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23 .TH TCPDUMP 1 "1 July 2003"
25 tcpdump \- dump traffic on a network
30 .B \-aAdDeflLnNOpqRStuUvxX
76 .I spi@ipaddr algo:secret,...
92 \fITcpdump\fP prints out the headers of packets on a network interface
93 that match the boolean \fIexpression\fP. It can also be run with the
95 flag, which causes it to save the packet data to a file for later
96 analysis, and/or with the
98 flag, which causes it to read from a saved packet file rather than to
99 read packets from a network interface. In all cases, only packets that
106 will, if not run with the
108 flag, continue capturing packets until it is interrupted by a SIGINT
109 signal (generated, for example, by typing your interrupt character,
110 typically control-C) or a SIGTERM signal (typically generated with the
112 command); if run with the
114 flag, it will capture packets until it is interrupted by a SIGINT or
115 SIGTERM signal or the specified number of packets have been processed.
119 finishes capturing packets, it will report counts of:
121 packets ``received by filter'' (the meaning of this depends on the OS on
124 and possibly on the way the OS was configured - if a filter was
125 specified on the command line, on some OSes it counts packets regardless
126 of whether they were matched by the filter expression, and on other OSes
127 it counts only packets that were matched by the filter expression and
131 packets ``dropped by kernel'' (this is the number of packets that were
132 dropped, due to a lack of buffer space, by the packet capture mechanism
135 is running, if the OS reports that information to applications; if not,
136 it will be reported as 0).
138 On platforms that support the SIGINFO signal, such as most BSDs, it will
139 report those counts when it receives a SIGINFO signal (generated, for
140 example, by typing your ``status'' character, typically control-T) and
141 will continue capturing packets.
143 Reading packets from a network interface may require that you have
146 .B Under SunOS 3.x or 4.x with NIT or BPF:
147 You must have read access to
152 .B Under Solaris with DLPI:
153 You must have read/write access to the network pseudo device, e.g.
155 On at least some versions of Solaris, however, this is not sufficient to
158 to capture in promiscuous mode; on those versions of Solaris, you must
161 must be installed setuid to root, in order to capture in promiscuous
162 mode. Note that, on many (perhaps all) interfaces, if you don't capture
163 in promiscuous mode, you will not see any outgoing packets, so a capture
164 not done in promiscuous mode may not be very useful.
166 .B Under HP-UX with DLPI:
169 must be installed setuid to root.
171 .B Under IRIX with snoop:
174 must be installed setuid to root.
179 must be installed setuid to root (unless your distribution has a kernel
180 that supports capability bits such as CAP_NET_RAW and code to allow
181 those capability bits to be given to particular accounts and to cause
182 those bits to be set on a user's initial processes when they log in, in
183 which case you must have CAP_NET_RAW in order to capture and
184 CAP_NET_ADMIN to enumerate network devices with, for example, the
188 .B Under Ultrix and Digital UNIX/Tru64 UNIX:
189 Any user may capture network traffic with
191 However, no user (not even the super-user) can capture in promiscuous
192 mode on an interface unless the super-user has enabled promiscuous-mode
193 operation on that interface using
195 and no user (not even the super-user) can capture unicast traffic
196 received by or sent by the machine on an interface unless the super-user
197 has enabled copy-all-mode operation on that interface using
201 packet capture on an interface probably requires that either
202 promiscuous-mode or copy-all-mode operation, or both modes of
203 operation, be enabled on that interface.
205 .B Under BSD (this includes Mac OS X):
206 You must have read access to
208 On BSDs with a devfs (this includes Mac OS X), this might involve more
209 than just having somebody with super-user access setting the ownership
210 or permissions on the BPF devices - it might involve configuring devfs
211 to set the ownership or permissions every time the system is booted,
212 if the system even supports that; if it doesn't support that, you might
213 have to find some other way to make that happen at boot time.
215 Reading a saved packet file doesn't require special privileges.
219 Print each packet (minus its link level header) in ASCII. Handy for
223 Attempt to convert network and broadcast addresses to names.
226 Exit after receiving \fIcount\fP packets.
229 Before writing a raw packet to a savefile, check whether the file is
230 currently larger than \fIfile_size\fP and, if so, close the current
231 savefile and open a new one. Savefiles after the first savefile will
232 have the name specified with the
234 flag, with a number after it, starting at 2 and continuing upward.
235 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
236 not 1,048,576 bytes).
239 Dump the compiled packet-matching code in a human readable form to
240 standard output and stop.
243 Dump packet-matching code as a
248 Dump packet-matching code as decimal numbers (preceded with a count).
251 Print the list of the network interfaces available on the system and on
254 can capture packets. For each network interface, a number and an
255 interface name, possibly followed by a text description of the
256 interface, is printed. The interface name or the number can be supplied
259 flag to specify an interface on which to capture.
261 This can be useful on systems that don't have a command to list them
262 (e.g., Windows systems, or UNIX systems lacking
263 .BR "ifconfig \-a" );
264 the number can be useful on Windows 2000 and later systems, where the
265 interface name is a somewhat complex string.
269 flag will not be supported if
271 was built with an older version of
274 .B pcap_findalldevs()
278 Print the link-level header on each dump line.
281 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
282 are addressed to \fIaddr\fP and contain Security Parameter Index value
283 \fIspi\fP. This combination may be repeated with comma or newline seperation.
285 Note that setting the secret for IPv4 ESP packets is supported at this time.
292 \fBcast128-cbc\fP, or
294 The default is \fBdes-cbc\fP.
295 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
296 with cryptography enabled.
298 \fIsecret\fP is the ASCII text for ESP secret key.
299 If preceeded by 0x, then a hex value will be read.
301 The option assumes RFC2406 ESP, not RFC1827 ESP.
302 The option is only for debugging purposes, and
303 the use of this option with a true `secret' key is discouraged.
304 By presenting IPsec secret key onto command line
305 you make it visible to others, via
309 In addition to the above syntax, the syntax \fIfile name\fP may be used
310 to have tcpdump read the provided file in. The file is opened upon
311 receiving the first ESP packet, so any special permissions that tcpdump
312 may have been given should already have been given up.
315 Print `foreign' IPv4 addresses numerically rather than symbolically
316 (this option is intended to get around serious brain damage in
317 Sun's NIS server \(em usually it hangs forever translating non-local
320 The test for `foreign' IPv4 addresses is done using the IPv4 address and
321 netmask of the interface on which capture is being done. If that
322 address or netmask are not available, available, either because the
323 interface on which capture is being done has no address or netmask or
324 because the capture is being done on the Linux "any" interface, which
325 can capture on more than one interface, this option will not work
329 Use \fIfile\fP as input for the filter expression.
330 An additional expression given on the command line is ignored.
333 Listen on \fIinterface\fP.
334 If unspecified, \fItcpdump\fP searches the system interface list for the
335 lowest numbered, configured up interface (excluding loopback).
