1 .\" @(#) $Header: /tcpdump/master/tcpdump/tcpdump.1.in,v 1.2 2008-11-09 23:35:03 mcr Exp $ (LBL)
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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.
348 This flag will affect the output of the
352 isn't specified, only those link-layer types available when not in
353 monitor mode will be shown; if
355 is specified, only those link-layer types available when in monitor mode
359 Don't attempt to verify TCP checksums. This is useful for interfaces
360 that perform the TCP checksum calculation in hardware; otherwise,
361 all outgoing TCP checksums will be flagged as bad.
364 Make stdout line buffered.
365 Useful if you want to see the data
369 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
370 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
373 List the known data link types for the interface, in the specified mode,
374 and exit. The list of known data link types may be dependent on the
375 specified mode; for example, on some platforms, a Wi-Fi interface might
376 support one set of data link types when not in monitor mode (for
377 example, it might support only fake Ethernet headers, or might support
378 802.11 headers but not support 802.11 headers with radio information)
379 and another set of data link types when in monitor mode (for example, it
380 might support 802.11 headers, or 802.11 headers with radio information,
381 only in monitor mode).
384 Load SMI MIB module definitions from file \fImodule\fR.
386 can be used several times to load several MIB modules into \fItcpdump\fP.
389 Use \fIsecret\fP as a shared secret for validating the digests found in
390 TCP segments with the TCP-MD5 option (RFC 2385), if present.
393 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
396 Don't print domain name qualification of host names.
398 if you give this flag then \fItcpdump\fP will print ``nic''
399 instead of ``nic.ddn.mil''.
402 Do not run the packet-matching code optimizer.
404 if you suspect a bug in the optimizer.
407 \fIDon't\fP put the interface
408 into promiscuous mode.
409 Note that the interface might be in promiscuous
410 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
411 `ether host {local-hw-addr} or ether broadcast'.
414 Quick (quiet?) output.
415 Print less protocol information so output
419 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
420 If specified, \fItcpdump\fP will not print replay prevention field.
421 Since there is no protocol version field in ESP/AH specification,
422 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
425 Read packets from \fIfile\fR (which was created with the
428 Standard input is used if \fIfile\fR is ``-''.
431 Print absolute, rather than relative, TCP sequence numbers.
434 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
435 default of 68 (with SunOS's NIT, the minimum is actually 96).
436 68 bytes is adequate for IP, ICMP, TCP
437 and UDP but may truncate protocol information from name server and NFS
439 Packets truncated because of a limited snapshot
440 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
441 is the name of the protocol level at which the truncation has occurred.
442 Note that taking larger snapshots both increases
443 the amount of time it takes to process packets and, effectively,
444 decreases the amount of packet buffering.
445 This may cause packets to be
447 You should limit \fIsnaplen\fP to the smallest number that will
448 capture the protocol information you're interested in.
450 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
453 Force packets selected by "\fIexpression\fP" to be interpreted the
454 specified \fItype\fR.
455 Currently known types are
456 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
457 \fBcnfp\fR (Cisco NetFlow protocol),
458 \fBrpc\fR (Remote Procedure Call),
459 \fBrtp\fR (Real-Time Applications protocol),
460 \fBrtcp\fR (Real-Time Applications control protocol),
461 \fBsnmp\fR (Simple Network Management Protocol),
462 \fBtftp\fR (Trivial File Transfer Protocol),
463 \fBvat\fR (Visual Audio Tool),
465 \fBwb\fR (distributed White Board).
468 \fIDon't\fP print a timestamp on each dump line.
471 Print an unformatted timestamp on each dump line.
474 Print a delta (micro-second resolution) between current and previous line
478 Print a timestamp in default format proceeded by date on each dump line.
481 Print a delta (micro-second resolution) between current and first line
485 Print undecoded NFS handles.
488 Make output saved via the
490 option ``packet-buffered''; i.e., as each packet is saved, it will be
491 written to the output file, rather than being written only when the
496 flag will not be supported if
498 was built with an older version of
505 When parsing and printing, produce (slightly more) verbose output.
506 For example, the time to live,
507 identification, total length and options in an IP packet are printed.
