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 "05 March 2009"
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 65535 bytes.
436 Packets truncated because of a limited snapshot
437 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
438 is the name of the protocol level at which the truncation has occurred.
439 Note that taking larger snapshots both increases
440 the amount of time it takes to process packets and, effectively,
441 decreases the amount of packet buffering.
442 This may cause packets to be
444 You should limit \fIsnaplen\fP to the smallest number that will
445 capture the protocol information you're interested in.
447 \fIsnaplen\fP to 0 sets it to the default of 65535,
448 for backwards compatibility with recent older versions of
452 Force packets selected by "\fIexpression\fP" to be interpreted the
453 specified \fItype\fR.
454 Currently known types are
455 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
456 \fBcnfp\fR (Cisco NetFlow protocol),
457 \fBrpc\fR (Remote Procedure Call),
458 \fBrtp\fR (Real-Time Applications protocol),
459 \fBrtcp\fR (Real-Time Applications control protocol),
460 \fBsnmp\fR (Simple Network Management Protocol),
461 \fBtftp\fR (Trivial File Transfer Protocol),
462 \fBvat\fR (Visual Audio Tool),
464 \fBwb\fR (distributed White Board).
467 \fIDon't\fP print a timestamp on each dump line.
470 Print an unformatted timestamp on each dump line.
473 Print a delta (micro-second resolution) between current and previous line
477 Print a timestamp in default format proceeded by date on each dump line.
480 Print a delta (micro-second resolution) between current and first line
484 Print undecoded NFS handles.
487 Make output saved via the
489 option ``packet-buffered''; i.e., as each packet is saved, it will be
490 written to the output file, rather than being written only when the
495 flag will not be supported if
497 was built with an older version of
504 When parsing and printing, produce (slightly more) verbose output.
505 For example, the time to live,
506 identification, total length and options in an IP packet are printed.
507 Also enables additional packet integrity checks such as verifying the
508 IP and ICMP header checksum.
510 When writing to a file with the
512 option, report, every 10 seconds, the number of packets captured.
515 Even more verbose output.
516 For example, additional fields are
517 printed from NFS reply packets, and SMB packets are fully decoded.
520 Even more verbose output.
522 telnet \fBSB\fP ... \fBSE\fP options
526 Telnet options are printed in hex as well.
529 Write the raw packets to \fIfile\fR rather than parsing and printing
531 They can later be printed with the \-r option.
532 Standard output is used if \fIfile\fR is ``-''.
534 .BR pcap-savefile (@MAN_FILE_FORMATS@)
535 for a description of the file format.
538 Used in conjunction with the
540 option, this will limit the number
541 of files created to the specified number, and begin overwriting files
542 from the beginning, thus creating a 'rotating' buffer.
543 In addition, it will name
544 the files with enough leading 0s to support the maximum number of
545 files, allowing them to sort correctly.
547 Used in conjunction with the
549 option, this will limit the number of rotated dump files that get
550 created, exiting with status 0 when reaching the limit. If used with
552 as well, the behavior will result in cyclical files per timeslice.
555 When parsing and printing,
556 in addition to printing the headers of each packet, print the data of
557 each packet (minus its link level header) in hex.
558 The smaller of the entire packet or
560 bytes will be printed. Note that this is the entire link-layer
561 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
562 will also be printed when the higher layer packet is shorter than the
566 When parsing and printing,
567 in addition to printing the headers of each packet, print the data of
570 its link level header, in hex.
573 When parsing and printing,
574 in addition to printing the headers of each packet, print the data of
575 each packet (minus its link level header) in hex and ASCII.
576 This is very handy for analysing new protocols.
579 When parsing and printing,
580 in addition to printing the headers of each packet, print the data of
583 its link level header, in hex and ASCII.
586 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
589 Used in conjunction with the
593 options, this will make
599 is the savefile being closed after each rotation. For example, specifying
603 will compress each savefile using gzip or bzip2.
605 Note that tcpdump will run the command in parallel to the capture, using
606 the lowest priority so that this doesn't disturb the capture process.
608 And in case you would like to use a command that itself takes flags or
609 different arguments, you can always write a shell script that will take the
610 savefile name as the only argument, make the flags & arguments arrangements
611 and execute the command that you want.
614 Drops privileges (if root) and changes user ID to
616 and the group ID to the primary group of
619 This behavior can also be enabled by default at compile time.
620 .IP "\fI expression\fP"
622 selects which packets will be dumped.
