TCPDUMP(1) User Commands TCPDUMP(1)

NAME

tcpdump - dump traffic on a network

SYNOPSIS

tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]
[ -c count ] [ --count ] [ -C file_size ]
[ -E spi@ipaddr algo:secret,... ]
[ -F file ] [ -G rotate_seconds ] [ -i interface ]
[ --immediate-mode ] [ -j tstamp_type ] [ -m module ]
[ -M secret ] [ --number ] [ --print ] [ -Q in|out|inout ]
[ -r file ] [ -s snaplen ] [ -T type ] [ --version ]
[ -V file ] [ -w file ] [ -W filecount ] [ -y datalinktype ]
[ -z postrotate-command ] [ -Z user ]
[ --time-stamp-precision=tstamp_precision ]
[ --micro ] [ --nano ]
[ expression ]

DESCRIPTION

Tcpdump prints out a description of the contents of packets on a
network interface that match the Boolean expression (see
pcap-filter(5) for the expression syntax); the description is
preceded by a time stamp, printed, by default, as hours, minutes,
seconds, and fractions of a second since midnight. It can also be
run with the -w flag, which causes it to save the packet data to a
file for later analysis, and/or with the -r flag, which causes it to
read from a saved packet file rather than to read packets from a
network interface. It can also be run with the -V flag, which causes
it to read a list of saved packet files. In all cases, only packets
that match expression will be processed by tcpdump.

Tcpdump will, if not run with the -c flag, continue capturing packets
until it is interrupted by a SIGINT signal (generated, for example,
by typing your interrupt character, typically control-C) or a SIGTERM
signal (typically generated with the kill(1) command); if run with
the -c flag, it will capture packets until it is interrupted by a
SIGINT or SIGTERM signal or the specified number of packets have been
processed.

When tcpdump finishes capturing packets, it will report counts of:

packets ``captured'' (this is the number of packets that
tcpdump has received and processed);

packets ``received by filter'' (the meaning of this depends on
the OS on which you're running tcpdump, and possibly on the
way the OS was configured - if a filter was specified on the
command line, on some OSes it counts packets regardless of
whether they were matched by the filter expression and, even
if they were matched by the filter expression, regardless of
whether tcpdump has read and processed them yet, on other OSes
it counts only packets that were matched by the filter
expression regardless of whether tcpdump has read and
processed them yet, and on other OSes it counts only packets
that were matched by the filter expression and were processed
by tcpdump);

packets ``dropped by kernel'' (this is the number of packets
that were dropped, due to a lack of buffer space, by the
packet capture mechanism in the OS on which tcpdump is
running, if the OS reports that information to applications;
if not, it will be reported as 0).

On platforms that support the SIGINFO signal, such as most BSDs
(including macOS) and Digital/Tru64 UNIX, it will report those counts
when it receives a SIGINFO signal (generated, for example, by typing
your ``status'' character, typically control-T, although on some
platforms, such as macOS, the ``status'' character is not set by
default, so you must set it with stty(1) in order to use it) and will
continue capturing packets. On platforms that do not support the
SIGINFO signal, the same can be achieved by using the SIGUSR1 signal.

Using the SIGUSR2 signal along with the -w flag will forcibly flush
the packet buffer into the output file.

Reading packets from a network interface may require that you have
special privileges; see the pcap(3PCAP) man page for details.
Reading a saved packet file doesn't require special privileges.

OPTIONS

-A Print each packet (minus its link level header) in ASCII.
Handy for capturing web pages.

-b Print the AS number in BGP packets in ASDOT notation rather
than ASPLAIN notation.

-B buffer_size
--buffer-size=buffer_size
Set the operating system capture buffer size to buffer_size,
in units of KiB (1024 bytes).

-c count
Exit after receiving count packets.

--count
Print only on stdout the packet count when reading capture
file(s) instead of parsing/printing the packets. If a filter
is specified on the command line, tcpdump counts only packets
that were matched by the filter expression.

-C file_size
Before writing a raw packet to a savefile, check whether the
file is currently larger than file_size and, if so, close the
current savefile and open a new one. Savefiles after the
first savefile will have the name specified with the -w flag,
with a number after it, starting at 1 and continuing upward.
The units of file_size are millions of bytes (1,000,000 bytes,
not 1,048,576 bytes).

-d Dump the compiled packet-matching code in a human readable
form to standard output and stop.

Please mind that although code compilation is always DLT-
specific, typically it is impossible (and unnecessary) to
specify which DLT to use for the dump because tcpdump uses
either the DLT of the input pcap file specified with -r, or
the default DLT of the network interface specified with -i, or
the particular DLT of the network interface specified with -y
and -i respectively. In these cases the dump shows the same
exact code that would filter the input file or the network
interface without -d.

However, when neither -r nor -i is specified, specifying -d
prevents tcpdump from guessing a suitable network interface
(see -i). In this case the DLT defaults to EN10MB and can be
set to another valid value manually with -y.

-dd Dump packet-matching code as a C program fragment.

-ddd Dump packet-matching code as decimal numbers (preceded with a
count).

-D
--list-interfaces
Print the list of the network interfaces available on the
system and on which tcpdump can capture packets. For each
network interface, a number and an interface name, possibly
followed by a text description of the interface, are printed.
The interface name or the number can be supplied to the -i
flag to specify an interface on which to capture.

This can be useful on systems that don't have a command to
list them (e.g., Windows systems, or UNIX systems lacking
ifconfig -a); the number can be useful on Windows 2000 and
later systems, where the interface name is a somewhat complex
string.

