1 .\" @(#) $Header: /tcpdump/master/tcpdump/tcpdump.1.in,v 1.2 2008-11-09 23:35:03 mcr 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 "05 March 2009"
27 tcpdump \- dump traffic on a network
32 .B \-AbdDefIKlLnNOpqRStuUvxX
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 Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
199 Set the operating system capture buffer size to \fIbuffer_size\fP.
202 Exit after receiving \fIcount\fP packets.
205 Before writing a raw packet to a savefile, check whether the file is
206 currently larger than \fIfile_size\fP and, if so, close the current
207 savefile and open a new one. Savefiles after the first savefile will
208 have the name specified with the
210 flag, with a number after it, starting at 1 and continuing upward.
211 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
212 not 1,048,576 bytes).
215 Dump the compiled packet-matching code in a human readable form to
216 standard output and stop.
219 Dump packet-matching code as a
224 Dump packet-matching code as decimal numbers (preceded with a count).
227 Print the list of the network interfaces available on the system and on
230 can capture packets. For each network interface, a number and an
231 interface name, possibly followed by a text description of the
232 interface, is printed. The interface name or the number can be supplied
235 flag to specify an interface on which to capture.
237 This can be useful on systems that don't have a command to list them
238 (e.g., Windows systems, or UNIX systems lacking
239 .BR "ifconfig \-a" );
240 the number can be useful on Windows 2000 and later systems, where the
241 interface name is a somewhat complex string.
245 flag will not be supported if
247 was built with an older version of
250 .B pcap_findalldevs()
254 Print the link-level header on each dump line.
257 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
258 are addressed to \fIaddr\fP and contain Security Parameter Index value
259 \fIspi\fP. This combination may be repeated with comma or newline seperation.
261 Note that setting the secret for IPv4 ESP packets is supported at this time.
268 \fBcast128-cbc\fP, or
270 The default is \fBdes-cbc\fP.
271 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
272 with cryptography enabled.
274 \fIsecret\fP is the ASCII text for ESP secret key.
275 If preceeded by 0x, then a hex value will be read.
277 The option assumes RFC2406 ESP, not RFC1827 ESP.
278 The option is only for debugging purposes, and
279 the use of this option with a true `secret' key is discouraged.
280 By presenting IPsec secret key onto command line
281 you make it visible to others, via
285 In addition to the above syntax, the syntax \fIfile name\fP may be used
286 to have tcpdump read the provided file in. The file is opened upon
287 receiving the first ESP packet, so any special permissions that tcpdump
288 may have been given should already have been given up.
291 Print `foreign' IPv4 addresses numerically rather than symbolically
292 (this option is intended to get around serious brain damage in
293 Sun's NIS server \(em usually it hangs forever translating non-local
296 The test for `foreign' IPv4 addresses is done using the IPv4 address and
297 netmask of the interface on which capture is being done. If that
298 address or netmask are not available, available, either because the
299 interface on which capture is being done has no address or netmask or
300 because the capture is being done on the Linux "any" interface, which
301 can capture on more than one interface, this option will not work
305 Use \fIfile\fP as input for the filter expression.
306 An additional expression given on the command line is ignored.
309 If specified, rotates the dump file specified with the
311 option every \fIrotate_seconds\fP seconds.
312 Savefiles will have the name specified by
314 which should include a time format as defined by
316 If no time format is specified, each new file will overwrite the previous.
318 If used in conjunction with the
320 option, filenames will take the form of `\fIfile\fP<count>'.
323 Listen on \fIinterface\fP.
324 If unspecified, \fItcpdump\fP searches the system interface list for the
325 lowest numbered, configured up interface (excluding loopback).
326 Ties are broken by choosing the earliest match.
328 On Linux systems with 2.2 or later kernels, an
330 argument of ``any'' can be used to capture packets from all interfaces.
331 Note that captures on the ``any'' device will not be done in promiscuous
336 flag is supported, an interface number as printed by that flag can be
342 Put the interface in "monitor mode"; this is supported only on IEEE
343 802.11 Wi-Fi interfaces, and supported only on some operating systems.
345 Note that in monitor mode the adapter might disassociate from the
346 network with which it's associated, so that you will not be able to use
347 any wireless networks with that adapter. This could prevent accessing
348 files on a network server, or resolving host names or network addresses,
349 if you are capturing in monitor mode and are not connected to another
350 network with another adapter.