508 Also enables additional packet integrity checks such as verifying the
509 IP and ICMP header checksum.
511 When writing to a file with the
513 option, report, every 10 seconds, the number of packets captured.
516 Even more verbose output.
517 For example, additional fields are
518 printed from NFS reply packets, and SMB packets are fully decoded.
521 Even more verbose output.
523 telnet \fBSB\fP ... \fBSE\fP options
527 Telnet options are printed in hex as well.
530 Write the raw packets to \fIfile\fR rather than parsing and printing
532 They can later be printed with the \-r option.
533 Standard output is used if \fIfile\fR is ``-''.
535 .BR pcap-savefile (@MAN_FILE_FORMATS@)
536 for a description of the file format.
539 Used in conjunction with the
541 option, this will limit the number
542 of files created to the specified number, and begin overwriting files
543 from the beginning, thus creating a 'rotating' buffer.
544 In addition, it will name
545 the files with enough leading 0s to support the maximum number of
546 files, allowing them to sort correctly.
548 Used in conjunction with the
550 option, this will limit the number of rotated dump files that get
551 created, exiting with status 0 when reaching the limit. If used with
553 as well, the behavior will result in cyclical files per timeslice.
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.
559 The smaller of the entire packet or
561 bytes will be printed. Note that this is the entire link-layer
562 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
563 will also be printed when the higher layer packet is shorter than the
567 When parsing and printing,
568 in addition to printing the headers of each packet, print the data of
571 its link level header, in hex.
574 When parsing and printing,
575 in addition to printing the headers of each packet, print the data of
576 each packet (minus its link level header) in hex and ASCII.
577 This is very handy for analysing new protocols.
580 When parsing and printing,
581 in addition to printing the headers of each packet, print the data of
584 its link level header, in hex and ASCII.
587 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
590 Used in conjunction with the
594 options, this will make
600 is the savefile being closed after each rotation. For example, specifying
604 will compress each savefile using gzip or bzip2.
606 Note that tcpdump will run the command in parallel to the capture, using
607 the lowest priority so that this doesn't disturb the capture process.
609 And in case you would like to use a command that itself takes flags or
610 different arguments, you can always write a shell script that will take the
611 savefile name as the only argument, make the flags & arguments arrangements
612 and execute the command that you want.
615 Drops privileges (if root) and changes user ID to
617 and the group ID to the primary group of
620 This behavior can also be enabled by default at compile time.
621 .IP "\fI expression\fP"
623 selects which packets will be dumped.
624 If no \fIexpression\fP
625 is given, all packets on the net will be dumped.
627 only packets for which \fIexpression\fP is `true' will be dumped.
629 For the \fIexpression\fP syntax, see
630 .BR pcap-filter (@MAN_MISC_INFO@).
632 Expression arguments can be passed to \fItcpdump\fP as either a single
633 argument or as multiple arguments, whichever is more convenient.
634 Generally, if the expression contains Shell metacharacters, it is
635 easier to pass it as a single, quoted argument.
636 Multiple arguments are concatenated with spaces before being parsed.
639 To print all packets arriving at or departing from \fIsundown\fP:
642 \fBtcpdump host sundown\fP
646 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
649 \fBtcpdump host helios and \\( hot or ace \\)\fP
653 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
656 \fBtcpdump ip host ace and not helios\fP
660 To print all traffic between local hosts and hosts at Berkeley:
664 tcpdump net ucb-ether
668 To print all ftp traffic through internet gateway \fIsnup\fP:
669 (note that the expression is quoted to prevent the shell from
670 (mis-)interpreting the parentheses):
674 tcpdump 'gateway snup and (port ftp or ftp-data)'
678 To print traffic neither sourced from nor destined for local hosts
679 (if you gateway to one other net, this stuff should never make it
680 onto your local net).