The -D flag will not be supported if tcpdump was built with an
older version of libpcap that lacks the
pcap_findalldevs(3PCAP) function.

-e Print the link-level header on each dump line. This can be
used, for example, to print MAC layer addresses for protocols
such as Ethernet and IEEE 802.11.

-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets
that are addressed to addr and contain Security Parameter
Index value spi. This combination may be repeated with comma
or newline separation.

Note that setting the secret for IPv4 ESP packets is supported
at this time.

Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
cast128-cbc, or none. The default is des-cbc. The ability to
decrypt packets is only present if tcpdump was compiled with
cryptography enabled.

secret is the ASCII text for ESP secret key. If preceded by
0x, then a hex value will be read.

The option assumes RFC 2406 ESP, not RFC 1827 ESP. The option
is only for debugging purposes, and the use of this option
with a true `secret' key is discouraged. By presenting IPsec
secret key onto command line you make it visible to others,
via ps(1) and other occasions.

In addition to the above syntax, the syntax file name may be
used to have tcpdump read the provided file in. The file is
opened upon receiving the first ESP packet, so any special
permissions that tcpdump may have been given should already
have been given up.

-f Print `foreign' IPv4 addresses numerically rather than
symbolically (this option is intended to get around serious
brain damage in Sun's NIS server -- usually it hangs forever
translating non-local internet numbers).

The test for `foreign' IPv4 addresses is done using the IPv4
address and netmask of the interface on that capture is being
done. If that address or netmask are not available, either
because the interface on that capture is being done has no
address or netmask or because it is the "any" pseudo-
interface, which is available in Linux and in recent versions
of macOS and Solaris, and which can capture on more than one
interface, this option will not work correctly.

-F file
Use file as input for the filter expression. An additional
expression given on the command line is ignored.

-G rotate_seconds
If specified, rotates the dump file specified with the -w
option every rotate_seconds seconds. Savefiles will have the
name specified by -w which should include a time format as
defined by strftime(3). If no time format is specified, each
new file will overwrite the previous. Whenever a generated
filename is not unique, tcpdump will overwrite the preexisting
data; providing a time specification that is coarser than the
capture period is therefore not advised.

If used in conjunction with the -C option, filenames will take
the form of `file<count>'.

-h
--help Print the tcpdump and libpcap version strings, print a usage
message, and exit.

--version
Print the tcpdump and libpcap version strings and exit.

-H Attempt to detect 802.11s draft mesh headers.

-i interface
--interface=interface
Listen, report the list of link-layer types, report the list
of time stamp types, or report the results of compiling a
filter expression on interface. If unspecified and if the -d
flag is not given, tcpdump searches the system interface list
for the lowest numbered, configured up interface (excluding
loopback), which may turn out to be, for example, ``eth0''.

On Linux systems with 2.2 or later kernels and on recent
versions of macOS and Solaris, an interface argument of
``any'' can be used to capture packets from all interfaces.
Note that captures on the ``any'' pseudo-interface will not be
done in promiscuous mode.

If the -D flag is supported, an interface number as printed by
that flag can be used as the interface argument, if no
interface on the system has that number as a name.

-I
--monitor-mode
Put the interface in "monitor mode"; this is supported only on
IEEE 802.11 Wi-Fi interfaces, and supported only on some
operating systems.

Note that in monitor mode the adapter might disassociate from
the network with which it's associated, so that you will not
be able to use any wireless networks with that adapter. This
could prevent accessing files on a network server, or
resolving host names or network addresses, if you are
capturing in monitor mode and are not connected to another
network with another adapter.

This flag will affect the output of the -L flag. If -I isn't
specified, only those link-layer types available when not in
monitor mode will be shown; if -I is specified, only those
link-layer types available when in monitor mode will be shown.

--immediate-mode
Capture in "immediate mode". In this mode, packets are
delivered to tcpdump as soon as they arrive, rather than being
buffered for efficiency. This is the default when printing
packets rather than saving packets to a ``savefile'' if the
packets are being printed to a terminal rather than to a file
or pipe.

-j tstamp_type
--time-stamp-type=tstamp_type
Set the time stamp type for the capture to tstamp_type. The
names to use for the time stamp types are given in
pcap-tstamp(5); not all the types listed there will
necessarily be valid for any given interface.

-J
--list-time-stamp-types
List the supported time stamp types for the interface and
exit. If the time stamp type cannot be set for the interface,
no time stamp types are listed.

--time-stamp-precision=tstamp_precision
When capturing, set the time stamp precision for the capture
to tstamp_precision. Note that availability of high precision
time stamps (nanoseconds) and their actual accuracy is
platform and hardware dependent. Also note that when writing
captures made with nanosecond accuracy to a savefile, the time
stamps are written with nanosecond resolution, and the file is
written with a different magic number, to indicate that the
time stamps are in seconds and nanoseconds; not all programs
that read pcap savefiles will be able to read those captures.

When reading a savefile, convert time stamps to the precision
specified by timestamp_precision, and display them with that
resolution. If the precision specified is less than the
precision of time stamps in the file, the conversion will lose
precision.

The supported values for timestamp_precision are micro for
microsecond resolution and nano for nanosecond resolution.
The default is microsecond resolution.

--micro
--nano Shorthands for --time-stamp-precision=micro or
--time-stamp-precision=nano, adjusting the time stamp
precision accordingly. When reading packets from a savefile,
using --micro truncates time stamps if the savefile was
created with nanosecond precision. In contrast, a savefile
created with microsecond precision will have trailing zeroes
added to the time stamp when --nano is used.