352 This flag will affect the output of the
356 isn't specified, only those link-layer types available when not in
357 monitor mode will be shown; if
359 is specified, only those link-layer types available when in monitor mode
363 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
364 interfaces that perform some or all of those checksum calculation in
365 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
368 Make stdout line buffered.
369 Useful if you want to see the data
373 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
374 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
377 List the known data link types for the interface, in the specified mode,
378 and exit. The list of known data link types may be dependent on the
379 specified mode; for example, on some platforms, a Wi-Fi interface might
380 support one set of data link types when not in monitor mode (for
381 example, it might support only fake Ethernet headers, or might support
382 802.11 headers but not support 802.11 headers with radio information)
383 and another set of data link types when in monitor mode (for example, it
384 might support 802.11 headers, or 802.11 headers with radio information,
385 only in monitor mode).
388 Load SMI MIB module definitions from file \fImodule\fR.
390 can be used several times to load several MIB modules into \fItcpdump\fP.
393 Use \fIsecret\fP as a shared secret for validating the digests found in
394 TCP segments with the TCP-MD5 option (RFC 2385), if present.
397 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
400 Don't print domain name qualification of host names.
402 if you give this flag then \fItcpdump\fP will print ``nic''
403 instead of ``nic.ddn.mil''.
406 Do not run the packet-matching code optimizer.
408 if you suspect a bug in the optimizer.
411 \fIDon't\fP put the interface
412 into promiscuous mode.
413 Note that the interface might be in promiscuous
414 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
415 `ether host {local-hw-addr} or ether broadcast'.
418 Quick (quiet?) output.
419 Print less protocol information so output
423 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
424 If specified, \fItcpdump\fP will not print replay prevention field.
425 Since there is no protocol version field in ESP/AH specification,
426 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
429 Read packets from \fIfile\fR (which was created with the
432 Standard input is used if \fIfile\fR is ``-''.
435 Print absolute, rather than relative, TCP sequence numbers.
438 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
439 default of 65535 bytes.
440 Packets truncated because of a limited snapshot
441 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
442 is the name of the protocol level at which the truncation has occurred.
443 Note that taking larger snapshots both increases
444 the amount of time it takes to process packets and, effectively,
445 decreases the amount of packet buffering.
446 This may cause packets to be
448 You should limit \fIsnaplen\fP to the smallest number that will
449 capture the protocol information you're interested in.
451 \fIsnaplen\fP to 0 sets it to the default of 65535,
452 for backwards compatibility with recent older versions of
456 Force packets selected by "\fIexpression\fP" to be interpreted the
457 specified \fItype\fR.
458 Currently known types are
459 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
460 \fBcnfp\fR (Cisco NetFlow protocol),
461 \fBrpc\fR (Remote Procedure Call),
462 \fBrtp\fR (Real-Time Applications protocol),
463 \fBrtcp\fR (Real-Time Applications control protocol),
464 \fBsnmp\fR (Simple Network Management Protocol),
465 \fBtftp\fR (Trivial File Transfer Protocol),
466 \fBvat\fR (Visual Audio Tool),
468 \fBwb\fR (distributed White Board).
471 \fIDon't\fP print a timestamp on each dump line.
474 Print an unformatted timestamp on each dump line.
477 Print a delta (micro-second resolution) between current and previous line
481 Print a timestamp in default format proceeded by date on each dump line.
484 Print a delta (micro-second resolution) between current and first line
488 Print undecoded NFS handles.
491 Make output saved via the
493 option ``packet-buffered''; i.e., as each packet is saved, it will be
494 written to the output file, rather than being written only when the
499 flag will not be supported if
501 was built with an older version of
508 When parsing and printing, produce (slightly more) verbose output.
509 For example, the time to live,
510 identification, total length and options in an IP packet are printed.
511 Also enables additional packet integrity checks such as verifying the
512 IP and ICMP header checksum.
514 When writing to a file with the
516 option, report, every 10 seconds, the number of packets captured.
519 Even more verbose output.
520 For example, additional fields are
521 printed from NFS reply packets, and SMB packets are fully decoded.
524 Even more verbose output.
526 telnet \fBSB\fP ... \fBSE\fP options
530 Telnet options are printed in hex as well.
533 Write the raw packets to \fIfile\fR rather than parsing and printing
535 They can later be printed with the \-r option.
536 Standard output is used if \fIfile\fR is ``-''.
538 .BR pcap-savefile (@MAN_FILE_FORMATS@)
539 for a description of the file format.
542 Used in conjunction with the
544 option, this will limit the number
545 of files created to the specified number, and begin overwriting files
546 from the beginning, thus creating a 'rotating' buffer.
547 In addition, it will name
548 the files with enough leading 0s to support the maximum number of
549 files, allowing them to sort correctly.
551 Used in conjunction with the
553 option, this will limit the number of rotated dump files that get
554 created, exiting with status 0 when reaching the limit. If used with
556 as well, the behavior will result in cyclical files per timeslice.
