1 .\" @(#) $Header: /tcpdump/master/tcpdump/tcpdump.1.in,v 1.1.2.2 2008-10-27 21:16:37 guy Exp $ (LBL)
3 .\" $NetBSD: tcpdump.8,v 1.9 2003/03/31 00:18:17 perry Exp $
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25 .TH TCPDUMP 1 "07 January 2008"
27 tcpdump \- dump traffic on a network
32 .B \-AdDefIKlLnNOpqRStuUvxX
94 .I spi@ipaddr algo:secret,...
104 .I postrotate-command
118 \fITcpdump\fP prints out a description of the contents of packets on a
119 network interface that match the boolean \fIexpression\fP. It can also
122 flag, which causes it to save the packet data to a file for later
123 analysis, and/or with the
125 flag, which causes it to read from a saved packet file rather than to
126 read packets from a network interface. In all cases, only packets that
133 will, if not run with the
135 flag, continue capturing packets until it is interrupted by a SIGINT
136 signal (generated, for example, by typing your interrupt character,
137 typically control-C) or a SIGTERM signal (typically generated with the
139 command); if run with the
141 flag, it will capture packets until it is interrupted by a SIGINT or
142 SIGTERM signal or the specified number of packets have been processed.
146 finishes capturing packets, it will report counts of:
148 packets ``captured'' (this is the number of packets that
150 has received and processed);
152 packets ``received by filter'' (the meaning of this depends on the OS on
155 and possibly on the way the OS was configured - if a filter was
156 specified on the command line, on some OSes it counts packets regardless
157 of whether they were matched by the filter expression and, even if they
158 were matched by the filter expression, regardless of whether
160 has read and processed them yet, on other OSes it counts only packets that were
161 matched by the filter expression regardless of whether
163 has read and processed them yet, and on other OSes it counts only
164 packets that were matched by the filter expression and were processed by
167 packets ``dropped by kernel'' (this is the number of packets that were
168 dropped, due to a lack of buffer space, by the packet capture mechanism
171 is running, if the OS reports that information to applications; if not,
172 it will be reported as 0).
174 On platforms that support the SIGINFO signal, such as most BSDs
175 (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
176 when it receives a SIGINFO signal (generated, for example, by typing
177 your ``status'' character, typically control-T, although on some
178 platforms, such as Mac OS X, the ``status'' character is not set by
179 default, so you must set it with
181 in order to use it) and will continue capturing packets.
183 Reading packets from a network interface may require that you have
184 special privileges; see the
186 man page for details. Reading a saved packet file doesn't require
191 Print each packet (minus its link level header) in ASCII. Handy for
195 Set the operating system capture buffer size to \fIbuffer_size\fP.
198 Exit after receiving \fIcount\fP packets.
201 Before writing a raw packet to a savefile, check whether the file is
202 currently larger than \fIfile_size\fP and, if so, close the current
203 savefile and open a new one. Savefiles after the first savefile will
204 have the name specified with the
206 flag, with a number after it, starting at 1 and continuing upward.
207 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
208 not 1,048,576 bytes).
211 Dump the compiled packet-matching code in a human readable form to
212 standard output and stop.
215 Dump packet-matching code as a
220 Dump packet-matching code as decimal numbers (preceded with a count).
223 Print the list of the network interfaces available on the system and on
226 can capture packets. For each network interface, a number and an
227 interface name, possibly followed by a text description of the
228 interface, is printed. The interface name or the number can be supplied
231 flag to specify an interface on which to capture.
233 This can be useful on systems that don't have a command to list them
234 (e.g., Windows systems, or UNIX systems lacking
235 .BR "ifconfig \-a" );
236 the number can be useful on Windows 2000 and later systems, where the
237 interface name is a somewhat complex string.
241 flag will not be supported if
243 was built with an older version of
246 .B pcap_findalldevs()
250 Print the link-level header on each dump line.
253 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
254 are addressed to \fIaddr\fP and contain Security Parameter Index value
255 \fIspi\fP. This combination may be repeated with comma or newline seperation.
257 Note that setting the secret for IPv4 ESP packets is supported at this time.
264 \fBcast128-cbc\fP, or
266 The default is \fBdes-cbc\fP.
267 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
268 with cryptography enabled.
270 \fIsecret\fP is the ASCII text for ESP secret key.
271 If preceeded by 0x, then a hex value will be read.
273 The option assumes RFC2406 ESP, not RFC1827 ESP.
274 The option is only for debugging purposes, and
275 the use of this option with a true `secret' key is discouraged.
276 By presenting IPsec secret key onto command line
277 you make it visible to others, via
281 In addition to the above syntax, the syntax \fIfile name\fP may be used
282 to have tcpdump read the provided file in. The file is opened upon
283 receiving the first ESP packet, so any special permissions that tcpdump
284 may have been given should already have been given up.