-K
--dont-verify-checksums
Don't attempt to verify IP, TCP, or UDP checksums. This is
useful for interfaces that perform some or all of those
checksum calculation in hardware; otherwise, all outgoing TCP
checksums will be flagged as bad.

-l Make stdout line buffered. Useful if you want to see the data
while capturing it. E.g.,
tcpdump -l | tee dat

or
tcpdump -l > dat & tail -f dat

Note that on Windows,``line buffered'' means ``unbuffered'',
so that WinDump will write each character individually if -l
is specified.

-U is similar to -l in its behavior, but it will cause output
to be ``packet-buffered'', so that the output is written to
stdout at the end of each packet rather than at the end of
each line; this is buffered on all platforms, including
Windows.

-L
--list-data-link-types
List the known data link types for the interface, in the
specified mode, and exit. The list of known data link types
may be dependent on the specified mode; for example, on some
platforms, a Wi-Fi interface might support one set of data
link types when not in monitor mode (for example, it might
support only fake Ethernet headers, or might support 802.11
headers but not support 802.11 headers with radio information)
and another set of data link types when in monitor mode (for
example, it might support 802.11 headers, or 802.11 headers
with radio information, only in monitor mode).

-m module
Load SMI MIB module definitions from file module. This option
can be used several times to load several MIB modules into
tcpdump.

-M secret
Use secret as a shared secret for validating the digests found
in TCP segments with the TCP-MD5 option (RFC 2385), if
present.

-n Don't convert addresses (i.e., host addresses, port numbers,
etc.) to names.

-N Don't print domain name qualification of host names. E.g., if
you give this flag then tcpdump will print ``nic'' instead of
``nic.ddn.mil''.

-#
--number
Print a packet number at the beginning of the line.

-O
--no-optimize
Do not run the packet-matching code optimizer. This is useful
only if you suspect a bug in the optimizer.

-p
--no-promiscuous-mode
Don't put the interface into promiscuous mode. Note that the
interface might be in promiscuous mode for some other reason;
hence, `-p' cannot be used as an abbreviation for `ether host
{local-hw-addr} or ether broadcast'.

--print
Print parsed packet output, even if the raw packets are being
saved to a file with the -w flag.

-Q direction
--direction=direction
Choose send/receive direction direction for which packets
should be captured. Possible values are `in', `out' and
`inout'. Not available on all platforms.

-q Quick (quiet?) output. Print less protocol information so
output lines are shorter.

-r file
Read packets from file (which was created with the -w option
or by other tools that write pcap or pcapng files). Standard
input is used if file is ``-''.

-S
--absolute-tcp-sequence-numbers
Print absolute, rather than relative, TCP sequence numbers.

-s snaplen
--snapshot-length=snaplen
Snarf snaplen bytes of data from each packet rather than the
default of 262144 bytes. Packets truncated because of a
limited snapshot are indicated in the output with
``[|proto]'', where proto is the name of the protocol level at
which the truncation has occurred.

Note that taking larger snapshots both increases the amount of
time it takes to process packets and, effectively, decreases
the amount of packet buffering. This may cause packets to be
lost. Note also that taking smaller snapshots will discard
data from protocols above the transport layer, which loses
information that may be important. NFS and AFS requests and
replies, for example, are very large, and much of the detail
won't be available if a too-short snapshot length is selected.

If you need to reduce the snapshot size below the default, you
should limit snaplen to the smallest number that will capture
the protocol information you're interested in. Setting
snaplen to 0 sets it to the default of 262144, for backwards
compatibility with recent older versions of tcpdump.

-T type
Force packets selected by "expression" to be interpreted the
specified type. Currently known types are aodv (Ad-hoc On-
demand Distance Vector protocol), carp (Common Address
Redundancy Protocol), cnfp (Cisco NetFlow protocol), domain
(Domain Name System), lmp (Link Management Protocol), pgm
(Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0 inside
PGM/EPGM), ptp (Precision Time Protocol), radius (RADIUS),
resp (REdis Serialization Protocol), rpc (Remote Procedure
Call), rtcp (Real-Time Applications control protocol), rtp
(Real-Time Applications protocol), snmp (Simple Network
Management Protocol), someip (SOME/IP), tftp (Trivial File
Transfer Protocol), vat (Visual Audio Tool), vxlan (Virtual
eXtensible Local Area Network), wb (distributed White Board)
and zmtp1 (ZeroMQ Message Transport Protocol 1.0).

Note that the pgm type above affects UDP interpretation only,
the native PGM is always recognised as IP protocol 113
regardless. UDP-encapsulated PGM is often called "EPGM" or
"PGM/UDP".

Note that the pgm_zmtp1 type above affects interpretation of
both native PGM and UDP at once. During the native PGM
decoding the application data of an ODATA/RDATA packet would
be decoded as a ZeroMQ datagram with ZMTP/1.0 frames. During
the UDP decoding in addition to that any UDP packet would be
treated as an encapsulated PGM packet.

-t Don't print a timestamp on each dump line.

-tt Print the timestamp, as seconds since January 1, 1970,
00:00:00, UTC, and fractions of a second since that time, on
each dump line.

-ttt Print a delta (microsecond or nanosecond resolution depending
on the --time-stamp-precision option) between current and
previous line on each dump line. The default is microsecond
resolution.

-tttt Print a timestamp, as hours, minutes, seconds, and fractions
of a second since midnight, preceded by the date, on each dump
line.

-ttttt Print a delta (microsecond or nanosecond resolution depending
on the --time-stamp-precision option) between current and
first line on each dump line. The default is microsecond
resolution.