623 If no \fIexpression\fP
624 is given, all packets on the net will be dumped.
626 only packets for which \fIexpression\fP is `true' will be dumped.
628 For the \fIexpression\fP syntax, see
629 .BR pcap-filter (@MAN_MISC_INFO@).
631 Expression arguments can be passed to \fItcpdump\fP as either a single
632 argument or as multiple arguments, whichever is more convenient.
633 Generally, if the expression contains Shell metacharacters, it is
634 easier to pass it as a single, quoted argument.
635 Multiple arguments are concatenated with spaces before being parsed.
638 To print all packets arriving at or departing from \fIsundown\fP:
641 \fBtcpdump host sundown\fP
645 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
648 \fBtcpdump host helios and \\( hot or ace \\)\fP
652 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
655 \fBtcpdump ip host ace and not helios\fP
659 To print all traffic between local hosts and hosts at Berkeley:
663 tcpdump net ucb-ether
667 To print all ftp traffic through internet gateway \fIsnup\fP:
668 (note that the expression is quoted to prevent the shell from
669 (mis-)interpreting the parentheses):
673 tcpdump 'gateway snup and (port ftp or ftp-data)'
677 To print traffic neither sourced from nor destined for local hosts
678 (if you gateway to one other net, this stuff should never make it
679 onto your local net).
683 tcpdump ip and not net \fIlocalnet\fP
687 To print the start and end packets (the SYN and FIN packets) of each
688 TCP conversation that involves a non-local host.
692 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
696 To print all IPv4 HTTP packets to and from port 80, i.e. print only
697 packets that contain data, not, for example, SYN and FIN packets and
698 ACK-only packets. (IPv6 is left as an exercise for the reader.)
702 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
706 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
710 tcpdump 'gateway snup and ip[2:2] > 576'
714 To print IP broadcast or multicast packets that were
716 sent via Ethernet broadcast or multicast:
720 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
724 To print all ICMP packets that are not echo requests/replies (i.e., not
729 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
734 The output of \fItcpdump\fP is protocol dependent.
736 gives a brief description and examples of most of the formats.
744 If the '-e' option is given, the link level header is printed out.
745 On Ethernets, the source and destination addresses, protocol,
746 and packet length are printed.
748 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
749 the `frame control' field, the source and destination addresses,
750 and the packet length.
751 (The `frame control' field governs the
752 interpretation of the rest of the packet.
754 as those containing IP datagrams) are `async' packets, with a priority
755 value between 0 and 7; for example, `\fBasync4\fR'.
757 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
758 the LLC header is printed if it is \fInot\fR an ISO datagram or a
759 so-called SNAP packet.
761 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
762 the `access control' and `frame control' fields, the source and
763 destination addresses, and the packet length.
765 packets are assumed to contain an LLC packet.
766 Regardless of whether
767 the '-e' option is specified or not, the source routing information is
768 printed for source-routed packets.
770 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
771 the `frame control' fields, all of the addresses in the 802.11 header,
772 and the packet length.
774 packets are assumed to contain an LLC packet.
776 \fI(N.B.: The following description assumes familiarity with
777 the SLIP compression algorithm described in RFC-1144.)\fP
779 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
780 packet type, and compression information are printed out.
781 The packet type is printed first.
782 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
783 No further link information is printed for \fIip\fR packets.
784 For TCP packets, the connection identifier is printed following the type.
785 If the packet is compressed, its encoded header is printed out.
786 The special cases are printed out as
787 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
788 the sequence number (or sequence number and ack) has changed.
789 If it is not a special case,
790 zero or more changes are printed.
791 A change is indicated by U (urgent pointer), W (window), A (ack),
792 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
794 Finally, the amount of data in the packet and compressed header length
797 For example, the following line shows an outbound compressed TCP packet,
798 with an implicit connection identifier; the ack has changed by 6,
799 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
800 data and 6 bytes of compressed header:
803 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
809 Arp/rarp output shows the type of request and its arguments.
811 format is intended to be self explanatory.
812 Here is a short sample taken from the start of an `rlogin' from
813 host \fIrtsg\fP to host \fIcsam\fP:
817 \f(CWarp who-has csam tell rtsg
818 arp reply csam is-at CSAM\fR
822 The first line says that rtsg sent an arp packet asking
823 for the Ethernet address of internet host csam.
