Linux Commands Examples

A great documentation place for Linux commands


dump traffic on a network

see also : stty


tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
[ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
[ expression ]

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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 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’

bool "tcpdump"
select USER_PCAP
Enable building of the tcpdump tools.
sudo tcpdump -i en1

Linux interface stops receiving packets as seen by tcpdump

This is probably down to your switch.

Sniffing a network in parallel like this either requires a hub or a switch that has the capability to have a port placed in 'monitor mode'. In this mode all traffic to other ports gets duplicated to the monitoring port for sniffing / capturing.

Without this monitor port the traffic just won't be coming from the switch - it knows of no valid reason to send it through that port.


How to make tcpdump (or other tool) to actually dump TCP stream?

tcpdump normally displays packet information, as opposed to actual data.

Use the -A flag to dump ASCII contents. It will still dump a lot of other data (like ARP and DNS packets, for example), but you should be able to get what you want through filters.


How to output a simple network activity plot in console in Linux?

Well, if you have perl and tcpdump installed it shouldn't be too hard to hack something up...
Following script will output something like:

# ./host_traffic
  30k ****************
  26k **************
  24k *************
  26k **************
  43k **********************
  39k ********************
  24k *************
  15k ********

I'll leave the sent/received bytes display as an exercise.



$interface = "eth0";
$interval = 1;
$target_host = $ARGV[0];
$bytes = 0;

sub print_stats
    $nstars = $bytes / 2000;   # diagram scale = 2k
    $stars = "";
    for ($i = 0; $i < $nstars; $i++)
    { $stars .= "*"; }
    printf("%4ik $stars\n", $bytes / 1000);

    $bytes = 0;
    alarm $interval;

$SIG{ALRM} = sub { print_stats(); };
alarm $interval;
$pcap_filter = "ip src or dst $target_host";
open(IN, "tcpdump -s 500 -l -n -i $interface '$pcap_filter' | ");

while (my $s = <IN>)
{   # parse tcpdump output
    if ($s =~ m|(.*) IP (.*)\.([^.]*) > (.*)\.([^.]*): Flags \[(.*)\],.*, length (.*)|)
        my ($timestamp, $src, $sport, $dst, $dport, $flags, $len) = ($1, $2, $3, $4, $5, $6, $7);   
        $bytes += $len;
    print $s;

Count exchanged bytes per TCP connection

If you want something fancy and graphical you might take a look at ntop - it's in most distro's apt / yum / whatever.

It monitors the network traffic creating nice graphs and charts on a per-connection, protocol, host, etc basis and displays it all through your web browser (has a built in web server).

It's nice, but fairly heavyweight.


TCPDUMP -G option

Any chance you have upper-case %S instead of %s? If so, it's doing exactly what you tell it to:

  • -w %S create a file named strftime("%S") seconds into each minute, i.e. 15 for arguments sake
  • -G 60 recreate it every 60 seconds, i.e. at 15 seconds into each minute

If you want to capture files in minute-sized chunks you can use %s lower-case, or better still something like %H%M%S.cap, these should work:

tcpdump -w %s -G 60
tcpdump -w %H%M%S.cap -G 60

See the strftime man page for details on the specifiers. Otherwise what you describe is normal behaviour for just -G with no %-specifiers in the -w option. I'm not aware of any bugs related to the -G option.

The %s specifier is a GNU extension to strftime(), it's been present in glibc since glibc-2.0 (1997), however it may cause a problem with non-glibc libraries, though both dietlibc and uClibc have supported it since 2006 and 2002 respectively. I cannot think of anything else that would cause strftime() to not support %s (unless you are not running linux: on Solaris I would expect to get a capture file called %s which was reset every 60 seconds).


tcpdump error: no suitable device found in Linux

You probably need to execute as superuser in order to access the network device directly.

Sudo the command.

Edit: To clarify, that tcpdump command won't give you much useful information either. Try this:

sudo tcpdump host

Or, if sudo is not available:

su - ; tcpdump host

Detect HTTP sessions using tcpdump

tcpdump -r file.trace - tcp dst port 80

is, as you've discovered, not a valid tcpdump command. If you remove the extra -, then you get

tcpdump -r file.trace tcp dst port 80

which is a valid tcpdump command, that will show you only traffic to TCP port 80. It will not show you traffic from TCP port 80, so, for example, if file.trace has HTTP traffic to and from port 80, it will show you HTTP request traffic but NOT HTTP response traffic.

