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23 .TH TCPDUMP 1 "3 January 2001"
25 tcpdump \- dump traffic on a network
30 .B \-aAdeflnNOpqRStuvxX
85 \fITcpdump\fP prints out the headers of packets on a network interface
86 that match the boolean \fIexpression\fP. It can also be run with the
88 flag, which causes it to save the packet data to a file for later
89 analysis, and/or with the
91 flag, which causes it to read from a saved packet file rather than to
92 read packets from a network interface. In all cases, only packets that
99 will, if not run with the
101 flag, continue capturing packets until it is interrupted by a SIGINT
102 signal (generated, for example, by typing your interrupt character,
103 typically control-C) or a SIGTERM signal (typically generated with the
105 command); if run with the
107 flag, it will capture packets until it is interrupted by a SIGINT or
108 SIGTERM signal or the specified number of packets have been processed.
112 finishes capturing packets, it will report counts of:
114 packets ``received by filter'' (the meaning of this depends on the OS on
117 and possibly on the way the OS was configured - if a filter was
118 specified on the command line, on some OSes it counts packets regardless
119 of whether they were matched by the filter expression, and on other OSes
120 it counts only packets that were matched by the filter expression and
124 packets ``dropped by kernel'' (this is the number of packets that were
125 dropped, due to a lack of buffer space, by the packet capture mechanism
128 is running, if the OS reports that information to applications; if not,
129 it will be reported as 0).
131 On platforms that support the SIGINFO signal, such as most BSDs, it will
132 report those counts when it receives a SIGINFO signal (generated, for
133 example, by typing your ``status'' character, typically control-T) and
134 will continue capturing packets.
136 Reading packets from a network interface may require that you have
139 .B Under SunOS 3.x or 4.x with NIT or BPF:
140 You must have read access to
145 .B Under Solaris with DLPI:
146 You must have read/write access to the network pseudo device, e.g.
148 On at least some versions of Solaris, however, this is not sufficient to
151 to capture in promiscuous mode; on those versions of Solaris, you must
154 must be installed setuid to root, in order to capture in promiscuous
155 mode. Note that, on many (perhaps all) interfaces, if you don't capture
156 in promiscuous mode, you will not see any outgoing packets, so a capture
157 not done in promiscuous mode may not be very useful.
159 .B Under HP-UX with DLPI:
162 must be installed setuid to root.
164 .B Under IRIX with snoop:
167 must be installed setuid to root.
172 must be installed setuid to root.
174 .B Under Ultrix and Digital UNIX/Tru64 UNIX:
175 Any user may capture network traffic with
177 However, no user (not even the super-user) can capture in promiscuous
178 mode on an interface unless the super-user has enabled promiscuous-mode
179 operation on that interface using
181 and no user (not even the super-user) can capture unicast traffic
182 received by or sent by the machine on an interface unless the super-user
183 has enabled copy-all-mode operation on that interface using
187 packet capture on an interface probably requires that either
188 promiscuous-mode or copy-all-mode operation, or both modes of
189 operation, be enabled on that interface.
192 You must have read access to
195 Reading a saved packet file doesn't require special privileges.
200 Print each packet (minus its link level header) in ASCII. Handy for
203 Attempt to convert network and broadcast addresses to names.
206 Exit after receiving \fIcount\fP packets.
209 Before writing a raw packet to a savefile, check whether the file is
210 currently larger than \fIfile_size\fP and, if so, close the current
211 savefile and open a new one. Savefiles after the first savefile will
212 have the name specified with the
214 flag, with a number after it, starting at 2 and continuing upward.
215 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
216 not 1,048,576 bytes).
219 Dump the compiled packet-matching code in a human readable form to
220 standard output and stop.
223 Dump packet-matching code as a
228 Dump packet-matching code as decimal numbers (preceded with a count).
231 Print the link-level header on each dump line.
234 Use \fIalgo:secret\fP for decrypting IPsec ESP packets.
240 \fBcast128-cbc\fP, or
242 The default is \fBdes-cbc\fP.
243 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
244 with cryptography enabled.
245 \fIsecret\fP the ascii text for ESP secret key.
246 We cannot take arbitrary binary value at this moment.
247 The option assumes RFC2406 ESP, not RFC1827 ESP.
248 The option is only for debugging purposes, and
249 the use of this option with truly `secret' key is discouraged.