825 replies with its Ethernet address (in this example, Ethernet addresses
826 are in caps and internet addresses in lower case).
828 This would look less redundant if we had done \fItcpdump \-n\fP:
832 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
833 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
837 If we had done \fItcpdump \-e\fP, the fact that the first packet is
838 broadcast and the second is point-to-point would be visible:
842 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
843 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
847 For the first packet this says the Ethernet source address is RTSG, the
848 destination is the Ethernet broadcast address, the type field
849 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
853 \fI(N.B.:The following description assumes familiarity with
854 the TCP protocol described in RFC-793.
855 If you are not familiar
856 with the protocol, neither this description nor \fItcpdump\fP will
857 be of much use to you.)\fP
859 The general format of a tcp protocol line is:
863 \fIsrc > dst: flags data-seqno ack window urgent options\fP
867 \fISrc\fP and \fIdst\fP are the source and destination IP
869 \fIFlags\fP are some combination of S (SYN),
870 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
872 \fIData-seqno\fP describes the portion of sequence space covered
873 by the data in this packet (see example below).
874 \fIAck\fP is sequence number of the next data expected the other
875 direction on this connection.
876 \fIWindow\fP is the number of bytes of receive buffer space available
877 the other direction on this connection.
878 \fIUrg\fP indicates there is `urgent' data in the packet.
879 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
881 \fISrc, dst\fP and \fIflags\fP are always present.
883 depend on the contents of the packet's tcp protocol header and
884 are output only if appropriate.
886 Here is the opening portion of an rlogin from host \fIrtsg\fP to
891 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
892 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
893 rtsg.1023 > csam.login: . ack 1 win 4096
894 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
895 csam.login > rtsg.1023: . ack 2 win 4096
896 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
897 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
898 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
899 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
903 The first line says that tcp port 1023 on rtsg sent a packet
906 The \fBS\fP indicates that the \fISYN\fP flag was set.
907 The packet sequence number was 768512 and it contained no data.
908 (The notation is `first:last(nbytes)' which means `sequence
910 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
911 There was no piggy-backed ack, the available receive window was 4096
912 bytes and there was a max-segment-size option requesting an mss of
915 Csam replies with a similar packet except it includes a piggy-backed
917 Rtsg then acks csam's SYN.
920 The packet contained no data so there is no data sequence number.
921 Note that the ack sequence
922 number is a small integer (1).
923 The first time \fItcpdump\fP sees a
924 tcp `conversation', it prints the sequence number from the packet.
925 On subsequent packets of the conversation, the difference between
926 the current packet's sequence number and this initial sequence number
928 This means that sequence numbers after the
929 first can be interpreted
930 as relative byte positions in the conversation's data stream (with the
931 first data byte each direction being `1').
932 `-S' will override this
933 feature, causing the original sequence numbers to be output.
935 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
936 in the rtsg \(-> csam side of the conversation).
937 The PUSH flag is set in the packet.
938 On the 7th line, csam says it's received data sent by rtsg up to
939 but not including byte 21.
940 Most of this data is apparently sitting in the
941 socket buffer since csam's receive window has gotten 19 bytes smaller.
942 Csam also sends one byte of data to rtsg in this packet.
943 On the 8th and 9th lines,
944 csam sends two bytes of urgent, pushed data to rtsg.
946 If the snapshot was small enough that \fItcpdump\fP didn't capture
947 the full TCP header, it interprets as much of the header as it can
948 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
950 If the header contains a bogus option (one with a length
951 that's either too small or beyond the end of the header), \fItcpdump\fP
952 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
953 options (since it's impossible to tell where they start).
955 length indicates options are present but the IP datagram length is not
956 long enough for the options to actually be there, \fItcpdump\fP reports
957 it as ``[\fIbad hdr length\fP]''.
959 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
961 There are 8 bits in the control bits section of the TCP header:
963 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
965 Let's assume that we want to watch packets used in establishing
967 Recall that TCP uses a 3-way handshake protocol
968 when it initializes a new connection; the connection sequence with
969 regard to the TCP control bits is
975 2) Recipient responds with SYN, ACK
981 Now we're interested in capturing packets that have only the
982 SYN bit set (Step 1).
983 Note that we don't want packets from step 2
984 (SYN-ACK), just a plain initial SYN.