-u Print undecoded NFS handles.

-U
--packet-buffered
If the -w option is not specified, or if it is specified but
the --print flag is also specified, make the printed packet
output ``packet-buffered''; i.e., as the description of the
contents of each packet is printed, it will be written to the
standard output, rather than, when not writing to a terminal,
being written only when the output buffer fills.

If the -w option is specified, make the saved raw packet
output ``packet-buffered''; i.e., as each packet is saved, it
will be written to the output file, rather than being written
only when the output buffer fills.

The -U flag will not be supported if tcpdump was built with an
older version of libpcap that lacks the pcap_dump_flush(3PCAP)
function.

-v When parsing and printing, produce (slightly more) verbose
output. For example, the time to live, identification, total
length and options in an IP packet are printed. Also enables
additional packet integrity checks such as verifying the IP
and ICMP header checksum.

When writing to a file with the -w option and at the same time
not reading from a file with the -r option, report to stderr,
once per second, the number of packets captured. In Solaris,
FreeBSD and possibly other operating systems this periodic
update currently can cause loss of captured packets on their
way from the kernel to tcpdump.

-vv Even more verbose output. For example, additional fields are
printed from NFS reply packets, and SMB packets are fully
decoded.

-vvv Even more verbose output. For example, telnet SB ... SE
options are printed in full. With -X Telnet options are
printed in hex as well.

-V file
Read a list of filenames from file. Standard input is used if
file is ``-''.

-w file
Write the raw packets to file rather than parsing and printing
them out. They can later be printed with the -r option.
Standard output is used if file is ``-''.

This output will be buffered if written to a file or pipe, so
a program reading from the file or pipe may not see packets
for an arbitrary amount of time after they are received. Use
the -U flag to cause packets to be written as soon as they are
received.

The MIME type application/vnd.tcpdump.pcap has been registered
with IANA for pcap files. The filename extension .pcap appears
to be the most commonly used along with .cap and .dmp. Tcpdump
itself doesn't check the extension when reading capture files
and doesn't add an extension when writing them (it uses magic
numbers in the file header instead). However, many operating
systems and applications will use the extension if it is
present and adding one (e.g. .pcap) is recommended.

See pcap-savefile(4) for a description of the file format.

-W filecount
Used in conjunction with the -C option, this will limit the
number of files created to the specified number, and begin
overwriting files from the beginning, thus creating a
'rotating' buffer. In addition, it will name the files with
enough leading 0s to support the maximum number of files,
allowing them to sort correctly.

Used in conjunction with the -G option, this will limit the
number of rotated dump files that get created, exiting with
status 0 when reaching the limit.

If used in conjunction with both -C and -G, the -W option will
currently be ignored, and will only affect the file name.

-x When parsing and printing, in addition to printing the headers
of each packet, print the data of each packet (minus its link
level header) in hex. The smaller of the entire packet or
snaplen bytes will be printed. Note that this is the entire
link-layer packet, so for link layers that pad (e.g.
Ethernet), the padding bytes will also be printed when the
higher layer packet is shorter than the required padding. In
the current implementation this flag may have the same effect
as -xx if the packet is truncated.

-xx When parsing and printing, in addition to printing the headers
of each packet, print the data of each packet, including its
link level header, in hex.

-X When parsing and printing, in addition to printing the headers
of each packet, print the data of each packet (minus its link
level header) in hex and ASCII. This is very handy for
analysing new protocols. In the current implementation this
flag may have the same effect as -XX if the packet is
truncated.

-XX When parsing and printing, in addition to printing the headers
of each packet, print the data of each packet, including its
link level header, in hex and ASCII.

-y datalinktype
--linktype=datalinktype
Set the data link type to use while capturing packets (see -L)
or just compiling and dumping packet-matching code (see -d) to
datalinktype.

-z postrotate-command
Used in conjunction with the -C or -G options, this will make
tcpdump run " postrotate-command file " where file is the
savefile being closed after each rotation. For example,
specifying -z gzip or -z bzip2 will compress each savefile
using gzip or bzip2.

Note that tcpdump will run the command in parallel to the
capture, using the lowest priority so that this doesn't
disturb the capture process.

And in case you would like to use a command that itself takes
flags or different arguments, you can always write a shell
script that will take the savefile name as the only argument,
make the flags & arguments arrangements and execute the
command that you want.

-Z user
--relinquish-privileges=user
If tcpdump is running as root, after opening the capture
device or input savefile, but before opening any savefiles for
output, change the user ID to user and the group ID to the
primary group of user.

This behavior can also be enabled by default at compile time.

expression
selects which packets will be dumped. If no expression is
given, all packets on the net will be dumped. Otherwise, only
packets for which expression is `true' will be dumped.

For the expression syntax, see pcap-filter(5).

The expression argument can be passed to tcpdump as either a
single Shell argument, or as multiple Shell arguments,
whichever is more convenient. Generally, if the expression
contains Shell metacharacters, such as backslashes used to
escape protocol names, it is easier to pass it as a single,
quoted argument rather than to escape the Shell
metacharacters. Multiple arguments are concatenated with
spaces before being parsed.

