the fchmod() system call after opening or connecting the
channel. This is useful only on file system entries.

append[=<bool>]
Always writes data to the actual end of file. If the address
is member of the OPEN option group, socat uses the O_APPEND
flag with the open() system call (example). Otherwise, socat
applies the fcntl(fd, F_SETFL, O_APPEND) call.

nonblock[=<bool>]
Tries to open or use file in nonblocking mode. Its only
effects are that the connect() call of TCP addresses does not
block, and that opening a named pipe for reading does not
block. If the address is member of the OPEN option group,
socat uses the O_NONBLOCK flag with the open() system call.
Otherwise, socat applies the fcntl(fd, F_SETFL, O_NONBLOCK)
call.

binary[=<bool>]
Opens the file in binary mode to avoid implicit line
terminator conversions (Cygwin).

text[=<bool>]
Opens the file in text mode to force implicit line terminator
conversions (Cygwin).

noinherit[=<bool>]
Does not keep this file open in a spawned process (Cygwin).

cool-write[=<bool>]
Takes it easy when write fails with EPIPE or ECONNRESET and
logs the message with notice level instead of error. This
prevents the log file from being filled with useless error
messages when socat is used as a high volume server or proxy
where clients often abort the connection. Use this option only
with option fork because otherwise it might cause socat to
exit with code 0 even on failure.
This option is deprecated, consider using option
children-shutup instead.

end-close[=<bool>]
Changes the (address dependent) method of ending a connection
to just close the file descriptors. This is useful when the
connection is to be reused by or shared with other processes
(example).
Normally, socket connections will be ended with shutdown(2)
which terminates the socket even if it is shared by multiple
processes. close(2) "unlinks" the socket from the process but
keeps it active as long as there are still links from other
processes.
Similarly, when an address of type EXEC or SYSTEM is ended,
socat usually will explicitly kill the sub process. With this
option, it will just close the file descriptors.

shut-none[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to not do anything.

shut-down[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to shutdown(fd, SHUT_WR). Is only
useful with sockets.

shut-close[=<bool>]
Changes the (address dependent) method of shutting down the
write part of a connection to close(fd).

shut-null[=<bool>]
When one address indicates EOF, socat will send a zero sized
packet to the write channel of the other address to transfer
the EOF condition. This is useful with UDP and other datagram
protocols. Has been tested against netcat and socat with
option null-eof.

null-eof[=<bool>]
Normally socat will ignore empty (zero size payload) packets
arriving on datagram sockets, so it survives port scans. With
this option socat interprets empty datagram packets as EOF
indicator (see shut-null).

ioctl-void=<request>
Calls ioctl() with the request value as second argument and
NULL as third argument. This option allows utilizing ioctls
that are not explicitly implemented in socat.

ioctl-int=<request>:<value>
Calls ioctl() with the request value as second argument and
the integer value as third argument.

ioctl-intp=<request>:<value>
Calls ioctl() with the request value as second argument and a
pointer to the integer value as third argument.

ioctl-bin=<request>:<value>
Calls ioctl() with the request value as second argument and a
pointer to the given data value as third argument. This data
must be specified in <dalan> form.

ioctl-string=<request>:<value>
Calls ioctl() with the request value as second argument and a
pointer to the given string as third argument. <dalan> form.


NAMED option group

These options work on file system entries.
Please note that, with UNIX domain client addresses, this means the
bind entry, not the target/peer entry.
See also options user, group, and mode.

user-early=<user>
Changes the <user> (owner) of the file system entry before
accessing it, using the chown() system call. This call might
require root privilege.

group-early=<group>
Changes the <group> of the file system entry before accessing
it, using the chown() system call. This call might require
group membership or root privilege.

perm-early=<mode>
Changes the <mode> [mode_t] of the file system entry before
accessing it, using the chmod() system call. This call might
require ownership or root privilege.

unlink-early[=<bool>]
Unlinks (removes) the file before opening it and even before
applying user-early etc.

unlink[=<bool>]
Unlinks (removes) the file before accessing it, but after
user-early etc.

unlink-late[=<bool>]
Unlinks (removes) the file after opening it to make it
inaccessible for other processes after a short race condition.

unlink-close[=<bool>]
Controls removal of the addresses file system entry when
closing the address. For named pipes, UNIX domain sockets,
and the symbolic links of pty addresses, the default is remove
(1); for created files, opened files, and generic opened files
the default is keep (0). Setting this option to 1 removes the
entry, 0 keeps it. No value means 1.


OPEN option group

The OPEN group options allow setting flags with the open() system
call. E.g., option `creat' sets the O_CREAT flag. When the used
address does not use open() (e.g.STDIO), the fcntl(..., F_SETFL, ...)
call is used instead.
See also options append and nonblock.

creat[=<bool>]
Creates the file if it does not exist (example).

dsync[=<bool>]
Blocks write() calls until metainfo is physically written to
media.

excl[=<bool>]
With option creat, if file exists this is an error.

largefile[=<bool>]
On 32 bit systems, allows a file larger than 2^31 bytes.

noatime[=<bool>]
Sets the O_NOATIME options, so reads do not change the access
timestamp.

noctty[=<bool>]
Does not make this file the controlling terminal.

nofollow[=<bool>]
Does not follow symbolic links.

nshare[=<bool>]
Does not allow sharing this file with other processes.

rshare[=<bool>]
Does not allow other processes to open this file for writing.

rsync[=<bool>]
Blocks write() until metainfo is physically written to media.

sync[=<bool>]
Blocks write() until data is physically written to media.

rdonly[=<bool>]
Opens the file for reading only.

wronly[=<bool>]
Opens the file for writing only.

trunc[=<bool>]
Truncates the file to size 0 during opening it.


REG and BLK option group

These options are usually applied to a UN*X file descriptor, but
their semantics make sense only on a file supporting random access.

seek=<offset>
Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64 ) system
call, thus positioning the file pointer absolutely to <offset>
[off_t or off64_t]. Please note that a missing value defaults
to 1, not 0.

seek-cur=<offset>
Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64 ) system
call, thus positioning the file pointer <offset> [off_t or
off64_t] bytes relatively to its current position (which is
usually 0). Please note that a missing value defaults to 1,
not 0.

seek-end=<offset>
Applies the lseek(fd, <offset>, SEEK_END) (or lseek64 ) system
call, thus positioning the file pointer <offset> [off_t or
off64_t] bytes relatively to the files current end. Please
note that a missing value defaults to 1, not 0.

ftruncate=<offset>
Applies the ftruncate(fd, <offset>) (or ftruncate64 if
available) system call, thus truncating the file at the
position <offset> [off_t or off64_t]. Please note that a
missing value defaults to 1, not 0.

secrm[=<bool>]

unrm[=<bool>]

compr[=<bool>]

fs-sync[=<bool>]

immutable[=<bool>]

fs-append[=<bool>]

nodump[=<bool>]

fs-noatime[=<bool>]

journal-data[=<bool>]

notail[=<bool>]

dirsync[=<bool>]
These options change non standard file attributes on operating
systems and file systems that support these features, like
Linux with ext2fs and successors, xfs, or reiserfs. See man 1
chattr for information on these options. Please note that
there might be a race condition between creating the file and
applying these options.


PIPE options

These options may be applied to pipes (fifos).

f-setpipe-sz=<int>

setpipe=<int>
Set the number of bytes a pipe can buffer. Where more bytes
are written the writing process might block. When more bytes
are written in a single write() the writing process blocks and
might never recover.


General address options

These options may be applied to all address types. They change some
process properties that are restored after opening the address.

chdir=<filename>

cd=<filename>
Changes the working directory. After opening the address the
master process changes back to the original working directory.
Sub processes inherit the temporary setting.

umask=<mode>
Sets the umask of the process to <mode> [mode_t] before
opening the address. Useful when file system entries are
created or a shell or program is invoked. Usually the value is
specified as octal number.
The processes umask value is inherited by child processes.
Note: umask is an inverted value: creating a file with
umask=0026 results in permissions 0640.


PROCESS option group

Options of this group change the process properties instead of just
affecting one data channel. For EXEC and SYSTEM addresses and for
LISTEN and CONNECT type addresses with option fork, these options
apply to the child processes instead of the main socat process.

chroot=<directory>
Performs a chroot() operation to <directory> after processing
the address (example). This call might require root privilege.

chroot-early=<directory>
Performs a chroot() operation to <directory> before opening
the address. This call might require root privilege.

setgid=<group>
Changes the primary <group> of the process after processing
the address. This call might require root privilege. Please
note that this option does not drop other group related
privileges.

setgid-early=<group>
Like setgit but is performed before opening the address.

setuid=<user>
Changes the <user> (owner) of the process after processing the
address. This call might require root privilege. Please note
that this option does not drop group related privileges. Check
if option su better fits your needs.

setuid-early=<user>
Like setuid but is performed before opening the address.

su=<user>
Changes the <user> (owner) and groups of the process after
processing the address (example). This call might require root
privilege.

su-d=<user>
Short name for substuser-delayed. Changes the <user> (owner)
and groups of the process after processing the address
(example). The user and his groups are retrieved before a
possible chroot() . This call might require root privilege.

setpgid=<pid_t>
Makes the process a member of the specified process group
<pid_t>. If no value is given, or if the value is 0 or 1, the
process becomes leader of a new process group.

setsid Makes the process the leader of a new session (example).

netns=<net-namespace-name>
Before opening the address it tries to switch to the named
network namespace. After opening the address it switches back
to the previous namespace. (Example with TCP forwarder,
example with virtual network connection.
Only on Linux; requires root; use option --experimental.