336 Ties are broken by choosing the earliest match.
338 On Linux systems with 2.2 or later kernels, an
340 argument of ``any'' can be used to capture packets from all interfaces.
341 Note that captures on the ``any'' device will not be done in promiscuous
346 flag is supported, an interface number as printed by that flag can be
352 Make stdout line buffered.
353 Useful if you want to see the data
357 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
358 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
361 List the known data link types for the interface and exit.
364 Load SMI MIB module definitions from file \fImodule\fR.
366 can be used several times to load several MIB modules into \fItcpdump\fP.
369 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
372 Don't print domain name qualification of host names.
374 if you give this flag then \fItcpdump\fP will print ``nic''
375 instead of ``nic.ddn.mil''.
378 Do not run the packet-matching code optimizer.
380 if you suspect a bug in the optimizer.
383 \fIDon't\fP put the interface
384 into promiscuous mode.
385 Note that the interface might be in promiscuous
386 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
387 `ether host {local-hw-addr} or ether broadcast'.
390 Quick (quiet?) output.
391 Print less protocol information so output
395 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
396 If specified, \fItcpdump\fP will not print replay prevention field.
397 Since there is no protocol version field in ESP/AH specification,
398 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
401 Read packets from \fIfile\fR (which was created with the
404 Standard input is used if \fIfile\fR is ``-''.
407 Print absolute, rather than relative, TCP sequence numbers.
410 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
411 default of 68 (with SunOS's NIT, the minimum is actually 96).
412 68 bytes is adequate for IP, ICMP, TCP
413 and UDP but may truncate protocol information from name server and NFS
415 Packets truncated because of a limited snapshot
416 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
417 is the name of the protocol level at which the truncation has occurred.
418 Note that taking larger snapshots both increases
419 the amount of time it takes to process packets and, effectively,
420 decreases the amount of packet buffering.
421 This may cause packets to be
423 You should limit \fIsnaplen\fP to the smallest number that will
424 capture the protocol information you're interested in.
426 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
429 Force packets selected by "\fIexpression\fP" to be interpreted the
430 specified \fItype\fR.
431 Currently known types are
432 \fBcnfp\fR (Cisco NetFlow protocol),
433 \fBrpc\fR (Remote Procedure Call),
434 \fBrtp\fR (Real-Time Applications protocol),
435 \fBrtcp\fR (Real-Time Applications control protocol),
436 \fBsnmp\fR (Simple Network Management Protocol),
437 \fBtftp\fR (Trivial File Transfer Protocol),
438 \fBvat\fR (Visual Audio Tool),
440 \fBwb\fR (distributed White Board).
443 \fIDon't\fP print a timestamp on each dump line.
446 Print an unformatted timestamp on each dump line.
449 Print a delta (in micro-seconds) between current and previous line
453 Print a timestamp in default format proceeded by date on each dump line.
456 Print undecoded NFS handles.
459 Make output saved via the
461 option ``packet-buffered''; i.e., as each packet is saved, it will be
462 written to the output file, rather than being written only when the
467 flag will not be supported if
469 was built with an older version of
476 (Slightly more) verbose output.
477 For example, the time to live,
478 identification, total length and options in an IP packet are printed.
479 Also enables additional packet integrity checks such as verifying the
480 IP and ICMP header checksum.
483 Even more verbose output.
484 For example, additional fields are
485 printed from NFS reply packets, and SMB packets are fully decoded.
488 Even more verbose output.
490 telnet \fBSB\fP ... \fBSE\fP options
494 Telnet options are printed in hex as well.
497 Write the raw packets to \fIfile\fR rather than parsing and printing
499 They can later be printed with the \-r option.
500 Standard output is used if \fIfile\fR is ``-''.
503 Print each packet (minus its link level header) in hex.
504 The smaller of the entire packet or
506 bytes will be printed. Note that this is the entire link-layer
507 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
508 will also be printed when the higher layer packet is shorter than the
514 its link level header, in hex.
517 Print each packet (minus its link level header) in hex and ASCII.
518 This is very handy for analysing new protocols.
523 its link level header, in hex and ASCII.
526 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
527 .IP "\fI expression\fP"
529 selects which packets will be dumped.
530 If no \fIexpression\fP
531 is given, all packets on the net will be dumped.
533 only packets for which \fIexpression\fP is `true' will be dumped.
535 The \fIexpression\fP consists of one or more
537 Primitives usually consist of an
539 (name or number) preceded by one or more qualifiers.
541 different kinds of qualifier:
543 qualifiers say what kind of thing the id name or number refers to.
549 E.g., `host foo', `net 128.3', `port 20'.
555 qualifiers specify a particular transfer direction to and/or from
557 Possible directions are
564 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.
566 there is no dir qualifier,
569 For some link layers, such as SLIP and the ``cooked'' Linux capture mode
570 used for the ``any'' device and for some other device types, the
574 qualifiers can be used to specify a desired direction.
576 qualifiers restrict the match to a particular protocol.
591 E.g., `ether src foo', `arp net 128.3', `tcp port 21'.
593 no proto qualifier, all protocols consistent with the type are
595 E.g., `src foo' means `(ip or arp or rarp) src foo'
596 (except the latter is not legal syntax), `net bar' means `(ip or
597 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
599 [`fddi' is actually an alias for `ether'; the parser treats them
600 identically as meaning ``the data link level used on the specified
601 network interface.'' FDDI headers contain Ethernet-like source
602 and destination addresses, and often contain Ethernet-like packet
603 types, so you can filter on these FDDI fields just as with the
604 analogous Ethernet fields.
605 FDDI headers also contain other fields,
606 but you cannot name them explicitly in a filter expression.
608 Similarly, `tr' and `wlan' are aliases for `ether'; the previous
609 paragraph's statements about FDDI headers also apply to Token Ring
610 and 802.11 wireless LAN headers. For 802.11 headers, the destination
611 address is the DA field and the source address is the SA field; the
612 BSSID, RA, and TA fields aren't tested.]
614 In addition to the above, there are some special `primitive' keywords
615 that don't follow the pattern:
620 and arithmetic expressions.
621 All of these are described below.
623 More complex filter expressions are built up by using the words
628 to combine primitives.
629 E.g., `host foo and not port ftp and not port ftp-data'.
630 To save typing, identical qualifier lists can be omitted.
632 `tcp dst port ftp or ftp-data or domain' is exactly the same as
633 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
635 Allowable primitives are:
636 .IP "\fBdst host \fIhost\fR"
637 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
638 which may be either an address or a name.
639 .IP "\fBsrc host \fIhost\fR"
640 True if the IPv4/v6 source field of the packet is \fIhost\fP.
641 .IP "\fBhost \fIhost\fP
642 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
643 Any of the above host expressions can be prepended with the keywords,
644 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
647 \fBip host \fIhost\fR
650 which is equivalent to:
653 \fBether proto \fI\\ip\fB and host \fIhost\fR
656 If \fIhost\fR is a name with multiple IP addresses, each address will
657 be checked for a match.