985 What we need is a correct filter
986 expression for \fItcpdump\fP.
988 Recall the structure of a TCP header without options:
992 -----------------------------------------------------------------
993 | source port | destination port |
994 -----------------------------------------------------------------
996 -----------------------------------------------------------------
997 | acknowledgment number |
998 -----------------------------------------------------------------
999 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1000 -----------------------------------------------------------------
1001 | TCP checksum | urgent pointer |
1002 -----------------------------------------------------------------
1005 A TCP header usually holds 20 octets of data, unless options are
1007 The first line of the graph contains octets 0 - 3, the
1008 second line shows octets 4 - 7 etc.
1010 Starting to count with 0, the relevant TCP control bits are contained
1015 ----------------|---------------|---------------|----------------
1016 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1017 ----------------|---------------|---------------|----------------
1018 | | 13th octet | | |
1021 Let's have a closer look at octet no. 13:
1031 These are the TCP control bits we are interested
1033 We have numbered the bits in this octet from 0 to 7, right to
1034 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1036 Recall that we want to capture packets with only SYN set.
1037 Let's see what happens to octet 13 if a TCP datagram arrives
1038 with the SYN bit set in its header:
1049 control bits section we see that only bit number 1 (SYN) is set.
1051 Assuming that octet number 13 is an 8-bit unsigned integer in
1052 network byte order, the binary value of this octet is
1056 and its decimal representation is
1060 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1063 We're almost done, because now we know that if only SYN is set,
1064 the value of the 13th octet in the TCP header, when interpreted
1065 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1067 This relationship can be expressed as
1073 We can use this expression as the filter for \fItcpdump\fP in order
1074 to watch packets which have only SYN set:
1077 tcpdump -i xl0 tcp[13] == 2
1080 The expression says "let the 13th octet of a TCP datagram have
1081 the decimal value 2", which is exactly what we want.
1083 Now, let's assume that we need to capture SYN packets, but we
1084 don't care if ACK or any other TCP control bit is set at the
1086 Let's see what happens to octet 13 when a TCP datagram
1087 with SYN-ACK set arrives:
1097 Now bits 1 and 4 are set in the 13th octet.
1103 which translates to decimal
1107 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1110 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1111 expression, because that would select only those packets that have
1112 SYN-ACK set, but not those with only SYN set.
1113 Remember that we don't care
1114 if ACK or any other control bit is set as long as SYN is set.
1116 In order to achieve our goal, we need to logically AND the
1117 binary value of octet 13 with some other value to preserve
1119 We know that we want SYN to be set in any case,
1120 so we'll logically AND the value in the 13th octet with
1121 the binary value of a SYN:
1125 00010010 SYN-ACK 00000010 SYN
1126 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1128 = 00000010 = 00000010
1131 We see that this AND operation delivers the same result
1132 regardless whether ACK or another TCP control bit is set.
1133 The decimal representation of the AND value as well as
1134 the result of this operation is 2 (binary 00000010),
1135 so we know that for packets with SYN set the following
1136 relation must hold true:
1138 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1140 This points us to the \fItcpdump\fP filter expression
1143 tcpdump -i xl0 'tcp[13] & 2 == 2'
1146 Note that you should use single quotes or a backslash
1147 in the expression to hide the AND ('&') special character
1153 UDP format is illustrated by this rwho packet:
1157 \f(CWactinide.who > broadcast.who: udp 84\fP
1161 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1162 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1164 The packet contained 84 bytes of user data.
1166 Some UDP services are recognized (from the source or destination
1167 port number) and the higher level protocol information printed.
1168 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1169 RPC calls (RFC-1050) to NFS.
1171 UDP Name Server Requests
1173 \fI(N.B.:The following description assumes familiarity with
1174 the Domain Service protocol described in RFC-1035.
1175 If you are not familiar
1176 with the protocol, the following description will appear to be written
1179 Name server requests are formatted as
1183 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1185 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1189 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1190 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1191 The query id was `3'.
1192 The `+' indicates the \fIrecursion desired\fP flag
1194 The query length was 37 bytes, not including the UDP and
1195 IP protocol headers.
1196 The query operation was the normal one, \fIQuery\fP,
1197 so the op field was omitted.
1198 If the op had been anything else, it would
1199 have been printed between the `3' and the `+'.
1200 Similarly, the qclass was the normal one,
1201 \fIC_IN\fP, and omitted.