READLINE option group

These options apply to the readline address type.

history=<filename>
Reads and writes history from/to <filename> (example).

noprompt
Since version 1.4.0, socat per default tries to determine a
prompt - that is then passed to the readline call - by
remembering the last incomplete line of the output. With this
option, socat does not pass a prompt to readline, so it begins
line editing in the first column of the terminal.

noecho=<pattern>
Specifies a regular pattern for a prompt that prevents the
following input line from being displayed on the screen and
from being added to the history. The prompt is defined as the
text that was output to the readline address after the lastest
newline character and before an input character was typed. The
pattern is a regular expression, e.g. "^[Pp]assword:.*$" or
"([Uu]ser:|[Pp]assword:)". See regex(7) for details.
(example)

prompt=<string>
Passes the string as prompt to the readline function. readline
prints this prompt when stepping through the history. If this
string matches a constant prompt issued by an interactive
program on the other socat address, consistent look and feel
can be achieved.


APPLICATION option group

This group contains options that work at data level. Note that these
options only apply to the "raw" data transferred by socat, but not to
protocol data used by addresses like PROXY.

cr Converts the default line termination character NL ('\n',
0x0a) to/from CR ('\r', 0x0d) when writing/reading on this
channel.

crnl Converts the default line termination character NL ('\n',
0x0a) to/from CRNL ("\r\n", 0x0d0a) when writing/reading on
this channel (example). Note: socat simply strips all CR
characters.

ignoreeof
When EOF occurs on this channel, socat ignores it and tries to
read more data (like "tail -f") (example).

readbytes=<bytes>
socat reads only so many bytes from this address (the address
provides only so many bytes for transfer and pretends to be at
EOF afterwards). Must be greater than 0.

lockfile=<filename>
If lockfile exists, exits with error. If lockfile does not
exist, creates it and continues, unlinks lockfile on exit.

waitlock=<filename>
If lockfile exists, waits until it disappears. When lockfile
does not exist, creates it and continues, unlinks lockfile on
exit.

escape=<int>
Specifies the numeric code of a character that triggers EOF on
the input stream. It is useful with a terminal in raw mode
(example).


SOCKET option group

These options are intended for all kinds of sockets, e.g. IP or UNIX
domain. Most are applied with a setsockopt() call.

bind=<sockname>
Binds the socket to the given socket address using the bind()
system call. The form of <sockname> is socket domain
dependent: IP4 and IP6 allow the form
[hostname|hostaddress][:(service|port)] (example), VSOCK
allows the form [cid][:(port)].
See also: unix-bind-tempname

connect-timeout=<seconds>
Abort the connection attempt after <seconds> [timeval] with
error status.

so-bindtodevice=<interface>
Binds the socket to the given <interface>. This option might
require root privilege.

broadcast
For datagram sockets, allows sending to broadcast addresses
and receiving packets addressed to broadcast addresses.

debug Enables socket debugging.

dontroute
Only communicates with directly connected peers, does not use
routers.

keepalive
Enables sending keepalives on the socket.

linger=<seconds>
Blocks shutdown() or close() until data transfers have
finished or the given timeout [int] expired.

oobinline
Places out-of-band data in the input data stream.

priority=<priority>
Sets the protocol defined <priority> [<int>] for outgoing
packets.

rcvbuf=<bytes>
Sets the size of the receive buffer after the socket() call to
<bytes> [int]. With TCP sockets, this value corresponds to
the socket's maximal window size.

rcvbuf-late=<bytes>
Sets the size of the receive buffer when the socket is already
connected to <bytes> [int]. With TCP sockets, this value
corresponds to the socket's maximal window size.

so-rcvtimeo=<time>, rcvtimeo=<time>
Specifies the time [int] until recv() , read() etc. functions
timeout when no data is received. Note that in the transfer
phase socat only calls these functions when select() has
reported that data is available. However this option is useful
with DTLS addresses to timeout during connection negotiation.

so-sndtimeo=<time>, sndtimeo=<time>
Like so-recvtimeo, but for send . Not usecase known.

rcvlowat=<bytes>
Specifies the minimum number of received bytes [int] until the
socket layer will pass the buffered data to socat.

reuseaddr[=[0|1]]
Allows other sockets to bind to an address even if parts of it
(e.g. the local port) are already in use by socat.
With version 1.8.0, this socket option is set automatically
for TCP LISTEN addresses. If you prefer the system default (no
related setsockopt(...SO_REUSEADDR...) call at all), use form
reuseaddr=.
(example).

sndbuf=<bytes>
Sets the size of the send buffer after the socket() call to
<bytes> [int].

sndbuf-late=<bytes>
Sets the size of the send buffer when the socket is connected
to <bytes> [int].

sndlowat=<bytes>
Specifies the minimum number of bytes in the send buffer until
the socket layer will send the data to <bytes> [int].

pf=<string>
Forces the use of the specified IP version or protocol.
<string> can be something like "ip4" or "ip6". The resulting
value is used as first argument to the socket() or
socketpair() calls. This option affects address resolution
and the required syntax of bind and range options.

socktype=<type>
Sets the type of the socket, specified as second argument to
the socket() or socketpair() calls, to <type> [int]. Address
resolution is not affected by this option. Under Linux, 1
means stream oriented socket, 2 means datagram socket, 3 means
raw socket, and 5 seqpacket (stream keeping packet
boundaries). Datagrams are useful when you want to keep
packet boundaries.

protocol
Sets the protocol of the socket, specified as third argument
to the socket() or socketpair() calls, to <protocol> [int].
Address resolution is not affected by this option. 6 means
TCP, 17 means UDP.

reuseport
Set the SO_REUSEPORT socket option.

so-timestamp
Sets the SO_TIMESTAMP socket option. This enables receiving
and logging of timestamp ancillary messages.

setsockopt=<level>:<optname>:<optval>
Invokes setsockopt() for the socket with the given parameters.
level [int] is used as second argument to setsockopt() and
specifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or
SOL_SOCKET for the socket layer (1 on Linux). optname [int] is
the third argument to setsockopt() and tells which socket
option is to be set. For the actual numbers you might have to
look up the appropriate include files of your system. For the
4th and 5th setsockopt() parameters, value [dalan] specifies
an arbitrary sequence of bytes that are passed to the function
per pointer, with the automatically derived length parameter.

setsockopt-int=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a pointer to int [int]

setsockopt-listen=<level>:<optname>:<optval>
Like setsockopt, but for listen type addresses it is applied
to the listening socket instead of the connected socket.

setsockopt-string=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a string. This string is
passed to the function with trailing null character, and the
length parameter is automatically derived from the data.


UNIX option group

These options apply to UNIX domain based addresses.

bind-tempname[=/tmp/pre-XXXXXX],
unix-bind-tempname[=/tmp/pre-XXXXXX]" Binds to a random path
or random address (on abstract namespace sockets). This is
useful with datagram client addresses (SENDTO, or CLIENT) that
are opened in child processes forked off from a common parent
process where the child processes cannot have different bind
options. In the path X 's get replaced with a random
character sequence similar to tempnam(3). When no argument is
given socat takes a default like /tmp/fileXXXXXX .

unix-tightsocklen[=(0|1)]
On socket operations, pass a socket address length that does
not include the whole struct sockaddr_un record but (besides
other components) only the relevant part of the filename or
abstract string. Default is 1.


IP4 and IP6 option groups

These options can be used with IPv4 and IPv6 based sockets.

tos=<tos>
Sets the TOS (type of service) field of outgoing packets to
<tos> [byte] (see RFC 791).

ttl=<ttl>
Sets the TTL (time to live) field of outgoing packets to <ttl>
[byte].

ip-options=<data>
Sets IP options like source routing. Must be given in binary
form, recommended format is a leading "x" followed by an even
number of hex digits. This option may be used multiple times,
data are appended. E.g., to connect to host 10.0.0.1 via some
gateway using a loose source route, use the gateway as address
parameter and set a loose source route using the option
ip-options=x8307040a000001 .
IP options are defined in RFC 791.

mtudiscover=<0|1|2>
Takes 0, 1, 2 to never, want, or always use path MTU discover
on this socket.

ip-pktinfo
Sets the IP_PKTINFO socket option. This enables receiving and
logging of ancillary messages containing destination address
and interface (Linux) (example).

ip-recverr
Sets the IP_RECVERR socket option. This enables receiving and
logging of ancillary messages containing detailed error
information.

ip-recvopts
Sets the IP_RECVOPTS socket option. This enables receiving and
logging of IP options ancillary messages (Linux, *BSD).

ip-recvtos
Sets the IP_RECVTOS socket option. This enables receiving and
logging of TOS (type of service) ancillary messages (Linux).

ip-recvttl
Sets the IP_RECVTTL socket option. This enables receiving and
logging of TTL (time to live) ancillary messages (Linux,
*BSD).

ip-recvdstaddr
Sets the IP_RECVDSTADDR socket option. This enables receiving
and logging of ancillary messages containing destination
address (*BSD) (example).

ip-recvif
Sets the IP_RECVIF socket option. This enables receiving and
logging of interface ancillary messages (*BSD) (example).

ip-add-membership=<multicast-address:interface-address>

ip-add-membership=<multicast-address:interface-name>

ip-add-membership=<multicast-address:interface-index>

ip-add-membership=<multicast-address:interface-address:interface-name>

ip-add-membership=<multicast-address:interface-address:interface-index>
Makes the socket member of the specified multicast group. This
only works for IPv4, see ipv6-join-group for the IPv6 variant.
The option takes the IP address of the multicast group and
info about the desired network interface. The most common
syntax is the first one, while the others are only available
on systems that provide struct mreqn (Linux).
The indices of active network interfaces can be shown using
the utility procan.