658 .IP "\fBether dst \fIehost\fP
659 True if the ethernet destination address is \fIehost\fP.
661 may be either a name from /etc/ethers or a number (see
664 .IP "\fBether src \fIehost\fP
665 True if the ethernet source address is \fIehost\fP.
666 .IP "\fBether host \fIehost\fP
667 True if either the ethernet source or destination address is \fIehost\fP.
668 .IP "\fBgateway\fP \fIhost\fP
669 True if the packet used \fIhost\fP as a gateway.
671 source or destination address was \fIhost\fP but neither the IP source
672 nor the IP destination was \fIhost\fP.
673 \fIHost\fP must be a name and
674 must be found both by the machine's host-name-to-IP-address resolution
675 mechanisms (host name file, DNS, NIS, etc.) and by the machine's
676 host-name-to-Ethernet-address resolution mechanism (/etc/ethers, etc.).
677 (An equivalent expression is
680 \fBether host \fIehost \fBand not host \fIhost\fR
683 which can be used with either names or numbers for \fIhost / ehost\fP.)
684 This syntax does not work in IPv6-enabled configuration at this moment.
685 .IP "\fBdst net \fInet\fR"
686 True if the IPv4/v6 destination address of the packet has a network
688 \fINet\fP may be either a name from /etc/networks
689 or a network number (see \fInetworks(4)\fP for details).
690 .IP "\fBsrc net \fInet\fR"
691 True if the IPv4/v6 source address of the packet has a network
693 .IP "\fBnet \fInet\fR"
694 True if either the IPv4/v6 source or destination address of the packet has a network
696 .IP "\fBnet \fInet\fR \fBmask \fInetmask\fR"
697 True if the IP address matches \fInet\fR with the specific \fInetmask\fR.
698 May be qualified with \fBsrc\fR or \fBdst\fR.
699 Note that this syntax is not valid for IPv6 \fInet\fR.
700 .IP "\fBnet \fInet\fR/\fIlen\fR"
701 True if the IPv4/v6 address matches \fInet\fR with a netmask \fIlen\fR
703 May be qualified with \fBsrc\fR or \fBdst\fR.
704 .IP "\fBdst port \fIport\fR"
705 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
706 destination port value of \fIport\fP.
707 The \fIport\fP can be a number or a name used in /etc/services (see
711 If a name is used, both the port
712 number and protocol are checked.
713 If a number or ambiguous name is used,
714 only the port number is checked (e.g., \fBdst port 513\fR will print both
715 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
716 both tcp/domain and udp/domain traffic).
717 .IP "\fBsrc port \fIport\fR"
718 True if the packet has a source port value of \fIport\fP.
719 .IP "\fBport \fIport\fR"
720 True if either the source or destination port of the packet is \fIport\fP.
721 Any of the above port expressions can be prepended with the keywords,
722 \fBtcp\fP or \fBudp\fP, as in:
725 \fBtcp src port \fIport\fR
728 which matches only tcp packets whose source port is \fIport\fP.
729 .IP "\fBless \fIlength\fR"
730 True if the packet has a length less than or equal to \fIlength\fP.
731 This is equivalent to:
734 \fBlen <= \fIlength\fP.
737 .IP "\fBgreater \fIlength\fR"
738 True if the packet has a length greater than or equal to \fIlength\fP.
739 This is equivalent to:
742 \fBlen >= \fIlength\fP.
745 .IP "\fBip proto \fIprotocol\fR"
746 True if the packet is an IP packet (see
748 of protocol type \fIprotocol\fP.
749 \fIProtocol\fP can be a number or one of the names
750 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
751 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
752 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
753 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
754 Note that this primitive does not chase the protocol header chain.
755 .IP "\fBip6 proto \fIprotocol\fR"
756 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
757 Note that this primitive does not chase the protocol header chain.
758 .IP "\fBip6 protochain \fIprotocol\fR"
759 True if the packet is IPv6 packet,
760 and contains protocol header with type \fIprotocol\fR
761 in its protocol header chain.
765 \fBip6 protochain 6\fR
768 matches any IPv6 packet with TCP protocol header in the protocol header chain.
769 The packet may contain, for example,
770 authentication header, routing header, or hop-by-hop option header,
771 between IPv6 header and TCP header.
772 The BPF code emitted by this primitive is complex and
773 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
774 so this can be somewhat slow.
775 .IP "\fBip protochain \fIprotocol\fR"
776 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
777 .IP "\fBether broadcast\fR"
778 True if the packet is an ethernet broadcast packet.
781 .IP "\fBip broadcast\fR"
782 True if the packet is an IPv4 broadcast packet.
783 It checks for both the all-zeroes and all-ones broadcast conventions,
784 and looks up the subnet mask on the interface on which the capture is
787 If the subnet mask of the interface on which the capture is being done
788 is not available, either because the interface on which capture is being
789 done has no netmask or because the capture is being done on the Linux
790 "any" interface, which can capture on more than one interface, this
791 check will not work correctly.
792 .IP "\fBether multicast\fR"
793 True if the packet is an ethernet multicast packet.
796 This is shorthand for `\fBether[0] & 1 != 0\fP'.
797 .IP "\fBip multicast\fR"
798 True if the packet is an IP multicast packet.
799 .IP "\fBip6 multicast\fR"
800 True if the packet is an IPv6 multicast packet.
801 .IP "\fBether proto \fIprotocol\fR"
802 True if the packet is of ether type \fIprotocol\fR.
803 \fIProtocol\fP can be a number or one of the names
804 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
805 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
806 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
807 Note these identifiers are also keywords
808 and must be escaped via backslash (\\).
810 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), Token Ring
811 (e.g., `\fBtr protocol arp\fR'), and IEEE 802.11 wireless LANS (e.g.,
812 `\fBwlan protocol arp\fR'), for most of those protocols, the
813 protocol identification comes from the 802.2 Logical Link Control (LLC)
814 header, which is usually layered on top of the FDDI, Token Ring, or
817 When filtering for most protocol identifiers on FDDI, Token Ring, or
818 802.11, \fItcpdump\fR checks only the protocol ID field of an LLC header
819 in so-called SNAP format with an Organizational Unit Identifier (OUI) of
820 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
821 is in SNAP format with an OUI of 0x000000.
826 \fItcpdump\fR checks the DSAP (Destination Service Access Point) and
827 SSAP (Source Service Access Point) fields of the LLC header;
829 \fBstp\fP and \fInetbeui\fP
830 \fItcpdump\fR checks the DSAP of the LLC header;
833 \fItcpdump\fR checks for a SNAP-format packet with an OUI of 0x080007
834 and the AppleTalk etype.