559 When parsing and printing,
560 in addition to printing the headers of each packet, print the data of
561 each packet (minus its link level header) in hex.
562 The smaller of the entire packet or
564 bytes will be printed. Note that this is the entire link-layer
565 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
566 will also be printed when the higher layer packet is shorter than the
570 When parsing and printing,
571 in addition to printing the headers of each packet, print the data of
574 its link level header, in hex.
577 When parsing and printing,
578 in addition to printing the headers of each packet, print the data of
579 each packet (minus its link level header) in hex and ASCII.
580 This is very handy for analysing new protocols.
583 When parsing and printing,
584 in addition to printing the headers of each packet, print the data of
587 its link level header, in hex and ASCII.
590 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
593 Used in conjunction with the
597 options, this will make
603 is the savefile being closed after each rotation. For example, specifying
607 will compress each savefile using gzip or bzip2.
609 Note that tcpdump will run the command in parallel to the capture, using
610 the lowest priority so that this doesn't disturb the capture process.
612 And in case you would like to use a command that itself takes flags or
613 different arguments, you can always write a shell script that will take the
614 savefile name as the only argument, make the flags & arguments arrangements
615 and execute the command that you want.
618 Drops privileges (if root) and changes user ID to
620 and the group ID to the primary group of
623 This behavior can also be enabled by default at compile time.
624 .IP "\fI expression\fP"
626 selects which packets will be dumped.
627 If no \fIexpression\fP
628 is given, all packets on the net will be dumped.
630 only packets for which \fIexpression\fP is `true' will be dumped.
632 For the \fIexpression\fP syntax, see
633 .BR pcap-filter (@MAN_MISC_INFO@).
635 Expression arguments can be passed to \fItcpdump\fP as either a single
636 argument or as multiple arguments, whichever is more convenient.
637 Generally, if the expression contains Shell metacharacters, it is
638 easier to pass it as a single, quoted argument.
639 Multiple arguments are concatenated with spaces before being parsed.
642 To print all packets arriving at or departing from \fIsundown\fP:
645 \fBtcpdump host sundown\fP
649 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
652 \fBtcpdump host helios and \\( hot or ace \\)\fP
656 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
659 \fBtcpdump ip host ace and not helios\fP
663 To print all traffic between local hosts and hosts at Berkeley:
667 tcpdump net ucb-ether
671 To print all ftp traffic through internet gateway \fIsnup\fP:
672 (note that the expression is quoted to prevent the shell from
673 (mis-)interpreting the parentheses):
677 tcpdump 'gateway snup and (port ftp or ftp-data)'
681 To print traffic neither sourced from nor destined for local hosts
682 (if you gateway to one other net, this stuff should never make it
683 onto your local net).
687 tcpdump ip and not net \fIlocalnet\fP
691 To print the start and end packets (the SYN and FIN packets) of each
692 TCP conversation that involves a non-local host.
696 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
700 To print all IPv4 HTTP packets to and from port 80, i.e. print only
701 packets that contain data, not, for example, SYN and FIN packets and
702 ACK-only packets. (IPv6 is left as an exercise for the reader.)
706 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
710 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
714 tcpdump 'gateway snup and ip[2:2] > 576'
718 To print IP broadcast or multicast packets that were
720 sent via Ethernet broadcast or multicast:
724 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
728 To print all ICMP packets that are not echo requests/replies (i.e., not
733 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
738 The output of \fItcpdump\fP is protocol dependent.
740 gives a brief description and examples of most of the formats.
748 If the '-e' option is given, the link level header is printed out.
749 On Ethernets, the source and destination addresses, protocol,
750 and packet length are printed.
752 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
753 the `frame control' field, the source and destination addresses,
754 and the packet length.
755 (The `frame control' field governs the
756 interpretation of the rest of the packet.
758 as those containing IP datagrams) are `async' packets, with a priority
759 value between 0 and 7; for example, `\fBasync4\fR'.
761 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
762 the LLC header is printed if it is \fInot\fR an ISO datagram or a
763 so-called SNAP packet.
765 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
766 the `access control' and `frame control' fields, the source and
767 destination addresses, and the packet length.
769 packets are assumed to contain an LLC packet.
770 Regardless of whether
771 the '-e' option is specified or not, the source routing information is
772 printed for source-routed packets.
774 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
775 the `frame control' fields, all of the addresses in the 802.11 header,
776 and the packet length.
778 packets are assumed to contain an LLC packet.
780 \fI(N.B.: The following description assumes familiarity with
781 the SLIP compression algorithm described in RFC-1144.)\fP
783 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
784 packet type, and compression information are printed out.