684 tcpdump ip and not net \fIlocalnet\fP
688 To print the start and end packets (the SYN and FIN packets) of each
689 TCP conversation that involves a non-local host.
693 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
697 To print all IPv4 HTTP packets to and from port 80, i.e. print only
698 packets that contain data, not, for example, SYN and FIN packets and
699 ACK-only packets. (IPv6 is left as an exercise for the reader.)
703 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
707 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
711 tcpdump 'gateway snup and ip[2:2] > 576'
715 To print IP broadcast or multicast packets that were
717 sent via Ethernet broadcast or multicast:
721 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
725 To print all ICMP packets that are not echo requests/replies (i.e., not
730 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
735 The output of \fItcpdump\fP is protocol dependent.
737 gives a brief description and examples of most of the formats.
745 If the '-e' option is given, the link level header is printed out.
746 On Ethernets, the source and destination addresses, protocol,
747 and packet length are printed.
749 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
750 the `frame control' field, the source and destination addresses,
751 and the packet length.
752 (The `frame control' field governs the
753 interpretation of the rest of the packet.
755 as those containing IP datagrams) are `async' packets, with a priority
756 value between 0 and 7; for example, `\fBasync4\fR'.
758 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
759 the LLC header is printed if it is \fInot\fR an ISO datagram or a
760 so-called SNAP packet.
762 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
763 the `access control' and `frame control' fields, the source and
764 destination addresses, and the packet length.
766 packets are assumed to contain an LLC packet.
767 Regardless of whether
768 the '-e' option is specified or not, the source routing information is
769 printed for source-routed packets.
771 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
772 the `frame control' fields, all of the addresses in the 802.11 header,
773 and the packet length.
775 packets are assumed to contain an LLC packet.
777 \fI(N.B.: The following description assumes familiarity with
778 the SLIP compression algorithm described in RFC-1144.)\fP
780 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
781 packet type, and compression information are printed out.
782 The packet type is printed first.
783 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
784 No further link information is printed for \fIip\fR packets.
785 For TCP packets, the connection identifier is printed following the type.
786 If the packet is compressed, its encoded header is printed out.
787 The special cases are printed out as
788 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
789 the sequence number (or sequence number and ack) has changed.
790 If it is not a special case,
791 zero or more changes are printed.
792 A change is indicated by U (urgent pointer), W (window), A (ack),
793 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
795 Finally, the amount of data in the packet and compressed header length
798 For example, the following line shows an outbound compressed TCP packet,
799 with an implicit connection identifier; the ack has changed by 6,
800 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
801 data and 6 bytes of compressed header:
804 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
810 Arp/rarp output shows the type of request and its arguments.
812 format is intended to be self explanatory.
813 Here is a short sample taken from the start of an `rlogin' from
814 host \fIrtsg\fP to host \fIcsam\fP:
818 \f(CWarp who-has csam tell rtsg
819 arp reply csam is-at CSAM\fR
823 The first line says that rtsg sent an arp packet asking
824 for the Ethernet address of internet host csam.
826 replies with its Ethernet address (in this example, Ethernet addresses
827 are in caps and internet addresses in lower case).
829 This would look less redundant if we had done \fItcpdump \-n\fP:
833 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
834 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
838 If we had done \fItcpdump \-e\fP, the fact that the first packet is
839 broadcast and the second is point-to-point would be visible:
843 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
844 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
848 For the first packet this says the Ethernet source address is RTSG, the
849 destination is the Ethernet broadcast address, the type field
850 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
854 \fI(N.B.:The following description assumes familiarity with
855 the TCP protocol described in RFC-793.
856 If you are not familiar
857 with the protocol, neither this description nor \fItcpdump\fP will
858 be of much use to you.)\fP
860 The general format of a tcp protocol line is:
864 \fIsrc > dst: flags data-seqno ack window urgent options\fP
868 \fISrc\fP and \fIdst\fP are the source and destination IP
870 \fIFlags\fP are some combination of S (SYN),
871 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
873 \fIData-seqno\fP describes the portion of sequence space covered
874 by the data in this packet (see example below).
875 \fIAck\fP is sequence number of the next data expected the other
876 direction on this connection.
877 \fIWindow\fP is the number of bytes of receive buffer space available
878 the other direction on this connection.
879 \fIUrg\fP indicates there is `urgent' data in the packet.
880 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
882 \fISrc, dst\fP and \fIflags\fP are always present.