837 In the case of Ethernet, \fItcpdump\fR checks the Ethernet type field
838 for most of those protocols. The exceptions are:
841 \fBiso\fP, \fBsap\fP, and \fBnetbeui\fP
842 \fItcpdump\fR checks for an 802.3 frame and then checks the LLC header as
843 it does for FDDI, Token Ring, and 802.11;
846 \fItcpdump\fR checks both for the AppleTalk etype in an Ethernet frame and
847 for a SNAP-format packet as it does for FDDI, Token Ring, and 802.11;
850 \fItcpdump\fR checks for the AppleTalk ARP etype in either an Ethernet
851 frame or an 802.2 SNAP frame with an OUI of 0x000000;
854 \fItcpdump\fR checks for the IPX etype in an Ethernet frame, the IPX
855 DSAP in the LLC header, the 802.3-with-no-LLC-header encapsulation of
856 IPX, and the IPX etype in a SNAP frame.
858 .IP "\fBdecnet src \fIhost\fR"
859 True if the DECNET source address is
861 which may be an address of the form ``10.123'', or a DECNET host
863 [DECNET host name support is only available on Ultrix systems
864 that are configured to run DECNET.]
865 .IP "\fBdecnet dst \fIhost\fR"
866 True if the DECNET destination address is
868 .IP "\fBdecnet host \fIhost\fR"
869 True if either the DECNET source or destination address is
871 .IP "\fBifname \fIinterface\fR"
872 True if the packet was logged as coming from the specified interface (applies
873 only to packets logged by OpenBSD's
875 .IP "\fBon \fIinterface\fR"
879 .IP "\fBrnr \fInum\fR"
880 True if the packet was logged as matching the specified PF rule number
881 (applies only to packets logged by OpenBSD's
883 .IP "\fBrulenum \fInum\fR"
887 .IP "\fBreason \fIcode\fR"
888 True if the packet was logged with the specified PF reason code. The known
897 (applies only to packets logged by OpenBSD's
899 .IP "\fBaction \fIact\fR"
900 True if PF took the specified action when the packet was logged. Known actions
905 (applies only to packets logged by OpenBSD's
907 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
911 \fBether proto \fIp\fR
914 where \fIp\fR is one of the above protocols.
915 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
919 \fBether proto \fIp\fR
922 where \fIp\fR is one of the above protocols.
924 \fItcpdump\fP does not currently know how to parse these protocols.
925 .IP "\fBvlan \fI[vlan_id]\fR"
926 True if the packet is an IEEE 802.1Q VLAN packet.
927 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
929 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
930 changes the decoding offsets for the remainder of \fIexpression\fR
931 on the assumption that the packet is a VLAN packet.
932 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
936 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
939 where \fIp\fR is one of the above protocols.
940 .IP "\fBiso proto \fIprotocol\fR"
941 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
942 \fIProtocol\fP can be a number or one of the names
943 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
944 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
951 where \fIp\fR is one of the above protocols.
952 .IP "\fBl1\fR, \fBl2\fR, \fBiih\fR, \fBlsp\fR, \fBsnp\fR, \fBcsnp\fR, \fBpsnp\fR"
953 Abbreviations for IS-IS PDU types.
954 .IP "\fBvpi\fP \fIn\fR
955 True if the packet is an ATM packet, for SunATM on Solaris, with a
956 virtual path identifier of
958 .IP "\fBvci\fP \fIn\fR
959 True if the packet is an ATM packet, for SunATM on Solaris, with a
960 virtual channel identifier of
963 True if the packet is an ATM packet, for SunATM on Solaris, and is
965 Note that the first \fBlane\fR keyword encountered in \fIexpression\fR
966 changes the tests done in the remainder of \fIexpression\fR
967 on the assumption that the packet is either a LANE emulated Ethernet
968 packet or a LANE LE Control packet. If \fBlane\fR isn't specified, the
969 tests are done under the assumption that the packet is an
970 LLC-encapsulated packet.
972 True if the packet is an ATM packet, for SunATM on Solaris, and is
973 an LLC-encapsulated packet.
975 True if the packet is an ATM packet, for SunATM on Solaris, and is
976 a segment OAM F4 flow cell (VPI=0 & VCI=3).
978 True if the packet is an ATM packet, for SunATM on Solaris, and is
979 an end-to-end OAM F4 flow cell (VPI=0 & VCI=4).
981 True if the packet is an ATM packet, for SunATM on Solaris, and is
982 a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 | VCI=4)).
984 True if the packet is an ATM packet, for SunATM on Solaris, and is
985 a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 | VCI=4)).
987 True if the packet is an ATM packet, for SunATM on Solaris, and is
988 on a meta signaling circuit (VPI=0 & VCI=1).
990 True if the packet is an ATM packet, for SunATM on Solaris, and is
991 on a broadcast signaling circuit (VPI=0 & VCI=2).
993 True if the packet is an ATM packet, for SunATM on Solaris, and is
994 on a signaling circuit (VPI=0 & VCI=5).
996 True if the packet is an ATM packet, for SunATM on Solaris, and is
997 on an ILMI circuit (VPI=0 & VCI=16).
999 True if the packet is an ATM packet, for SunATM on Solaris, and is
1000 on a signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect,
1001 Connect Ack, Release, or Release Done message.
1002 .IP \fBmetaconnect\fP
1003 True if the packet is an ATM packet, for SunATM on Solaris, and is
1004 on a meta signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect,
1005 Release, or Release Done message.
1006 .IP "\fIexpr relop expr\fR"
1007 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
1008 and \fIexpr\fR is an arithmetic expression composed of integer constants
1009 (expressed in standard C syntax), the normal binary operators
1010 [+, -, *, /, &, |], a length operator, and special packet data accessors.
1012 data inside the packet, use the following syntax:
1015 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
1018 \fIProto\fR is one of \fBether, fddi, tr, wlan, ppp, slip, link,
1019 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
1020 indicates the protocol layer for the index operation.
1021 (\fBether, fddi, wlan, tr, ppp, slip\fR and \fBlink\fR all refer to the
1023 Note that \fItcp, udp\fR and other upper-layer protocol types only
1024 apply to IPv4, not IPv6 (this will be fixed in the future).
1025 The byte offset, relative to the indicated protocol layer, is
1026 given by \fIexpr\fR.
1027 \fISize\fR is optional and indicates the number of bytes in the
1028 field of interest; it can be either one, two, or four, and defaults to one.
1029 The length operator, indicated by the keyword \fBlen\fP, gives the
1030 length of the packet.
1032 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
1033 The expression `\fBip[0] & 0xf != 5\fP'
1034 catches all IP packets with options.
1036 `\fBip[6:2] & 0x1fff = 0\fP'
1037 catches only unfragmented datagrams and frag zero of fragmented datagrams.
1038 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
1040 For instance, \fBtcp[0]\fP always means the first
1041 byte of the TCP \fIheader\fP, and never means the first byte of an
1042 intervening fragment.