287 Print `foreign' IPv4 addresses numerically rather than symbolically
288 (this option is intended to get around serious brain damage in
289 Sun's NIS server \(em usually it hangs forever translating non-local
292 The test for `foreign' IPv4 addresses is done using the IPv4 address and
293 netmask of the interface on which capture is being done. If that
294 address or netmask are not available, available, either because the
295 interface on which capture is being done has no address or netmask or
296 because the capture is being done on the Linux "any" interface, which
297 can capture on more than one interface, this option will not work
301 Use \fIfile\fP as input for the filter expression.
302 An additional expression given on the command line is ignored.
305 If specified, rotates the dump file specified with the
307 option every \fIrotate_seconds\fP seconds.
308 Savefiles will have the name specified by
310 which should include a time format as defined by
312 If no time format is specified, each new file will overwrite the previous.
314 If used in conjunction with the
316 option, filenames will take the form of `\fIfile\fP<count>'.
319 Listen on \fIinterface\fP.
320 If unspecified, \fItcpdump\fP searches the system interface list for the
321 lowest numbered, configured up interface (excluding loopback).
322 Ties are broken by choosing the earliest match.
324 On Linux systems with 2.2 or later kernels, an
326 argument of ``any'' can be used to capture packets from all interfaces.
327 Note that captures on the ``any'' device will not be done in promiscuous
332 flag is supported, an interface number as printed by that flag can be
338 Put the interface in "monitor mode"; this is supported only on IEEE
339 802.11 Wi-Fi interfaces, and supported only on some operating systems.
341 Note that in monitor mode the adapter might disassociate from the
342 network with which it's associated, so that you will not be able to use
343 any wireless networks with that adapter. This could prevent accessing
344 files on a network server, or resolving host names or network addresses,
345 if you are capturing in monitor mode and are not connected to another
346 network with another adapter.
349 Don't attempt to verify TCP checksums. This is useful for interfaces
350 that perform the TCP checksum calculation in hardware; otherwise,
351 all outgoing TCP checksums will be flagged as bad.
354 Make stdout line buffered.
355 Useful if you want to see the data
359 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
360 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
363 List the known data link types for the interface and exit.
366 Load SMI MIB module definitions from file \fImodule\fR.
368 can be used several times to load several MIB modules into \fItcpdump\fP.
371 Use \fIsecret\fP as a shared secret for validating the digests found in
372 TCP segments with the TCP-MD5 option (RFC 2385), if present.
375 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
378 Don't print domain name qualification of host names.
380 if you give this flag then \fItcpdump\fP will print ``nic''
381 instead of ``nic.ddn.mil''.
384 Do not run the packet-matching code optimizer.
386 if you suspect a bug in the optimizer.
389 \fIDon't\fP put the interface
390 into promiscuous mode.
391 Note that the interface might be in promiscuous
392 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
393 `ether host {local-hw-addr} or ether broadcast'.
396 Quick (quiet?) output.
397 Print less protocol information so output
401 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
402 If specified, \fItcpdump\fP will not print replay prevention field.
403 Since there is no protocol version field in ESP/AH specification,
404 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
407 Read packets from \fIfile\fR (which was created with the
410 Standard input is used if \fIfile\fR is ``-''.
413 Print absolute, rather than relative, TCP sequence numbers.
416 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
417 default of 68 (with SunOS's NIT, the minimum is actually 96).
418 68 bytes is adequate for IP, ICMP, TCP
419 and UDP but may truncate protocol information from name server and NFS
421 Packets truncated because of a limited snapshot
422 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
423 is the name of the protocol level at which the truncation has occurred.
424 Note that taking larger snapshots both increases
425 the amount of time it takes to process packets and, effectively,
426 decreases the amount of packet buffering.
427 This may cause packets to be
429 You should limit \fIsnaplen\fP to the smallest number that will
430 capture the protocol information you're interested in.
432 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
435 Force packets selected by "\fIexpression\fP" to be interpreted the
436 specified \fItype\fR.
437 Currently known types are
438 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
439 \fBcnfp\fR (Cisco NetFlow protocol),
440 \fBrpc\fR (Remote Procedure Call),
441 \fBrtp\fR (Real-Time Applications protocol),
442 \fBrtcp\fR (Real-Time Applications control protocol),
443 \fBsnmp\fR (Simple Network Management Protocol),
444 \fBtftp\fR (Trivial File Transfer Protocol),
445 \fBvat\fR (Visual Audio Tool),
447 \fBwb\fR (distributed White Board).
450 \fIDon't\fP print a timestamp on each dump line.