884 depend on the contents of the packet's tcp protocol header and
885 are output only if appropriate.
887 Here is the opening portion of an rlogin from host \fIrtsg\fP to
892 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
893 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
894 rtsg.1023 > csam.login: . ack 1 win 4096
895 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
896 csam.login > rtsg.1023: . ack 2 win 4096
897 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
898 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
899 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
900 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
904 The first line says that tcp port 1023 on rtsg sent a packet
907 The \fBS\fP indicates that the \fISYN\fP flag was set.
908 The packet sequence number was 768512 and it contained no data.
909 (The notation is `first:last(nbytes)' which means `sequence
911 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
912 There was no piggy-backed ack, the available receive window was 4096
913 bytes and there was a max-segment-size option requesting an mss of
916 Csam replies with a similar packet except it includes a piggy-backed
918 Rtsg then acks csam's SYN.
921 The packet contained no data so there is no data sequence number.
922 Note that the ack sequence
923 number is a small integer (1).
924 The first time \fItcpdump\fP sees a
925 tcp `conversation', it prints the sequence number from the packet.
926 On subsequent packets of the conversation, the difference between
927 the current packet's sequence number and this initial sequence number
929 This means that sequence numbers after the
930 first can be interpreted
931 as relative byte positions in the conversation's data stream (with the
932 first data byte each direction being `1').
933 `-S' will override this
934 feature, causing the original sequence numbers to be output.
936 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
937 in the rtsg \(-> csam side of the conversation).
938 The PUSH flag is set in the packet.
939 On the 7th line, csam says it's received data sent by rtsg up to
940 but not including byte 21.
941 Most of this data is apparently sitting in the
942 socket buffer since csam's receive window has gotten 19 bytes smaller.
943 Csam also sends one byte of data to rtsg in this packet.
944 On the 8th and 9th lines,
945 csam sends two bytes of urgent, pushed data to rtsg.
947 If the snapshot was small enough that \fItcpdump\fP didn't capture
948 the full TCP header, it interprets as much of the header as it can
949 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
951 If the header contains a bogus option (one with a length
952 that's either too small or beyond the end of the header), \fItcpdump\fP
953 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
954 options (since it's impossible to tell where they start).
956 length indicates options are present but the IP datagram length is not
957 long enough for the options to actually be there, \fItcpdump\fP reports
958 it as ``[\fIbad hdr length\fP]''.
960 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
962 There are 8 bits in the control bits section of the TCP header:
964 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
966 Let's assume that we want to watch packets used in establishing
968 Recall that TCP uses a 3-way handshake protocol
969 when it initializes a new connection; the connection sequence with
970 regard to the TCP control bits is
976 2) Recipient responds with SYN, ACK
982 Now we're interested in capturing packets that have only the
983 SYN bit set (Step 1).
984 Note that we don't want packets from step 2
985 (SYN-ACK), just a plain initial SYN.
986 What we need is a correct filter
987 expression for \fItcpdump\fP.
989 Recall the structure of a TCP header without options:
993 -----------------------------------------------------------------
994 | source port | destination port |
995 -----------------------------------------------------------------
997 -----------------------------------------------------------------
998 | acknowledgment number |
999 -----------------------------------------------------------------
1000 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1001 -----------------------------------------------------------------
1002 | TCP checksum | urgent pointer |
1003 -----------------------------------------------------------------
1006 A TCP header usually holds 20 octets of data, unless options are
1008 The first line of the graph contains octets 0 - 3, the
1009 second line shows octets 4 - 7 etc.
1011 Starting to count with 0, the relevant TCP control bits are contained
1016 ----------------|---------------|---------------|----------------
1017 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1018 ----------------|---------------|---------------|----------------
1019 | | 13th octet | | |
1022 Let's have a closer look at octet no. 13:
1032 These are the TCP control bits we are interested
1034 We have numbered the bits in this octet from 0 to 7, right to
1035 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1037 Recall that we want to capture packets with only SYN set.
1038 Let's see what happens to octet 13 if a TCP datagram arrives
1039 with the SYN bit set in its header:
1050 control bits section we see that only bit number 1 (SYN) is set.