1044 Some offsets and field values may be expressed as names rather than
1046 The following protocol header field offsets are
1047 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
1048 code field), and \fBtcpflags\fP (TCP flags field).
1050 The following ICMP type field values are available: \fBicmp-echoreply\fP,
1051 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
1052 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
1053 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
1054 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
1055 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
1057 The following TCP flags field values are available: \fBtcp-fin\fP,
1058 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP,
1059 \fBtcp-ack\fP, \fBtcp-urg\fP.
1061 Primitives may be combined using:
1063 A parenthesized group of primitives and operators
1064 (parentheses are special to the Shell and must be escaped).
1066 Negation (`\fB!\fP' or `\fBnot\fP').
1068 Concatenation (`\fB&&\fP' or `\fBand\fP').
1070 Alternation (`\fB||\fP' or `\fBor\fP').
1072 Negation has highest precedence.
1073 Alternation and concatenation have equal precedence and associate
1075 Note that explicit \fBand\fR tokens, not juxtaposition,
1076 are now required for concatenation.
1078 If an identifier is given without a keyword, the most recent keyword
1083 \fBnot host vs and ace\fR
1089 \fBnot host vs and host ace\fR
1092 which should not be confused with
1095 \fBnot ( host vs or ace )\fR
1099 Expression arguments can be passed to \fItcpdump\fP as either a single
1100 argument or as multiple arguments, whichever is more convenient.
1101 Generally, if the expression contains Shell metacharacters, it is
1102 easier to pass it as a single, quoted argument.
1103 Multiple arguments are concatenated with spaces before being parsed.
1106 To print all packets arriving at or departing from \fIsundown\fP:
1109 \fBtcpdump host sundown\fP
1113 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
1116 \fBtcpdump host helios and \\( hot or ace \\)\fP
1120 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
1123 \fBtcpdump ip host ace and not helios\fP
1127 To print all traffic between local hosts and hosts at Berkeley:
1131 tcpdump net ucb-ether
1135 To print all ftp traffic through internet gateway \fIsnup\fP:
1136 (note that the expression is quoted to prevent the shell from
1137 (mis-)interpreting the parentheses):
1141 tcpdump 'gateway snup and (port ftp or ftp-data)'
1145 To print traffic neither sourced from nor destined for local hosts
1146 (if you gateway to one other net, this stuff should never make it
1147 onto your local net).
1151 tcpdump ip and not net \fIlocalnet\fP
1155 To print the start and end packets (the SYN and FIN packets) of each
1156 TCP conversation that involves a non-local host.
1160 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
1164 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
1168 tcpdump 'gateway snup and ip[2:2] > 576'
1172 To print IP broadcast or multicast packets that were
1174 sent via ethernet broadcast or multicast:
1178 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
1182 To print all ICMP packets that are not echo requests/replies (i.e., not
1187 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
1192 The output of \fItcpdump\fP is protocol dependent.
1194 gives a brief description and examples of most of the formats.
1202 If the '-e' option is given, the link level header is printed out.
1203 On ethernets, the source and destination addresses, protocol,
1204 and packet length are printed.
1206 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
1207 the `frame control' field, the source and destination addresses,
1208 and the packet length.
1209 (The `frame control' field governs the
1210 interpretation of the rest of the packet.
1211 Normal packets (such
1212 as those containing IP datagrams) are `async' packets, with a priority
1213 value between 0 and 7; for example, `\fBasync4\fR'.
1215 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
1216 the LLC header is printed if it is \fInot\fR an ISO datagram or a
1217 so-called SNAP packet.
1219 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
1220 the `access control' and `frame control' fields, the source and
1221 destination addresses, and the packet length.
1222 As on FDDI networks,
1223 packets are assumed to contain an LLC packet.
1224 Regardless of whether
1225 the '-e' option is specified or not, the source routing information is
1226 printed for source-routed packets.
1228 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
1229 the `frame control' fields, all of the addresses in the 802.11 header,
1230 and the packet length.
1231 As on FDDI networks,
1232 packets are assumed to contain an LLC packet.
1234 \fI(N.B.: The following description assumes familiarity with
1235 the SLIP compression algorithm described in RFC-1144.)\fP
1237 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
1238 packet type, and compression information are printed out.
1239 The packet type is printed first.
1240 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
1241 No further link information is printed for \fIip\fR packets.
1242 For TCP packets, the connection identifier is printed following the type.
1243 If the packet is compressed, its encoded header is printed out.
1244 The special cases are printed out as
1245 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
1246 the sequence number (or sequence number and ack) has changed.
1247 If it is not a special case,
1248 zero or more changes are printed.
1249 A change is indicated by U (urgent pointer), W (window), A (ack),
1250 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1251 or a new value (=n).
1252 Finally, the amount of data in the packet and compressed header length
1255 For example, the following line shows an outbound compressed TCP packet,
1256 with an implicit connection identifier; the ack has changed by 6,
1257 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1258 data and 6 bytes of compressed header:
1261 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1267 Arp/rarp output shows the type of request and its arguments.
1269 format is intended to be self explanatory.
1270 Here is a short sample taken from the start of an `rlogin' from
1271 host \fIrtsg\fP to host \fIcsam\fP:
1275 \f(CWarp who-has csam tell rtsg
1276 arp reply csam is-at CSAM\fR
1280 The first line says that rtsg sent an arp packet asking
1281 for the ethernet address of internet host csam.
1283 replies with its ethernet address (in this example, ethernet addresses
1284 are in caps and internet addresses in lower case).
1286 This would look less redundant if we had done \fItcpdump \-n\fP:
1290 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1291 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1295 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1296 broadcast and the second is point-to-point would be visible:
1300 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1301 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1305 For the first packet this says the ethernet source address is RTSG, the
1306 destination is the ethernet broadcast address, the type field
1307 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1311 \fI(N.B.:The following description assumes familiarity with
1312 the TCP protocol described in RFC-793.
1313 If you are not familiar
1314 with the protocol, neither this description nor \fItcpdump\fP will
1315 be of much use to you.)\fP
1317 The general format of a tcp protocol line is:
1321 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1325 \fISrc\fP and \fIdst\fP are the source and destination IP
1326 addresses and ports.
1327 \fIFlags\fP are some combination of S (SYN),
1328 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
1330 \fIData-seqno\fP describes the portion of sequence space covered
1331 by the data in this packet (see example below).
1332 \fIAck\fP is sequence number of the next data expected the other
1333 direction on this connection.
1334 \fIWindow\fP is the number of bytes of receive buffer space available
1335 the other direction on this connection.
1336 \fIUrg\fP indicates there is `urgent' data in the packet.
1337 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1339 \fISrc, dst\fP and \fIflags\fP are always present.
1341 depend on the contents of the packet's tcp protocol header and
1342 are output only if appropriate.