If you want to see traffic to and from port 80, use

tcpdump -r file.trace tcp port 80

However, if there's TCP traffic other than HTTP traffic to or from port 80, that will also be shown.

In addition, HTTP traffic not going to or from port 80, such as HTTP-over-SSL/TLS ("https") traffic, or traffic to ports such as port 8080, will not be shown. To, for example, see traffic to or from port 80 (for HTTP) and port 443 (for HTTPS), do

tcpdump -r file.trace tcp port 80 or 443

So what's "not working"?

Is it not showing, for example, HTTPS traffic? If so, you need to add port 443.

Is it not showing traffic to other ports? If so, you need to add those ports as well. Tcpdump can't recognize HTTP traffic (and Wireshark only recognizes it by port number).

Is it showing only requests, not responses? If you want to see responses, use tcp port rather than tcp dst port. If you only want to see requests, use tcp dst port.


how to make tcpdump to display ip and port number but not hostname and protocol

Add -n to your tcpdump command line.

From the tcpdump manpage:

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


Trouble with mergecap [ concatenation of pcap files ] - undesired info in output file

I think you must have done something strange, it seems to work perfectly well for me:

$ head oo.pcap.merged | hexdump -C

00000000  0a 0d 0d 0a 84 00 00 00  4d 3c 2b 1a 01 00 00 00  |........M<+.....|
00000010  ff ff ff ff ff ff ff ff  01 00 52 00 46 69 6c 65  |..........R.File|
00000020  20 63 72 65 61 74 65 64  20 62 79 20 6d 65 72 67  | created by merg|
00000030  69 6e 67 3a 20 0a 46 69  6c 65 31 3a 20 61 61 61  |ing: .File1: aaa|
00000040  61 2e 70 63 61 70 20 0a  46 69 6c 65 32 3a 20 62  |a.pcap .File2: b|
00000050  62 62 62 62 2e 70 63 61  70 20 0a 46 69 6c 65 33  |bbbb.pcap .File3|
00000060  3a 20 63 63 63 63 63 2e  70 63 61 70 20 0a 00 00  |: ccccc.pcap ...|
00000070  04 00 08 00 6d 65 72 67  65 63 61 70 00 00 00 00  |....mergecap....|

So, it is displaying the same behavior you describe, but then when I try to use tcpdump on the file, it seems to work:

$ tcpdump -r oo.pcap.merged | head
reading from file oo.pcap.merged, link-type EN10MB (Ethernet)

And then continues normally for 369 lines.

tcpdump version 4.3.0

Mergecap 1.8.0


Is there a tool to decrypt all SSL/SSH traffic originating from this computer?

I believe encryption keys are not static and they change during the course of a session. So would be almost impossible to achieve this.


Tcpdump prints out a description of the contents of packets on a network interface that match the boolean expression. 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 (please note tcpdump is protected via an enforcing apparmor(7) profile in Ubuntu which limits the files tcpdump may access). 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 Mac OS X) 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 Mac OS X, 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.

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.



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


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


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


Exit after receiving count packets.


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).


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


Dump packet-matching code as a C program fragment.


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


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, is 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() function.


Print the link-level header on each dump line.


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 RFC2406 ESP, not RFC1827 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.


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 which capture is being done. If that address or netmask are not available, available, either because the interface on which capture is being done has no address or netmask or because the capture is being done on the Linux "any" interface, which can capture on more than one interface, this option will not work correctly.


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


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.

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


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


Attempt to detect 802.11s draft mesh headers.


Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback). Ties are broken by choosing the earliest match.

On Linux systems with 2.2 or later kernels, an interface argument of ’’any’’ can be used to capture packets from all interfaces. Note that captures on the ’’any’’ device 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.


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.


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-type(7); not all the types listed there will necessarily be valid for any given interface.


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.


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.


Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,

tcpdump -l | tee dat


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.


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).


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


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


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


Don’t print domain name qualification of host names. E.g., if you give this flag then tcpdump will print ’’nic’’ instead of ’’’’.


Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.


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’.


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


Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified, tcpdump will not print replay prevention field. Since there is no protocol version field in ESP/AH specification, tcpdump cannot deduce the version of ESP/AH protocol.


Read packets from file (which was created with the -w option). Standard input is used if file is ’’-’’.


Print absolute, rather than relative, TCP sequence numbers.


Snarf snaplen bytes of data from each packet rather than the default of 65535 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. 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 65535, for backwards compatibility with recent older versions of tcpdump.


Force packets selected by "expression" to be interpreted the specified type. Currently known types are aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc (Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Protocol), vat (Visual Audio Tool), and wb (distributed White Board).