EXAMPLES

To print all packets arriving at or departing from sundown:
tcpdump host sundown

To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether

To print all ftp traffic through internet gateway snup: (note that
the expression is quoted to prevent the shell from (mis-)interpreting
the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'

To print traffic neither sourced from nor destined for local hosts
(if you gateway to one other net, this stuff should never make it
onto your local net).
tcpdump ip and not net localnet

To print the start and end packets (the SYN and FIN packets) of each
TCP conversation that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

To print the TCP packets with flags RST and ACK both set. (i.e.
select only the RST and ACK flags in the flags field, and if the
result is "RST and ACK both set", match)
tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'

To print all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data, not, for example, SYN and FIN packets and
ACK-only packets. (IPv6 is left as an exercise for the reader.)
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'

To print IP broadcast or multicast packets that were not sent via
Ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

To print all ICMP packets that are not echo requests/replies (i.e.,
not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

OUTPUT FORMAT

The output of tcpdump is protocol dependent. The following gives a
brief description and examples of most of the formats.

Timestamps

By default, all output lines are preceded by a timestamp. The
timestamp is the current clock time in the form
hh:mm:ss.frac
and is as accurate as the kernel's clock. The timestamp reflects the
time the kernel applied a time stamp to the packet. No attempt is
made to account for the time lag between when the network interface
finished receiving the packet from the network and when the kernel
applied a time stamp to the packet; that time lag could include a
delay between the time when the network interface finished receiving
a packet from the network and the time when an interrupt was
delivered to the kernel to get it to read the packet and a delay
between the time when the kernel serviced the `new packet' interrupt
and the time when it applied a time stamp to the packet.

Interface

When the any interface is selected on capture or when a link-type
LINUX_SLL2 capture file is read the interface name is printed after
the timestamp. This is followed by the packet type with In and Out
denoting a packet destined for this host or originating from this
host respectively. Other possible values are B for broadcast packets,
M for multicast packets, and P for packets destined for other hosts.

Link Level Headers

If the '-e' option is given, the link level header is printed out.
On Ethernets, the source and destination addresses, protocol, and
packet length are printed.

On FDDI networks, the '-e' option causes tcpdump to print the `frame
control' field, the source and destination addresses, and the packet
length. (The `frame control' field governs the interpretation of the
rest of the packet. Normal packets (such as those containing IP
datagrams) are `async' packets, with a priority value between 0 and
7; for example, `async4'. Such packets are assumed to contain an
802.2 Logical Link Control (LLC) packet; the LLC header is printed if
it is not an ISO datagram or a so-called SNAP packet.

On Token Ring networks, the '-e' option causes tcpdump to print the
`access control' and `frame control' fields, the source and
destination addresses, and the packet length. As on FDDI networks,
packets are assumed to contain an LLC packet. Regardless of whether
the '-e' option is specified or not, the source routing information
is printed for source-routed packets.

On 802.11 networks, the '-e' option causes tcpdump to print the
`frame control' fields, all of the addresses in the 802.11 header,
and the packet length. As on FDDI networks, packets are assumed to
contain an LLC packet.

(N.B.: The following description assumes familiarity with the SLIP
compression algorithm described in RFC 1144.)

On SLIP links, a direction indicator (``I'' for inbound, ``O'' for
outbound), packet type, and compression information are printed out.
The packet type is printed first. The three types are ip, utcp, and
ctcp. No further link information is printed for ip packets. For
TCP packets, the connection identifier is printed following the type.
If the packet is compressed, its encoded header is printed out. The
special cases are printed out as *S+n and *SA+n, where n is the
amount by which the sequence number (or sequence number and ack) has
changed. If it is not a special case, zero or more changes are
printed. A change is indicated by U (urgent pointer), W (window), A
(ack), S (sequence number), and I (packet ID), followed by a delta
(+n or -n), or a new value (=n). Finally, the amount of data in the
packet and compressed header length are printed.

For example, the following line shows an outbound compressed TCP
packet, with an implicit connection identifier; the ack has changed
by 6, the sequence number by 49, and the packet ID by 6; there are 3
bytes of data and 6 bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)

ARP/RARP Packets
ARP/RARP output shows the type of request and its arguments. The
format is intended to be self explanatory. Here is a short sample
taken from the start of an `rlogin' from host rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an ARP packet asking for the
Ethernet address of internet host csam. Csam replies with its
Ethernet address (in this example, Ethernet addresses are in caps and
internet addresses in lower case).

This would look less redundant if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

If we had done tcpdump -e, the fact that the first packet is
broadcast and the second is point-to-point would be visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the Ethernet source address is RTSG,
the destination is the Ethernet broadcast address, the type field
contained hex 0806 (type ETHER_ARP) and the total length was 64
bytes.

IPv4 Packets
If the link-layer header is not being printed, for IPv4 packets, IP
is printed after the time stamp.

If the -v flag is specified, information from the IPv4 header is
shown in parentheses after the IP or the link-layer header. The
general format of this information is:
tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
tos is the type of service field; if the ECN bits are non-zero, those
are reported as ECT(1), ECT(0), or CE. ttl is the time-to-live; it
is not reported if it is zero. id is the IP identification field.
offset is the fragment offset field; it is printed whether this is
part of a fragmented datagram or not. flags are the MF and DF flags;
+ is reported if MF is set, and DF is reported if F is set. If
neither are set, . is reported. proto is the protocol ID field.
length is the total length field; if the packet is a presumed TSO
(TCP Segmentation Offload) send, [was 0, presumed TSO] is reported.
options are the IP options, if any.

Next, for TCP and UDP packets, the source and destination IP
addresses and TCP or UDP ports, with a dot between each IP address
and its corresponding port, will be printed, with a > separating the
source and destination. For other protocols, the addresses will be
printed, with a > separating the source and destination. Higher
level protocol information, if any, will be printed after that.

For fragmented IP datagrams, the first fragment contains the higher
level protocol header; fragments after the first contain no higher
level protocol header. Fragmentation information will be printed
only with the -v flag, in the IP header information, as described
above.