1344 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1349 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1350 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1351 rtsg.1023 > csam.login: . ack 1 win 4096
1352 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1353 csam.login > rtsg.1023: . ack 2 win 4096
1354 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1355 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1356 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1357 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1361 The first line says that tcp port 1023 on rtsg sent a packet
1364 The \fBS\fP indicates that the \fISYN\fP flag was set.
1365 The packet sequence number was 768512 and it contained no data.
1366 (The notation is `first:last(nbytes)' which means `sequence
1368 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1369 There was no piggy-backed ack, the available receive window was 4096
1370 bytes and there was a max-segment-size option requesting an mss of
1373 Csam replies with a similar packet except it includes a piggy-backed
1375 Rtsg then acks csam's SYN.
1378 The packet contained no data so there is no data sequence number.
1379 Note that the ack sequence
1380 number is a small integer (1).
1381 The first time \fItcpdump\fP sees a
1382 tcp `conversation', it prints the sequence number from the packet.
1383 On subsequent packets of the conversation, the difference between
1384 the current packet's sequence number and this initial sequence number
1386 This means that sequence numbers after the
1387 first can be interpreted
1388 as relative byte positions in the conversation's data stream (with the
1389 first data byte each direction being `1').
1390 `-S' will override this
1391 feature, causing the original sequence numbers to be output.
1393 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1394 in the rtsg \(-> csam side of the conversation).
1395 The PUSH flag is set in the packet.
1396 On the 7th line, csam says it's received data sent by rtsg up to
1397 but not including byte 21.
1398 Most of this data is apparently sitting in the
1399 socket buffer since csam's receive window has gotten 19 bytes smaller.
1400 Csam also sends one byte of data to rtsg in this packet.
1401 On the 8th and 9th lines,
1402 csam sends two bytes of urgent, pushed data to rtsg.
1404 If the snapshot was small enough that \fItcpdump\fP didn't capture
1405 the full TCP header, it interprets as much of the header as it can
1406 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1408 If the header contains a bogus option (one with a length
1409 that's either too small or beyond the end of the header), \fItcpdump\fP
1410 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1411 options (since it's impossible to tell where they start).
1413 length indicates options are present but the IP datagram length is not
1414 long enough for the options to actually be there, \fItcpdump\fP reports
1415 it as ``[\fIbad hdr length\fP]''.
1417 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1419 There are 8 bits in the control bits section of the TCP header:
1421 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1423 Let's assume that we want to watch packets used in establishing
1425 Recall that TCP uses a 3-way handshake protocol
1426 when it initializes a new connection; the connection sequence with
1427 regard to the TCP control bits is
1433 2) Recipient responds with SYN, ACK
1439 Now we're interested in capturing packets that have only the
1440 SYN bit set (Step 1).
1441 Note that we don't want packets from step 2
1442 (SYN-ACK), just a plain initial SYN.
1443 What we need is a correct filter
1444 expression for \fItcpdump\fP.
1446 Recall the structure of a TCP header without options:
1450 -----------------------------------------------------------------
1451 | source port | destination port |
1452 -----------------------------------------------------------------
1454 -----------------------------------------------------------------
1455 | acknowledgment number |
1456 -----------------------------------------------------------------
1457 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1458 -----------------------------------------------------------------
1459 | TCP checksum | urgent pointer |
1460 -----------------------------------------------------------------
1463 A TCP header usually holds 20 octets of data, unless options are
1465 The first line of the graph contains octets 0 - 3, the
1466 second line shows octets 4 - 7 etc.
1468 Starting to count with 0, the relevant TCP control bits are contained
1473 ----------------|---------------|---------------|----------------
1474 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1475 ----------------|---------------|---------------|----------------
1476 | | 13th octet | | |
1479 Let's have a closer look at octet no. 13:
1489 These are the TCP control bits we are interested
1491 We have numbered the bits in this octet from 0 to 7, right to
1492 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1494 Recall that we want to capture packets with only SYN set.
1495 Let's see what happens to octet 13 if a TCP datagram arrives
1496 with the SYN bit set in its header:
1507 control bits section we see that only bit number 1 (SYN) is set.
1509 Assuming that octet number 13 is an 8-bit unsigned integer in
1510 network byte order, the binary value of this octet is
1514 and its decimal representation is
1518 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1521 We're almost done, because now we know that if only SYN is set,
1522 the value of the 13th octet in the TCP header, when interpreted
1523 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1525 This relationship can be expressed as
1531 We can use this expression as the filter for \fItcpdump\fP in order
1532 to watch packets which have only SYN set:
1535 tcpdump -i xl0 tcp[13] == 2
1538 The expression says "let the 13th octet of a TCP datagram have
1539 the decimal value 2", which is exactly what we want.
1541 Now, let's assume that we need to capture SYN packets, but we
1542 don't care if ACK or any other TCP control bit is set at the
1544 Let's see what happens to octet 13 when a TCP datagram
1545 with SYN-ACK set arrives:
1555 Now bits 1 and 4 are set in the 13th octet.
1561 which translates to decimal
1565 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1568 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1569 expression, because that would select only those packets that have
1570 SYN-ACK set, but not those with only SYN set.
1571 Remember that we don't care
1572 if ACK or any other control bit is set as long as SYN is set.
1574 In order to achieve our goal, we need to logically AND the
1575 binary value of octet 13 with some other value to preserve
1577 We know that we want SYN to be set in any case,
1578 so we'll logically AND the value in the 13th octet with
1579 the binary value of a SYN:
1583 00010010 SYN-ACK 00000010 SYN
1584 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1586 = 00000010 = 00000010
1589 We see that this AND operation delivers the same result
1590 regardless whether ACK or another TCP control bit is set.
1591 The decimal representation of the AND value as well as
1592 the result of this operation is 2 (binary 00000010),
1593 so we know that for packets with SYN set the following
1594 relation must hold true:
1596 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1598 This points us to the \fItcpdump\fP filter expression
1601 tcpdump -i xl0 'tcp[13] & 2 == 2'
1604 Note that you should use single quotes or a backslash
1605 in the expression to hide the AND ('&') special character
1611 UDP format is illustrated by this rwho packet:
1615 \f(CWactinide.who > broadcast.who: udp 84\fP
1619 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1620 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1622 The packet contained 84 bytes of user data.
1624 Some UDP services are recognized (from the source or destination
1625 port number) and the higher level protocol information printed.
1626 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1627 RPC calls (RFC-1050) to NFS.
1629 UDP Name Server Requests
1631 \fI(N.B.:The following description assumes familiarity with
1632 the Domain Service protocol described in RFC-1035.
1633 If you are not familiar
1634 with the protocol, the following description will appear to be written
1637 Name server requests are formatted as
1641 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1643 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1647 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1648 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1649 The query id was `3'.