1052 Assuming that octet number 13 is an 8-bit unsigned integer in
1053 network byte order, the binary value of this octet is
1057 and its decimal representation is
1061 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1064 We're almost done, because now we know that if only SYN is set,
1065 the value of the 13th octet in the TCP header, when interpreted
1066 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1068 This relationship can be expressed as
1074 We can use this expression as the filter for \fItcpdump\fP in order
1075 to watch packets which have only SYN set:
1078 tcpdump -i xl0 tcp[13] == 2
1081 The expression says "let the 13th octet of a TCP datagram have
1082 the decimal value 2", which is exactly what we want.
1084 Now, let's assume that we need to capture SYN packets, but we
1085 don't care if ACK or any other TCP control bit is set at the
1087 Let's see what happens to octet 13 when a TCP datagram
1088 with SYN-ACK set arrives:
1098 Now bits 1 and 4 are set in the 13th octet.
1104 which translates to decimal
1108 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1111 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1112 expression, because that would select only those packets that have
1113 SYN-ACK set, but not those with only SYN set.
1114 Remember that we don't care
1115 if ACK or any other control bit is set as long as SYN is set.
1117 In order to achieve our goal, we need to logically AND the
1118 binary value of octet 13 with some other value to preserve
1120 We know that we want SYN to be set in any case,
1121 so we'll logically AND the value in the 13th octet with
1122 the binary value of a SYN:
1126 00010010 SYN-ACK 00000010 SYN
1127 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1129 = 00000010 = 00000010
1132 We see that this AND operation delivers the same result
1133 regardless whether ACK or another TCP control bit is set.
1134 The decimal representation of the AND value as well as
1135 the result of this operation is 2 (binary 00000010),
1136 so we know that for packets with SYN set the following
1137 relation must hold true:
1139 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1141 This points us to the \fItcpdump\fP filter expression
1144 tcpdump -i xl0 'tcp[13] & 2 == 2'
1147 Note that you should use single quotes or a backslash
1148 in the expression to hide the AND ('&') special character
1154 UDP format is illustrated by this rwho packet:
1158 \f(CWactinide.who > broadcast.who: udp 84\fP
1162 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1163 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1165 The packet contained 84 bytes of user data.
1167 Some UDP services are recognized (from the source or destination
1168 port number) and the higher level protocol information printed.
1169 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1170 RPC calls (RFC-1050) to NFS.
1172 UDP Name Server Requests
1174 \fI(N.B.:The following description assumes familiarity with
1175 the Domain Service protocol described in RFC-1035.
1176 If you are not familiar
1177 with the protocol, the following description will appear to be written
1180 Name server requests are formatted as
1184 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1186 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1190 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1191 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1192 The query id was `3'.
1193 The `+' indicates the \fIrecursion desired\fP flag
1195 The query length was 37 bytes, not including the UDP and
1196 IP protocol headers.
1197 The query operation was the normal one, \fIQuery\fP,
1198 so the op field was omitted.
1199 If the op had been anything else, it would
1200 have been printed between the `3' and the `+'.
1201 Similarly, the qclass was the normal one,
1202 \fIC_IN\fP, and omitted.
1203 Any other qclass would have been printed
1204 immediately after the `A'.
1206 A few anomalies are checked and may result in extra fields enclosed in
1207 square brackets: If a query contains an answer, authority records or
1208 additional records section,
1213 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1214 is the appropriate count.
1215 If any of the response bits are set (AA, RA or rcode) or any of the
1216 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1217 is printed, where \fIx\fP is the hex value of header bytes two and three.
1219 UDP Name Server Responses
1221 Name server responses are formatted as
1225 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1227 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1228 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1232 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1233 with 3 answer records, 3 name server records and 7 additional records.
1234 The first answer record is type A (address) and its data is internet
1235 address 128.32.137.3.
1236 The total size of the response was 273 bytes,
1237 excluding UDP and IP headers.
1238 The op (Query) and response code
1239 (NoError) were omitted, as was the class (C_IN) of the A record.