453 Print an unformatted timestamp on each dump line.
456 Print a delta (micro-second resolution) between current and previous line
460 Print a timestamp in default format proceeded by date on each dump line.
463 Print a delta (micro-second resolution) between current and first line
467 Print undecoded NFS handles.
470 Make output saved via the
472 option ``packet-buffered''; i.e., as each packet is saved, it will be
473 written to the output file, rather than being written only when the
478 flag will not be supported if
480 was built with an older version of
487 When parsing and printing, produce (slightly more) verbose output.
488 For example, the time to live,
489 identification, total length and options in an IP packet are printed.
490 Also enables additional packet integrity checks such as verifying the
491 IP and ICMP header checksum.
493 When writing to a file with the
495 option, report, every 10 seconds, the number of packets captured.
498 Even more verbose output.
499 For example, additional fields are
500 printed from NFS reply packets, and SMB packets are fully decoded.
503 Even more verbose output.
505 telnet \fBSB\fP ... \fBSE\fP options
509 Telnet options are printed in hex as well.
512 Write the raw packets to \fIfile\fR rather than parsing and printing
514 They can later be printed with the \-r option.
515 Standard output is used if \fIfile\fR is ``-''.
517 .BR pcap-savefile (@MAN_FILE_FORMATS@)
518 for a description of the file format.
521 Used in conjunction with the
523 option, this will limit the number
524 of files created to the specified number, and begin overwriting files
525 from the beginning, thus creating a 'rotating' buffer.
526 In addition, it will name
527 the files with enough leading 0s to support the maximum number of
528 files, allowing them to sort correctly.
530 Used in conjunction with the
532 option, this will limit the number of rotated dump files that get
533 created, exiting with status 0 when reaching the limit. If used with
535 as well, the behavior will result in cyclical files per timeslice.
538 When parsing and printing,
539 in addition to printing the headers of each packet, print the data of
540 each packet (minus its link level header) in hex.
541 The smaller of the entire packet or
543 bytes will be printed. Note that this is the entire link-layer
544 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
545 will also be printed when the higher layer packet is shorter than the
549 When parsing and printing,
550 in addition to printing the headers of each packet, print the data of
553 its link level header, in hex.
556 When parsing and printing,
557 in addition to printing the headers of each packet, print the data of
558 each packet (minus its link level header) in hex and ASCII.
559 This is very handy for analysing new protocols.
562 When parsing and printing,
563 in addition to printing the headers of each packet, print the data of
566 its link level header, in hex and ASCII.
569 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
572 Used in conjunction with the
576 options, this will make
582 is the savefile being closed after each rotation. For example, specifying
586 will compress each savefile using gzip or bzip2.
588 Note that tcpdump will run the command in parallel to the capture, using
589 the lowest priority so that this doesn't disturb the capture process.
591 And in case you would like to use a command that itself takes flags or
592 different arguments, you can always write a shell script that will take the
593 savefile name as the only argument, make the flags & arguments arrangements
594 and execute the command that you want.
597 Drops privileges (if root) and changes user ID to
599 and the group ID to the primary group of
602 This behavior can also be enabled by default at compile time.
603 .IP "\fI expression\fP"
605 selects which packets will be dumped.
606 If no \fIexpression\fP
607 is given, all packets on the net will be dumped.
609 only packets for which \fIexpression\fP is `true' will be dumped.
611 For the \fIexpression\fP syntax, see
612 .BR pcap-filter (@MAN_MISC_INFO@).
614 Expression arguments can be passed to \fItcpdump\fP as either a single
615 argument or as multiple arguments, whichever is more convenient.
616 Generally, if the expression contains Shell metacharacters, it is
617 easier to pass it as a single, quoted argument.
618 Multiple arguments are concatenated with spaces before being parsed.
621 To print all packets arriving at or departing from \fIsundown\fP:
624 \fBtcpdump host sundown\fP
628 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
631 \fBtcpdump host helios and \\( hot or ace \\)\fP
635 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
638 \fBtcpdump ip host ace and not helios\fP
642 To print all traffic between local hosts and hosts at Berkeley:
646 tcpdump net ucb-ether
650 To print all ftp traffic through internet gateway \fIsnup\fP:
651 (note that the expression is quoted to prevent the shell from
652 (mis-)interpreting the parentheses):
656 tcpdump 'gateway snup and (port ftp or ftp-data)'
660 To print traffic neither sourced from nor destined for local hosts
661 (if you gateway to one other net, this stuff should never make it
662 onto your local net).
666 tcpdump ip and not net \fIlocalnet\fP
670 To print the start and end packets (the SYN and FIN packets) of each
671 TCP conversation that involves a non-local host.
675 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
679 To print all IPv4 HTTP packets to and from port 80, i.e. print only
680 packets that contain data, not, for example, SYN and FIN packets and
681 ACK-only packets. (IPv6 is left as an exercise for the reader.)
685 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
689 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
693 tcpdump 'gateway snup and ip[2:2] > 576'
697 To print IP broadcast or multicast packets that were
699 sent via Ethernet broadcast or multicast:
703 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
707 To print all ICMP packets that are not echo requests/replies (i.e., not
712 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
717 The output of \fItcpdump\fP is protocol dependent.
719 gives a brief description and examples of most of the formats.
