TCPDUMP(1)

📦
version 4.99.7 · 3 versions
, in arch rolling, nixpkgs unstable

okay bestie, welcome to the tcpdump(1) page from the General Commands Manual — it's packet-capture lore, but make it cute. 💅✨

✨ NAME ✨

tcpdump — dump traffic on a network. that's the whole assignment. 👑

🦋 SYNOPSIS 🦋

tcpdump [ -AbdDefghHIJKlLnNOpqStuUvxX# ] [ -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 💖

okay so tcpdump prints out a description of the contents of packets on a network interface that match the Boolean expression (peep pcap-filter(7) 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 makes it save the packet data to a file for later analysis, and/or with the -r flag, which makes it read from a saved packet file rather than read packets from a network interface. it can also be run with the -V flag, which makes it read a list of saved packet files. in all cases, only packets that match expression will be processed by tcpdump. no cap. 🫶

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. no effect when -x[x] or -X[X] options are used. 🎀
  • -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 default unit of file_size is millions of bytes (1,000,000 bytes, not 1,048,576 bytes). 💜

by adding a suffix of k/K, m/M or g/G to the value, the unit can be changed to 1,024 (KiB), 1,048,576 (MiB), or 1,073,741,824 (GiB) respectively. 👑

  • -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 (see the -i flag below), 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, --ip-oneline — do not insert a line break after the IP header in verbose mode. 🎀
  • -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*** — okay so this is your "which network thing am I even looking at" flag, and it's lowkey the most important one. It makes tcpdump listen on *interface*, report the list of link-layer types, report the list of time stamp types, or report the results of compiling a filter expression — all on that interface. If you leave it unspecified and you didn't pass the -dflag, tcpdump goes snooping through the system interface list and grabs the **lowest numbered, configured up interface** (loopback is excluded, bestie, don't worry), which might turn out to be, like,eth0`.

On all supported Linux systems, plus recent versions of macOS and Solaris, an interface argument of any means a special pseudo-interface that captures packets from all the regular network interfaces of the OS. Now, what counts as a "regular network interface" is an implementation detail of the OS (for example, on Linux this includes SocketCAN devices), so packets captured on the any pseudo-interface may represent way more different network protocols than you'd expect. Also, all regular network interfaces are a subset of all available capture devices as printed by the -D flag — that bigger list may also include capture devices that libpcap implements by means other than the OS network stack: Bluetooth, DAG, D-Bus, SNF and USB. Quick heads-up: captures on the any pseudo-interface will not be done in promiscuous mode, periodt.

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 — puts the interface in "monitor mode" 🕵️‍♀️ and no, it's not available everywhere: this is supported only on IEEE 802.11 Wi-Fi interfaces, and supported only on some operating systems.

Real talk though: in monitor mode the adapter might disassociate from the network it's associated with, so you won't be able to use any wireless networks with that adapter. That could straight-up prevent you from accessing files on a network server, or resolving host names or network addresses, if you're capturing in monitor mode and you're not connected to another network with another adapter. 💀

This flag also affects 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 — captures in "immediate mode" ⚡ where packets are delivered to tcpdump as soon as they arrive, instead of being buffered for efficiency. This is the default when you're 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*** — sets the time stamp type for the capture to *tstamp_type*. The names to use for the time stamp types are given in pcap-tstamp(7)`; and ngl, not all the types listed there will necessarily be valid for any given interface.
  • -J, --list-time-stamp-types — lists the supported time stamp types for the interface and dips ✌️. If the time stamp type cannot be set for the interface, no time stamp types are listed.
  • --time-stamp-precision=tstamp_precision — when capturing, sets 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 — and not all programs that read pcap savefiles will be able to read those captures 🫠.