1650 The `+' indicates the \fIrecursion desired\fP flag
1652 The query length was 37 bytes, not including the UDP and
1653 IP protocol headers.
1654 The query operation was the normal one, \fIQuery\fP,
1655 so the op field was omitted.
1656 If the op had been anything else, it would
1657 have been printed between the `3' and the `+'.
1658 Similarly, the qclass was the normal one,
1659 \fIC_IN\fP, and omitted.
1660 Any other qclass would have been printed
1661 immediately after the `A'.
1663 A few anomalies are checked and may result in extra fields enclosed in
1664 square brackets: If a query contains an answer, authority records or
1665 additional records section,
1670 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1671 is the appropriate count.
1672 If any of the response bits are set (AA, RA or rcode) or any of the
1673 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1674 is printed, where \fIx\fP is the hex value of header bytes two and three.
1676 UDP Name Server Responses
1678 Name server responses are formatted as
1682 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1684 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1685 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1689 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1690 with 3 answer records, 3 name server records and 7 additional records.
1691 The first answer record is type A (address) and its data is internet
1692 address 128.32.137.3.
1693 The total size of the response was 273 bytes,
1694 excluding UDP and IP headers.
1695 The op (Query) and response code
1696 (NoError) were omitted, as was the class (C_IN) of the A record.
1698 In the second example, \fIhelios\fP responds to query 2 with a
1699 response code of non-existent domain (NXDomain) with no answers,
1700 one name server and no authority records.
1701 The `*' indicates that
1702 the \fIauthoritative answer\fP bit was set.
1704 answers, no type, class or data were printed.
1706 Other flag characters that might appear are `\-' (recursion available,
1707 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1709 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1712 Note that name server requests and responses tend to be large and the
1713 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1715 Use the \fB\-s\fP flag to increase the snaplen if you
1716 need to seriously investigate name server traffic.
1718 has worked well for me.
1723 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1724 on UDP/137, UDP/138 and TCP/139.
1725 Some primitive decoding of IPX and
1726 NetBEUI SMB data is also done.
1728 By default a fairly minimal decode is done, with a much more detailed
1729 decode done if -v is used.
1730 Be warned that with -v a single SMB packet
1731 may take up a page or more, so only use -v if you really want all the
1734 If you are decoding SMB sessions containing unicode strings then you
1735 may wish to set the environment variable USE_UNICODE to 1.
1737 auto-detect unicode strings would be welcome.
1739 For information on SMB packet formats and what all te fields mean see
1740 www.cifs.org or the pub/samba/specs/ directory on your favorite
1741 samba.org mirror site.
1742 The SMB patches were written by Andrew Tridgell
1746 NFS Requests and Replies
1748 Sun NFS (Network File System) requests and replies are printed as:
1752 \fIsrc.xid > dst.nfs: len op args\fP
1753 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1756 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1757 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1758 sushi.201b > wrl.nfs:
1759 144 lookup fh 9,74/4096.6878 "xcolors"
1760 wrl.nfs > sushi.201b:
1761 reply ok 128 lookup fh 9,74/4134.3150
1766 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1767 to \fIwrl\fP (note that the number following the src host is a
1768 transaction id, \fInot\fP the source port).
1769 The request was 112 bytes,
1770 excluding the UDP and IP headers.
1771 The operation was a \fIreadlink\fP
1772 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1773 (If one is lucky, as in this case, the file handle can be interpreted
1774 as a major,minor device number pair, followed by the inode number and
1776 \fIWrl\fP replies `ok' with the contents of the link.
1778 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1779 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1780 Note that the data printed
1781 depends on the operation type.
1782 The format is intended to be self
1783 explanatory if read in conjunction with
1784 an NFS protocol spec.
1786 If the \-v (verbose) flag is given, additional information is printed.
1792 sushi.1372a > wrl.nfs:
1793 148 read fh 21,11/12.195 8192 bytes @ 24576
1794 wrl.nfs > sushi.1372a:
1795 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1800 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1801 which have been omitted from this example.) In the first line,
1802 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1803 at byte offset 24576.
1804 \fIWrl\fP replies `ok'; the packet shown on the
1805 second line is the first fragment of the reply, and hence is only 1472
1806 bytes long (the other bytes will follow in subsequent fragments, but
1807 these fragments do not have NFS or even UDP headers and so might not be
1808 printed, depending on the filter expression used).
1809 Because the \-v flag
1810 is given, some of the file attributes (which are returned in addition
1811 to the file data) are printed: the file type (``REG'', for regular file),
1812 the file mode (in octal), the uid and gid, and the file size.
1814 If the \-v flag is given more than once, even more details are printed.
1816 Note that NFS requests are very large and much of the detail won't be printed
1817 unless \fIsnaplen\fP is increased.
1818 Try using `\fB\-s 192\fP' to watch
1821 NFS reply packets do not explicitly identify the RPC operation.
1823 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1824 replies using the transaction ID.
1825 If a reply does not closely follow the
1826 corresponding request, it might not be parsable.
1828 AFS Requests and Replies
1830 Transarc AFS (Andrew File System) requests and replies are printed
1836 \fIsrc.sport > dst.dport: rx packet-type\fP
1837 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1838 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1841 elvis.7001 > pike.afsfs:
1842 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1843 new fid 536876964/1/1 ".newsrc"
1844 pike.afsfs > elvis.7001: rx data fs reply rename
1849 In the first line, host elvis sends a RX packet to pike.
1851 a RX data packet to the fs (fileserver) service, and is the start of
1853 The RPC call was a rename, with the old directory file id
1854 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1855 file id of 536876964/1/1 and a new filename of `.newsrc'.
1857 responds with a RPC reply to the rename call (which was successful, because
1858 it was a data packet and not an abort packet).
1860 In general, all AFS RPCs are decoded at least by RPC call name.
1862 AFS RPCs have at least some of the arguments decoded (generally only
1863 the `interesting' arguments, for some definition of interesting).
1865 The format is intended to be self-describing, but it will probably
1866 not be useful to people who are not familiar with the workings of
1869 If the -v (verbose) flag is given twice, acknowledgement packets and
1870 additional header information is printed, such as the the RX call ID,
1871 call number, sequence number, serial number, and the RX packet flags.
1873 If the -v flag is given twice, additional information is printed,
1874 such as the the RX call ID, serial number, and the RX packet flags.
1875 The MTU negotiation information is also printed from RX ack packets.
1877 If the -v flag is given three times, the security index and service id
1880 Error codes are printed for abort packets, with the exception of Ubik
1881 beacon packets (because abort packets are used to signify a yes vote
1882 for the Ubik protocol).
1884 Note that AFS requests are very large and many of the arguments won't
1885 be printed unless \fIsnaplen\fP is increased.
1886 Try using `\fB-s 256\fP'
1887 to watch AFS traffic.
1889 AFS reply packets do not explicitly identify the RPC operation.
1891 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1892 replies using the call number and service ID.