ip-add-source-membership=<multicast-address:interface-address:source-address>
Makes the socket member of the specified multicast group for
the specified source, i.e. only multicast traffic from this
address is to be delivered. This only works for IPv4, see
ipv6-join-source-group for the IPv6 variant. The option takes
the IP address of the multicast group, the IP address of the
desired network interface and the source IP address of the
multicast traffic.

ipv6-join-group=<multicast-address:interface-name>

ipv6-join-group=<multicast-address:interface-index>
Makes the socket member of the specified multicast group. This
only works for IPv6, see ip-add-membership for the IPv4
variant. The option takes the IP address of the multicast
group and info about the desired network interface. The
indices of active network interfaces can be shown using the
utility procan.

ipv6-join-source-group=<multicast-address:interface-name:source-address>

ipv6-join-source-group=<multicast-address:interface-index:source-address>
Makes the socket member of the specified multicast group for
the specified source, i.e. only multicast traffic from this
address is to be delivered. This only works for IPv6, see
ip-add-source-membership for the IPv4 variant. The option
takes the IP address of the multicast group, info about the
desired network interface and the source IP address of the
multicast traffic. The indices of active network interfaces
can be shown using the utility procan.

ip-multicast-if=<hostname>
Specifies hostname or address of the network interface to be
used for multicast traffic.

ip-multicast-loop[=<bool>]
Specifies if outgoing multicast traffic should loop back to
the interface.

ip-multicast-ttl=<byte>
Sets the TTL used for outgoing multicast traffic. Default is
1.

ip-transparent
Sets the IP_TRANSPARENT socket option. This option might
require root privilege.


Resolver options

These options temporarily change the behaviour of hostname
resolution. The options of form ai-* affect behaviour of the
getaddrinfo() function that includes /etc/hosts and NIS based
lookups.

The addresses of form res-* only affect DNS lookups, and only when
the result is not cached in nscd . These options might not work on
all operating systems or libc implementations.

ai-addrconfig[=0|1]

addrconfig[=0|1]
Sets or unsets the AI_ADDRCONFIG flag to prevent name
resolution to address families that are not available on the
computer (e.g. IPv6). Default value is 1 in case the resolver
does not get an address family hint from Socat address or
defaults.

ai-passive[=0|1]

passive[=0|1]
Sets of unsets the AI_PASSIVE flag for getaddrinfo() calls.
Default is 1 for LISTEN, RECV, and RECVFROM type addresses,
and with bind option.

ai-v4mapped[=0|1]

v4mapped[=0|1]
Sets or unsets the AI_V4MAPPED flag for getaddrinfo() . With
socat addresses requiring IPv6 addresses, this resolves IPv4
addresses to the appropriate IPv6 address [::ffff:*:*]. For
IPv6 socat addresses, the default is 1.

ai-all[=0|1]
Sets or unsets the AI_ALL flag for getaddrinfo() .

res-debug

res-aaonly

res-usevc

res-primary

res-igntc

res-recurse

res-defnames

res-stayopen

res-dnsrch
These options set the corresponding resolver (name resolution)
option flags. Append "=0" to clear a default option. See man
resolver(5) for more information on these options. Socat
restores the old values after finishing the open phase of the
address, so these options are valid just for the address they
are applied to.
Please note that these flags only affect DNS resolution, but
not hosts or NIS based name resolution, and they have no
effect when (g)libc retrieves the results from nscd .

res-retrans=<int>
Sets the retransmission time interval of the DNS resolver
(based on an undocumented feature).

res-retry=<int>
Sets the number of retransmits of the DNS resolver (based on
an undocumented feature).

res-nsaddr=<ipaddr>[:<port>]
Tries to overwrite nameserver settings loaded from
/etc/resolv.conf by writing the given IPv4 address into the
undocumented _res:nsaddr_list[0] field. /etc/hosts is still
checked by resolver. Please note that glibc's nscd is always
queried first when it is running!


IP6 option group

These options can only be used on IPv6 based sockets. See IP options
for options that can be applied to both IPv4 and IPv6 sockets.

ipv6only[=<bool>]
Sets the IPV6_V6ONLY socket option. If 0, the TCP stack will
also accept connections using IPv4 protocol on the same port.
The default is system dependent.

ipv6-recvdstopts
Sets the IPV6_RECVDSTOPTS socket option. This enables
receiving and logging of ancillary messages containing the
destination options.

ipv6-recvhoplimit
Sets the IPV6_RECVHOPLIMIT socket option. This enables
receiving and logging of ancillary messages containing the
hoplimit.

ipv6-recvhopopts
Sets the IPV6_RECVHOPOPTS socket option. This enables
receiving and logging of ancillary messages containing the hop
options.

ipv6-recvpktinfo
Sets the IPV6_RECVPKTINFO socket option. This enables
receiving and logging of ancillary messages containing
destination address and interface.

ipv6-unicast-hops=link(TYPE_INT)(<int>)
Sets the IPV6_UNICAST_HOPS socket option. This sets the hop
count limit (TTL) for outgoing unicast packets.

ipv6-recvrthdr
Sets the IPV6_RECVRTHDR socket option. This enables receiving
and logging of ancillary messages containing routing
information.

ipv6-tclass
Sets the IPV6_TCLASS socket option. This sets the transfer
class of outgoing packets.

ipv6-recvtclass
Sets the IPV6_RECVTCLASS socket option. This enables receiving
and logging of ancillary messages containing the transfer
class.


TCP option group

These options may be applied to TCP sockets. They work by invoking
setsockopt() with the appropriate parameters.

cork Doesn't send packets smaller than MSS (maximal segment size).

defer-accept
While listening, accepts connections only when data from the
peer arrived.

keepcnt=<count>
Sets the number of keepalives before shutting down the socket
to <count> [int].

keepidle=<seconds>
Sets the idle time before sending the first keepalive to
<seconds> [int].

keepintvl=<seconds>
Sets the interval between two keepalives to <seconds> [int].

linger2=<seconds>
Sets the time to keep the socket in FIN-WAIT-2 state to
<seconds> [int].

mss=<bytes>
Sets the MSS (maximum segment size) after the socket() call to
<bytes> [int]. This value is then proposed to the peer with
the SYN or SYN/ACK packet (example).

mss-late=<bytes>
Sets the MSS of the socket after connection has been
established to <bytes> [int].

nodelay
Turns off the Nagle algorithm for measuring the RTT (round
trip time).

rfc1323
Enables RFC1323 TCP options: TCP window scale, round-trip time
measurement (RTTM), and protect against wrapped sequence
numbers (PAWS) (AIX).

stdurg Enables RFC1122 compliant urgent pointer handling (AIX).

syncnt=<count>
Sets the maximal number of SYN retransmits during connect to
<count> [int].

md5sig Enables generation of MD5 digests on the packets (FreeBSD).

noopt Disables use of TCP options (FreeBSD, MacOSX).

nopush sets the TCP_NOPUSH socket option (FreeBSD, MacOSX).

sack-disable
Disables use the selective acknowledge feature (OpenBSD).

signature-enable
Enables generation of MD5 digests on the packets (OpenBSD).

abort-threshold=<milliseconds>
Sets the time to wait for an answer of the peer on an
established connection (HP-UX).

conn-abort-threshold=<milliseconds>
Sets the time to wait for an answer of the server during the
initial connect (HP-UX).

keepinit
Sets the time to wait for an answer of the server during
connect() before giving up. Value in half seconds, default is
150 (75s) (Tru64).

paws Enables the "protect against wrapped sequence numbers" feature
(Tru64).

sackena
Enables selective acknowledge (Tru64).

tsoptena
Enables the time stamp option that allows RTT recalculation on
existing connections (Tru64).


UDP option group

This option may be applied to UDP datagram sockets.

udp-ignore-peerport>
Address UDP-DATAGRAM expects incoming responses to come from
the port specified in its second parameter. With this option,
it accepts packets coming from any port.


UDPLITE option group

These options may be applied to UDPLITE addresses:

udplite-send-cscov
Sets the number of bytes for which the checksum is calculated
and sent ("checksum coverage").

udplite-recv-cscov
Sets the number of bytes for which the checksum is checked
("checksum coverage").


SCTP option group

These options may be applied to SCTP stream sockets.

sctp-nodelay
Sets the SCTP_NODELAY socket option that disables the Nagle
algorithm.

sctp-maxseg=<bytes>
Sets the SCTP_MAXSEG socket option to <bytes> [int]. This
value is then proposed to the peer with the SYN or SYN/ACK
packet.


DCCP option group

These options may be applied to DCCP sockets.

dccp-set-ccid=<int>

ccid=<int>
Selects the desired congestion control mechanism (CCID).


UDP, TCP, SCTP, DCCP, and UDPLITE option group

Here we find options that are related to the network port mechanism
and thus can be used with UDP, TCP, SCTP, DCCP, and UDP-Lite client
and server addresses.

sourceport=<port>
For outgoing (client) connections, it sets the source <port>
using an extra bind() call. With TCP or UDP listen addresses,
socat immediately shuts down the connection if the client does
not use this sourceport. UDP-RECV, UDP-RECVFROM, UDP-SENDTO,
and UDP-DATAGRAM addresses ignore the packet when it does not
match. (example).

lowport
Outgoing (client) connections with this option use an unused
random source port between 640 and 1023 incl. On UNIX class
operating systems, this requires root privilege, and thus
indicates that the client process is authorized by local root.
TCP and UDP listen addresses with this option immediately shut
down the connection if the client does not use a sourceport <=
1023. This mechanism can provide limited authorization under
some circumstances.