When reading a savefile, it converts time stamps to the precision specified by timestamp_precision, and displays 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 genuinely useful for interfaces that perform some or all of those checksum calculations in hardware; otherwise, all outgoing TCP checksums will be flagged as bad, and you'll be sitting there like "why is everything broken" 😭.
  • -l — makes 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 — lists the known data link types for the interface, in the specified mode, and exits. 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). It's giving "it depends" 💅.
  • -m module — loads SMI MIB module definitions from file module. This option can be used several times to load several MIB modules into tcpdump. Stack 'em up 🎀.
  • -M secret — uses 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. Numbers only, no DNS side quests 🔥.
  • -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 — prints a packet number at the beginning of the line. Little numbered receipts for every packet 👑
  • -O, --no-optimize — do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer. Like, only then, bestie.
  • -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. Important distinction, don't get it twisted 🫶.
  • --print — prints parsed packet output, even if the raw packets are being saved to a file with the -w flag.
  • *-Q *direction, --direction=*direction*** — chooses the send/receive direction *direction* for which packets should be captured. Possible values are in, outandinout`. Not available on all platforms. 🙈
  • -q — Quick (quiet?) output. Prints less protocol information so output lines are shorter. Minimalist queen behavior.
  • -r file — reads 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 — prints 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 262144bytes. 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 — forces packets selected by expression to be interpreted as 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). That's a whole alphabet of vibes 🌈.

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".

Ok so, real quick, heads-up about that pgm_zmtp1 type we just mentioned — it's a whole double-dipper, because it messes with both native PGM and UDP at the same time. 🫠 When we're doing the native PGM decoding, the application data of an ODATA/RDATA packet gets decoded as a ZeroMQ datagram with ZMTP/1.0 frames. And during the UDP decoding, on top of all that, any UDP packet gets treated as an encapsulated PGM packet. Two-for-one special, no cap.

  • -t — don't print a timestamp on each dump line. that's literally it. no clock, no stress 💅
  • -tt — print the timestamp as seconds since January 1, 1970, 00:00:00, UTC, plus fractions of a second since that time, on each dump line. epoch girlie behavior ⏳
  • -ttt — print a delta on each dump line, between the current line and the previous line. Resolution is microsecond or nanosecond depending on the --time-stamp-precision option, and the default is microsecond resolution.
  • -tttt — print a timestamp of hours, minutes, seconds, and fractions of a second since midnight, preceded by the date, on each dump line. very "good morning bestie" of it 🌸
  • -ttttt — print a delta on each dump line between the current line and the first line. Again, microsecond or nanosecond resolution depending on --time-stamp-precision, default microsecond.
  • -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, this makes the printed packet output ``packet-buffered''. Which means: as the description of the contents of each packet is printed, it gets written to standard output right then, instead of — when not writing to a terminal — only being written once the output buffer fills.

If the -w option is specified, this makes the saved raw packet output ``packet-buffered'' instead. Same deal: as each packet is saved, it gets written to the output file, rather than only being written when the output buffer fills. 🫶

Fr fr heads-up: 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. Like, for example, the time to live, identification, total length and options in an IP packet get printed. It also switches on additional packet integrity checks, such as verifying the IP and ICMP header checksum.

When you're writing to a file with the -w option and at the same time not reading from a file with the -r option, it reports 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. 😭 (not the update causing problems…)

  • -vv — even more verbose output. For example, additional fields are printed from NFS reply packets, and SMB packets are fully decoded. it's giving thorough 🔥
  • -vvv — even more verbose output, again. For example, telnet SB ... SE options are printed in full. And 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 instead of parsing and printing them out. You can print them later 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(5) for a description of the file format.

  • -W filecount — used in conjunction with the -C option, this limits the number of files created to the specified filecount, and starts overwriting files from the beginning, thus creating a 'rotating' buffer. On top of that, it names the files with enough leading 0s to support the maximum number of files, so they sort correctly. 🔄

Used in conjunction with the -G option, this limits 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. (yes, bestie, it's one of those flags 🙈)

  • -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. No effect when -X[X] option is used.

  • -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. No effect when -X[X] option is used.
  • -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 makes 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. 🗜️

This option can only be used if fork subprocess is implemented (e.g. not on Windows systems).

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. iykyk 💜

  • -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(7).