785 The packet type is printed first.
786 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
787 No further link information is printed for \fIip\fR packets.
788 For TCP packets, the connection identifier is printed following the type.
789 If the packet is compressed, its encoded header is printed out.
790 The special cases are printed out as
791 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
792 the sequence number (or sequence number and ack) has changed.
793 If it is not a special case,
794 zero or more changes are printed.
795 A change is indicated by U (urgent pointer), W (window), A (ack),
796 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
798 Finally, the amount of data in the packet and compressed header length
801 For example, the following line shows an outbound compressed TCP packet,
802 with an implicit connection identifier; the ack has changed by 6,
803 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
804 data and 6 bytes of compressed header:
807 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
813 Arp/rarp output shows the type of request and its arguments.
815 format is intended to be self explanatory.
816 Here is a short sample taken from the start of an `rlogin' from
817 host \fIrtsg\fP to host \fIcsam\fP:
821 \f(CWarp who-has csam tell rtsg
822 arp reply csam is-at CSAM\fR
826 The first line says that rtsg sent an arp packet asking
827 for the Ethernet address of internet host csam.
829 replies with its Ethernet address (in this example, Ethernet addresses
830 are in caps and internet addresses in lower case).
832 This would look less redundant if we had done \fItcpdump \-n\fP:
836 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
837 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
841 If we had done \fItcpdump \-e\fP, the fact that the first packet is
842 broadcast and the second is point-to-point would be visible:
846 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
847 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
851 For the first packet this says the Ethernet source address is RTSG, the
852 destination is the Ethernet broadcast address, the type field
853 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
857 \fI(N.B.:The following description assumes familiarity with
858 the TCP protocol described in RFC-793.
859 If you are not familiar
860 with the protocol, neither this description nor \fItcpdump\fP will
861 be of much use to you.)\fP
863 The general format of a tcp protocol line is:
867 \fIsrc > dst: flags data-seqno ack window urgent options\fP
871 \fISrc\fP and \fIdst\fP are the source and destination IP
873 \fIFlags\fP are some combination of S (SYN),
874 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
876 \fIData-seqno\fP describes the portion of sequence space covered
877 by the data in this packet (see example below).
878 \fIAck\fP is sequence number of the next data expected the other
879 direction on this connection.
880 \fIWindow\fP is the number of bytes of receive buffer space available
881 the other direction on this connection.
882 \fIUrg\fP indicates there is `urgent' data in the packet.
883 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
885 \fISrc, dst\fP and \fIflags\fP are always present.
887 depend on the contents of the packet's tcp protocol header and
888 are output only if appropriate.
890 Here is the opening portion of an rlogin from host \fIrtsg\fP to
895 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
896 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
897 rtsg.1023 > csam.login: . ack 1 win 4096
898 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
899 csam.login > rtsg.1023: . ack 2 win 4096
900 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
901 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
902 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
903 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
907 The first line says that tcp port 1023 on rtsg sent a packet
910 The \fBS\fP indicates that the \fISYN\fP flag was set.
911 The packet sequence number was 768512 and it contained no data.
912 (The notation is `first:last(nbytes)' which means `sequence
914 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
915 There was no piggy-backed ack, the available receive window was 4096
916 bytes and there was a max-segment-size option requesting an mss of
919 Csam replies with a similar packet except it includes a piggy-backed
921 Rtsg then acks csam's SYN.
924 The packet contained no data so there is no data sequence number.
925 Note that the ack sequence
926 number is a small integer (1).
927 The first time \fItcpdump\fP sees a
928 tcp `conversation', it prints the sequence number from the packet.
929 On subsequent packets of the conversation, the difference between
930 the current packet's sequence number and this initial sequence number
932 This means that sequence numbers after the
933 first can be interpreted
934 as relative byte positions in the conversation's data stream (with the
935 first data byte each direction being `1').
936 `-S' will override this
937 feature, causing the original sequence numbers to be output.
939 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
940 in the rtsg \(-> csam side of the conversation).
941 The PUSH flag is set in the packet.
942 On the 7th line, csam says it's received data sent by rtsg up to
943 but not including byte 21.
944 Most of this data is apparently sitting in the
945 socket buffer since csam's receive window has gotten 19 bytes smaller.
946 Csam also sends one byte of data to rtsg in this packet.
947 On the 8th and 9th lines,
948 csam sends two bytes of urgent, pushed data to rtsg.
950 If the snapshot was small enough that \fItcpdump\fP didn't capture
951 the full TCP header, it interprets as much of the header as it can
952 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
954 If the header contains a bogus option (one with a length
955 that's either too small or beyond the end of the header), \fItcpdump\fP
956 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
957 options (since it's impossible to tell where they start).