1241 In the second example, \fIhelios\fP responds to query 2 with a
1242 response code of non-existent domain (NXDomain) with no answers,
1243 one name server and no authority records.
1244 The `*' indicates that
1245 the \fIauthoritative answer\fP bit was set.
1247 answers, no type, class or data were printed.
1249 Other flag characters that might appear are `\-' (recursion available,
1250 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1252 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1255 Note that name server requests and responses tend to be large and the
1256 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1258 Use the \fB\-s\fP flag to increase the snaplen if you
1259 need to seriously investigate name server traffic.
1261 has worked well for me.
1266 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1267 on UDP/137, UDP/138 and TCP/139.
1268 Some primitive decoding of IPX and
1269 NetBEUI SMB data is also done.
1271 By default a fairly minimal decode is done, with a much more detailed
1272 decode done if -v is used.
1273 Be warned that with -v a single SMB packet
1274 may take up a page or more, so only use -v if you really want all the
1277 For information on SMB packet formats and what all te fields mean see
1278 www.cifs.org or the pub/samba/specs/ directory on your favorite
1279 samba.org mirror site.
1280 The SMB patches were written by Andrew Tridgell
1284 NFS Requests and Replies
1286 Sun NFS (Network File System) requests and replies are printed as:
1290 \fIsrc.xid > dst.nfs: len op args\fP
1291 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1294 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1295 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1296 sushi.201b > wrl.nfs:
1297 144 lookup fh 9,74/4096.6878 "xcolors"
1298 wrl.nfs > sushi.201b:
1299 reply ok 128 lookup fh 9,74/4134.3150
1304 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1305 to \fIwrl\fP (note that the number following the src host is a
1306 transaction id, \fInot\fP the source port).
1307 The request was 112 bytes,
1308 excluding the UDP and IP headers.
1309 The operation was a \fIreadlink\fP
1310 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1311 (If one is lucky, as in this case, the file handle can be interpreted
1312 as a major,minor device number pair, followed by the inode number and
1314 \fIWrl\fP replies `ok' with the contents of the link.
1316 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1317 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1318 Note that the data printed
1319 depends on the operation type.
1320 The format is intended to be self
1321 explanatory if read in conjunction with
1322 an NFS protocol spec.
1324 If the \-v (verbose) flag is given, additional information is printed.
1330 sushi.1372a > wrl.nfs:
1331 148 read fh 21,11/12.195 8192 bytes @ 24576
1332 wrl.nfs > sushi.1372a:
1333 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1338 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1339 which have been omitted from this example.) In the first line,
1340 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1341 at byte offset 24576.
1342 \fIWrl\fP replies `ok'; the packet shown on the
1343 second line is the first fragment of the reply, and hence is only 1472
1344 bytes long (the other bytes will follow in subsequent fragments, but
1345 these fragments do not have NFS or even UDP headers and so might not be
1346 printed, depending on the filter expression used).
1347 Because the \-v flag
1348 is given, some of the file attributes (which are returned in addition
1349 to the file data) are printed: the file type (``REG'', for regular file),
1350 the file mode (in octal), the uid and gid, and the file size.
1352 If the \-v flag is given more than once, even more details are printed.
1354 Note that NFS requests are very large and much of the detail won't be printed
1355 unless \fIsnaplen\fP is increased.
1356 Try using `\fB\-s 192\fP' to watch
1359 NFS reply packets do not explicitly identify the RPC operation.
1361 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1362 replies using the transaction ID.
1363 If a reply does not closely follow the
1364 corresponding request, it might not be parsable.
1366 AFS Requests and Replies
1368 Transarc AFS (Andrew File System) requests and replies are printed
1374 \fIsrc.sport > dst.dport: rx packet-type\fP
1375 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1376 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1379 elvis.7001 > pike.afsfs:
1380 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1381 new fid 536876964/1/1 ".newsrc"
1382 pike.afsfs > elvis.7001: rx data fs reply rename
1387 In the first line, host elvis sends a RX packet to pike.
1389 a RX data packet to the fs (fileserver) service, and is the start of
1391 The RPC call was a rename, with the old directory file id
1392 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1393 file id of 536876964/1/1 and a new filename of `.newsrc'.