SOCKS option group

When using SOCKS type addresses, some socks specific options can be
set.

socksport=<tcp service>
Overrides the default "socks" service or port 1080 for the
socks server port with <TCP service>.

socksuser=<user>
Sends the <user> [string] in the username field to the socks
server. Default is the actual user name ($LOGNAME or $USER)
(example).


HTTP option group

Options that can be provided with HTTP type addresses. The only HTTP
address currently implemented is proxy-connect.

http-version=<string>
Changes the default "1.0" that is sent to the server in the
initial HTTP request. Currently it has not other effect, in
particular it does not provide any means to send a Host
header.

proxyport=<TCP service>
Overrides the default HTTP proxy port 8080 with <TCP service>.

ignorecr
The HTTP protocol requires the use of CR+NL as line
terminator. When a proxy server violates this standard, socat
might not understand its answer. This option directs socat to
interpret NL as line terminator and to ignore CR in the
answer. Nevertheless, socat sends CR+NL to the proxy.

proxy-authorization=<username>:<password>
Provide "basic" authentication to the proxy server. The
argument to the option is used with a "Proxy-Authorization:
Basic" header in base64 encoded form.
Note: username and password are visible for every user on the
local machine in the process list; username and password are
transferred to the proxy server unencrypted (base64 encoded)
and might be sniffed.

proxy-authorization-file=<filename>
Like option proxy-authorization, but the credentials are read
from the file and therefore not visible in the process list.

resolve
Per default, socat sends to the proxy a CONNECT request
containing the target hostname. With this option, socat
resolves the hostname locally and sends the IP address. Please
note that, according to RFC 2396, only name resolution to IPv4
addresses is implemented.


RANGE option group

These options check if a connecting client should be granted access.
They can be applied to listening and receiving network sockets.
tcp-wrappers options fall into this group.

range=<address-range>
After accepting a connection, tests if the peer is within
range. For IPv4 addresses, address-range takes the form
address/bits, e.g. 10.0.0.0/8, or address:mask, e.g.
10.0.0.0:255.0.0.0 (example); for IPv6, it is
[ip6-address]/bits, e.g. [::1]/128. If the client address
does not match, socat refuses the connection attempt, issues a
warning, and keeps listening/receiving.

tcpwrap[=<name>]
Uses Wietse Venema's libwrap (tcpd) library to determine if
the client is allowed to connect. The configuration files are
/etc/hosts.allow and /etc/hosts.deny per default, see "man 5
hosts_access" for more information. The optional <name> (type
string) is passed to the wrapper functions as daemon process
name (example). If omitted, the basename of socats invocation
(argv[0]) is passed. If both tcpwrap and range options are
applied to an address, both conditions must be fulfilled to
allow the connection.

allow-table=<filename>
Takes the specified file instead of /etc/hosts.allow.

deny-table=<filename>
Takes the specified file instead of /etc/hosts.deny.

tcpwrap-etc=<directoryname>
Looks for hosts.allow and hosts.deny in the specified
directory. Is overridden by options hosts-allow and
hosts-deny.


LISTEN option group

Options specific to listening sockets.

backlog=<count>
Sets the backlog value passed with the listen() system call to
<count> [int]. Default is 5.

accept-timeout=<seconds>
End waiting for a connection after <seconds> [timeval] with
error status.


CHILD option group

Addresses of LISTEN and CONNECT type take the fork option to handle
multiple connections via child processes.

fork After establishing a connection, handles its channel in a
child process and keeps the parent process attempting to
produce more connections, either by listening or by connecting
in a loop (example).
OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they
actually fork off the child: OPENSSL-LISTEN forks before the
SSL handshake, while OPENSSL-CONNECT forks afterwards. retry
and forever options are not inherited by the child process.
On some operating systems (e.g. FreeBSD) this option does not
work for UDP-LISTEN addresses.

max-children=<count>
Limits the number of concurrent child processes [int].
Default is no limit.

children-shutup[=1|2|..]
Decreases the severity of log messages produced by child
processes. For example, with value 1 notices are logged as
info (or dropped depending on option -dX), and errors are
logged as warnings but still cause termination of the child
process.
This option is intended to reduce logging of high volume
servers or proxies.
This option succeeds option cool-write.


EXEC option group

Options for addresses that invoke a program.

path=<string>
Overrides the PATH environment variable for searching the
program with <string>. This $PATH value is effective in the
child process too.

login Prefixes argv[0] for the execvp() call with '-', thus making a
shell behave as login shell.


FORK option group

EXEC or SYSTEM addresses invoke a program using a child process and
transfer data between socat and the program. The interprocess
communication mechanism can be influenced with the following options.
Per default, a socketpair() is created and assigned to stdin and
stdout of the child process, while stderr is inherited from the socat
process, and the child process uses file descriptors 0 and 1 for
communicating with the main socat process.

nofork Does not fork a subprocess for executing the program, instead
calls execvp() or system() directly from the actual socat
instance. This avoids the overhead of another process between
the program and its peer, but introduces a lot of
restrictions:

o this option can only be applied to the second socat address.

o it cannot be applied to a part of a dual address.

o the first socat address cannot be OPENSSL or READLINE

o socat options -b, -t, -D, -l, -v, -x become useless

o for both addresses, options ignoreeof, cr, and crnl become
useless

o for the second address (the one with option nofork), options
append, cloexec, flock, user, group, mode, nonblock,
perm-late, setlk, and setpgid cannot be applied. Some of these
could be used on the first address though.

pipes Creates a pair of unnamed pipes for interprocess communication
instead of a socket pair.

openpty
Establishes communication with the sub process using a pseudo
terminal created with openpty() instead of the default
(socketpair or ptmx).

ptmx Establishes communication with the sub process using a pseudo
terminal created by opening /dev/ptmx or /dev/ptc instead of
the default (socketpair).

pty Establishes communication with the sub process using a pseudo
terminal instead of a socket pair. Creates the pty with an
available mechanism. If openpty and ptmx are both available,
it uses ptmx because this is POSIX compliant (example).

ctty Makes the pty the controlling tty of the sub process
(example).

stderr Directs stderr of the sub process to its output channel by
making stderr a dup() of stdout (example).

fdin=<fdnum>
Assigns the sub processes input channel to its file descriptor
<fdnum> instead of stdin (0). The program started from the
subprocess has to use this fd for reading data from socat
(example).

fdout=<fdnum>
Assigns the sub processes output channel to its file
descriptor <fdnum> instead of stdout (1). The program started
from the subprocess has to use this fd for writing data to
socat (example).

sighup, sigint, sigquit
Has socat pass signals of this type to the sub process. If no
address has this option, socat terminates on these signals.


Options for address SHELL

shell=<filename>
Overwrites use the default shell with the named executable,
e.g. /bin/dash. Also sets the SHELL environment variable.


TERMIOS option group

For addresses that work on a tty (e.g., stdio, file:/dev/tty,
exec:...,pty), the terminal parameters defined in the UN*X termios
mechanism are made available as address option parameters. Please
note that changes of the parameters of your interactive terminal
remain effective after socat's termination, so you might have to
enter "reset" or "stty sane" in your shell afterwards. For EXEC and
SYSTEM addresses with option PTY, these options apply to the pty by
the child processes.

b0 Disconnects the terminal.

b19200 Sets the serial line speed to 19200 baud. Some other rates are
possible; use something like socat -hh |grep ' b[1-9]' to find
all speeds supported by your implementation.
Note: On some operating systems, these options may not be
available. Use ispeed or ospeed instead.

echo[=<bool>]
Enables or disables local echo.

icanon[=<bool>]
Sets or clears canonical mode, enabling line buffering and
some special characters.

raw Sets raw mode, thus passing input and output almost
unprocessed. This option is obsolete, use option rawer or
cfmakeraw instead.

rawer Makes terminal rawer than raw option. This option implicitly
turns off echo. (example).

cfmakeraw
Sets raw mode by invoking cfmakeraw() or by simulating this
call. This option implicitly turns off echo.

ignbrk[=<bool>]
Ignores or interprets the BREAK character (e.g., ^C)

brkint[=<bool>]

bs0

bs1

bsdly=<0|1>

clocal[=<bool>]


cr0
cr1
cr2
cr3

Sets the carriage return delay to 0, 1, 2, or 3, respectively.
0 means no delay, the other values are terminal dependent.

crdly=<0|1|2|3>

cread[=<bool>]

crtscts[=<bool>]


cs5
cs6
cs7
cs8

Sets the character size to 5, 6, 7, or 8 bits, respectively.

csize=<0|1|2|3>

cstopb[=<bool>]
Sets two stop bits, rather than one.

dsusp=<byte>
Sets the value for the VDSUSP character that suspends the
current foreground process and reactivates the shell (all
except Linux).

echoctl[=<bool>]
Echos control characters in hat notation (e.g. ^A)

echoe[=<bool>]

echok[=<bool>]

echoke[=<bool>]

echonl[=<bool>]

echoprt[=<bool>]

eof=<byte>

eol=<byte>

eol2=<byte>

erase=<byte>

discard=<byte>

ff0

ff1

ffdly[=<bool>]

flusho[=<bool>]

hupcl[=<bool>]

icrnl[=<bool>]

iexten[=<bool>]

igncr[=<bool>]

ignpar[=<bool>]

imaxbel[=<bool>]

inlcr[=<bool>]

inpck[=<bool>]

intr=<byte>

isig[=<bool>]

ispeed=<unsigned-int>
Set the baud rate for incoming data on this line.
See also: ospeed, b19200

istrip[=<bool>]

iuclc[=<bool>]

ixany[=<bool>]

ixoff[=<bool>]

ixon[=<bool>]

kill=<byte>

lnext=<byte>

min=<byte>

nl0 Sets the newline delay to 0.