250 By presenting IPsec secret key onto command line
251 you make it visible to others, via
256 Print `foreign' internet addresses numerically rather than symbolically
257 (this option is intended to get around serious brain damage in
258 Sun's yp server \(em usually it hangs forever translating non-local
262 Use \fIfile\fP as input for the filter expression.
263 An additional expression given on the command line is ignored.
266 Listen on \fIinterface\fP.
267 If unspecified, \fItcpdump\fP searches the system interface list for the
268 lowest numbered, configured up interface (excluding loopback).
269 Ties are broken by choosing the earliest match.
271 On Linux systems with 2.2 or later kernels, an
273 argument of ``any'' can be used to capture packets from all interfaces.
274 Note that captures on the ``any'' device will not be done in promiscuous
278 Make stdout line buffered.
279 Useful if you want to see the data
283 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
284 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
287 Load SMI MIB module definitions from file \fImodule\fR.
289 can be used several times to load several MIB modules into \fItcpdump\fP.
292 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
295 Don't print domain name qualification of host names.
297 if you give this flag then \fItcpdump\fP will print ``nic''
298 instead of ``nic.ddn.mil''.
301 Do not run the packet-matching code optimizer.
303 if you suspect a bug in the optimizer.
306 \fIDon't\fP put the interface
307 into promiscuous mode.
308 Note that the interface might be in promiscuous
309 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
310 `ether host {local-hw-addr} or ether broadcast'.
313 Quick (quiet?) output.
314 Print less protocol information so output
318 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
319 If specified, \fItcpdump\fP will not print replay prevention field.
320 Since there is no protocol version field in ESP/AH specification,
321 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
324 Read packets from \fIfile\fR (which was created with the -w option).
325 Standard input is used if \fIfile\fR is ``-''.
328 Print absolute, rather than relative, TCP sequence numbers.
331 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
332 default of 68 (with SunOS's NIT, the minimum is actually 96).
333 68 bytes is adequate for IP, ICMP, TCP
334 and UDP but may truncate protocol information from name server and NFS
336 Packets truncated because of a limited snapshot
337 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
338 is the name of the protocol level at which the truncation has occurred.
339 Note that taking larger snapshots both increases
340 the amount of time it takes to process packets and, effectively,
341 decreases the amount of packet buffering.
342 This may cause packets to be
344 You should limit \fIsnaplen\fP to the smallest number that will
345 capture the protocol information you're interested in.
347 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
350 Force packets selected by "\fIexpression\fP" to be interpreted the
351 specified \fItype\fR.
352 Currently known types are
353 \fBcnfp\fR (Cisco NetFlow protocol),
354 \fBrpc\fR (Remote Procedure Call),
355 \fBrtp\fR (Real-Time Applications protocol),
356 \fBrtcp\fR (Real-Time Applications control protocol),
357 \fBsnmp\fR (Simple Network Management Protocol),
358 \fBvat\fR (Visual Audio Tool),
360 \fBwb\fR (distributed White Board).
363 \fIDon't\fP print a timestamp on each dump line.
366 Print an unformatted timestamp on each dump line.
369 Print a delta (in micro-seconds) between current and previous line
373 Print a timestamp in default format proceeded by date on each dump line.
376 Print undecoded NFS handles.
379 (Slightly more) verbose output.
380 For example, the time to live,
381 identification, total length and options in an IP packet are printed.
382 Also enables additional packet integrity checks such as verifying the
383 IP and ICMP header checksum.
386 Even more verbose output.
387 For example, additional fields are
388 printed from NFS reply packets, and SMB packets are fully decoded.
391 Even more verbose output.
393 telnet \fBSB\fP ... \fBSE\fP options
397 telnet options are printed in hex as well.
400 Write the raw packets to \fIfile\fR rather than parsing and printing
402 They can later be printed with the \-r option.
403 Standard output is used if \fIfile\fR is ``-''.
406 Print each packet (minus its link level header) in hex.
407 The smaller of the entire packet or
409 bytes will be printed. Note that this is the entire link-layer
410 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
411 will also be printed when the higher layer packet is shorter than the
415 When printing hex, print ascii too.
418 is also set, the packet is printed in hex/ascii.
419 This is very handy for analysing new protocols.
422 is not also set, some parts of some packets may be printed
424 .IP "\fI expression\fP"
426 selects which packets will be dumped.
427 If no \fIexpression\fP
428 is given, all packets on the net will be dumped.
430 only packets for which \fIexpression\fP is `true' will be dumped.
432 The \fIexpression\fP consists of one or more
434 Primitives usually consist of an
436 (name or number) preceded by one or more qualifiers.