TCP Packets

(N.B.:The following description assumes familiarity with the TCP
protocol described in RFC 793. If you are not familiar with the
protocol, this description will not be of much use to you.)

The general format of a TCP protocol line is:
src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
Src and dst are the source and destination IP addresses and ports.
Tcpflags are some combination of S (SYN), F (FIN), P (PSH), R (RST),
U (URG), W (CWR), E (ECE) or `.' (ACK), or `none' if no flags are
set. Data-seqno describes the portion of sequence space covered by
the data in this packet (see example below). Ackno is sequence
number of the next data expected the other direction on this
connection. Window is the number of bytes of receive buffer space
available the other direction on this connection. Urg indicates
there is `urgent' data in the packet. Opts are TCP options (e.g.,
mss 1024). Len is the length of payload data.

Iptype, Src, dst, and flags are always present. The other fields
depend on the contents of the packet's TCP protocol header and are
output only if appropriate.

Here is the opening portion of an rlogin from host rtsg to host csam.
IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1
The first line says that TCP port 1023 on rtsg sent a packet to port
login on csam. The S indicates that the SYN flag was set. The
packet sequence number was 768512 and it contained no data. (The
notation is `first:last' which means `sequence numbers first up to
but not including last'.) There was no piggy-backed ACK, the
available receive window was 4096 bytes and there was a max-segment-
size option requesting an MSS of 1024 bytes.

Csam replies with a similar packet except it includes a piggy-backed
ACK for rtsg's SYN. Rtsg then ACKs csam's SYN. The `.' means the
ACK flag was set. The packet contained no data so there is no data
sequence number or length. Note that the ACK sequence number is a
small integer (1). The first time tcpdump sees a TCP `conversation',
it prints the sequence number from the packet. On subsequent packets
of the conversation, the difference between the current packet's
sequence number and this initial sequence number is printed. This
means that sequence numbers after the first can be interpreted as
relative byte positions in the conversation's data stream (with the
first data byte each direction being `1'). `-S' will override this
feature, causing the original sequence numbers to be output.

On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
in the rtsg -> csam side of the conversation). The PSH flag is set
in the packet. On the 7th line, csam says it's received data sent by
rtsg up to but not including byte 21. Most of this data is
apparently sitting in the socket buffer since csam's receive window
has gotten 19 bytes smaller. Csam also sends one byte of data to
rtsg in this packet. On the 8th and 9th lines, csam sends two bytes
of urgent, pushed data to rtsg.

If the snapshot was small enough that tcpdump didn't capture the full
TCP header, it interprets as much of the header as it can and then
reports ``[|tcp]'' to indicate the remainder could not be
interpreted. If the header contains a bogus option (one with a
length that's either too small or beyond the end of the header),
tcpdump reports it as ``[bad opt]'' and does not interpret any
further options (since it's impossible to tell where they start). If
the header length indicates options are present but the IP datagram
length is not long enough for the options to actually be there,
tcpdump reports it as ``[bad hdr length]''.

Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.)
There are 8 bits in the control bits section of the TCP header:

CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

Let's assume that we want to watch packets used in establishing a TCP
connection. Recall that TCP uses a 3-way handshake protocol when it
initializes a new connection; the connection sequence with regard to
the TCP control bits is

1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK

Now we're interested in capturing packets that have only the SYN bit
set (Step 1). Note that we don't want packets from step 2 (SYN-ACK),
just a plain initial SYN. What we need is a correct filter
expression for tcpdump.

Recall the structure of a TCP header without options:

0 15 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless options are
present. The first line of the graph contains octets 0 - 3, the
second line shows octets 4 - 7 etc.

Starting to count with 0, the relevant TCP control bits are contained
in octet 13:

0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |

Let's have a closer look at octet no. 13:

| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|

These are the TCP control bits we are interested in. We have
numbered the bits in this octet from 0 to 7, right to left, so the
PSH bit is bit number 3, while the URG bit is number 5.

Recall that we want to capture packets with only SYN set. Let's see
what happens to octet 13 if a TCP datagram arrives with the SYN bit
set in its header:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Looking at the control bits section we see that only bit number 1
(SYN) is set.

Assuming that octet number 13 is an 8-bit unsigned integer in network
byte order, the binary value of this octet is

00000010

and its decimal representation is

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2

We're almost done, because now we know that if only SYN is set, the
value of the 13th octet in the TCP header, when interpreted as a
8-bit unsigned integer in network byte order, must be exactly 2.

This relationship can be expressed as
tcp[13] == 2

We can use this expression as the filter for tcpdump in order to
watch packets which have only SYN set:
tcpdump -i xl0 'tcp[13] == 2'

The expression says "let the 13th octet of a TCP datagram have the
decimal value 2", which is exactly what we want.

Now, let's assume that we need to capture SYN packets, but we don't
care if ACK or any other TCP control bit is set at the same time.
Let's see what happens to octet 13 when a TCP datagram with SYN-ACK
set arrives:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Now bits 1 and 4 are set in the 13th octet. The binary value of
octet 13 is

00010010

which translates to decimal

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump filter
expression, because that would select only those packets that have
SYN-ACK set, but not those with only SYN set. Remember that we don't
care if ACK or any other control bit is set as long as SYN is set.