1202 Any other qclass would have been printed
1203 immediately after the `A'.
1205 A few anomalies are checked and may result in extra fields enclosed in
1206 square brackets: If a query contains an answer, authority records or
1207 additional records section,
1212 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1213 is the appropriate count.
1214 If any of the response bits are set (AA, RA or rcode) or any of the
1215 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1216 is printed, where \fIx\fP is the hex value of header bytes two and three.
1218 UDP Name Server Responses
1220 Name server responses are formatted as
1224 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1226 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1227 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1231 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1232 with 3 answer records, 3 name server records and 7 additional records.
1233 The first answer record is type A (address) and its data is internet
1234 address 128.32.137.3.
1235 The total size of the response was 273 bytes,
1236 excluding UDP and IP headers.
1237 The op (Query) and response code
1238 (NoError) were omitted, as was the class (C_IN) of the A record.
1240 In the second example, \fIhelios\fP responds to query 2 with a
1241 response code of non-existent domain (NXDomain) with no answers,
1242 one name server and no authority records.
1243 The `*' indicates that
1244 the \fIauthoritative answer\fP bit was set.
1246 answers, no type, class or data were printed.
1248 Other flag characters that might appear are `\-' (recursion available,
1249 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1251 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1257 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1258 on UDP/137, UDP/138 and TCP/139.
1259 Some primitive decoding of IPX and
1260 NetBEUI SMB data is also done.
1262 By default a fairly minimal decode is done, with a much more detailed
1263 decode done if -v is used.
1264 Be warned that with -v a single SMB packet
1265 may take up a page or more, so only use -v if you really want all the
1268 For information on SMB packet formats and what all te fields mean see
1269 www.cifs.org or the pub/samba/specs/ directory on your favorite
1270 samba.org mirror site.
1271 The SMB patches were written by Andrew Tridgell
1275 NFS Requests and Replies
1277 Sun NFS (Network File System) requests and replies are printed as:
1281 \fIsrc.xid > dst.nfs: len op args\fP
1282 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1285 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1286 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1287 sushi.201b > wrl.nfs:
1288 144 lookup fh 9,74/4096.6878 "xcolors"
1289 wrl.nfs > sushi.201b:
1290 reply ok 128 lookup fh 9,74/4134.3150
1295 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1296 to \fIwrl\fP (note that the number following the src host is a
1297 transaction id, \fInot\fP the source port).
1298 The request was 112 bytes,
1299 excluding the UDP and IP headers.
1300 The operation was a \fIreadlink\fP
1301 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1302 (If one is lucky, as in this case, the file handle can be interpreted
1303 as a major,minor device number pair, followed by the inode number and
1305 \fIWrl\fP replies `ok' with the contents of the link.
1307 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1308 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1309 Note that the data printed
1310 depends on the operation type.
1311 The format is intended to be self
1312 explanatory if read in conjunction with
1313 an NFS protocol spec.
1315 If the \-v (verbose) flag is given, additional information is printed.
1321 sushi.1372a > wrl.nfs:
1322 148 read fh 21,11/12.195 8192 bytes @ 24576
1323 wrl.nfs > sushi.1372a:
1324 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1329 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1330 which have been omitted from this example.) In the first line,
1331 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1332 at byte offset 24576.
1333 \fIWrl\fP replies `ok'; the packet shown on the
1334 second line is the first fragment of the reply, and hence is only 1472
1335 bytes long (the other bytes will follow in subsequent fragments, but
1336 these fragments do not have NFS or even UDP headers and so might not be
1337 printed, depending on the filter expression used).
1338 Because the \-v flag
1339 is given, some of the file attributes (which are returned in addition
1340 to the file data) are printed: the file type (``REG'', for regular file),
1341 the file mode (in octal), the uid and gid, and the file size.
1343 If the \-v flag is given more than once, even more details are printed.
1345 Note that NFS requests are very large and much of the detail won't be printed
1346 unless \fIsnaplen\fP is increased.
1347 Try using `\fB\-s 192\fP' to watch
1350 NFS reply packets do not explicitly identify the RPC operation.
1352 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1353 replies using the transaction ID.
1354 If a reply does not closely follow the
1355 corresponding request, it might not be parsable.
