Tribblix: manual page: nc.1

NC(1) User Commands NC(1)

NAME

nc - arbitrary TCP and UDP connections and listens

SYNOPSIS

nc -h
nc [-46BdnrStuvz] [-i interval] [-M ttl] [-m minttl]
[-P proxy_username] [-p port] [-s source_ip_address] [-T ToS]
[-w timeout] [-X proxy_protocol] [-x proxy_address[&: port]]
hostname port_list
nc -l [-46DdnrStuvz] [-i interval] [-M ttl] [-m minttl] [-T ToS]
[hostname] port
nc -l [-46DdnrStuvz] [-i interval] [-M ttl] [-m minttl] [-T ToS]
-p port
nc -U [-Ddtvz] [-i interval] [-w timeout] -p path
nc -Ul [-46Ddktv] [-i interval] path

DESCRIPTION

The nc (or netcat) utility is used for a variety of tasks associated
with TCP or UDP. nc can open TCP connections, send UDP packets, listen
on arbitrary TCP and UDP ports, perform port scanning, and deal with
both IPv4 and IPv6. Unlike telnet(1), nc scripts nicely, and separates
error messages onto standard error instead of sending them to standard
output.

The nc command is often used for the following tasks:

+o simple TCP proxies

+o shell-script based HTTP clients and servers

+o network daemon testing

+o a SOCKS or HTTP ProxyCommand for ssh(1)

OPTIONS

The following options are supported:

-4 Force nc to use IPv4 addresses only.

-6 Force nc to use IPv6 addresses only.

-B Attempt to bypass IPsec policy and force cleartext. Will fail
without the sys_ip_config privilege, or with the -l or -U
flags. If no IPsec policy exists, this flag is extraneous.

-D Enable debugging on the socket.

-d Do not attempt to read from stdin.

-h Print nc help.

-i interval
Specify a delay time of interval between lines of text sent and
received. This option also causes a delay time between
connections to multiple ports.

-k Force nc to listen for another connection after its current
connection is closed.

It is an error to use this option without the -l option.

-l Listen for an incoming connection rather than initiate a
connection to a remote host.

It is an error to use this option in conjunction with the -s or
-z options. Additionally, any timeout specified with the -w
option is ignored.

-M Set the default IPv4 TTL or IPv6 hop count that should be used
in outgoing packets. This corresponds to the IP_TTL and
IPV6_UNICAST_HOPS socket options.

-m Set the minimum IPv4 TTL or IPv6 hop count that must be present
on a packet for a packet to be received. This corresponds to
the IP_MINTTL and IPV6_MINHOPCOUNT socket options.

-n Do not do any naming or service lookups on any addresses,
hostnames, or ports.

Use of this option means that hostname and port arguments are
restricted to numeric values.

If used with -v option all addresses and ports are printed in
numeric form, in addition to the restriction imposed on the
arguments. This option does not have any effect when used in
conjunction with the -U option.

-P proxy_username
Specify a username (proxy_username) to present to a proxy
server that requires authentication. If proxy_username is not
specified, authentication is not attempted. Proxy
authentication is only supported for HTTP CONNECT proxies at
present.

It is an error to use this option in conjunction with the -l
option.

-p port
When used without -l option, specify the source port nc should
use, subject to privilege restrictions and availability. When
used with the -l option, set the listen port.

This option can be used with -l option only provided global
port argument is not specified.

-r Choose source or destination ports randomly instead of
sequentially within a range or in the order that the system
assigns them.

It is an error to use this option in conjunction with the -l
option.

-S Enables the RFC 2385 TCP MD5 signature option.

In order for packets to be sent or received in conjunction with
this option, a security association that matches the traffic
must also be created using tcpkey(8).

-s source_ip_address
Specify the IP of the interface which is used to send the
packets.

It is an error to use this option in conjunction with the -l
option.

-T ToS Specify IP Type of Service (ToS) or IPv6 traffic class for the
connection. Valid values are the tokens: lowdelay, throughput,
reliability, or any decimal or hexadecimal integer between 0
and 255.

-t Cause nc to send RFC 854 "DON'T" and "WON'T" responses to RFC
854 "DO" and "WILL" requests. This makes it possible to use nc
to script telnet sessions.

-U Specify the use of Unix Domain Sockets. If you specify this
option without -l, it becomes an AF_UNIX client. If you
specify this option with the -l option, a AF_UNIX server is
created.

Use of this option requires that a single argument of a valid
Unix domain path has to be provided to nc, not a host name or
port.

-u Use UDP instead of the default option of TCP.

-v Specify verbose output.