In order to achieve our goal, we need to logically AND the binary
value of octet 13 with some other value to preserve the SYN bit. We
know that we want SYN to be set in any case, so we'll logically AND
the value in the 13th octet with the binary value of a SYN:


00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010

We see that this AND operation delivers the same result regardless
whether ACK or another TCP control bit is set. The decimal
representation of the AND value as well as the result of this
operation is 2 (binary 00000010), so we know that for packets with
SYN set the following relation must hold true:

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'

Some offsets and field values may be expressed as names rather than
as numeric values. For example tcp[13] may be replaced with
tcp[tcpflags]. The following TCP flag field values are also
available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg,
tcp-ece and tcp-cwr.

This can be demonstrated as:
tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

Note that you should use single quotes or a backslash in the
expression to hide the AND ('&') special character from the shell.

UDP Packets

UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a UDP datagram to port
who on host broadcast, the Internet broadcast address. The packet
contained 84 bytes of user data.

Some UDP services are recognized (from the source or destination port
number) and the higher level protocol information printed. In
particular, Domain Name service requests (RFC 1034/1035) and Sun RPC
calls (RFC 1050) to NFS.

TCP or UDP Name Server Requests

(N.B.:The following description assumes familiarity with the Domain
Service protocol described in RFC 1035. If you are not familiar with
the protocol, the following description will appear to be written in
Greek.)

Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an address record
(qtype=A) associated with the name ucbvax.berkeley.edu. The query id
was `3'. The `+' indicates the recursion desired flag was set. The
query length was 37 bytes, excluding the TCP or UDP and IP protocol
headers. The query operation was the normal one, Query, so the op
field was omitted. If the op had been anything else, it would have
been printed between the `3' and the `+'. Similarly, the qclass was
the normal one, C_IN, and omitted. Any other qclass would have been
printed immediately after the `A'.

A few anomalies are checked and may result in extra fields enclosed
in square brackets: If a query contains an answer, authority records
or additional records section, ancount, nscount, or arcount are
printed as `[na]', `[nn]' or `[nau]' where n is the appropriate
count. If any of the response bits are set (AA, RA or rcode) or any
of the `must be zero' bits are set in bytes two and three, `[b2&3=x]'
is printed, where x is the hex value of header bytes two and three.

TCP or UDP Name Server Responses

Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo with
3 answer records, 3 name server records and 7 additional records.
The first answer record is type A (address) and its data is internet
address 128.32.137.3. The total size of the response was 273 bytes,
excluding TCP or UDP and IP headers. The op (Query) and response
code (NoError) were omitted, as was the class (C_IN) of the A record.

In the second example, helios responds to query 2 with a response
code of nonexistent domain (NXDomain) with no answers, one name
server and no authority records. The `*' indicates that the
authoritative answer bit was set. Since there were no answers, no
type, class or data were printed.

Other flag characters that might appear are `-' (recursion available,
RA, not set) and `|' (truncated message, TC, set). If the `question'
section doesn't contain exactly one entry, `[nq]' is printed.

SMB/CIFS Decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
NetBEUI SMB data is also done.

By default a fairly minimal decode is done, with a much more detailed
decode done if -v is used. Be warned that with -v a single SMB
packet may take up a page or more, so only use -v if you really want
all the gory details.

For information on SMB packet formats and what all the fields mean
see https://download.samba.org/pub/samba/specs/ and other online
resources. The SMB patches were written by Andrew Tridgell
(tridge@samba.org).

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as:
src.sport > dst.nfs: NFS request xid xid len op args
src.nfs > dst.dport: NFS reply xid xid reply stat len op results

sushi.1023 > wrl.nfs: NFS request xid 26377
112 readlink fh 21,24/10.73165
wrl.nfs > sushi.1023: NFS reply xid 26377
reply ok 40 readlink "../var"
sushi.1022 > wrl.nfs: NFS request xid 8219
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.1022: NFS reply xid 8219
reply ok 128 lookup fh 9,74/4134.3150

In the first line, host sushi sends a transaction with id 26377 to
wrl. The request was 112 bytes, excluding the UDP and IP headers.
The operation was a readlink (read symbolic link) on file handle (fh)
21,24/10.731657119. (If one is lucky, as in this case, the file
handle can be interpreted as a major,minor device number pair,
followed by the inode number and generation number.) In the second
line, wrl replies `ok' with the same transaction id and the contents
of the link.

In the third line, sushi asks (using a new transaction id) wrl to
lookup the name `xcolors' in directory file 9,74/4096.6878. In the
fourth line, wrl sends a reply with the respective transaction id.

Note that the data printed depends on the operation type. The format
is intended to be self explanatory if read in conjunction with an NFS
protocol spec. Also note that older versions of tcpdump printed NFS
packets in a slightly different format: the transaction id (xid)
would be printed instead of the non-NFS port number of the packet.

If the -v (verbose) flag is given, additional information is printed.
For example:

sushi.1023 > wrl.nfs: NFS request xid 79658
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1023: NFS reply xid 79658
reply ok 1472 read REG 100664 ids 417/0 sz 29388

(-v also prints the IP header TTL, ID, length, and fragmentation
fields, which have been omitted from this example.) In the first
line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at
byte offset 24576. Wrl replies `ok'; the packet shown on the second
line is the first fragment of the reply, and hence is only 1472 bytes
long (the other bytes will follow in subsequent fragments, but these
fragments do not have NFS or even UDP headers and so might not be
printed, depending on the filter expression used). Because the -v
flag is given, some of the file attributes (which are returned in
addition to the file data) are printed: the file type (``REG'', for
regular file), the file mode (in octal), the UID and GID, and the
file size.

If the -v flag is given more than once, even more details are
printed.

NFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump keeps track of ``recent'' requests, and matches them
to the replies using the transaction ID. If a reply does not closely
follow the corresponding request, it might not be parsable.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are printed
as:

src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args

elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename

In the first line, host elvis sends a RX packet to pike. This was a
RX data packet to the fs (fileserver) service, and is the start of an
RPC call. The RPC call was a rename, with the old directory file id
of 536876964/1/1 and an old filename of `.newsrc.new', and a new
directory file id of 536876964/1/1 and a new filename of `.newsrc'.
The host pike responds with a RPC reply to the rename call (which was
successful, because it was a data packet and not an abort packet).

In general, all AFS RPCs are decoded at least by RPC call name. Most
AFS RPCs have at least some of the arguments decoded (generally only
the `interesting' arguments, for some definition of interesting).

The format is intended to be self-describing, but it will probably
not be useful to people who are not familiar with the workings of AFS
and RX.

If the -v (verbose) flag is given twice, acknowledgement packets and
additional header information is printed, such as the RX call ID,
call number, sequence number, serial number, and the RX packet flags.

If the -v flag is given twice, additional information is printed,
such as the RX call ID, serial number, and the RX packet flags. The
MTU negotiation information is also printed from RX ack packets.

If the -v flag is given three times, the security index and service
id are printed.

Error codes are printed for abort packets, with the exception of Ubik
beacon packets (because abort packets are used to signify a yes vote
for the Ubik protocol).

AFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump keeps track of ``recent'' requests, and matches them
to the replies using the call number and service ID. If a reply does
not closely follow the corresponding request, it might not be
parsable.


KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-
encapsulated and dumped as DDP packets (i.e., all the UDP header
information is discarded). The file /etc/atalk.names is used to
translate AppleTalk net and node numbers to names. Lines in this
file have the form
number name

1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of AppleTalk networks. The third
line gives the name of a particular host (a host is distinguished
from a net by the 3rd octet in the number - a net number must have
two octets and a host number must have three octets.) The number and
name should be separated by whitespace (blanks or tabs). The
/etc/atalk.names file may contain blank lines or comment lines (lines
starting with a `#').

AppleTalk addresses are printed in the form
net.host.port

144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn't exist or doesn't contain an entry
for some AppleTalk host/net number, addresses are printed in numeric
form.) In the first example, NBP (DDP port 2) on net 144.1 node 209
is sending to whatever is listening on port 220 of net icsd node 112.
The second line is the same except the full name of the source node
is known (`office'). The third line is a send from port 235 on net
jssmag node 149 to broadcast on the icsd-net NBP port (note that the
broadcast address (255) is indicated by a net name with no host
number - for this reason it's a good idea to keep node names and net
names distinct in /etc/atalk.names).

NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
packets have their contents interpreted. Other protocols just dump
the protocol name (or number if no name is registered for the
protocol) and packet size.

NBP Packets

NBP packets are formatted like the following examples:
icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
The first line is a name lookup request for laserwriters sent by net
icsd host 112 and broadcast on net jssmag. The nbp id for the lookup
is 190. The second line shows a reply for this request (note that it
has the same id) from host jssmag.209 saying that it has a
laserwriter resource named "RM1140" registered on port 250. The
third line is another reply to the same request saying host techpit
has laserwriter "techpit" registered on port 186.

ATP Packets

ATP packet formatting is demonstrated by the following example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by
requesting up to 8 packets (the `<0-7>'). The hex number at the end
of the line is the value of the `userdata' field in the request.

Helios responds with 8 512-byte packets. The `:digit' following the
transaction id gives the packet sequence number in the transaction
and the number in parens is the amount of data in the packet,
excluding the ATP header. The `*' on packet 7 indicates that the EOM
bit was set.

Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
resends them then jssmag.209 releases the transaction. Finally,
jssmag.209 initiates the next request. The `*' on the request
indicates that XO (`exactly once') was not set.

BACKWARD COMPATIBILITY

The TCP flag names tcp-ece and tcp-cwr became available when linking
with libpcap 1.9.0 or later.

SEE ALSO

stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(4),
pcap-filter(5), pcap-tstamp(5)

https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap

AUTHORS

The original authors are:

Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
Berkeley National Laboratory, University of California, Berkeley, CA.

It is currently maintained by The Tcpdump Group.

The current version is available via HTTPS:

https://www.tcpdump.org/

The original distribution is available via anonymous ftp:

ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z

IPv6/IPsec support is added by WIDE/KAME project. This program uses
OpenSSL/LibreSSL, under specific configurations.

BUGS

To report a security issue please send an e-mail to
security@tcpdump.org.

To report bugs and other problems, contribute patches, request a
feature, provide generic feedback etc. please see the file
CONTRIBUTING.md in the tcpdump source tree root.

NIT doesn't let you watch your own outbound traffic, BPF will. We
recommend that you use the latter.

Some attempt should be made to reassemble IP fragments or, at least
to compute the right length for the higher level protocol.

Name server inverse queries are not dumped correctly: the (empty)
question section is printed rather than real query in the answer
section. Some believe that inverse queries are themselves a bug and
prefer to fix the program generating them rather than tcpdump.

A packet trace that crosses a daylight savings time change will give
skewed time stamps (the time change is ignored).

Filter expressions on fields other than those in Token Ring headers
will not correctly handle source-routed Token Ring packets.

Filter expressions on fields other than those in 802.11 headers will
not correctly handle 802.11 data packets with both To DS and From DS
set.

ip6 proto should chase header chain, but at this moment it does not.
ip6 protochain is supplied for this behavior.

Arithmetic expression against transport layer headers, like tcp[0],
does not work against IPv6 packets. It only looks at IPv4 packets.

26 March 2024 TCPDUMP(1)