Don’t print a timestamp on each dump line.


Print an unformatted timestamp on each dump line.


Print a delta (micro-second resolution) between current and previous line on each dump line.


Print a timestamp in default format proceeded by date on each dump line.


Print a delta (micro-second resolution) between current and first line on each dump line.


Print undecoded NFS handles.


If the -w option is not 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() function.


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, report, every 10 seconds, the number of packets captured.


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


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


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.

See pcap-savefile(5) for a description of the file format.


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 with -C as well, the behavior will result in cyclical files per timeslice.


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.


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.


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.


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.


Set the data link type to use while capturing packets to datalinktype.


Used in conjunction with the -C or -G options, this will make tcpdump run " 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.


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.


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).

Expression arguments can be passed to tcpdump as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains Shell metacharacters, it is easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed.

output format

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

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 tell
arp reply 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.

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, neither this description nor tcpdump will be of much use to you.)

The general format of a tcp protocol line is:

src > dst: flags data-seqno ack window urgent options

Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) 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). Ack 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. Options are tcp options enclosed in angle brackets (e.g., <mss 1024>).

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.

rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
rtsg.1023 > csam.login: . ack 1 win 4096
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
csam.login > rtsg.1023: . ack 2 win 4096
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 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(nbytes)’ which means ’sequence numbers first up to but not including last which is nbytes bytes of user data’.) 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. 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 PUSH 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]’’.

Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)

There are 8 bits in the control bits section of the TCP header:


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:

| |
|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:

|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


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:

|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


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-act, tcp-urg.

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.

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? (37)

Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name The query id was ’3’. The ’+’ indicates the recursion desired flag was set. The query length was 37 bytes, not including the 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.

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 (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 The total size of the response was 273 bytes, excluding 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 non-existent 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 or the pub/samba/specs/ directory on your favorite mirror site. The SMB patches were written by Andrew Tridgell (tridge[:at:]samba[:dot:]org).

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as:

src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results

sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
     144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
     reply ok 128 lookup fh 9,74/4134.3150

In the first line, host sushi sends a transaction with id 6709 to wrl (note that the number following the src host is a transaction id, not the source port). 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.) Wrl replies ’ok’ with the contents of the link.

In the third line, sushi asks wrl to lookup the name ’xcolors’ in directory file 9,74/4096.6878. 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.

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

sushi.1372a > wrl.nfs:
     148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
     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.

Note that NFS requests are very large and much of the detail won’t be printed unless snaplen is increased. Try using ’-s 192’ to watch NFS traffic.

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

elvis.7001 > pike.afsfs:
     rx data fs call rename old fid 536876964/1/1 ""
     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 ’’, 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 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 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).

Note that AFS requests are very large and many of the arguments won’t be printed unless snaplen is increased. Try using ’-s 256’ to watch AFS traffic.

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 > 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 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 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.

IP Fragmentation

Fragmented Internet datagrams are printed as

(frag id:size@offset+)

(The first form indicates there are more fragments. The second indicates this is the last fragment.)

Id is the fragment id. Size is the fragment size (in bytes) excluding the IP header. Offset is this fragment’s offset (in bytes) in the original datagram.

The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from to over a CSNET connection that doesn’t appear to handle 576 byte datagrams:

arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560

There are a couple of things to note here: First, addresses in the 2nd line don’t include port numbers. This is because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you.

A packet with the IP don’t fragment flag is marked with a trailing (DF).


By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form


and is as accurate as the kernel’s clock. The timestamp reflects the time the kernel first saw the packet. No attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire and when the kernel serviced the ’new packet’ interrupt.


Please send problems, bugs, questions, desirable enhancements, patches etc. to:


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

On Linux systems with 2.0[.x] kernels:

packets on the loopback device will be seen twice;

packet filtering cannot be done in the kernel, so that all packets must be copied from the kernel in order to be filtered in user mode;

all of a packet, not just the part that’s within the snapshot length, will be copied from the kernel (the 2.0[.x] packet capture mechanism, if asked to copy only part of a packet to userland, will not report the true length of the packet; this would cause most IP packets to get an error from tcpdump);

capturing on some PPP devices won’t work correctly.

We recommend that you upgrade to a 2.2 or later kernel.

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.

see also

stty , pcap(3PCAP), bpf, nit(4P), pcap-savefile, pcap-filter, pcap-tstamp-type, apparmor


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 being maintained by

The current version is available via http:

The original distribution is available via anonymous ftp:

IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young’s SSLeay library, under specific configurations.

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