438 different kinds of qualifier:
440 qualifiers say what kind of thing the id name or number refers to.
446 E.g., `host foo', `net 128.3', `port 20'.
452 qualifiers specify a particular transfer direction to and/or from
454 Possible directions are
461 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.
463 there is no dir qualifier,
466 For `null' link layers (i.e. point to point protocols such as slip) the
470 qualifiers can be used to specify a desired direction.
472 qualifiers restrict the match to a particular protocol.
486 E.g., `ether src foo', `arp net 128.3', `tcp port 21'.
488 no proto qualifier, all protocols consistent with the type are
490 E.g., `src foo' means `(ip or arp or rarp) src foo'
491 (except the latter is not legal syntax), `net bar' means `(ip or
492 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
494 [`fddi' is actually an alias for `ether'; the parser treats them
495 identically as meaning ``the data link level used on the specified
496 network interface.'' FDDI headers contain Ethernet-like source
497 and destination addresses, and often contain Ethernet-like packet
498 types, so you can filter on these FDDI fields just as with the
499 analogous Ethernet fields.
500 FDDI headers also contain other fields,
501 but you cannot name them explicitly in a filter expression.
503 Similarly, `tr' is an alias for `ether'; the previous paragraph's
504 statements about FDDI headers also apply to Token Ring headers.]
506 In addition to the above, there are some special `primitive' keywords
507 that don't follow the pattern:
512 and arithmetic expressions.
513 All of these are described below.
515 More complex filter expressions are built up by using the words
520 to combine primitives.
521 E.g., `host foo and not port ftp and not port ftp-data'.
522 To save typing, identical qualifier lists can be omitted.
524 `tcp dst port ftp or ftp-data or domain' is exactly the same as
525 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
527 Allowable primitives are:
528 .IP "\fBdst host \fIhost\fR"
529 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
530 which may be either an address or a name.
531 .IP "\fBsrc host \fIhost\fR"
532 True if the IPv4/v6 source field of the packet is \fIhost\fP.
533 .IP "\fBhost \fIhost\fP
534 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
535 Any of the above host expressions can be prepended with the keywords,
536 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
539 \fBip host \fIhost\fR
542 which is equivalent to:
545 \fBether proto \fI\\ip\fB and host \fIhost\fR
548 If \fIhost\fR is a name with multiple IP addresses, each address will
549 be checked for a match.
550 .IP "\fBether dst \fIehost\fP
551 True if the ethernet destination address is \fIehost\fP.
553 may be either a name from /etc/ethers or a number (see
556 .IP "\fBether src \fIehost\fP
557 True if the ethernet source address is \fIehost\fP.
558 .IP "\fBether host \fIehost\fP
559 True if either the ethernet source or destination address is \fIehost\fP.
560 .IP "\fBgateway\fP \fIhost\fP
561 True if the packet used \fIhost\fP as a gateway.
563 source or destination address was \fIhost\fP but neither the IP source
564 nor the IP destination was \fIhost\fP.
565 \fIHost\fP must be a name and
566 must be found both by the machine's host-name-to-IP-address resolution
567 mechanisms (host name file, DNS, NIS, etc.) and by the machine's
568 host-name-to-Ethernet-address resolution mechanism (/etc/ethers, etc.).
569 (An equivalent expression is
572 \fBether host \fIehost \fBand not host \fIhost\fR
575 which can be used with either names or numbers for \fIhost / ehost\fP.)
576 This syntax does not work in IPv6-enabled configuration at this moment.
577 .IP "\fBdst net \fInet\fR"
578 True if the IPv4/v6 destination address of the packet has a network
580 \fINet\fP may be either a name from /etc/networks
581 or a network number (see \fInetworks(4)\fP for details).
582 .IP "\fBsrc net \fInet\fR"
583 True if the IPv4/v6 source address of the packet has a network
585 .IP "\fBnet \fInet\fR"
586 True if either the IPv4/v6 source or destination address of the packet has a network
588 .IP "\fBnet \fInet\fR \fBmask \fInetmask\fR"
589 True if the IP address matches \fInet\fR with the specific \fInetmask\fR.
590 May be qualified with \fBsrc\fR or \fBdst\fR.
591 Note that this syntax is not valid for IPv6 \fInet\fR.
592 .IP "\fBnet \fInet\fR/\fIlen\fR"
593 True if the IPv4/v6 address matches \fInet\fR with a netmask \fIlen\fR
595 May be qualified with \fBsrc\fR or \fBdst\fR.
596 .IP "\fBdst port \fIport\fR"
597 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
598 destination port value of \fIport\fP.