959 length indicates options are present but the IP datagram length is not
960 long enough for the options to actually be there, \fItcpdump\fP reports
961 it as ``[\fIbad hdr length\fP]''.
963 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
965 There are 8 bits in the control bits section of the TCP header:
967 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
969 Let's assume that we want to watch packets used in establishing
971 Recall that TCP uses a 3-way handshake protocol
972 when it initializes a new connection; the connection sequence with
973 regard to the TCP control bits is
979 2) Recipient responds with SYN, ACK
985 Now we're interested in capturing packets that have only the
986 SYN bit set (Step 1).
987 Note that we don't want packets from step 2
988 (SYN-ACK), just a plain initial SYN.
989 What we need is a correct filter
990 expression for \fItcpdump\fP.
992 Recall the structure of a TCP header without options:
996 -----------------------------------------------------------------
997 | source port | destination port |
998 -----------------------------------------------------------------
1000 -----------------------------------------------------------------
1001 | acknowledgment number |
1002 -----------------------------------------------------------------
1003 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1004 -----------------------------------------------------------------
1005 | TCP checksum | urgent pointer |
1006 -----------------------------------------------------------------
1009 A TCP header usually holds 20 octets of data, unless options are
1011 The first line of the graph contains octets 0 - 3, the
1012 second line shows octets 4 - 7 etc.
1014 Starting to count with 0, the relevant TCP control bits are contained
1019 ----------------|---------------|---------------|----------------
1020 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1021 ----------------|---------------|---------------|----------------
1022 | | 13th octet | | |
1025 Let's have a closer look at octet no. 13:
1035 These are the TCP control bits we are interested
1037 We have numbered the bits in this octet from 0 to 7, right to
1038 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1040 Recall that we want to capture packets with only SYN set.
1041 Let's see what happens to octet 13 if a TCP datagram arrives
1042 with the SYN bit set in its header:
1053 control bits section we see that only bit number 1 (SYN) is set.
1055 Assuming that octet number 13 is an 8-bit unsigned integer in
1056 network byte order, the binary value of this octet is
1060 and its decimal representation is
1064 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1067 We're almost done, because now we know that if only SYN is set,
1068 the value of the 13th octet in the TCP header, when interpreted
1069 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1071 This relationship can be expressed as
1077 We can use this expression as the filter for \fItcpdump\fP in order
1078 to watch packets which have only SYN set:
1081 tcpdump -i xl0 tcp[13] == 2
1084 The expression says "let the 13th octet of a TCP datagram have
1085 the decimal value 2", which is exactly what we want.
1087 Now, let's assume that we need to capture SYN packets, but we
1088 don't care if ACK or any other TCP control bit is set at the
1090 Let's see what happens to octet 13 when a TCP datagram
1091 with SYN-ACK set arrives:
1101 Now bits 1 and 4 are set in the 13th octet.
1107 which translates to decimal
1111 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1114 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1115 expression, because that would select only those packets that have
1116 SYN-ACK set, but not those with only SYN set.
1117 Remember that we don't care
1118 if ACK or any other control bit is set as long as SYN is set.
1120 In order to achieve our goal, we need to logically AND the
1121 binary value of octet 13 with some other value to preserve
1123 We know that we want SYN to be set in any case,
1124 so we'll logically AND the value in the 13th octet with
1125 the binary value of a SYN:
1129 00010010 SYN-ACK 00000010 SYN
1130 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1132 = 00000010 = 00000010
1135 We see that this AND operation delivers the same result
1136 regardless whether ACK or another TCP control bit is set.
1137 The decimal representation of the AND value as well as
1138 the result of this operation is 2 (binary 00000010),
1139 so we know that for packets with SYN set the following
1140 relation must hold true:
1142 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1144 This points us to the \fItcpdump\fP filter expression
1147 tcpdump -i xl0 'tcp[13] & 2 == 2'
1150 Some offsets and field values may be expressed as names
1151 rather than as numeric values. For example tcp[13] may
1152 be replaced with tcp[tcpflags]. The following TCP flag
1153 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1154 tcp-push, tcp-act, tcp-urg.
1156 This can be demonstrated as:
1159 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1162 Note that you should use single quotes or a backslash
1163 in the expression to hide the AND ('&') special character
1169 UDP format is illustrated by this rwho packet:
1173 \f(CWactinide.who > broadcast.who: udp 84\fP
1177 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1178 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1180 The packet contained 84 bytes of user data.
1182 Some UDP services are recognized (from the source or destination
1183 port number) and the higher level protocol information printed.