727 If the '-e' option is given, the link level header is printed out.
728 On Ethernets, the source and destination addresses, protocol,
729 and packet length are printed.
731 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
732 the `frame control' field, the source and destination addresses,
733 and the packet length.
734 (The `frame control' field governs the
735 interpretation of the rest of the packet.
737 as those containing IP datagrams) are `async' packets, with a priority
738 value between 0 and 7; for example, `\fBasync4\fR'.
740 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
741 the LLC header is printed if it is \fInot\fR an ISO datagram or a
742 so-called SNAP packet.
744 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
745 the `access control' and `frame control' fields, the source and
746 destination addresses, and the packet length.
748 packets are assumed to contain an LLC packet.
749 Regardless of whether
750 the '-e' option is specified or not, the source routing information is
751 printed for source-routed packets.
753 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
754 the `frame control' fields, all of the addresses in the 802.11 header,
755 and the packet length.
757 packets are assumed to contain an LLC packet.
759 \fI(N.B.: The following description assumes familiarity with
760 the SLIP compression algorithm described in RFC-1144.)\fP
762 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
763 packet type, and compression information are printed out.
764 The packet type is printed first.
765 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
766 No further link information is printed for \fIip\fR packets.
767 For TCP packets, the connection identifier is printed following the type.
768 If the packet is compressed, its encoded header is printed out.
769 The special cases are printed out as
770 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
771 the sequence number (or sequence number and ack) has changed.
772 If it is not a special case,
773 zero or more changes are printed.
774 A change is indicated by U (urgent pointer), W (window), A (ack),
775 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
777 Finally, the amount of data in the packet and compressed header length
780 For example, the following line shows an outbound compressed TCP packet,
781 with an implicit connection identifier; the ack has changed by 6,
782 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
783 data and 6 bytes of compressed header:
786 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
792 Arp/rarp output shows the type of request and its arguments.
794 format is intended to be self explanatory.
795 Here is a short sample taken from the start of an `rlogin' from
796 host \fIrtsg\fP to host \fIcsam\fP:
800 \f(CWarp who-has csam tell rtsg
801 arp reply csam is-at CSAM\fR
805 The first line says that rtsg sent an arp packet asking
806 for the Ethernet address of internet host csam.
808 replies with its Ethernet address (in this example, Ethernet addresses
809 are in caps and internet addresses in lower case).
811 This would look less redundant if we had done \fItcpdump \-n\fP:
815 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
816 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
820 If we had done \fItcpdump \-e\fP, the fact that the first packet is
821 broadcast and the second is point-to-point would be visible:
825 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
826 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
830 For the first packet this says the Ethernet source address is RTSG, the
831 destination is the Ethernet broadcast address, the type field
832 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
836 \fI(N.B.:The following description assumes familiarity with
837 the TCP protocol described in RFC-793.
838 If you are not familiar
839 with the protocol, neither this description nor \fItcpdump\fP will
840 be of much use to you.)\fP
842 The general format of a tcp protocol line is:
846 \fIsrc > dst: flags data-seqno ack window urgent options\fP
850 \fISrc\fP and \fIdst\fP are the source and destination IP
852 \fIFlags\fP are some combination of S (SYN),
853 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
855 \fIData-seqno\fP describes the portion of sequence space covered
856 by the data in this packet (see example below).
857 \fIAck\fP is sequence number of the next data expected the other
858 direction on this connection.
859 \fIWindow\fP is the number of bytes of receive buffer space available
860 the other direction on this connection.
861 \fIUrg\fP indicates there is `urgent' data in the packet.
862 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
864 \fISrc, dst\fP and \fIflags\fP are always present.
866 depend on the contents of the packet's tcp protocol header and
867 are output only if appropriate.
869 Here is the opening portion of an rlogin from host \fIrtsg\fP to
874 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
875 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
876 rtsg.1023 > csam.login: . ack 1 win 4096
877 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
878 csam.login > rtsg.1023: . ack 2 win 4096
879 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
880 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
881 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
882 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
886 The first line says that tcp port 1023 on rtsg sent a packet
889 The \fBS\fP indicates that the \fISYN\fP flag was set.
890 The packet sequence number was 768512 and it contained no data.
891 (The notation is `first:last(nbytes)' which means `sequence
893 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
894 There was no piggy-backed ack, the available receive window was 4096
895 bytes and there was a max-segment-size option requesting an mss of
898 Csam replies with a similar packet except it includes a piggy-backed
900 Rtsg then acks csam's SYN.
903 The packet contained no data so there is no data sequence number.
904 Note that the ack sequence
905 number is a small integer (1).
906 The first time \fItcpdump\fP sees a
907 tcp `conversation', it prints the sequence number from the packet.