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 🎬

okay these are the good part. copy-paste responsibly, bestie 💖

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 — and 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. 😌 good luck bestie)

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 (aka the little clock thing at the start of every line)

Okay so bestie, by default, every single output line gets a timestamp in front of it. Every one. It's the current clock time, and it looks like this:

hh:mm:ss.frac

It's as accurate as the kernel's clock, periodt. And here's the tea: the timestamp reflects the time the kernel applied a time stamp to the packet — not when the packet actually showed up. tcpdump makes no attempt 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 it. 😭

And that lag is not a vibe check you can skip — it 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.

Like, the kernel is doing its best, ok? 🫶

🦋 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. Slay.

Then comes the packet type, where:

  • In = a packet destined for this host 👑
  • Out = a packet originating from this host 🚀
  • B = broadcast packets 📢
  • M = multicast packets 🌈
  • P = packets destined for other hosts 👀

If you hand it the -e option, the link level header gets printed out. Boom. On Ethernets, that means the source and destination addresses, protocol, and packet length are printed. Easy.

On FDDI networks, the -e option makes tcpdump 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 — it's lowkey the boss of the packet. Normal packets, like the ones containing IP datagrams, are async packets with a priority value between 0 and 7 — for example, async4. Those packets are assumed to contain an 802.2 Logical Link Control (LLC) packet, and the LLC header is printed if it's not an ISO datagram or a so-called SNAP packet.)

On Token Ring networks, the -e option makes tcpdump print the access control and frame control fields, the source and destination addresses, and the packet length. Just like on FDDI networks, packets are assumed to contain an LLC packet. And here's the fun part: regardless of whether the -e option is specified or not, the source routing information is printed for source-routed packets. It simply does not care. 💅

On 802.11 networks, the -e option makes tcpdump print the frame control fields, all of the addresses in the 802.11 header, and the packet length. Same deal as FDDI: packets are assumed to contain an LLC packet.

N.B.: The following description assumes familiarity with the SLIP compression algorithm described in RFC 1144. So like, if you haven't read that, this next bit might be giving cryptic, ngl.

On SLIP links, a direction indicator (I for inbound, O for outbound), packet type, and compression information are printed out. The packet type gets 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)
  • I (packet ID)

...followed by a delta (+n or -n), or a new value (=n). Finally, the amount of data in the packet and the compressed header length are printed. Chef's kiss. 👨‍🍳💋

For example, this 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, and the format is intended to be self explanatory. Iconic behaviour honestly.

Here's 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 — and in this example, Ethernet addresses are in caps and internet addresses are in lower case. Cute, right?

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

And 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. We love a fully explained packet bestie. 💖

💜 IPv4 Packets

If the link-layer header is not being printed, then 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)

Let's break down every single one of those, because knowing is half the battle:

  • 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; if the packet is a presumed BIG TCP, [was 0, presumed BIG TCP] 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 get printed, with a dot between each IP address and its corresponding port, and a > separating the source and destination. For other protocols, the addresses are printed with a > separating the source and destination. Higher level protocol information, if any, gets printed after that. Slay, hierarchy. 🏆

For fragmented IP datagrams: the first fragment contains the higher level protocol header, and 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. That's the rules, bestie.

🔥 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. Like, fr fr, it's giving "you had to be there". 😭

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

So what's what:

  • 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), e (AE) 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 the 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. Main character fields only. ✨

Here's 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. Obsessed with the clarity. 💅

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).

Here's the why: 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). And -S will override this feature, causing the original sequence numbers to be output. Your choice, bestie. 🎀

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. And on the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg. Look at them go. 🦋

Okay so if the snapshot was too small and tcpdump didn't get the whole TCP header? It'll interpret as much as it physically can and then hit you with [|tcp] to say "bestie the rest is unreadable, sorry 💀". And if the header has a bogus option — like one with a length that's either too small or goes past the end of the header — tcpdump flags it as [bad opt] and just stops interpreting any further options (because honestly how is it supposed to know where the next one even starts?). And if the header length says "yeah there are options here" but the IP datagram length isn't long enough for those options to actually exist? You get [bad hdr length]. Simple as that. 🎀

✨ Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.) ✨

So the control bits section of the TCP header has 9 bits in it, and they are, in order:

AE(e) CWR(W) ECE(E) URG(U) ACK(.) PSH(P) RST(R) SYN(S) FIN(F)