1395 responds with a RPC reply to the rename call (which was successful, because
1396 it was a data packet and not an abort packet).
1398 In general, all AFS RPCs are decoded at least by RPC call name.
1400 AFS RPCs have at least some of the arguments decoded (generally only
1401 the `interesting' arguments, for some definition of interesting).
1403 The format is intended to be self-describing, but it will probably
1404 not be useful to people who are not familiar with the workings of
1407 If the -v (verbose) flag is given twice, acknowledgement packets and
1408 additional header information is printed, such as the the RX call ID,
1409 call number, sequence number, serial number, and the RX packet flags.
1411 If the -v flag is given twice, additional information is printed,
1412 such as the the RX call ID, serial number, and the RX packet flags.
1413 The MTU negotiation information is also printed from RX ack packets.
1415 If the -v flag is given three times, the security index and service id
1418 Error codes are printed for abort packets, with the exception of Ubik
1419 beacon packets (because abort packets are used to signify a yes vote
1420 for the Ubik protocol).
1422 Note that AFS requests are very large and many of the arguments won't
1423 be printed unless \fIsnaplen\fP is increased.
1424 Try using `\fB-s 256\fP'
1425 to watch AFS traffic.
1427 AFS reply packets do not explicitly identify the RPC operation.
1429 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1430 replies using the call number and service ID.
1431 If a reply does not closely
1433 corresponding request, it might not be parsable.
1436 KIP AppleTalk (DDP in UDP)
1438 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1439 and dumped as DDP packets (i.e., all the UDP header information is
1443 is used to translate AppleTalk net and node numbers to names.
1444 Lines in this file have the form
1456 The first two lines give the names of AppleTalk networks.
1458 line gives the name of a particular host (a host is distinguished
1459 from a net by the 3rd octet in the number \-
1460 a net number \fImust\fP have two octets and a host number \fImust\fP
1461 have three octets.) The number and name should be separated by
1462 whitespace (blanks or tabs).
1465 file may contain blank lines or comment lines (lines starting with
1468 AppleTalk addresses are printed in the form
1474 \f(CW144.1.209.2 > icsd-net.112.220
1475 office.2 > icsd-net.112.220
1476 jssmag.149.235 > icsd-net.2\fR
1482 doesn't exist or doesn't contain an entry for some AppleTalk
1483 host/net number, addresses are printed in numeric form.)
1484 In the first example, NBP (DDP port 2) on net 144.1 node 209
1485 is sending to whatever is listening on port 220 of net icsd node 112.
1486 The second line is the same except the full name of the source node
1487 is known (`office').
1488 The third line is a send from port 235 on
1489 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1490 the broadcast address (255) is indicated by a net name with no host
1491 number \- for this reason it's a good idea to keep node names and
1492 net names distinct in /etc/atalk.names).
1494 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1495 packets have their contents interpreted.
1496 Other protocols just dump
1497 the protocol name (or number if no name is registered for the
1498 protocol) and packet size.
1500 \fBNBP packets\fP are formatted like the following examples:
1504 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1505 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1506 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1510 The first line is a name lookup request for laserwriters sent by net icsd host
1511 112 and broadcast on net jssmag.
1512 The nbp id for the lookup is 190.
1513 The second line shows a reply for this request (note that it has the
1514 same id) from host jssmag.209 saying that it has a laserwriter
1515 resource named "RM1140" registered on port 250.
1517 another reply to the same request saying host techpit has laserwriter
1518 "techpit" registered on port 186.
1520 \fBATP packet\fP formatting is demonstrated by the following example:
1524 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1525 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1526 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1527 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1528 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1529 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1530 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1531 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1532 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1533 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1534 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1535 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1536 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1537 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1541 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1542 up to 8 packets (the `<0-7>').
1543 The hex number at the end of the line
1544 is the value of the `userdata' field in the request.
1546 Helios responds with 8 512-byte packets.
1547 The `:digit' following the
1548 transaction id gives the packet sequence number in the transaction
1549 and the number in parens is the amount of data in the packet,
1550 excluding the atp header.