908 On subsequent packets of the conversation, the difference between
909 the current packet's sequence number and this initial sequence number
911 This means that sequence numbers after the
912 first can be interpreted
913 as relative byte positions in the conversation's data stream (with the
914 first data byte each direction being `1').
915 `-S' will override this
916 feature, causing the original sequence numbers to be output.
918 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
919 in the rtsg \(-> csam side of the conversation).
920 The PUSH flag is set in the packet.
921 On the 7th line, csam says it's received data sent by rtsg up to
922 but not including byte 21.
923 Most of this data is apparently sitting in the
924 socket buffer since csam's receive window has gotten 19 bytes smaller.
925 Csam also sends one byte of data to rtsg in this packet.
926 On the 8th and 9th lines,
927 csam sends two bytes of urgent, pushed data to rtsg.
929 If the snapshot was small enough that \fItcpdump\fP didn't capture
930 the full TCP header, it interprets as much of the header as it can
931 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
933 If the header contains a bogus option (one with a length
934 that's either too small or beyond the end of the header), \fItcpdump\fP
935 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
936 options (since it's impossible to tell where they start).
938 length indicates options are present but the IP datagram length is not
939 long enough for the options to actually be there, \fItcpdump\fP reports
940 it as ``[\fIbad hdr length\fP]''.
942 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
944 There are 8 bits in the control bits section of the TCP header:
946 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
948 Let's assume that we want to watch packets used in establishing
950 Recall that TCP uses a 3-way handshake protocol
951 when it initializes a new connection; the connection sequence with
952 regard to the TCP control bits is
958 2) Recipient responds with SYN, ACK
964 Now we're interested in capturing packets that have only the
965 SYN bit set (Step 1).
966 Note that we don't want packets from step 2
967 (SYN-ACK), just a plain initial SYN.
968 What we need is a correct filter
969 expression for \fItcpdump\fP.
971 Recall the structure of a TCP header without options:
975 -----------------------------------------------------------------
976 | source port | destination port |
977 -----------------------------------------------------------------
979 -----------------------------------------------------------------
980 | acknowledgment number |
981 -----------------------------------------------------------------
982 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
983 -----------------------------------------------------------------
984 | TCP checksum | urgent pointer |
985 -----------------------------------------------------------------
988 A TCP header usually holds 20 octets of data, unless options are
990 The first line of the graph contains octets 0 - 3, the
991 second line shows octets 4 - 7 etc.
993 Starting to count with 0, the relevant TCP control bits are contained
998 ----------------|---------------|---------------|----------------
999 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1000 ----------------|---------------|---------------|----------------
1001 | | 13th octet | | |
1004 Let's have a closer look at octet no. 13:
1014 These are the TCP control bits we are interested
1016 We have numbered the bits in this octet from 0 to 7, right to
1017 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1019 Recall that we want to capture packets with only SYN set.
1020 Let's see what happens to octet 13 if a TCP datagram arrives
1021 with the SYN bit set in its header:
1032 control bits section we see that only bit number 1 (SYN) is set.
1034 Assuming that octet number 13 is an 8-bit unsigned integer in
1035 network byte order, the binary value of this octet is
1039 and its decimal representation is
1043 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1046 We're almost done, because now we know that if only SYN is set,
1047 the value of the 13th octet in the TCP header, when interpreted
1048 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1050 This relationship can be expressed as
1056 We can use this expression as the filter for \fItcpdump\fP in order
1057 to watch packets which have only SYN set:
1060 tcpdump -i xl0 tcp[13] == 2
1063 The expression says "let the 13th octet of a TCP datagram have
1064 the decimal value 2", which is exactly what we want.
1066 Now, let's assume that we need to capture SYN packets, but we
1067 don't care if ACK or any other TCP control bit is set at the
1069 Let's see what happens to octet 13 when a TCP datagram
1070 with SYN-ACK set arrives:
1080 Now bits 1 and 4 are set in the 13th octet.
1086 which translates to decimal
1090 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1093 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1094 expression, because that would select only those packets that have
1095 SYN-ACK set, but not those with only SYN set.
1096 Remember that we don't care
1097 if ACK or any other control bit is set as long as SYN is set.
1099 In order to achieve our goal, we need to logically AND the
1100 binary value of octet 13 with some other value to preserve
1102 We know that we want SYN to be set in any case,
1103 so we'll logically AND the value in the 13th octet with
1104 the binary value of a SYN:
1108 00010010 SYN-ACK 00000010 SYN
1109 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1111 = 00000010 = 00000010
1114 We see that this AND operation delivers the same result
1115 regardless whether ACK or another TCP control bit is set.