nl1

nldly[=<bool>]

noflsh[=<bool>]

ocrnl[=<bool>]

ofdel[=<bool>]

ofill[=<bool>]

olcuc[=<bool>]

onlcr[=<bool>]

onlret[=<bool>]

onocr[=<bool>]

opost[=<bool>]
Enables or disables output processing; e.g., converts NL to
CR-NL.

ospeed=<unsigned-int>
Set the baud rate for outgoing data on this line.
See also: ispeed, b19200

parenb[=<bool>]
Enable parity generation on output and parity checking for
input.

parmrk[=<bool>]

parodd[=<bool>]

pendin[=<bool>]

quit=<byte>

reprint=<byte>

sane Brings the terminal to something like a useful default state.

start=<byte>

stop=<byte>

susp=<byte>

swtc=<byte>

tab0

tab1

tab2

tab3

tabdly=<unsigned-int>

time=<byte>

tostop[=<bool>]

vt0

vt1

vtdly[=<bool>]

werase=<byte>

xcase[=<bool>]

xtabs

i-pop-all
With UNIX System V STREAMS, removes all drivers from the
stack.

i-push=<string>
With UNIX System V STREAMS, pushes the driver (module) with
the given name (string) onto the stack. For example, to make
sure that a character device on Solaris supports termios etc,
use the following options:
i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat


PTY option group

These options are intended for use with the pty address type.

link=<filename>
Generates a symbolic link that points to the actual pseudo
terminal (pty). This might help to solve the problem that ptys
are generated with more or less unpredictable names, making it
difficult to directly access the socat generated pty
automatically. With this option, the user can specify a "fix"
point in the file hierarchy that helps him to access the
actual pty (example). Beginning with socat version 1.4.3, the
symbolic link is removed when the address is closed (but see
option unlink-close).

wait-slave
Blocks the open phase until a process opens the slave side of
the pty. Usually, socat continues after generating the pty
with opening the next address or with entering the transfer
loop. With the wait-slave option, socat waits until some
process opens the slave side of the pty before continuing.
This option only works if the operating system provides the
poll() system call. And it depends on an undocumented
behaviour of pty's, so it does not work on all operating
systems. It has successfully been tested on Linux, FreeBSD,
NetBSD, and on Tru64 with openpty.

pty-interval=<seconds>
When the wait-slave option is set, socat periodically checks
the HUP condition using poll() to find if the pty's slave side
has been opened. The default polling interval is 1s. Use the
pty-interval option [timeval] to change this value.

sitout-eio=<timeval>
The login program in Linux closes its tty/pty and reopens it
for security reasons. During this time the pty master would
get EIO on I/O operations and might terminate. With this
option socat tolerates EIO for the specified time. Please note
that in this state socat blocks traffic in both directions,
even when it is not related to this channel.


OPENSSL option group

These options apply to the openssl and openssl-listen address types.

cipher=<cipherlist>
Specifies the list of ciphers that may be used for the
connection. See the man page of ciphers , section CIPHER LIST
FORMAT, for detailed information about syntax, values, and
default of <cipherlist>.
Several cipher strings may be given, separated by ':'. Some
simple cipher strings:

3DES Uses a cipher suite with triple DES.

MD5 Uses a cipher suite with MD5.

aNULL Uses a cipher suite without authentication.

NULL Does not use encryption.

HIGH Uses a cipher suite with "high" encryption. Note that the
peer must support the selected property, or the negotiation
will fail.

method=<ssl-method>
This option is based on deprecated functions and is only
available when socat was build with option
--with-openssl-method. Use option min-proto-version and maybe
max-proto-version instead. Sets the protocol version to be
used. Valid strings (not case sensitive) are:

SSL2 Select SSL protocol version 2.

SSL3 Select SSL protocol version 3.

SSL23 Select the best available SSL or TLS protocol.

TLS1 Select TLS protocol version 1.

TLS1.1 Select TLS protocol version 1.1.

TLS1.2 Select TLS protocol version 1.2. When this option is not
provided OpenSSL negotiates the method with its peer.

min-proto-version
This option tells OpenSSL to use this or a later SSL/TLS
protocol version and refuses to accept a lower/older protocol.
Valid syntax is:

SSL2 Select SSL protocol version 2.

SSL3 Select SSL protocol version 3.

TLS1

TLS1.0 Select TLS protocol version 1.

TLS1.1 Select TLS protocol version 1.1.

TLS1.2 Select TLS protocol version 1.2.

TLS1.3 Select TLS protocol version 1.3.

openssl-max-proto-version
This option is similar to min-proto-version, however, it
disallows use of a higher protocol version. Useful for testing
the peer.

verify[=<bool>]
Controls check of the peer's certificate. Default is 1 (true).
Disabling verify might open your socket for everyone, making
the encryption useless!

cert=<filename>
Specifies the file with the certificate and private key for
authentication. The certificate must be in OpenSSL format
(*.pem). With openssl-listen, use of this option is strongly
recommended. Except with cipher aNULL, "no shared ciphers"
error will occur when no certificate is given.

key=<filename>
Specifies the file with the private key. The private key may
be in this file or in the file given with the cert option. The
party that has to proof that it is the owner of a certificate
needs the private key.

dhparams=<filename>
Specifies the file with the Diffie Hellman parameters. These
parameters may also be in the file given with the cert option
in which case the dhparams option is not needed.

cafile=<filename>
Specifies the file with the trusted (root) authority
certificates. The file must be in PEM format and should
contain one or more certificates. The party that checks the
authentication of its peer trusts only certificates that are
in this file.

capath=<dirname>
Specifies the directory with the trusted (root) certificates.
The directory must contain certificates in PEM format and
their hashes (see OpenSSL documentation)

egd=<filename>
On some systems, openssl requires an explicit source of random
data. Specify the socket name where an entropy gathering
daemon like egd provides random data, e.g. /dev/egd-pool.

openssl-maxfraglen=<int>, maxfraglen=<int>
For client connections, make a Max Fragment Length Negotiation
Request to the server to limit the maximum size fragment the
server will send to us. Supported lengths are: 512, 1024,
2048, or 4096. Note that this option is not applicable for
OPENSSL-LISTEN.

openssl-maxsendfrag=<int>, maxsendfrag=<int>
Limit the maximum size of the fragment we will send to the
other side. Supported length range: 512 - 16384. Note that
under OPENSSL-LISTEN, the maximum fragment size may be further
limited by the client's Maximum Fragment Length Negotiation
Request, if it makes one.

pseudo On systems where openssl cannot find an entropy source and
where no entropy gathering daemon can be utilized, this option
activates a mechanism for providing pseudo entropy. This is
achieved by taking the current time in microseconds for
feeding the libc pseudo random number generator with an
initial value. openssl is then feeded with output from
random() calls.
NOTE:This mechanism is not sufficient for generation of secure
keys!

compress
Enable or disable the use of compression for a connection.
Setting this to "none" disables compression, setting it to
"auto" lets OpenSSL choose the best available algorithm
supported by both parties. The default is to not touch any
compression-related settings. NOTE: Requires OpenSSL 0.9.8 or
higher and disabling compression with OpenSSL 0.9.8 affects
all new connections in the process.

commonname=<string>
Specify the commonname that the peer certificate must match.
With OPENSSL-CONNECT address this overrides the given hostname
or IP target address; with OPENSSL-LISTEN this turns on check
of peer certificates commonname. This option has only meaning
when option verify is not disabled and the chosen cipher
provides a peer certificate.

no-sni[=<bool>]
Do not use the client side Server Name Indication (SNI)
feature that selects the desired server certificate.
Note: SNI is automatically used since socat version 1.7.4.0
and uses commonname or the given host name.

snihost=<string>
Set the client side Server Name Indication (SNI) host name
different from the addressed server name or common name. This
might be useful when the server certificate has multiple host
names or wildcard names because the SNI host name is passed in
cleartext to the server and might be eavesdropped; with this
option a mock name of the desired certificate may be
transferred.

fips Enables FIPS mode if compiled in. For info about the FIPS
encryption implementation standard see
http://oss-institute.org/fips-faq.html. This mode might
require that the involved certificates are generated with a
FIPS enabled version of openssl. Setting or clearing this
option on one socat address affects all OpenSSL addresses of
this process.


RETRY option group

Options that control retry of some system calls, especially
connection attempts.

retry=<num>
Number of retries before the connection or listen attempt is
aborted. Default is 0, which means just one attempt.

interval=<timespec>
Time between consecutive attempts (seconds, [timespec]).
Default is 1 second.

forever
Performs an unlimited number of retry attempts.


INTERFACE option group

These options may be applied to addresses INTERFACE and TUN. These
address types and options are currently only implemented on Linux
operating system.

Note regarding VLANs: On incoming packets the Linux kernel strips off
the VLAN tag before passing the data to the user space program on raw
sockets. Special measures are required to get the VLAN information,
see packet(7) PACKET_AUXDATA, and to optionally insert the tag into
the packet again, use option retrieve-vlan when you need this.

retrieve-vlan
On packets incoming on raw sockets, retrieve the VLAN
information and insert it into the packets for further
processing (Linux)

iff-up Sets the TUN network interface status UP. Strongly
recommended.

iff-broadcast
Sets the BROADCAST flag of the TUN network interface.

iff-debug
Sets the DEBUG flag of the TUN network interface.

iff-loopback
Sets the LOOPBACK flag of the TUN network interface.

iff-pointopoint
Sets the POINTOPOINT flag of the TUN device.

iff-notrailers
Sets the NOTRAILERS flag of the TUN device.

iff-running
Sets the RUNNING flag of the TUN device.

iff-noarp
Sets the NOARP flag of the TUN device.

iff-promisc
Sets the PROMISC flag of the TUN device.

iff-allmulti
Sets the ALLMULTI flag of the TUN device.

iff-master
Sets the MASTER flag of the TUN device.

iff-slave
Sets the SLAVE flag of the TUN device.

iff-multicast
Sets the MULTICAST flag of the TUN device.

iff-portsel
Sets the PORTSEL flag of the TUN device.

iff-automedia
Sets the AUTOMEDIA flag of the TUN device.

iff-dynamic
Sets the DYNAMIC flag of the TUN device.