1551 The `*' on packet 7 indicates that the
1554 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1556 resends them then jssmag.209 releases the transaction.
1558 jssmag.209 initiates the next request.
1559 The `*' on the request
1560 indicates that XO (`exactly once') was \fInot\fP set.
1565 Fragmented Internet datagrams are printed as
1569 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1570 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1574 (The first form indicates there are more fragments.
1576 indicates this is the last fragment.)
1578 \fIId\fP is the fragment id.
1579 \fISize\fP is the fragment
1580 size (in bytes) excluding the IP header.
1581 \fIOffset\fP is this
1582 fragment's offset (in bytes) in the original datagram.
1584 The fragment information is output for each fragment.
1586 fragment contains the higher level protocol header and the frag
1587 info is printed after the protocol info.
1589 after the first contain no higher level protocol header and the
1590 frag info is printed after the source and destination addresses.
1591 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1592 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1596 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1597 arizona > rtsg: (frag 595a:204@328)
1598 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1602 There are a couple of things to note here: First, addresses in the
1603 2nd line don't include port numbers.
1604 This is because the TCP
1605 protocol information is all in the first fragment and we have no idea
1606 what the port or sequence numbers are when we print the later fragments.
1607 Second, the tcp sequence information in the first line is printed as if there
1608 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1609 the first frag and 204 in the second).
1610 If you are looking for holes
1611 in the sequence space or trying to match up acks
1612 with packets, this can fool you.
1614 A packet with the IP \fIdon't fragment\fP flag is marked with a
1615 trailing \fB(DF)\fP.
1619 By default, all output lines are preceded by a timestamp.
1621 is the current clock time in the form
1627 and is as accurate as the kernel's clock.
1628 The timestamp reflects the time the kernel first saw the packet.
1630 is made to account for the time lag between when the
1631 Ethernet interface removed the packet from the wire and when the kernel
1632 serviced the `new packet' interrupt.
1634 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1635 pcap-filter(@MAN_MISC_INFO@)
1637 The original authors are:
1641 Steven McCanne, all of the
1642 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1644 It is currently being maintained by tcpdump.org.
1646 The current version is available via http:
1649 .I http://www.tcpdump.org/
1652 The original distribution is available via anonymous ftp:
1655 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1658 IPv6/IPsec support is added by WIDE/KAME project.
1659 This program uses Eric Young's SSLeay library, under specific configurations.
1661 Please send problems, bugs, questions, desirable enhancements, patches
1665 tcpdump-workers@lists.tcpdump.org
1668 NIT doesn't let you watch your own outbound traffic, BPF will.
1669 We recommend that you use the latter.
1671 On Linux systems with 2.0[.x] kernels:
1673 packets on the loopback device will be seen twice;
1675 packet filtering cannot be done in the kernel, so that all packets must
1676 be copied from the kernel in order to be filtered in user mode;
1678 all of a packet, not just the part that's within the snapshot length,
1679 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1680 asked to copy only part of a packet to userland, will not report the
1681 true length of the packet; this would cause most IP packets to get an
1685 capturing on some PPP devices won't work correctly.
1687 We recommend that you upgrade to a 2.2 or later kernel.
1689 Some attempt should be made to reassemble IP fragments or, at least
1690 to compute the right length for the higher level protocol.
1692 Name server inverse queries are not dumped correctly: the (empty)
1693 question section is printed rather than real query in the answer
1695 Some believe that inverse queries are themselves a bug and
1696 prefer to fix the program generating them rather than \fItcpdump\fP.
1698 A packet trace that crosses a daylight savings time change will give
1699 skewed time stamps (the time change is ignored).
1701 Filter expressions on fields other than those in Token Ring headers will
1702 not correctly handle source-routed Token Ring packets.
1704 Filter expressions on fields other than those in 802.11 headers will not
1705 correctly handle 802.11 data packets with both To DS and From DS set.
1708 should chase header chain, but at this moment it does not.
1709 .BR "ip6 protochain"
1710 is supplied for this behavior.
1712 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1713 does not work against IPv6 packets.
1714 It only looks at IPv4 packets.
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