1184 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1185 RPC calls (RFC-1050) to NFS.
1187 UDP Name Server Requests
1189 \fI(N.B.:The following description assumes familiarity with
1190 the Domain Service protocol described in RFC-1035.
1191 If you are not familiar
1192 with the protocol, the following description will appear to be written
1195 Name server requests are formatted as
1199 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1201 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1205 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1206 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1207 The query id was `3'.
1208 The `+' indicates the \fIrecursion desired\fP flag
1210 The query length was 37 bytes, not including the UDP and
1211 IP protocol headers.
1212 The query operation was the normal one, \fIQuery\fP,
1213 so the op field was omitted.
1214 If the op had been anything else, it would
1215 have been printed between the `3' and the `+'.
1216 Similarly, the qclass was the normal one,
1217 \fIC_IN\fP, and omitted.
1218 Any other qclass would have been printed
1219 immediately after the `A'.
1221 A few anomalies are checked and may result in extra fields enclosed in
1222 square brackets: If a query contains an answer, authority records or
1223 additional records section,
1228 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1229 is the appropriate count.
1230 If any of the response bits are set (AA, RA or rcode) or any of the
1231 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1232 is printed, where \fIx\fP is the hex value of header bytes two and three.
1234 UDP Name Server Responses
1236 Name server responses are formatted as
1240 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1242 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1243 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1247 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1248 with 3 answer records, 3 name server records and 7 additional records.
1249 The first answer record is type A (address) and its data is internet
1250 address 128.32.137.3.
1251 The total size of the response was 273 bytes,
1252 excluding UDP and IP headers.
1253 The op (Query) and response code
1254 (NoError) were omitted, as was the class (C_IN) of the A record.
1256 In the second example, \fIhelios\fP responds to query 2 with a
1257 response code of non-existent domain (NXDomain) with no answers,
1258 one name server and no authority records.
1259 The `*' indicates that
1260 the \fIauthoritative answer\fP bit was set.
1262 answers, no type, class or data were printed.
1264 Other flag characters that might appear are `\-' (recursion available,
1265 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1267 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1273 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1274 on UDP/137, UDP/138 and TCP/139.
1275 Some primitive decoding of IPX and
1276 NetBEUI SMB data is also done.
1278 By default a fairly minimal decode is done, with a much more detailed
1279 decode done if -v is used.
1280 Be warned that with -v a single SMB packet
1281 may take up a page or more, so only use -v if you really want all the
1284 For information on SMB packet formats and what all te fields mean see
1285 www.cifs.org or the pub/samba/specs/ directory on your favorite
1286 samba.org mirror site.
1287 The SMB patches were written by Andrew Tridgell
1291 NFS Requests and Replies
1293 Sun NFS (Network File System) requests and replies are printed as:
1297 \fIsrc.xid > dst.nfs: len op args\fP
1298 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1301 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1302 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1303 sushi.201b > wrl.nfs:
1304 144 lookup fh 9,74/4096.6878 "xcolors"
1305 wrl.nfs > sushi.201b:
1306 reply ok 128 lookup fh 9,74/4134.3150
1311 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1312 to \fIwrl\fP (note that the number following the src host is a
1313 transaction id, \fInot\fP the source port).
1314 The request was 112 bytes,
1315 excluding the UDP and IP headers.
1316 The operation was a \fIreadlink\fP
1317 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1318 (If one is lucky, as in this case, the file handle can be interpreted
1319 as a major,minor device number pair, followed by the inode number and
1321 \fIWrl\fP replies `ok' with the contents of the link.
1323 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1324 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1325 Note that the data printed
1326 depends on the operation type.
1327 The format is intended to be self
1328 explanatory if read in conjunction with
1329 an NFS protocol spec.
1331 If the \-v (verbose) flag is given, additional information is printed.
1337 sushi.1372a > wrl.nfs:
1338 148 read fh 21,11/12.195 8192 bytes @ 24576
1339 wrl.nfs > sushi.1372a:
1340 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1345 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1346 which have been omitted from this example.) In the first line,
1347 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1348 at byte offset 24576.
1349 \fIWrl\fP replies `ok'; the packet shown on the
1350 second line is the first fragment of the reply, and hence is only 1472
1351 bytes long (the other bytes will follow in subsequent fragments, but
1352 these fragments do not have NFS or even UDP headers and so might not be
1353 printed, depending on the filter expression used).
1354 Because the \-v flag
1355 is given, some of the file attributes (which are returned in addition
1356 to the file data) are printed: the file type (``REG'', for regular file),
1357 the file mode (in octal), the uid and gid, and the file size.