1116 The decimal representation of the AND value as well as
1117 the result of this operation is 2 (binary 00000010),
1118 so we know that for packets with SYN set the following
1119 relation must hold true:
1121 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1123 This points us to the \fItcpdump\fP filter expression
1126 tcpdump -i xl0 'tcp[13] & 2 == 2'
1129 Note that you should use single quotes or a backslash
1130 in the expression to hide the AND ('&') special character
1136 UDP format is illustrated by this rwho packet:
1140 \f(CWactinide.who > broadcast.who: udp 84\fP
1144 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1145 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1147 The packet contained 84 bytes of user data.
1149 Some UDP services are recognized (from the source or destination
1150 port number) and the higher level protocol information printed.
1151 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1152 RPC calls (RFC-1050) to NFS.
1154 UDP Name Server Requests
1156 \fI(N.B.:The following description assumes familiarity with
1157 the Domain Service protocol described in RFC-1035.
1158 If you are not familiar
1159 with the protocol, the following description will appear to be written
1162 Name server requests are formatted as
1166 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1168 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1172 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1173 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1174 The query id was `3'.
1175 The `+' indicates the \fIrecursion desired\fP flag
1177 The query length was 37 bytes, not including the UDP and
1178 IP protocol headers.
1179 The query operation was the normal one, \fIQuery\fP,
1180 so the op field was omitted.
1181 If the op had been anything else, it would
1182 have been printed between the `3' and the `+'.
1183 Similarly, the qclass was the normal one,
1184 \fIC_IN\fP, and omitted.
1185 Any other qclass would have been printed
1186 immediately after the `A'.
1188 A few anomalies are checked and may result in extra fields enclosed in
1189 square brackets: If a query contains an answer, authority records or
1190 additional records section,
1195 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1196 is the appropriate count.
1197 If any of the response bits are set (AA, RA or rcode) or any of the
1198 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1199 is printed, where \fIx\fP is the hex value of header bytes two and three.
1201 UDP Name Server Responses
1203 Name server responses are formatted as
1207 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1209 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1210 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1214 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1215 with 3 answer records, 3 name server records and 7 additional records.
1216 The first answer record is type A (address) and its data is internet
1217 address 128.32.137.3.
1218 The total size of the response was 273 bytes,
1219 excluding UDP and IP headers.
1220 The op (Query) and response code
1221 (NoError) were omitted, as was the class (C_IN) of the A record.
1223 In the second example, \fIhelios\fP responds to query 2 with a
1224 response code of non-existent domain (NXDomain) with no answers,
1225 one name server and no authority records.
1226 The `*' indicates that
1227 the \fIauthoritative answer\fP bit was set.
1229 answers, no type, class or data were printed.
1231 Other flag characters that might appear are `\-' (recursion available,
1232 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1234 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1237 Note that name server requests and responses tend to be large and the
1238 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1240 Use the \fB\-s\fP flag to increase the snaplen if you
1241 need to seriously investigate name server traffic.
1243 has worked well for me.
1248 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1249 on UDP/137, UDP/138 and TCP/139.
1250 Some primitive decoding of IPX and
1251 NetBEUI SMB data is also done.
1253 By default a fairly minimal decode is done, with a much more detailed
1254 decode done if -v is used.
1255 Be warned that with -v a single SMB packet
1256 may take up a page or more, so only use -v if you really want all the
1259 For information on SMB packet formats and what all te fields mean see
1260 www.cifs.org or the pub/samba/specs/ directory on your favorite
1261 samba.org mirror site.
1262 The SMB patches were written by Andrew Tridgell
1266 NFS Requests and Replies
1268 Sun NFS (Network File System) requests and replies are printed as:
1272 \fIsrc.xid > dst.nfs: len op args\fP
1273 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1276 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1277 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1278 sushi.201b > wrl.nfs:
1279 144 lookup fh 9,74/4096.6878 "xcolors"
1280 wrl.nfs > sushi.201b:
1281 reply ok 128 lookup fh 9,74/4134.3150
1286 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1287 to \fIwrl\fP (note that the number following the src host is a
1288 transaction id, \fInot\fP the source port).
1289 The request was 112 bytes,
1290 excluding the UDP and IP headers.
1291 The operation was a \fIreadlink\fP
1292 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1293 (If one is lucky, as in this case, the file handle can be interpreted
1294 as a major,minor device number pair, followed by the inode number and
1296 \fIWrl\fP replies `ok' with the contents of the link.
1298 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1299 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1300 Note that the data printed
1301 depends on the operation type.
1302 The format is intended to be self
1303 explanatory if read in conjunction with
1304 an NFS protocol spec.
1306 If the \-v (verbose) flag is given, additional information is printed.
1312 sushi.1372a > wrl.nfs:
1313 148 read fh 21,11/12.195 8192 bytes @ 24576
1314 wrl.nfs > sushi.1372a:
1315 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1320 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1321 which have been omitted from this example.) In the first line,
1322 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1323 at byte offset 24576.