1357 AFS Requests and Replies
1359 Transarc AFS (Andrew File System) requests and replies are printed
1365 \fIsrc.sport > dst.dport: rx packet-type\fP
1366 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1367 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1370 elvis.7001 > pike.afsfs:
1371 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1372 new fid 536876964/1/1 ".newsrc"
1373 pike.afsfs > elvis.7001: rx data fs reply rename
1378 In the first line, host elvis sends a RX packet to pike.
1380 a RX data packet to the fs (fileserver) service, and is the start of
1382 The RPC call was a rename, with the old directory file id
1383 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1384 file id of 536876964/1/1 and a new filename of `.newsrc'.
1386 responds with a RPC reply to the rename call (which was successful, because
1387 it was a data packet and not an abort packet).
1389 In general, all AFS RPCs are decoded at least by RPC call name.
1391 AFS RPCs have at least some of the arguments decoded (generally only
1392 the `interesting' arguments, for some definition of interesting).
1394 The format is intended to be self-describing, but it will probably
1395 not be useful to people who are not familiar with the workings of
1398 If the -v (verbose) flag is given twice, acknowledgement packets and
1399 additional header information is printed, such as the the RX call ID,
1400 call number, sequence number, serial number, and the RX packet flags.
1402 If the -v flag is given twice, additional information is printed,
1403 such as the the RX call ID, serial number, and the RX packet flags.
1404 The MTU negotiation information is also printed from RX ack packets.
1406 If the -v flag is given three times, the security index and service id
1409 Error codes are printed for abort packets, with the exception of Ubik
1410 beacon packets (because abort packets are used to signify a yes vote
1411 for the Ubik protocol).
1413 Note that AFS requests are very large and many of the arguments won't
1414 be printed unless \fIsnaplen\fP is increased.
1415 Try using `\fB-s 256\fP'
1416 to watch AFS traffic.
1418 AFS reply packets do not explicitly identify the RPC operation.
1420 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1421 replies using the call number and service ID.
1422 If a reply does not closely
1424 corresponding request, it might not be parsable.
1427 KIP AppleTalk (DDP in UDP)
1429 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1430 and dumped as DDP packets (i.e., all the UDP header information is
1434 is used to translate AppleTalk net and node numbers to names.
1435 Lines in this file have the form
1447 The first two lines give the names of AppleTalk networks.
1449 line gives the name of a particular host (a host is distinguished
1450 from a net by the 3rd octet in the number \-
1451 a net number \fImust\fP have two octets and a host number \fImust\fP
1452 have three octets.) The number and name should be separated by
1453 whitespace (blanks or tabs).
1456 file may contain blank lines or comment lines (lines starting with
1459 AppleTalk addresses are printed in the form
1465 \f(CW144.1.209.2 > icsd-net.112.220
1466 office.2 > icsd-net.112.220
1467 jssmag.149.235 > icsd-net.2\fR
1473 doesn't exist or doesn't contain an entry for some AppleTalk
1474 host/net number, addresses are printed in numeric form.)
1475 In the first example, NBP (DDP port 2) on net 144.1 node 209
1476 is sending to whatever is listening on port 220 of net icsd node 112.
1477 The second line is the same except the full name of the source node
1478 is known (`office').
1479 The third line is a send from port 235 on
1480 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1481 the broadcast address (255) is indicated by a net name with no host
1482 number \- for this reason it's a good idea to keep node names and
1483 net names distinct in /etc/atalk.names).
1485 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1486 packets have their contents interpreted.
1487 Other protocols just dump
1488 the protocol name (or number if no name is registered for the
1489 protocol) and packet size.
1491 \fBNBP packets\fP are formatted like the following examples:
1495 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1496 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1497 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1501 The first line is a name lookup request for laserwriters sent by net icsd host
1502 112 and broadcast on net jssmag.
1503 The nbp id for the lookup is 190.
1504 The second line shows a reply for this request (note that it has the
1505 same id) from host jssmag.209 saying that it has a laserwriter
1506 resource named "RM1140" registered on port 250.
1508 another reply to the same request saying host techpit has laserwriter
1509 "techpit" registered on port 186.
1511 \fBATP packet\fP formatting is demonstrated by the following example:
1515 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1516 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1517 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1518 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1519 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1520 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1521 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1522 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1523 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1524 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1525 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1526 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1527 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1528 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1532 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1533 up to 8 packets (the `<0-7>').
1534 The hex number at the end of the line
1535 is the value of the `userdata' field in the request.