1359 If the \-v flag is given more than once, even more details are printed.
1361 Note that NFS requests are very large and much of the detail won't be printed
1362 unless \fIsnaplen\fP is increased.
1363 Try using `\fB\-s 192\fP' to watch
1366 NFS reply packets do not explicitly identify the RPC operation.
1368 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1369 replies using the transaction ID.
1370 If a reply does not closely follow the
1371 corresponding request, it might not be parsable.
1373 AFS Requests and Replies
1375 Transarc AFS (Andrew File System) requests and replies are printed
1381 \fIsrc.sport > dst.dport: rx packet-type\fP
1382 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1383 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1386 elvis.7001 > pike.afsfs:
1387 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1388 new fid 536876964/1/1 ".newsrc"
1389 pike.afsfs > elvis.7001: rx data fs reply rename
1394 In the first line, host elvis sends a RX packet to pike.
1396 a RX data packet to the fs (fileserver) service, and is the start of
1398 The RPC call was a rename, with the old directory file id
1399 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1400 file id of 536876964/1/1 and a new filename of `.newsrc'.
1402 responds with a RPC reply to the rename call (which was successful, because
1403 it was a data packet and not an abort packet).
1405 In general, all AFS RPCs are decoded at least by RPC call name.
1407 AFS RPCs have at least some of the arguments decoded (generally only
1408 the `interesting' arguments, for some definition of interesting).
1410 The format is intended to be self-describing, but it will probably
1411 not be useful to people who are not familiar with the workings of
1414 If the -v (verbose) flag is given twice, acknowledgement packets and
1415 additional header information is printed, such as the the RX call ID,
1416 call number, sequence number, serial number, and the RX packet flags.
1418 If the -v flag is given twice, additional information is printed,
1419 such as the the RX call ID, serial number, and the RX packet flags.
1420 The MTU negotiation information is also printed from RX ack packets.
1422 If the -v flag is given three times, the security index and service id
1425 Error codes are printed for abort packets, with the exception of Ubik
1426 beacon packets (because abort packets are used to signify a yes vote
1427 for the Ubik protocol).
1429 Note that AFS requests are very large and many of the arguments won't
1430 be printed unless \fIsnaplen\fP is increased.
1431 Try using `\fB-s 256\fP'
1432 to watch AFS traffic.
1434 AFS reply packets do not explicitly identify the RPC operation.
1436 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1437 replies using the call number and service ID.
1438 If a reply does not closely
1440 corresponding request, it might not be parsable.
1443 KIP AppleTalk (DDP in UDP)
1445 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1446 and dumped as DDP packets (i.e., all the UDP header information is
1450 is used to translate AppleTalk net and node numbers to names.
1451 Lines in this file have the form
1463 The first two lines give the names of AppleTalk networks.
1465 line gives the name of a particular host (a host is distinguished
1466 from a net by the 3rd octet in the number \-
1467 a net number \fImust\fP have two octets and a host number \fImust\fP
1468 have three octets.) The number and name should be separated by
1469 whitespace (blanks or tabs).
1472 file may contain blank lines or comment lines (lines starting with
1475 AppleTalk addresses are printed in the form
1481 \f(CW144.1.209.2 > icsd-net.112.220
1482 office.2 > icsd-net.112.220
1483 jssmag.149.235 > icsd-net.2\fR
1489 doesn't exist or doesn't contain an entry for some AppleTalk
1490 host/net number, addresses are printed in numeric form.)
1491 In the first example, NBP (DDP port 2) on net 144.1 node 209
1492 is sending to whatever is listening on port 220 of net icsd node 112.
1493 The second line is the same except the full name of the source node
1494 is known (`office').
1495 The third line is a send from port 235 on
1496 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1497 the broadcast address (255) is indicated by a net name with no host
1498 number \- for this reason it's a good idea to keep node names and
1499 net names distinct in /etc/atalk.names).
1501 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1502 packets have their contents interpreted.
1503 Other protocols just dump
1504 the protocol name (or number if no name is registered for the
1505 protocol) and packet size.
1507 \fBNBP packets\fP are formatted like the following examples:
1511 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1512 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1513 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1517 The first line is a name lookup request for laserwriters sent by net icsd host
1518 112 and broadcast on net jssmag.
1519 The nbp id for the lookup is 190.
1520 The second line shows a reply for this request (note that it has the
1521 same id) from host jssmag.209 saying that it has a laserwriter
1522 resource named "RM1140" registered on port 250.
1524 another reply to the same request saying host techpit has laserwriter
1525 "techpit" registered on port 186.