Okay so picture this: we wanna watch the packets that set up a TCP connection. Remember TCP does that iconic 3-way handshake protocol when it spins up a new connection — and the connection sequence, in terms of the TCP control bits, goes like this:

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

Now we only want to capture the packets that have only the SYN bit set (that's step 1, the main character). Note — we do not want the step 2 packets (SYN-ACK), just the plain original SYN. So what we need is a correct filter expression for tcpdump. Let's go. 💅

Remember the structure of a TCP header without options:

 0                            15                              31
-----------------------------------------------------------------
|          source port          |       destination port        |
-----------------------------------------------------------------
|                        sequence number                        |
-----------------------------------------------------------------
|                     acknowledgment number                     |
-----------------------------------------------------------------
|header |re   |A|C|E|U|A|P|R|S|F|                               |
|length |serv |E|W|C|R|C|S|S|Y|I|        window size            |
|       |  ed | |R|E|G|K|H|T|N|N|                               |
-----------------------------------------------------------------
|         TCP checksum          |       urgent pointer          |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless options are present. The first line of that graph is octets 0 - 3, the second line is octets 4 - 7, etc. (Math! We love her. 🔥)

Start counting at 0, and the relevant TCP control bits live in octets 12 and 13:

 0             7|             15|             23|             31
----------------|---------------|---------------|----------------
|header |re   |A|C|E|U|A|P|R|S|F|                               |
|length |serv |E|W|C|R|C|S|S|Y|I|        window size            |
|       |  ed | |R|E|G|K|H|T|N|N|                               |
----------------|---------------|---------------|----------------
|               |  13th octet   |               |               |

Now let's zoom in on octet no. 13, bestie:

                |               |
                |---------------|
                |C|E|U|A|P|R|S|F|
                |W|C|R|C|S|S|Y|I|
                |R|E|G|K|H|T|N|N|
                |---------------|
                |7   5   3     0|

These are THE bits we care about. We numbered the bits in this octet from 0 to 7, right to left, so the PSH bit is bit number 3 and the URG bit is bit number 5. Okay? Okay. 💖

Recall (again, sorry, but it's important) that we want the packets with only SYN set. So watch what happens to octet 13 when a TCP datagram shows up with the SYN bit set in its header:

                |C|E|U|A|P|R|S|F|
                |W|C|R|C|S|S|Y|I|
                |R|E|G|K|H|T|N|N|
                |---------------|
                |0 0 0 0 0 0 1 0|
                |---------------|
                |7 6 5 4 3 2 1 0|

Squint at the control bits section and you'll see only bit number 1 (SYN) is set. Slay. ✨

So assuming 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

Almost done, we promise. Because now we know: if only SYN is set, then the value of the 13th octet in the TCP header — interpreted as an 8-bit unsigned integer in network byte order — has to be exactly 2.

That relationship can be written as:

tcp[13] == 2

And we can drop that expression straight into tcpdump's filter to watch only the packets with SYN set:

tcpdump -i xl0 'tcp[13] == 2'

That expression is literally saying "let the 13th octet of a TCP datagram have the decimal value 2" — which is exactly the vibe we were going for. 💅

Now. Plot twist 🫶 — let's say we need to capture SYN packets but we genuinely do not care whether ACK or any other control bit is set at the same time. So let's look at octet 13 when a TCP datagram with SYN-ACK set rolls in:

     |C|E|U|A|P|R|S|F|
     |W|C|R|C|S|S|Y|I|
     |R|E|G|K|H|T|N|N|
     |---------------|
     |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. So 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

And here's the thing — we can't just use tcp[13] == 18 in the tcpdump filter expression, because that would select only the packets with SYN-ACK set and miss the ones with only SYN set. And remember: we don't care if ACK or any other control bit is set, as long as SYN is set. Periodt.