599 The \fIport\fP can be a number or a name used in /etc/services (see
603 If a name is used, both the port
604 number and protocol are checked.
605 If a number or ambiguous name is used,
606 only the port number is checked (e.g., \fBdst port 513\fR will print both
607 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
608 both tcp/domain and udp/domain traffic).
609 .IP "\fBsrc port \fIport\fR"
610 True if the packet has a source port value of \fIport\fP.
611 .IP "\fBport \fIport\fR"
612 True if either the source or destination port of the packet is \fIport\fP.
613 Any of the above port expressions can be prepended with the keywords,
614 \fBtcp\fP or \fBudp\fP, as in:
617 \fBtcp src port \fIport\fR
620 which matches only tcp packets whose source port is \fIport\fP.
621 .IP "\fBless \fIlength\fR"
622 True if the packet has a length less than or equal to \fIlength\fP.
623 This is equivalent to:
626 \fBlen <= \fIlength\fP.
629 .IP "\fBgreater \fIlength\fR"
630 True if the packet has a length greater than or equal to \fIlength\fP.
631 This is equivalent to:
634 \fBlen >= \fIlength\fP.
637 .IP "\fBip proto \fIprotocol\fR"
638 True if the packet is an IP packet (see
640 of protocol type \fIprotocol\fP.
641 \fIProtocol\fP can be a number or one of the names
642 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
643 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
644 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
645 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
646 Note that this primitive does not chase the protocol header chain.
647 .IP "\fBip6 proto \fIprotocol\fR"
648 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
649 Note that this primitive does not chase the protocol header chain.
650 .IP "\fBip6 protochain \fIprotocol\fR"
651 True if the packet is IPv6 packet,
652 and contains protocol header with type \fIprotocol\fR
653 in its protocol header chain.
657 \fBip6 protochain 6\fR
660 matches any IPv6 packet with TCP protocol header in the protocol header chain.
661 The packet may contain, for example,
662 authentication header, routing header, or hop-by-hop option header,
663 between IPv6 header and TCP header.
664 The BPF code emitted by this primitive is complex and
665 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
666 so this can be somewhat slow.
667 .IP "\fBip protochain \fIprotocol\fR"
668 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
669 .IP "\fBether broadcast\fR"
670 True if the packet is an ethernet broadcast packet.
673 .IP "\fBip broadcast\fR"
674 True if the packet is an IP broadcast packet.
676 the all-zeroes and all-ones broadcast conventions, and looks up
677 the local subnet mask.
678 .IP "\fBether multicast\fR"
679 True if the packet is an ethernet multicast packet.
682 This is shorthand for `\fBether[0] & 1 != 0\fP'.
683 .IP "\fBip multicast\fR"
684 True if the packet is an IP multicast packet.
685 .IP "\fBip6 multicast\fR"
686 True if the packet is an IPv6 multicast packet.
687 .IP "\fBether proto \fIprotocol\fR"
688 True if the packet is of ether type \fIprotocol\fR.
689 \fIProtocol\fP can be a number or one of the names
690 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
691 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
692 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
693 Note these identifiers are also keywords
694 and must be escaped via backslash (\\).
696 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR') and Token Ring
697 (e.g., `\fBtr protocol arp\fR'), for most of those protocols, the
698 protocol identification comes from the 802.2 Logical Link Control (LLC)
699 header, which is usually layered on top of the FDDI or Token Ring
702 When filtering for most protocol identifiers on FDDI or Token Ring,
703 \fItcpdump\fR checks only the protocol ID field of an LLC header in
704 so-called SNAP format with an Organizational Unit Identifier (OUI) of
705 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
706 is in SNAP format with an OUI of 0x000000.
708 The exceptions are \fIiso\fP, for which it checks the DSAP (Destination
709 Service Access Point) and SSAP (Source Service Access Point) fields of
710 the LLC header, \fIstp\fP and \fInetbeui\fP, where it checks the DSAP of
711 the LLC header, and \fIatalk\fP, where it checks for a SNAP-format
712 packet with an OUI of 0x080007 and the Appletalk etype.