-w timeout
Silently close the connection if a connection and stdin are
idle for more than timeout seconds.

This option has no effect on the -l option, that is, nc listens
forever for a connection, with or without the -w flag. The
default is no timeout.

-X proxy_protocol
Use the specified protocol when talking to the proxy server.
Supported protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and
connect (HTTP proxy). If the protocol is not specified, SOCKS
v.5 is used.

It is an error to use this option in conjunction with the -l
option.

-x proxy_address[&: port]
Request connection to hostname using a proxy at proxy_address
and port. If port is not specified, the well-known port for
the proxy protocol is used (1080 for SOCKS, 3128 for HTTP).

It is an error to use this option in conjunction with the -l
option.

-z Scan for listening daemons, without sending any data to them.

It is an error to use this option in conjunction with the -l
option.

OPERANDS

The following operands are supported:

hostname
Specify host name.

hostname can be a numerical IP address or a symbolic hostname
(unless the -n option is specified).

In general, hostname must be specified, unless the -l option is
given or -U is used (in which case the argument is a path). If
hostname argument is specified with -l option then port
argument must be given as well and nc tries to bind to that
address and port. If hostname argument is not specified with
-l option then nc tries to listen on a wildcard socket for
given port.

path Specify pathname.

port | port_list
Specify port.

port_list can be specified as single integers, ranges or
combinations of both. Specify ranges in the form of nn-mm.
The port_list must have at least one member, but can have
multiple ports/ranges separated by commas.

In general, a destination port must be specified, unless the -U
option is given, in which case a Unix Domain Socket path must
be specified instead of hostname.

USAGE

Client/Server Model
It is quite simple to build a very basic client/server model using nc.
On one console, start nc listening on a specific port for a connection.
For example, the command:

$ nc -l 1234

listens on port 1234 for a connection. On a second console (or a
second machine), connect to the machine and port to which nc is
listening:

$ nc 127.0.0.1 1234

There should now be a connection between the ports. Anything typed at
the second console is concatenated to the first, and vice-versa. After
the connection has been set up, nc does not really care which side is
being used as a server and which side is being used as a client. The
connection can be terminated using an EOF (Ctrl/d).

Data Transfer

The example in the previous section can be expanded to build a basic
data transfer model. Any information input into one end of the
connection is output to the other end, and input and output can be
easily captured in order to emulate file transfer.

Start by using nc to listen on a specific port, with output captured
into a file:

$ nc -l 1234 > filename.out

Using a second machine, connect to the listening nc process, feeding it
the file which is to be transferred:

$ nc host.example.com 1234 < filename.in

After the file has been transferred, the connection closes
automatically.

Talking to Servers

It is sometimes useful to talk to servers by hand rather than through a
user interface. It can aid in troubleshooting, when it might be
necessary to verify what data a server is sending in response to
commands issued by the client.

For example, to retrieve the home page of a web site:

$ echo -n "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80

This also displays the headers sent by the web server. They can be
filtered, if necessary, by using a tool such as sed(1).

More complicated examples can be built up when the user knows the
format of requests required by the server. As another example, an
email can be submitted to an SMTP server using:

$ nc localhost 25 << EOF
HELO host.example.com
MAIL FROM: <user@host.example.com>
RCPT TO: <user2@host.example.com>
DATA
Body of email.
.
QUIT
EOF

Port Scanning

It can be useful to know which ports are open and running services on a
target machine. The -z flag can be used to tell nc to report open
ports, rather than to initiate a connection.

In this example:

$ nc -z host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!

The port range was specified to limit the search to ports 20 - 30.

Alternatively, it might be useful to know which server software is
running, and which versions. This information is often contained
within the greeting banners. In order to retrieve these, it is
necessary to first make a connection, and then break the connection
when the banner has been retrieved. This can be accomplished by
specifying a small timeout with the -w flag, or perhaps by issuing a
QUIT command to the server:

$ echo "QUIT" | nc host.example.com 20-30
SSH-2.0-Sun_SSH_1.1
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84 Ready

inetd Capabilities
One of the possible uses is to create simple services by using
inetd(8).