So to actually get what we want, we've gotta logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know we want SYN set no matter what, so we 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

Look at that 😭 — this AND operation delivers the same result whether ACK or another TCP control bit is set or not. And the decimal representation of the AND value, plus the result of the operation, is 2 (binary 00000010). So we know that for packets with SYN set, this relation has to hold true:

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

Which points us right at the tcpdump filter expression:

tcpdump -i xl0 'tcp[13] & 2 == 2'

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

Demonstration time, watch this:

tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
⚠️ Heads up bestie: you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell. The shell will absolutely eat it otherwise and you'll be sad. 💀

🌸 UDP Packets 🌸

UDP format is illustrated by this rwho packet:

actinide.who > broadcast.who: udp 84

Translation: port who on host actinide sent a UDP datagram to port who on host broadcast — that's the Internet broadcast address. The packet contained 84 bytes of user data. Simple, clean, no notes. 💜

Some UDP services get recognized (from the source or destination port number) and the higher level protocol info gets printed. Specifically: Domain Name service requests (RFC 1034/1035) and Sun RPC calls (RFC 1050) to NFS. We love a protocol that knows itself. ✨

🦋 TCP or UDP Name Server Requests 🦋

(N.B.: The following description assumes you're familiar with the Domain Service protocol described in RFC 1035. If you're not familiar with the protocol, the following description will appear to be written in Greek. No cap. 😭)

Name server requests are formatted like:

src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)

So 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. That + means the recursion desired flag was set. The query length was 37 bytes, excluding the TCP or UDP and IP protocol headers. And the query operation was the normal one, Query, so the op field was omitted. If the op had been literally anything else, it would've been printed between the 3 and the +. Same energy for qclass — it was the normal one, C_IN, so it got omitted. Any other qclass would've been printed immediately after the A.

A few anomalies get checked and may result in extra fields wrapped in square brackets:

  • If a query contains an answer, authority records or additional records section, then ancount, nscount, or arcount get 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, then [b2&3=x] gets printed, where x is the hex value of header bytes two and three.

🎀 TCP or UDP Name Server Responses 🎀

Name server responses are formatted like:

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) got omitted, and so did the class (C_IN) of the A record. Efficient. We respect it. 👑

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. That * indicates the authoritative answer bit was set. And since there were no answers, no type, class or data got printed.

Other flag characters that might show up:

  • - — recursion available, RA, not set
  • | — truncated message, TC, set

And if the "question" section doesn't contain exactly one entry, [nq] gets 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. Look at her go. 💖

By default a fairly minimal decode is done, with a much more detailed decode happening if -v is used.

⚠️ Be warned bestie: 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. Like genuinely. It's giving essay. 😭

For the deets on SMB packet formats and what all those fields actually mean, go peek at https://download.samba.org/pub/samba/specs/ and the other online resources out there. The SMB patches were written by Andrew Tridgell (tridge@samba.org) — king behavior, honestly. 👑

🍣 NFS Requests and Replies 🍣

Sun NFS (Network File System) requests and replies print out like this:

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

Okay so line one: host sushi is sending a transaction with id 26377 to wrl. The request was 112 bytes, and that's excluding the UDP and IP headers. The operation was a readlink (that's read symbolic link, bestie) on file handle (fh) 21,24/10.731657119. And if you're lucky — like genuinely blessed in this particular case — that file handle can be read as a major,minor device number pair, followed by the inode number and generation number. Cute, right? In line two, wrl replies ok with the same transaction id and the contents of the link. 🎀

Line three: sushi asks (new transaction id, who dis) wrl to lookup the name xcolors in directory file 9,74/4096.6878. And line four, wrl sends a reply with the matching transaction id. Matching IDs is the whole love language here. 💖

Heads up: the data that gets printed depends on the operation type. The format is meant to be self explanatory if you read it alongside 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 you pass the -v (verbose) flag, you get extra info too. Like:

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 — they're just omitted from this example to keep it cute and readable.) First line: sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. wrl replies ok; the packet on the second line is the first fragment of the reply, which is why it's only 1472 bytes long. The other bytes are coming in subsequent fragments, but those fragments don't have NFS or even UDP headers, so depending on the filter expression you used, they might not get printed at all. 😭 Because -v is on, some of the file attributes (returned in addition to the file data) get printed: the file type (REG, for regular file), the file mode (in octal), the UID and GID, and the file size.

Give -v more than once and you get even more details. Iconic.