1527 \fBATP packet\fP formatting is demonstrated by the following example:
1531 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1532 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1533 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1534 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1535 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1536 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1537 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1538 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1539 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1540 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1541 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1542 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1543 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1544 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1548 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1549 up to 8 packets (the `<0-7>').
1550 The hex number at the end of the line
1551 is the value of the `userdata' field in the request.
1553 Helios responds with 8 512-byte packets.
1554 The `:digit' following the
1555 transaction id gives the packet sequence number in the transaction
1556 and the number in parens is the amount of data in the packet,
1557 excluding the atp header.
1558 The `*' on packet 7 indicates that the
1561 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1563 resends them then jssmag.209 releases the transaction.
1565 jssmag.209 initiates the next request.
1566 The `*' on the request
1567 indicates that XO (`exactly once') was \fInot\fP set.
1572 Fragmented Internet datagrams are printed as
1576 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1577 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1581 (The first form indicates there are more fragments.
1583 indicates this is the last fragment.)
1585 \fIId\fP is the fragment id.
1586 \fISize\fP is the fragment
1587 size (in bytes) excluding the IP header.
1588 \fIOffset\fP is this
1589 fragment's offset (in bytes) in the original datagram.
1591 The fragment information is output for each fragment.
1593 fragment contains the higher level protocol header and the frag
1594 info is printed after the protocol info.
1596 after the first contain no higher level protocol header and the
1597 frag info is printed after the source and destination addresses.
1598 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1599 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1603 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1604 arizona > rtsg: (frag 595a:204@328)
1605 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1609 There are a couple of things to note here: First, addresses in the
1610 2nd line don't include port numbers.
1611 This is because the TCP
1612 protocol information is all in the first fragment and we have no idea
1613 what the port or sequence numbers are when we print the later fragments.
1614 Second, the tcp sequence information in the first line is printed as if there
1615 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1616 the first frag and 204 in the second).
1617 If you are looking for holes
1618 in the sequence space or trying to match up acks
1619 with packets, this can fool you.
1621 A packet with the IP \fIdon't fragment\fP flag is marked with a
1622 trailing \fB(DF)\fP.
1626 By default, all output lines are preceded by a timestamp.
1628 is the current clock time in the form
1634 and is as accurate as the kernel's clock.
1635 The timestamp reflects the time the kernel first saw the packet.
1637 is made to account for the time lag between when the
1638 Ethernet interface removed the packet from the wire and when the kernel
1639 serviced the `new packet' interrupt.
1641 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1642 pcap-filter(@MAN_MISC_INFO@)
1644 The original authors are:
1648 Steven McCanne, all of the
1649 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1651 It is currently being maintained by tcpdump.org.
1653 The current version is available via http:
1656 .I http://www.tcpdump.org/
1659 The original distribution is available via anonymous ftp:
1662 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1665 IPv6/IPsec support is added by WIDE/KAME project.
1666 This program uses Eric Young's SSLeay library, under specific configurations.
1668 Please send problems, bugs, questions, desirable enhancements, patches
1672 tcpdump-workers@lists.tcpdump.org
1675 NIT doesn't let you watch your own outbound traffic, BPF will.
1676 We recommend that you use the latter.
1678 On Linux systems with 2.0[.x] kernels:
1680 packets on the loopback device will be seen twice;
1682 packet filtering cannot be done in the kernel, so that all packets must
1683 be copied from the kernel in order to be filtered in user mode;
1685 all of a packet, not just the part that's within the snapshot length,
1686 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1687 asked to copy only part of a packet to userland, will not report the
1688 true length of the packet; this would cause most IP packets to get an
1692 capturing on some PPP devices won't work correctly.
1694 We recommend that you upgrade to a 2.2 or later kernel.
1696 Some attempt should be made to reassemble IP fragments or, at least
1697 to compute the right length for the higher level protocol.
1699 Name server inverse queries are not dumped correctly: the (empty)
1700 question section is printed rather than real query in the answer
1702 Some believe that inverse queries are themselves a bug and
1703 prefer to fix the program generating them rather than \fItcpdump\fP.
1705 A packet trace that crosses a daylight savings time change will give
1706 skewed time stamps (the time change is ignored).
1708 Filter expressions on fields other than those in Token Ring headers will
1709 not correctly handle source-routed Token Ring packets.
1711 Filter expressions on fields other than those in 802.11 headers will not
1712 correctly handle 802.11 data packets with both To DS and From DS set.
1715 should chase header chain, but at this moment it does not.
1716 .BR "ip6 protochain"
1717 is supplied for this behavior.
1719 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1720 does not work against IPv6 packets.
1721 It only looks at IPv4 packets.
projects / FreeBSD / releng / 9.0.git / blob