The following example creates a redirect from TCP port 8080 to port 80
on host realwww:

# cat << EOF >> /etc/services
wwwredir 8080/tcp # WWW redirect EOF
# cat << EOF > /tmp/wwwredir.conf
wwwredir stream tcp nowait nobody /usr/bin/nc /usr/bin/nc -w 3 realwww 80
EOF
# inetconv -i /tmp/wwwredir.conf
wwwredir -> /var/svc/manifest/network/wwwredir-tcp.xml
Importing wwwredir-tcp.xml ...Done
# inetadm -l wwwredir/tcp
SCOPE NAME=VALUE
name="wwwredir"
endpoint_type="stream"
proto="tcp"
isrpc=FALSE
wait=FALSE
exec="/usr/bin/nc -w 3 realwww 80"
arg0="/usr/bin/nc"
user="nobody"
default bind_addr=""
default bind_fail_max=-1
default bind_fail_interval=-1
default max_con_rate=-1
default max_copies=-1
default con_rate_offline=-1
default failrate_cnt=40
default failrate_interval=60
default inherit_env=TRUE
default tcp_trace=TRUE
default tcp_wrappers=FALSE

Privileges

To bind to a privileged port number nc needs to be granted the
net_privaddr privilege. Bypassing IPsec policy requires the
sys_ip_config privilege. If Trusted Extensions are configured and the
port nc should listen on is configured as a multi-level port nc also
needs the net_bindmlp privilege.

Privileges can be assigned to the user or role directly, by specifying
them in the account's default privilege set in user_attr(5). However,
this means that any application that this user or role starts have
these additional privileges. To only grant the privileges(7) when nc
is invoked, the recommended approach is to create and assign an rbac(7)
rights profile. See EXAMPLES for additional information.

EXAMPLES

Open a TCP connection to port 42 of host.example.com, using port 3141
as the source port, with a timeout of 5 seconds:

$ nc -p 3141 -w 5 host.example.com 42

Open a UDP connection to port 53 of host.example.com:

$ nc -u host.example.com 53

Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
the IP for the local end of the connection:

$ nc -s 10.1.2.3 host.example.com 42

Use a list of ports and port ranges for a port scan on various ports:

$ nc -z host.example.com 21-25,53,80,110-120,443

Create and listen on a Unix Domain Socket:

$ nc -lU /var/tmp/dsocket

Create and listen on a UDP socket with associated port 8888:

$ nc -u -l -p 8888

which is the same as:

$ nc -u -l 8888

Create and listen on a TCP socket with associated port 2222 and bind to
address 127.0.0.1 only:

$ nc -l 127.0.0.1 2222

Connect to port 42 of host.example.com using an HTTP proxy at 10.2.3.4,
port 8080. This example could also be used by ssh(1). See the
ProxyCommand directive in ssh_config(5) for more information.

$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

The same example again, this time enabling proxy authentication with
username ruser if the proxy requires it:

$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

To run nc with the smallest possible set of privileges as a user or
role that has additional privileges (such as the default root account)
it can be invoked using ppriv(1) as well. For example, limiting it to
only run with the privilege to bind to a privileged port:

$ ppriv -e -sA=basic,!file_link_any,!proc_exec,!proc_fork,\
!proc_info,!proc_session,net_privaddr nc -l 42

To allow a user or role to use only nc with the net_privaddr privilege,
a rights profile needs to be created:

/etc/security/exec_attr:
Netcat privileged:solaris:cmd:::/usr/bin/nc:privs=net_privaddr
/etc/security/prof_attr
Netcat privileged:::Allow nc to bind to privileged
ports:help=None.html

Assigning this rights profile using user_attr(5) permits the user or
role to run nc allowing it to listen on any port. To permit a user or
role to use nc only to listen on specific ports a wrapper script should
be specified in the rights profiles:

/etc/security/exec_attr
Netcat
restricted:solaris:cmd:::/usr/bin/nc-restricted:privs=net_privaddr
/etc/security/prof_attr
Netcat restricted:::Allow nc to bind to privileged
ports:help=None.html

and write a shell script that restricts the permissible options, for
example, one that permits one to bind only on ports between 42 and 64
non-inclusive:

#!/bin/ksh

(( $# == 1 )) || exit 1
(( $1 > 42 && $1 < 64 )) || exit 1
exec /usr/bin/nc -l -p "$1"

This grants the extra privileges when the user or role invokes nc using
the wrapper script from a profile shell. See pfsh(1), pfksh(1),
pfcsh(1), and pfexec(1).

Invoking nc directly does not run it with the additional privileges,
and neither does invoking the script without using pfexec or a profile
shell.

INTERFACE STABILITY

The command line syntax is Committed for the -4, -6, -l, -n, -p, -u,
and -w options and their arguments (if any). The name and port list
arguments are Committed. The port range syntax is Uncommitted. The
interface stability level for all other command line options and their
arguments is Uncommitted.

AUTHORS

The original implementation of nc was written by Hobbit
<hobbit@avian.org>

nc was rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>

NOTES

UDP port scans always succeed, that is, report the port as open,
rendering the -uz combination of flags relatively useless.

illumos February 10, 2025 illumos