Real talk: NFS reply packets don't explicitly identify the RPC operation. Instead, tcpdump keeps track of ``recent'' requests and matches them to the replies using the transaction ID. So if a reply doesn't closely follow the corresponding request, it might not be parsable. Not a bug, just vibes.

🦋 AFS Requests and Replies 🦋

Transarc AFS (Andrew File System) requests and replies print like this:

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

Line one: host elvis sends a RX packet to pike. It was a RX data packet to the fs (fileserver) service, and it's the start of an RPC call. The call was a rename, with the old directory file id 536876964/1/1 and an old filename of .newsrc.new, plus a new directory file id of 536876964/1/1 and a new filename of .newsrc. Host pike responds with an RPC reply to the rename call — and it was a success, because it came back as a data packet and not an abort packet. Slay. ✨

Generally speaking, all AFS RPCs get decoded at least by RPC call name. Most AFS RPCs have at least some of their arguments decoded too — generally only the `interesting' arguments, for some definition of interesting. iykyk.

The format is meant to be self-describing, but ngl it probably won't be useful to people who aren't familiar with how AFS and RX actually work.

If the -v (verbose) flag is given twice, acknowledgement packets and additional header information get 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 get printed. 💅

Error codes get 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. (Yes, a yes vote. Democracy but make it packets.)

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

🍎 KIP AppleTalk (DDP in UDP) 🍎

AppleTalk DDP packets that are encapsulated in UDP datagrams get de-encapsulated and dumped as DDP packets — meaning all the UDP header information is just yeeted. The file /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file look like:

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 — and you can tell a host 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, bestie, either is fine). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a #).

AppleTalk addresses print 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 /etc/atalk.names doesn't exist, or doesn't contain an entry for some AppleTalk host/net number, addresses get printed in numeric form. No name, no problem.)

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. Second line is the same thing except the full name of the source node is known (office). Third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port — and note that the broadcast address (255) is indicated by a net name with no host number. Which is exactly why it's a good idea to keep your 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 the packet size. Simple as.

🎀 NBP Packets 🎀

NBP packets are formatted like these 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 (see, same id 💖) from host jssmag.209, saying it has a laserwriter resource named "RM1140" registered on port 250. The third line is another reply to the same request, this time saying host techpit has laserwriter "techpit" registered on port 186.

🌈 ATP Packets 🌈

ATP packet formatting is demonstrated by this 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 (that's 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 packets of 512 bytes each. 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 means the EOM bit was set. ⭐

Then jssmag.209 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 means 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. So if you're on something ancient, don't come crying to me about it. 😭

👑 SEE ALSO 👑

stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp(7)

       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. Legends, fr fr. 🧚

It's 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 was added by the WIDE/KAME project. This program uses OpenSSL/LibreSSL, under specific configurations. 💜

💀 BUGS 💀

Okay bestie, listen — nothing is perfect, not even tcpdump, and honestly that's kind of iconic of it. Here's the receipts:

  • Found a security issue? Do NOT just drop it in the group chat — send an e-mail to security@tcpdump.org instead. 🔒
  • Got bugs and other problems, wanna contribute patches, request a feature, or just provide some generic feedback? Say less — check out the file CONTRIBUTING.md in the tcpdump source tree root. That's the whole vibe. 🫶

NIT doesn't let you watch your own outbound traffic. BPF will. We recommend that you use the latter. Like, that's the recommendation, no cap. 💅

Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol. It's giving "unfinished homework" but okay, we said what we said. 😭

Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. And some people believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than tcpdump — honestly? valid take, lowkey. 🙈

A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored). So yeah, your timestamps are gonna be a little zooted twice a year. ⏰💀

Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token Ring packets. Noted, flagged, whatever — Token Ring girlies stay strong. 🎀

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. 802.11 besties, you've been warned. 🌸

ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behavior — so like, use that one instead, iykyk. ✨

Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only looks at IPv4 packets. Fr fr, IPv6 said "not my problem." 🦋

And that's the whole manpage, bestie! 🎉 This little gem is from the source/version stamped 2 February 2026, and its footer tag says TCPDUMP(1) — go forth, capture some packets, slay, and remember: the vibe check is eternal. 💖👑⭐

📦 where this page comes from