TUN option group

Options that control Linux TUN/TAP interface device addresses.

tun-device=<device-file>
Instructs socat to take another path for the TUN clone device.
Default is /dev/net/tun.

tun-name=<if-name>
Gives the resulting network interface a specific name instead
of the system generated (tun0, tun1, etc.)

tun-type=[tun|tap]
Sets the type of the TUN device; use this option to generate a
TAP device. See the Linux docu for the difference between
these types. When you try to establish a tunnel between two
TUN devices, their types should be the same.

iff-no-pi
Sets the IFF_NO_PI flag which controls if the device includes
additional packet information in the tunnel. When you try to
establish a tunnel between two TUN devices, these flags should
have the same values.


POSIX-MQ option group

Options that may be applied to POSIX-MQ addresses.

posixmq-priority (mq-prio)
Sets the priority of messages (packets) written to the queue,
or the minimal priority of packet read from the queue.

DATA VALUES

This section explains the different data types that address
parameters and address options can take.

address-range
Is currently only implemented for IPv4 and IPv6. See
address-option `range'

bool "0" or "1"; if value is omitted, "1" is taken.

byte An unsigned int number, read with strtoul() , lower or equal
to UCHAR_MAX .

command-line
A string specifying a program name and its arguments,
separated by single spaces.

data This is a more general data specification. The given text
string contains information about the target data type and
value. Generally a leading character specifies the type of the
following data item. In its specific context a default data
type may exist.
Currently only the following specifications are implemented:

i A signed integer number, stored in host byte order.
Example: i-1000 (Integer number -1000)

I An unsigned integer number, stored in host byte order.

l A signed long integer number, stored in host byte order.

L An unsigned long integer number, stored in host byte order.

s A signed short integer number, stored in host byte order.

S An unsigned short integer number, stored in host byte order.

b A signed byte (signed char).

B An unsigned byte (unsigned char).

x Following is an even number of hex digits, stored as sequence
of bytes.
Example: x7f000001 (IP address 127.0.0.1)

" Following is a string that is used with the common conversions
\n \r \t \f \b \a \e \0; the string must be closed with '"'.
Please note that the quotes and backslashes need to be escaped
from shell and socat conversion.
Example: "Hello world!\n"

' A single char, with the usual conversions. Please note that
the quotes and backslashes need to be escaped from shell and
socat conversion.
Example: 'a' Data items may be separated with white space
without need to repeat the type specifier again.

directory
A string with usual UN*X directory name semantics.

facility
The name of a syslog facility in lower case characters.

fdnum An unsigned int type, read with strtoul() , specifying a UN*X
file descriptor.

filename
A string with usual UN*X filename semantics.

group If the first character is a decimal digit, the value is read
with strtoul() as unsigned integer specifying a group id.
Otherwise, it must be an existing group name.

int A number following the rules of the strtol() function with
base "0", i.e. decimal number, octal number with leading "0",
or hexadecimal number with leading "0x". The value must fit
into a C int.

interface
A string specifying the device name of a network interface as
shown by ifconfig or procan, e.g. "eth0".

IP address
An IPv4 address in numbers-and-dots notation, an IPv6 address
in hex notation enclosed in brackets, or a hostname that
resolves to an IPv4 or an IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1

IPv4 address
An IPv4 address in numbers-and-dots notation or a hostname
that resolves to an IPv4 address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2

IPv6 address
An IPv6 address in hexnumbers-and-colons notation enclosed in
brackets, or a hostname that resolves to an IPv6 address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0],
ip6name.domain.org

long A number read with strtol() . The value must fit into a C
long.

long long
A number read with strtoll() . The value must fit into a C
long long.

off_t An implementation dependend signed number, usually 32 bits,
read with strtol or strtoll.

off64_t
An implementation dependend signed number, usually 64 bits,
read with strtol or strtoll.

mode_t An unsigned integer, read with strtoul() , specifying mode
(permission) bits.

pid_t A number, read with strtol() , specifying a process id.

port A uint16_t (16 bit unsigned number) specifying a TCP or UDP
port, read with strtoul() .

protocol
An unsigned 8 bit number, read with strtoul() .

size_t An unsigned number with size_t limitations, read with strtoul
.

sockname
A socket address. See address-option `bind'

string A sequence of characters, not containing '\0' and, depending
on the position within the command line, ':', ',', or "!!".
Note that you might have to escape shell meta characters in
the command line.

TCP service
A service name, not starting with a digit, that is resolved by
getservbyname() , or an unsigned int 16 bit number read with
strtoul() .

timeval
A double float specifying seconds; the number is mapped into a
struct timeval, consisting of seconds and microseconds.

timespec
A double float specifying seconds; the number is mapped into a
struct timespec, consisting of seconds and nanoseconds.

UDP service
A service name, not starting with a digit, that is resolved by
getservbyname() , or an unsigned int 16 bit number read with
strtoul() .

unsigned int
A number read with strtoul() . The value must fit into a C
unsigned int.

user If the first character is a decimal digit, the value is read
with strtoul() as unsigned integer specifying a user id.
Otherwise, it must be an existing user name.

VSOCK cid
A uint32_t (32 bit unsigned number) specifying a VSOCK Context
Identifier (CID), read with strtoul() . There are several
special addresses: VMADDR_CID_ANY (-1U) means any address for
binding; VMADDR_CID_HOST (2) is the well-known address of the
host.

VSOCK port
A uint32_t (32 bit unsigned number) specifying a VSOCK port,
read with strtoul() .

EXAMPLES

socat - TCP4:www.domain.org:80

transfers data between STDIO (-) and a TCP4 connection to port
80 of host www.domain.org. This example results in an
interactive connection similar to telnet or netcat. The stdin
terminal parameters are not changed, so you may close the
relay with ^D or abort it with ^C.

socat -d -d \
READLINE,history=$HOME/.http_history \
TCP4:www.domain.org:www,crnl

this is similar to the previous example, but you can edit the
current line in a bash like manner (READLINE) and use the
history file .http_history; socat prints messages about
progress (-d -d). The port is specified by service name
(www), and correct network line termination characters (crnl)
instead of NL are used.

socat \
TCP4-LISTEN:www \
TCP4:www.domain.org:www

installs a simple TCP port forwarder. With TCP4-LISTEN it
listens on local port "www" until a connection comes in,
accepts it, then connects to the remote host (TCP4) and starts
data transfer. It will not accept a second connection.

socat -d -d -lmlocal2 \
TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \
TCP4:www.domain.org:80,bind=myaddr2

TCP port forwarder, each side bound to another local IP
address (bind). This example handles an almost arbitrary
number of parallel or consecutive connections by fork'ing a
new process after each accept() . It provides a little
security by su'ing to user nobody after forking; it only
permits connections from the private 10 network (range); due
to reuseaddr, it allows immediate restart after master
process's termination, even if some child sockets are not
completely shut down. With -lmlocal2, socat logs to stderr
until successfully reaching the accept loop. Further logging
is directed to syslog with facility local2.

socat \
TCP4-LISTEN:5555,fork,tcpwrap=script \
EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr

a simple server that accepts connections (TCP4-LISTEN) and
fork's a new child process for each connection; every child
acts as single relay. The client must match the rules for
daemon process name "script" in /etc/hosts.allow and
/etc/hosts.deny, otherwise it is refused access (see "man 5
hosts_access"). For EXEC'uting the program, the child process
chroot's to /home/sandbox, su's to user sandbox, and then
starts the program /home/sandbox/bin/myscript. Socat and
myscript communicate via a pseudo tty (pty); myscript's stderr
is redirected to stdout, so its error messages are transferred
via socat to the connected client.

socat \
EXEC:"mail.sh target@domain.com",fdin=3,fdout=4 \
TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512

mail.sh is a shell script, distributed with socat, that
implements a simple SMTP client. It is programmed to "speak"
SMTP on its FDs 3 (in) and 4 (out). The fdin and fdout
options tell socat to use these FDs for communication with the
program. Because mail.sh inherits stdin and stdout while socat
does not use them, the script can read a mail body from stdin.
Socat makes alias1 your local source address (bind), cares for
correct network line termination (crnl) and sends at most 512
data bytes per packet (mss).

socat \
-,escape=0x0f \
/dev/ttyS0,rawer,crnl

opens an interactive connection via the serial line, e.g. for
talking with a modem. rawer sets the console's and ttyS0's
terminal parameters to practicable values, crnl converts to
correct newline characters. escape allows terminating the
socat process with character control-O. Consider using
READLINE instead of the first address.

socat \
UNIX-LISTEN:/tmp/.X11-unix/X1,fork \
SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20

with UNIX-LISTEN, socat opens a listening UNIX domain socket
/tmp/.X11-unix/X1. This path corresponds to local XWindow
display :1 on your machine, so XWindow client connections to
DISPLAY=:1 are accepted. Socat then speaks with the SOCKS4
server host.victim.org that might permit sourceport 20 based
connections due to an FTP related weakness in its static IP
filters. Socat pretends to be invoked by socksuser nobody, and
requests to be connected to loopback port 6000 (only weak
sockd configurations will allow this). So we get a connection
to the victims XWindow server and, if it does not require MIT
cookies or Kerberos authentication, we can start work. Please
note that there can only be one connection at a time, because
TCP can establish only one session with a given set of
addresses and ports.

socat -u \
/tmp/readdata,seek-end=0,ignoreeof \
STDIO

this is an example for unidirectional data transfer (-u).
Socat transfers data from file /tmp/readdata (implicit address
GOPEN), starting at its current end (seek-end=0 lets socat
start reading at current end of file; use seek=0 or no seek
option to first read the existing data) in a "tail -f" like
mode (ignoreeof). The "file" might also be a listening UNIX
domain socket (do not use a seek option then).

(sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) | \
socat - \
EXEC:'ssh -l user server',pty,setsid,ctty

EXEC'utes an ssh session to server. Uses a pty for
communication between socat and ssh, makes it ssh's
controlling tty (ctty), and makes this pty the owner of a new
process group (setsid), so ssh accepts the password from
socat.

socat -u \
TCP4-LISTEN:3334,reuseaddr,fork \
OPEN:/tmp/in.log,creat,append

implements a simple network based message collector. For each
client connecting to port 3334, a new child process is
generated (option fork). All data sent by the clients are
append'ed to the file /tmp/in.log. If the file does not
exist, socat creat's it. Option reuseaddr allows immediate
restart of the server process.

socat \
READLINE,noecho='[Pp]assword:' \
EXEC:'ftp ftp.server.com',pty,setsid,ctty

wraps a command line history (READLINE) around the EXEC'uted
ftp client utility. This allows editing and reuse of FTP
commands for relatively comfortable browsing through the ftp
directory hierarchy. The password is echoed! pty is required
to have ftp issue a prompt. Nevertheless, there may occur
some confusion with the password and FTP prompts.

socat \
PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \
EXEC:'"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer"'

generates a pseudo terminal device (PTY) on the client that
can be reached under the symbolic link $HOME/dev/vmodem0. An
application that expects a serial line or modem can be
configured to use $HOME/dev/vmodem0; its traffic will be
directed to a modemserver via ssh where another socat instance
links it to /dev/ttyS0.

sudo socat --experimental \
TCP4-LISTEN:8000,reuseaddr,fork,netns=namespace1 \
TCP4-CONNECT:server2:8000

creates a listener in the given network namespace that accepts
TCP connections on port 8000 and forwards them to server2.

sudo socat --experimental \
TUN:192.168.2.1/24,up \
TUN:192.168.2.2/24,up,netns=namespace2

creates two virtual network interfaces, one in default
namespace, the other one in namespace2, and forwards packets
between them, acting as a virtual network connection.

socat \
TCP4-LISTEN:2022,reuseaddr,fork \
PROXY:proxy.local:www.domain.org:22,proxyport=3128,proxyauth=username:s3cr3t

starts a forwarder that accepts connections on port 2022, and
directs them through the proxy daemon listening on port 3128
(proxyport) on host proxy.local, using the CONNECT method,
where they are authenticated as "username" with "s3cr3t"
(proxyauth). proxy.local should establish connections to host
www.domain.org on port 22 then.

socat - \
SSL:server:4443,cafile=./server.crt,cert=./client.pem

is an OpenSSL client that tries to establish a secure
connection to an SSL server. Option cafile specifies a file
that contains trust certificates: we trust the server only
when it presents one of these certificates and proofs that it
owns the related private key. Otherwise the connection is
terminated. With cert a file containing the client
certificate and the associated private key is specified. This
is required in case the server wishes a client authentication;
many Internet servers do not.
The first address ('-') can be replaced by almost any other
socat address.

socat \
OPENSSL-LISTEN:4443,reuseaddr,pf=ip4,fork,cert=./server.pem,cafile=./client.crt \
PIPE

is an OpenSSL server that accepts TCP connections, presents
the certificate from the file server.pem and forces the client
to present a certificate that is verified against cafile.crt.
The second address ('PIPE') can be replaced by almost any
other socat address.
For instructions on generating and distributing OpenSSL keys
and certificates see the additional socat docu
socat-openssl.txt.

echo |
socat -u - \
FILE:/tmp/bigfile,create,largefile,seek=100000000000

creates a 100GB+1B sparse file; this requires a file system
type that supports this (ext2, ext3, ext4, reiserfs, xfs; not
minix, vfat). The operation of writing 1 byte might take long
(reiserfs: some minutes; ext2: "no" time), and the resulting
file can consume some disk space with just its inodes
(reiserfs: 2MB; ext2: 16KB).

socat \
TCP-L:7777,reuseaddr,fork \
SYSTEM:'filan -i 0 -s >&2',nofork

listens for incoming TCP connections on port 7777. For each
accepted connection, invokes a shell. This shell has its stdin
and stdout directly connected to the TCP socket (nofork). The
shell starts filan and lets it print the socket addresses to
stderr (your terminal window).

echo -e "\0\14\0\0\c" |
socat -u - \
FILE:/usr/bin/squid.exe,seek=0x00074420

functions as primitive binary editor: it writes the 4 bytes
000 014 000 000 to the executable /usr/bin/squid.exe at offset
0x00074420 (this was a real world patch to make the squid
executable from Cygwin run under Windows, in 2004).

socat - \
TCP:www.blackhat.org:31337,readbytes=1000

connects to an unknown service and prevents being flooded.

socat -U \
TCP:target:9999,end-close \
TCP-L:8888,reuseaddr,fork

merges data arriving from different TCP streams on port 8888
to just one stream to target:9999. The end-close option
prevents the child processes forked off by the second address
from terminating the shared connection to 9999 (close(2) just
unlinks the inode which stays active as long as the parent
process lives; shutdown(2) would actively terminate the
connection).

socat \
TCP-LISTEN:10021,reuseaddr,socktype=6,protocol=33,fork PIPE

is a simple DCCP echo server. DCCP is now directly provisioned
in socat, however this example shows how use socats TCP
procedures and change the socket type to SOCK_DCCP=6 (on
Linux) and the IP protocol to IPPROTO_DCCP=33.

socat - \
TCP:<server>:10021,reuseaddr,socktype=6,protocol=33,fork

is a simple DCCP client. DCCP is now directly provisioned in
socat, however this example shows how use socats TCP
procedures, but changes the socket type to SOCK_DCCP=6 (on
Linux) and the IP protocol to IPPROTO_DCCP=33.

socat - \
UDP4-DATAGRAM:192.168.1.0:123,sp=123,broadcast,range=192.168.1.0/24

sends a broadcast to the network 192.168.1.0/24 and receives
the replies of the timeservers there. Ignores NTP packets from
hosts outside this network.

socat - \
SOCKET-DATAGRAM:2:2:17:x007bxc0a80100x0000000000000000,bind=x007bx00000000x0000000000000000,setsockopt-int=1:6:1,range=x0000xc0a80100x0000000000000000:x0000xffffff00x0000000000000000

is semantically equivalent to the previous example, but all
parameters are specified in generic form. the value 6 of
setsockopt-int is the Linux value for SO_BROADCAST.

socat - \
IP4-DATAGRAM:255.255.255.255:44,broadcast,range=10.0.0.0/8

sends a broadcast to the local network(s) using protocol 44.
Accepts replies from the private address range only.

socat - \
UDP4-DATAGRAM:224.255.0.1:6666,bind=:6666,ip-add-membership=224.255.0.1:eth0

transfers data from stdin to the specified multicast address
using UDP. Both local and remote ports are 6666. Tells the
interface eth0 to also accept multicast packets of the given
group. Multiple hosts on the local network can run this
command, so all data sent by any of the hosts will be received
by all the other ones. Note that there are many possible
reasons for failure, including IP-filters, routing issues,
wrong interface selection by the operating system, bridges, or
a badly configured switch.

socat \
UDP:host2:4443 \
TUN:192.168.255.1/24,up

establishes one side of a virtual (but not private!) network
with host2 where a similar process might run, with UDP-L and
tun address 192.168.255.2. They can reach each other using the
addresses 192.168.255.1 and 192.168.255.2. Note that streaming
eg.via TCP or SSL does not guarantee to retain packet
boundaries and might thus cause packet loss.

socat - \
VSOCK-CONNECT:2:1234

establishes a VSOCK connection with the host (host is always
reachable with the well-know CID=2) on 1234 port.

socat - \
VSOCK-LISTEN:1234

listens for a VSOCK connection on 1234 port.

socat - \
VSOCK-CONNECT:31:4321,bind:5555

establishes a VSOCK connection with the guest that have CID=31
on 1234 port, binding the local socket to the 5555 port.

socat \
VSOCK-LISTEN:3333,reuseaddr,fork \
VSOCK-CONNECT:42,3333

starts a forwarder that accepts VSOCK connections on port
3333, and directs them to the guest with CID=42 on the same
port.

socat \
VSOCK-LISTEN:22,reuseaddr,fork \
TCP:localhost:22

forwards VSOCK connections from 22 port to the local SSH
server. Running this in a VM allows you to connect via SSH
from the host using VSOCK, as in the example below.

socat \
TCP4-LISTEN:22222,reuseaddr,fork \
VSOCK-CONNECT:33:22

forwards TCP connections from 22222 port to the guest with
CID=33 listening on VSOCK port 22. Running this in the host,
allows you to connect via SSH running "ssh -p 22222
user@localhost", if the guest runs the example above.

socat \
PTY,link=/var/run/ppp,rawer \
INTERFACE:hdlc0

circumvents the problem that pppd requires a serial device and
thus might not be able to work on a synchronous line that is
represented by a network device. socat creates a PTY to make
pppd happy, binds to the network interface hdlc0, and can
transfer data between both devices. Use pppd on device
/var/run/ppp then.