1537 Helios responds with 8 512-byte packets.
1538 The `:digit' following the
1539 transaction id gives the packet sequence number in the transaction
1540 and the number in parens is the amount of data in the packet,
1541 excluding the atp header.
1542 The `*' on packet 7 indicates that the
1545 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1547 resends them then jssmag.209 releases the transaction.
1549 jssmag.209 initiates the next request.
1550 The `*' on the request
1551 indicates that XO (`exactly once') was \fInot\fP set.
1556 Fragmented Internet datagrams are printed as
1560 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1561 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1565 (The first form indicates there are more fragments.
1567 indicates this is the last fragment.)
1569 \fIId\fP is the fragment id.
1570 \fISize\fP is the fragment
1571 size (in bytes) excluding the IP header.
1572 \fIOffset\fP is this
1573 fragment's offset (in bytes) in the original datagram.
1575 The fragment information is output for each fragment.
1577 fragment contains the higher level protocol header and the frag
1578 info is printed after the protocol info.
1580 after the first contain no higher level protocol header and the
1581 frag info is printed after the source and destination addresses.
1582 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1583 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1587 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1588 arizona > rtsg: (frag 595a:204@328)
1589 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1593 There are a couple of things to note here: First, addresses in the
1594 2nd line don't include port numbers.
1595 This is because the TCP
1596 protocol information is all in the first fragment and we have no idea
1597 what the port or sequence numbers are when we print the later fragments.
1598 Second, the tcp sequence information in the first line is printed as if there
1599 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1600 the first frag and 204 in the second).
1601 If you are looking for holes
1602 in the sequence space or trying to match up acks
1603 with packets, this can fool you.
1605 A packet with the IP \fIdon't fragment\fP flag is marked with a
1606 trailing \fB(DF)\fP.
1610 By default, all output lines are preceded by a timestamp.
1612 is the current clock time in the form
1618 and is as accurate as the kernel's clock.
1619 The timestamp reflects the time the kernel first saw the packet.
1621 is made to account for the time lag between when the
1622 Ethernet interface removed the packet from the wire and when the kernel
1623 serviced the `new packet' interrupt.
1625 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1626 pcap-filter(@MAN_MISC_INFO@)
1628 The original authors are:
1632 Steven McCanne, all of the
1633 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1635 It is currently being maintained by tcpdump.org.
1637 The current version is available via http:
1640 .I http://www.tcpdump.org/
1643 The original distribution is available via anonymous ftp:
1646 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1649 IPv6/IPsec support is added by WIDE/KAME project.
1650 This program uses Eric Young's SSLeay library, under specific configurations.
1652 Please send problems, bugs, questions, desirable enhancements, patches
1656 tcpdump-workers@lists.tcpdump.org
1659 NIT doesn't let you watch your own outbound traffic, BPF will.
1660 We recommend that you use the latter.
1662 On Linux systems with 2.0[.x] kernels:
1664 packets on the loopback device will be seen twice;
1666 packet filtering cannot be done in the kernel, so that all packets must
1667 be copied from the kernel in order to be filtered in user mode;
1669 all of a packet, not just the part that's within the snapshot length,
1670 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1671 asked to copy only part of a packet to userland, will not report the
1672 true length of the packet; this would cause most IP packets to get an
1676 capturing on some PPP devices won't work correctly.
1678 We recommend that you upgrade to a 2.2 or later kernel.
1680 Some attempt should be made to reassemble IP fragments or, at least
1681 to compute the right length for the higher level protocol.
1683 Name server inverse queries are not dumped correctly: the (empty)
1684 question section is printed rather than real query in the answer
1686 Some believe that inverse queries are themselves a bug and
1687 prefer to fix the program generating them rather than \fItcpdump\fP.
1689 A packet trace that crosses a daylight savings time change will give
1690 skewed time stamps (the time change is ignored).
1692 Filter expressions on fields other than those in Token Ring headers will
1693 not correctly handle source-routed Token Ring packets.
1695 Filter expressions on fields other than those in 802.11 headers will not
1696 correctly handle 802.11 data packets with both To DS and From DS set.
1699 should chase header chain, but at this moment it does not.
1700 .BR "ip6 protochain"
1701 is supplied for this behavior.
1703 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1704 does not work against IPv6 packets.
1705 It only looks at IPv4 packets.
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