1324 \fIWrl\fP replies `ok'; the packet shown on the
1325 second line is the first fragment of the reply, and hence is only 1472
1326 bytes long (the other bytes will follow in subsequent fragments, but
1327 these fragments do not have NFS or even UDP headers and so might not be
1328 printed, depending on the filter expression used).
1329 Because the \-v flag
1330 is given, some of the file attributes (which are returned in addition
1331 to the file data) are printed: the file type (``REG'', for regular file),
1332 the file mode (in octal), the uid and gid, and the file size.
1334 If the \-v flag is given more than once, even more details are printed.
1336 Note that NFS requests are very large and much of the detail won't be printed
1337 unless \fIsnaplen\fP is increased.
1338 Try using `\fB\-s 192\fP' to watch
1341 NFS reply packets do not explicitly identify the RPC operation.
1343 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1344 replies using the transaction ID.
1345 If a reply does not closely follow the
1346 corresponding request, it might not be parsable.
1348 AFS Requests and Replies
1350 Transarc AFS (Andrew File System) requests and replies are printed
1356 \fIsrc.sport > dst.dport: rx packet-type\fP
1357 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1358 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1361 elvis.7001 > pike.afsfs:
1362 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1363 new fid 536876964/1/1 ".newsrc"
1364 pike.afsfs > elvis.7001: rx data fs reply rename
1369 In the first line, host elvis sends a RX packet to pike.
1371 a RX data packet to the fs (fileserver) service, and is the start of
1373 The RPC call was a rename, with the old directory file id
1374 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1375 file id of 536876964/1/1 and a new filename of `.newsrc'.
1377 responds with a RPC reply to the rename call (which was successful, because
1378 it was a data packet and not an abort packet).
1380 In general, all AFS RPCs are decoded at least by RPC call name.
1382 AFS RPCs have at least some of the arguments decoded (generally only
1383 the `interesting' arguments, for some definition of interesting).
1385 The format is intended to be self-describing, but it will probably
1386 not be useful to people who are not familiar with the workings of
1389 If the -v (verbose) flag is given twice, acknowledgement packets and
1390 additional header information is printed, such as the the RX call ID,
1391 call number, sequence number, serial number, and the RX packet flags.
1393 If the -v flag is given twice, additional information is printed,
1394 such as the the RX call ID, serial number, and the RX packet flags.
1395 The MTU negotiation information is also printed from RX ack packets.
1397 If the -v flag is given three times, the security index and service id
1400 Error codes are printed for abort packets, with the exception of Ubik
1401 beacon packets (because abort packets are used to signify a yes vote
1402 for the Ubik protocol).
1404 Note that AFS requests are very large and many of the arguments won't
1405 be printed unless \fIsnaplen\fP is increased.
1406 Try using `\fB-s 256\fP'
1407 to watch AFS traffic.
1409 AFS reply packets do not explicitly identify the RPC operation.
1411 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1412 replies using the call number and service ID.
1413 If a reply does not closely
1415 corresponding request, it might not be parsable.
1418 KIP AppleTalk (DDP in UDP)
1420 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1421 and dumped as DDP packets (i.e., all the UDP header information is
1425 is used to translate AppleTalk net and node numbers to names.
1426 Lines in this file have the form
1438 The first two lines give the names of AppleTalk networks.
1440 line gives the name of a particular host (a host is distinguished
1441 from a net by the 3rd octet in the number \-
1442 a net number \fImust\fP have two octets and a host number \fImust\fP
1443 have three octets.) The number and name should be separated by
1444 whitespace (blanks or tabs).
1447 file may contain blank lines or comment lines (lines starting with
1450 AppleTalk addresses are printed in the form
1456 \f(CW144.1.209.2 > icsd-net.112.220
1457 office.2 > icsd-net.112.220
1458 jssmag.149.235 > icsd-net.2\fR
1464 doesn't exist or doesn't contain an entry for some AppleTalk
1465 host/net number, addresses are printed in numeric form.)
1466 In the first example, NBP (DDP port 2) on net 144.1 node 209
1467 is sending to whatever is listening on port 220 of net icsd node 112.
1468 The second line is the same except the full name of the source node
1469 is known (`office').
1470 The third line is a send from port 235 on
1471 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1472 the broadcast address (255) is indicated by a net name with no host
1473 number \- for this reason it's a good idea to keep node names and
1474 net names distinct in /etc/atalk.names).
1476 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1477 packets have their contents interpreted.
1478 Other protocols just dump
1479 the protocol name (or number if no name is registered for the
1480 protocol) and packet size.
1482 \fBNBP packets\fP are formatted like the following examples:
1486 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1487 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1488 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1492 The first line is a name lookup request for laserwriters sent by net icsd host
1493 112 and broadcast on net jssmag.
1494 The nbp id for the lookup is 190.
1495 The second line shows a reply for this request (note that it has the
1496 same id) from host jssmag.209 saying that it has a laserwriter
1497 resource named "RM1140" registered on port 250.