socat --experimental -u \
STDIO \
POSIXMQ-SEND:/queue1,unlink-early,mq-prio=10

Writes packets read from stdio (i.e., lines of input when run
interactively) into POSIX message queue, with priority 10.

socat --experimental -u \
POSIXMQ-RECV:/queue1,fork,max-children=3 \
SYSTEM:"worker.sh"

Receives messages (packets) from POSIX message queue and, for
each message, forks a sub process that reads and processes the
message. At most 3 sub processes are allowed at the same time.

socat -T 1 -d -d \
TCP-L:10081,reuseaddr,fork,crlf \
SYSTEM:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/plain\\\n\\\ndate: \$\(date\)\\\nserver:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT\\\nclient: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n\\\"\"; cat; echo -e \"\\\"\\\n\\\"\""

creates a very primitive HTTP echo server: each HTTP client
that connects gets a valid HTTP reply that contains
information about the client address and port as it is seen by
the server host, the host address (which might vary on
multihomed servers), and the original client request.

socat -d -d \
UDP4-RECVFROM:9999,so-broadcast,so-timestamp,ip-pktinfo,ip-recverr,ip-recvopts,ip-recvtos,ip-recvttl!!- \
SYSTEM:'export; sleep 1' |
grep SOCAT

waits for an incoming UDP packet on port 9999 and prints the
environment variables provided by socat. On BSD based systems
you have to replace ip-pktinfo with ip-recvdstaddr,ip-recvif.
Especially of interest is SOCAT_IP_DSTADDR: it contains the
target address of the packet which may be a unicast,
multicast, or broadcast address.

echo -e "M-SEARCH * HTTP/1.1\nHOST: 239.255.255.250:1900\nMAN: \"ssdp:discover\"\nMX: 4\nST: \"ssdp:all\"\n" |
socat - \
UDP-DATAGRAM:239.255.255.250:1900,crlf

sends an SSDP (Simple Service Discovery Protocol) query to the
local network and collects and outputs the answers received.

systemd-socket-activate -l 1077 --inetd socat ACCEPT:0,fork PIPE

systemd-socket-activate is a program for testing systemd
socket activation of daemons. With --inetd it waits for a
connection on the specified port. It does not accept the
connection but passes the listening file descriptor as FDs 0
and 1. Socat accepts the waiting connection and starts data
transfer.

DIAGNOSTICS

Socat uses a logging mechanism that allows filtering messages by
severity. The severities provided are more or less compatible to the
appropriate syslog priority. With one or up to four occurrences of
the -d command line option, the lowest priority of messages that are
issued can be selected. Each message contains a single uppercase
character specifying the messages severity (one of F, E, W, N, I, or
D)

FATAL: Conditions that require unconditional and immediate program
termination.

ERROR: Conditions that prevent proper program processing. Usually the
program is terminated (see option -s).

WARNING:
Something did not function correctly or is in a state where
correct further processing cannot be guaranteed, but might be
possible.

NOTICE:
Interesting actions of the program, e.g. for supervising socat
in some kind of server mode.

INFO: Description of what the program does, and maybe why it
happens. Allows monitoring the lifecycles of file descriptors.

DEBUG: Description of how the program works, all system or library
calls and their results.


Log messages can be written to stderr, to a file, or to syslog.

On exit, socat gives status 0 if it terminated due to EOF or
inactivity timeout, with a positive value on error, and with a
negative value on fatal error.

FILES

/usr/bin/socat
/usr/bin/filan
/usr/bin/procan

SIGNALS

SIGUSR1:
Causes logging of current transfer statistics.
See also option --statistics

ENVIRONMENT VARIABLES

Input variables carry information from the environment to socat,
output variables are set by socat for use in executed scripts and
programs.

In the output variables beginning with "SOCAT" this prefix is
actually replaced by the upper case name of the executable or the
value of option -lp.

SOCAT_DEFAULT_LISTEN_IP (input)
(Values 4 or 6) Sets the IP version to be used for listen,
recv, and recvfrom addresses if no pf (protocol-family) option
is given. Is overridden by socat options -4 or -6.

SOCAT_PREFERRED_RESOLVE_IP (input)
(Values 0, 4, or 6) Sets the IP version to be used when
resolving target host names when version is not specified by
address type, option pf (protocol-family), or address format.
If name resolution does not return a matching entry, the first
result (with differing IP version) is taken. With value 0,
socat always selects the first record and its IP version.

SOCAT_MAIN_WAIT (input)
Specifies the time (seconds) to sleep the main process on
begin of main\(). Useful for debugging.

SOCAT_TRANSFER_WAIT (input)
Specifies the time (seconds) to sleep the process after
opening addresses before entering the transfer loop. Useful
for debugging.

SOCAT_FORK_WAIT (input)
Specifies the time (seconds) to sleep the parent and child
processes after successful fork(). Useful for debugging.

SOCAT_VERSION (output)
Socat sets this variable to its version string, e.g. "1.7.0.0"
for released versions or e.g. "1.6.0.1+envvar" for temporary
versions; can be used in scripts invoked by socat.

SOCAT_PID (output)
Socat sets this variable to its process id. In case of fork
address option, SOCAT_PID gets the child processes id. Forking
for exec, system, and SHELL does not change SOCAT_PID.

SOCAT_PPID (output)
Socat sets this variable to its process id. In case of fork,
SOCAT_PPID keeps the pid of the master process.

SOCAT_PEERADDR (output)
With passive socket addresses (all LISTEN and RECVFROM
addresses), this variable is set to a string describing the
peers socket address. Port information is not included.

SOCAT_PEERPORT (output)
With appropriate passive socket addresses (TCP, UDP, and SCTP
- LISTEN and RECVFROM), this variable is set to a string
containing the number of the peer port.

SOCAT_SOCKADDR (output)
With all LISTEN addresses, this variable is set to a string
describing the local socket address. Port information is not
included example

SOCAT_SOCKPORT (output)
With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses, this
variable is set to the local port.

SOCAT_TIMESTAMP (output)
With all RECVFROM addresses where address option so-timestamp
is applied, socat sets this variable to the resulting
timestamp.

SOCAT_IP_OPTIONS (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvopts is applied, socat fills this variable with the IP
options of the received packet.

SOCAT_IP_DSTADDR (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvdstaddr (BSD) or ip-pktinfo (other platforms) is
applied, socat sets this variable to the destination address
of the received packet. This is particularly useful to
identify broadcast and multicast addressed packets.

SOCAT_IP_IF (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvif (BSD) or ip-pktinfo (other platforms) is applied,
socat sets this variable to the name of the interface where
the packet was received.

SOCAT_IP_LOCADDR (output)
With all IPv4 based RECVFROM addresses where address option
ip-pktinfo is applied, socat sets this variable to the address
of the interface where the packet was received.

SOCAT_IP_TOS (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvtos is applied, socat sets this variable to the TOS
(type of service) of the received packet.

SOCAT_IP_TTL (output)
With all IPv4 based RECVFROM addresses where address option
ip-recvttl is applied, socat sets this variable to the TTL
(time to live) of the received packet.

SOCAT_IPV6_HOPLIMIT (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvhoplimit is applied, socat sets this variable to the
hoplimit value of the received packet.

SOCAT_IPV6_DSTADDR (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvpktinfo is applied, socat sets this variable to the
destination address of the received packet.

SOCAT_IPV6_TCLASS (output)
With all IPv6 based RECVFROM addresses where address option
ipv6-recvtclass is applied, socat sets this variable to the
transfer class of the received packet.

SOCAT_OPENSSL_X509_ISSUER (output)
Issuer field from peer certificate

SOCAT_OPENSSL_X509_SUBJECT (output)
Subject field from peer certificate

SOCAT_OPENSSL_X509_COMMONNAME (output)
commonName entries from peer certificates subject. Multiple
values are separated by " // ".

SOCAT_OPENSSL_X509_* (output)
all other entries from peer certificates subject

SOCAT_OPENSSL_X509V3_DNS (output)
DNS entries from peer certificates extensions - subjectAltName
field. Multiple values are separated by " // ".

HOSTNAME (input)
Is used to determine the hostname for logging (see -lh).

LOGNAME (input)
Is used as name for the socks client user name if no socksuser
is given.
With options su and su-d, LOGNAME is set to the given user
name.

USER (input)
Is used as name for the socks client user name if no socksuser
is given and LOGNAME is empty.
With options su and su-d, USER is set to the given user name.

SHELL (output)
With options su and su-d, SHELL is set to the login shell of
the given user.

PATH (output)
Can be set with option path for exec, system, and SHELL
addresses.

HOME (output)
With options su and su-d, HOME is set to the home directory of
the given user.

CREDITS

The work of the following groups and organizations was invaluable for
this project:

The FSF (GNU, http://www.fsf.org/) project with their free and
portable development software and lots of other useful tools and
libraries.

The Linux developers community (http://www.linux.org/) for providing
a free, open source operating system.

The Open Group (http://www.unix-systems.org/) for making their
standard specifications available on the Internet for free.

VERSION

This man page describes version 1.8.0 of socat.

BUGS

Addresses cannot be nested, so a single socat process cannot, e.g.,
drive ssl over socks.

Address option ftruncate without value uses default 1 instead of 0.

Verbose modes (-x and/or -v) display line termination characters
inconsistently when address options cr or crnl are used: They show
the data after conversion in either direction.

The data transfer blocksize setting (-b) is ignored with address
readline.

Send bug reports to <socat@dest-unreach.org>

SEE ALSO

nc(1), rinetd(8), openssl(1), stunnel(8), rlwrap(1), setsid(1)

Socat home page http://www.dest-unreach.org/socat/

AUTHOR

Gerhard Rieger <rieger@dest-unreach.org> and contributors

socat(1)