1893 If a reply does not closely
1895 corresponding request, it might not be parsable.
1898 KIP AppleTalk (DDP in UDP)
1900 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1901 and dumped as DDP packets (i.e., all the UDP header information is
1905 is used to translate appletalk net and node numbers to names.
1906 Lines in this file have the form
1918 The first two lines give the names of appletalk networks.
1920 line gives the name of a particular host (a host is distinguished
1921 from a net by the 3rd octet in the number \-
1922 a net number \fImust\fP have two octets and a host number \fImust\fP
1923 have three octets.) The number and name should be separated by
1924 whitespace (blanks or tabs).
1927 file may contain blank lines or comment lines (lines starting with
1930 AppleTalk addresses are printed in the form
1936 \f(CW144.1.209.2 > icsd-net.112.220
1937 office.2 > icsd-net.112.220
1938 jssmag.149.235 > icsd-net.2\fR
1944 doesn't exist or doesn't contain an entry for some appletalk
1945 host/net number, addresses are printed in numeric form.)
1946 In the first example, NBP (DDP port 2) on net 144.1 node 209
1947 is sending to whatever is listening on port 220 of net icsd node 112.
1948 The second line is the same except the full name of the source node
1949 is known (`office').
1950 The third line is a send from port 235 on
1951 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1952 the broadcast address (255) is indicated by a net name with no host
1953 number \- for this reason it's a good idea to keep node names and
1954 net names distinct in /etc/atalk.names).
1956 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1957 packets have their contents interpreted.
1958 Other protocols just dump
1959 the protocol name (or number if no name is registered for the
1960 protocol) and packet size.
1962 \fBNBP packets\fP are formatted like the following examples:
1966 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1967 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1968 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1972 The first line is a name lookup request for laserwriters sent by net icsd host
1973 112 and broadcast on net jssmag.
1974 The nbp id for the lookup is 190.
1975 The second line shows a reply for this request (note that it has the
1976 same id) from host jssmag.209 saying that it has a laserwriter
1977 resource named "RM1140" registered on port 250.
1979 another reply to the same request saying host techpit has laserwriter
1980 "techpit" registered on port 186.
1982 \fBATP packet\fP formatting is demonstrated by the following example:
1986 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1987 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1988 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1989 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1990 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1991 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1992 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1993 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1994 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1995 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1996 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1997 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1998 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1999 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
2003 Jssmag.209 initiates transaction id 12266 with host helios by requesting
2004 up to 8 packets (the `<0-7>').
2005 The hex number at the end of the line
2006 is the value of the `userdata' field in the request.
2008 Helios responds with 8 512-byte packets.
2009 The `:digit' following the
2010 transaction id gives the packet sequence number in the transaction
2011 and the number in parens is the amount of data in the packet,
2012 excluding the atp header.
2013 The `*' on packet 7 indicates that the
2016 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
2018 resends them then jssmag.209 releases the transaction.
2020 jssmag.209 initiates the next request.
2021 The `*' on the request
2022 indicates that XO (`exactly once') was \fInot\fP set.
2027 Fragmented Internet datagrams are printed as
2031 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
2032 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
2036 (The first form indicates there are more fragments.
2038 indicates this is the last fragment.)
2040 \fIId\fP is the fragment id.
2041 \fISize\fP is the fragment
2042 size (in bytes) excluding the IP header.
2043 \fIOffset\fP is this
2044 fragment's offset (in bytes) in the original datagram.
2046 The fragment information is output for each fragment.
2048 fragment contains the higher level protocol header and the frag
2049 info is printed after the protocol info.
2051 after the first contain no higher level protocol header and the
2052 frag info is printed after the source and destination addresses.
2053 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
2054 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
2058 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
2059 arizona > rtsg: (frag 595a:204@328)
2060 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
2064 There are a couple of things to note here: First, addresses in the
2065 2nd line don't include port numbers.
2066 This is because the TCP
2067 protocol information is all in the first fragment and we have no idea
2068 what the port or sequence numbers are when we print the later fragments.
2069 Second, the tcp sequence information in the first line is printed as if there
2070 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
2071 the first frag and 204 in the second).
2072 If you are looking for holes
2073 in the sequence space or trying to match up acks
2074 with packets, this can fool you.
2076 A packet with the IP \fIdon't fragment\fP flag is marked with a
2077 trailing \fB(DF)\fP.
2081 By default, all output lines are preceded by a timestamp.
2083 is the current clock time in the form
2089 and is as accurate as the kernel's clock.
2090 The timestamp reflects the time the kernel first saw the packet.
2092 is made to account for the time lag between when the
2093 ethernet interface removed the packet from the wire and when the kernel
2094 serviced the `new packet' interrupt.
2096 traffic(1C), nit(4P), bpf(4), pcap(3)
2098 The original authors are:
2102 Steven McCanne, all of the
2103 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
2105 It is currently being maintained by tcpdump.org.
2107 The current version is available via http:
2110 .I http://www.tcpdump.org/
2113 The original distribution is available via anonymous ftp:
2116 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
2119 IPv6/IPsec support is added by WIDE/KAME project.
2120 This program uses Eric Young's SSLeay library, under specific configuration.
2122 Please send problems, bugs, questions, desirable enhancements, etc. to:
2125 tcpdump-workers@tcpdump.org
2128 Please send source code contributions, etc. to:
2134 NIT doesn't let you watch your own outbound traffic, BPF will.
2135 We recommend that you use the latter.
2137 On Linux systems with 2.0[.x] kernels:
2139 packets on the loopback device will be seen twice;
2141 packet filtering cannot be done in the kernel, so that all packets must
2142 be copied from the kernel in order to be filtered in user mode;
2144 all of a packet, not just the part that's within the snapshot length,
2145 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
2146 asked to copy only part of a packet to userland, will not report the
2147 true length of the packet; this would cause most IP packets to get an
2151 capturing on some PPP devices won't work correctly.
2153 We recommend that you upgrade to a 2.2 or later kernel.
2155 Some attempt should be made to reassemble IP fragments or, at least
2156 to compute the right length for the higher level protocol.
2158 Name server inverse queries are not dumped correctly: the (empty)
2159 question section is printed rather than real query in the answer
2161 Some believe that inverse queries are themselves a bug and
2162 prefer to fix the program generating them rather than \fItcpdump\fP.
2164 A packet trace that crosses a daylight savings time change will give
2165 skewed time stamps (the time change is ignored).
2167 Filter expressions on fields other than those in Token Ring headers will
2168 not correctly handle source-routed Token Ring packets.
2170 Filter expressions on fields other than those in 802.11 headers will not
2171 correctly handle 802.11 data packets with both To DS and From DS set.
2174 should chase header chain, but at this moment it does not.
2175 .BR "ip6 protochain"
2176 is supplied for this behavior.
2178 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
2179 does not work against IPv6 packets.
2180 It only looks at IPv4 packets.
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