1499 another reply to the same request saying host techpit has laserwriter
1500 "techpit" registered on port 186.
1502 \fBATP packet\fP formatting is demonstrated by the following example:
1506 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1507 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1508 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1509 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1510 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1511 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1512 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1513 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1514 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1515 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1516 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1517 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1518 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1519 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1523 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1524 up to 8 packets (the `<0-7>').
1525 The hex number at the end of the line
1526 is the value of the `userdata' field in the request.
1528 Helios responds with 8 512-byte packets.
1529 The `:digit' following the
1530 transaction id gives the packet sequence number in the transaction
1531 and the number in parens is the amount of data in the packet,
1532 excluding the atp header.
1533 The `*' on packet 7 indicates that the
1536 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1538 resends them then jssmag.209 releases the transaction.
1540 jssmag.209 initiates the next request.
1541 The `*' on the request
1542 indicates that XO (`exactly once') was \fInot\fP set.
1547 Fragmented Internet datagrams are printed as
1551 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1552 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1556 (The first form indicates there are more fragments.
1558 indicates this is the last fragment.)
1560 \fIId\fP is the fragment id.
1561 \fISize\fP is the fragment
1562 size (in bytes) excluding the IP header.
1563 \fIOffset\fP is this
1564 fragment's offset (in bytes) in the original datagram.
1566 The fragment information is output for each fragment.
1568 fragment contains the higher level protocol header and the frag
1569 info is printed after the protocol info.
1571 after the first contain no higher level protocol header and the
1572 frag info is printed after the source and destination addresses.
1573 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1574 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1578 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1579 arizona > rtsg: (frag 595a:204@328)
1580 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1584 There are a couple of things to note here: First, addresses in the
1585 2nd line don't include port numbers.
1586 This is because the TCP
1587 protocol information is all in the first fragment and we have no idea
1588 what the port or sequence numbers are when we print the later fragments.
1589 Second, the tcp sequence information in the first line is printed as if there
1590 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1591 the first frag and 204 in the second).
1592 If you are looking for holes
1593 in the sequence space or trying to match up acks
1594 with packets, this can fool you.
1596 A packet with the IP \fIdon't fragment\fP flag is marked with a
1597 trailing \fB(DF)\fP.
1601 By default, all output lines are preceded by a timestamp.
1603 is the current clock time in the form
1609 and is as accurate as the kernel's clock.
1610 The timestamp reflects the time the kernel first saw the packet.
1612 is made to account for the time lag between when the
1613 Ethernet interface removed the packet from the wire and when the kernel
1614 serviced the `new packet' interrupt.
1616 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1617 pcap-filter(@MAN_MISC_INFO@)
1619 The original authors are:
1623 Steven McCanne, all of the
1624 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1626 It is currently being maintained by tcpdump.org.
1628 The current version is available via http:
1631 .I http://www.tcpdump.org/
1634 The original distribution is available via anonymous ftp:
1637 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1640 IPv6/IPsec support is added by WIDE/KAME project.
1641 This program uses Eric Young's SSLeay library, under specific configurations.
1643 Please send problems, bugs, questions, desirable enhancements, patches
1647 tcpdump-workers@lists.tcpdump.org
1650 NIT doesn't let you watch your own outbound traffic, BPF will.
1651 We recommend that you use the latter.
1653 On Linux systems with 2.0[.x] kernels:
1655 packets on the loopback device will be seen twice;
1657 packet filtering cannot be done in the kernel, so that all packets must
1658 be copied from the kernel in order to be filtered in user mode;
1660 all of a packet, not just the part that's within the snapshot length,
1661 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1662 asked to copy only part of a packet to userland, will not report the
1663 true length of the packet; this would cause most IP packets to get an
1667 capturing on some PPP devices won't work correctly.
1669 We recommend that you upgrade to a 2.2 or later kernel.
1671 Some attempt should be made to reassemble IP fragments or, at least
1672 to compute the right length for the higher level protocol.
1674 Name server inverse queries are not dumped correctly: the (empty)
1675 question section is printed rather than real query in the answer
1677 Some believe that inverse queries are themselves a bug and
1678 prefer to fix the program generating them rather than \fItcpdump\fP.
1680 A packet trace that crosses a daylight savings time change will give
1681 skewed time stamps (the time change is ignored).
1683 Filter expressions on fields other than those in Token Ring headers will
1684 not correctly handle source-routed Token Ring packets.
1686 Filter expressions on fields other than those in 802.11 headers will not
1687 correctly handle 802.11 data packets with both To DS and From DS set.
1690 should chase header chain, but at this moment it does not.
1691 .BR "ip6 protochain"
1692 is supplied for this behavior.
1694 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1695 does not work against IPv6 packets.
1696 It only looks at IPv4 packets.
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