714 In the case of Ethernet, \fItcpdump\fR checks the Ethernet type field
715 for most of those protocols; the exceptions are \fIiso\fP, \fIsap\fP,
716 and \fInetbeui\fP, for which it checks for an 802.3 frame and then
717 checks the LLC header as it does for FDDI and Token Ring, \fIatalk\fP,
718 where it checks both for the Appletalk etype in an Ethernet frame and
719 for a SNAP-format packet as it does for FDDI and Token Ring, \fIaarp\fP,
720 where it checks for the Appletalk ARP etype in either an Ethernet frame
721 or an 802.2 SNAP frame with an OUI of 0x000000, and \fIipx\fP, where it
722 checks for the IPX etype in an Ethernet frame, the IPX DSAP in the LLC
723 header, the 802.3 with no LLC header encapsulation of IPX, and the IPX
724 etype in a SNAP frame.]
725 .IP "\fBdecnet src \fIhost\fR"
726 True if the DECNET source address is
728 which may be an address of the form ``10.123'', or a DECNET host
730 [DECNET host name support is only available on Ultrix systems
731 that are configured to run DECNET.]
732 .IP "\fBdecnet dst \fIhost\fR"
733 True if the DECNET destination address is
735 .IP "\fBdecnet host \fIhost\fR"
736 True if either the DECNET source or destination address is
738 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
742 \fBether proto \fIp\fR
745 where \fIp\fR is one of the above protocols.
746 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
750 \fBether proto \fIp\fR
753 where \fIp\fR is one of the above protocols.
755 \fItcpdump\fP does not currently know how to parse these protocols.
756 .IP "\fBvlan \fI[vlan_id]\fR"
757 True if the packet is an IEEE 802.1Q VLAN packet.
758 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
760 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
761 changes the decoding offsets for the remainder of \fIexpression\fR
762 on the assumption that the packet is a VLAN packet.
763 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
767 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
770 where \fIp\fR is one of the above protocols.
771 .IP "\fBiso proto \fIprotocol\fR"
772 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
773 \fIProtocol\fP can be a number or one of the names
774 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
775 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
782 where \fIp\fR is one of the above protocols.
783 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
784 .IP "\fBvpi\fP \fIn\fR
785 True if the packet is an ATM packet, for SunATM on Solaris, with a
786 virtual path identifier of
788 .IP "\fBvci\fP \fIn\fR
789 True if the packet is an ATM packet, for SunATM on Solaris, with a
790 virtual channel identifier of
793 True if the packet is an ATM packet, for SunATM on Solaris, and is
796 True if the packet is an ATM packet, for SunATM on Solaris, and is
797 an LLC-encapsulated packet.
799 True if the packet is an ATM packet, for SunATM on Solaris, and is
800 a segment OAM F4 flow cell (VPI=0 & VCI=3).
802 True if the packet is an ATM packet, for SunATM on Solaris, and is
803 an end-to-end OAM F4 flow cell (VPI=0 & VCI=4).
805 True if the packet is an ATM packet, for SunATM on Solaris, and is
806 a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 | VCI=4)).
808 True if the packet is an ATM packet, for SunATM on Solaris, and is
809 a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 | VCI=4)).
811 True if the packet is an ATM packet, for SunATM on Solaris, and is
812 on a meta signaling circuit (VPI=0 & VCI=1).
814 True if the packet is an ATM packet, for SunATM on Solaris, and is
815 on a broadcast signaling circuit (VPI=0 & VCI=2).
817 True if the packet is an ATM packet, for SunATM on Solaris, and is
818 on a signaling circuit (VPI=0 & VCI=5).
820 True if the packet is an ATM packet, for SunATM on Solaris, and is
821 on an ILMI circuit (VPI=0 & VCI=16).
823 True if the packet is an ATM packet, for SunATM on Solaris, and is
824 on a signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect,
825 Connect Ack, Release, or Release Done message.
826 .IP \fBmetaconnect\fP
827 True if the packet is an ATM packet, for SunATM on Solaris, and is
828 on a meta signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect,
829 Release, or Release Done message.
830 .IP "\fIexpr relop expr\fR"
831 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
832 and \fIexpr\fR is an arithmetic expression composed of integer constants
833 (expressed in standard C syntax), the normal binary operators
834 [+, -, *, /, &, |], a length operator, and special packet data accessors.
836 data inside the packet, use the following syntax:
839 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
842 \fIProto\fR is one of \fBether, fddi, tr, ppp, slip, link,
843 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
844 indicates the protocol layer for the index operation.
845 (\fBether, fddi, tr, ppp, slip\fR and \fBlink\fR all refer to the link
847 Note that \fItcp, udp\fR and other upper-layer protocol types only
848 apply to IPv4, not IPv6 (this will be fixed in the future).
849 The byte offset, relative to the indicated protocol layer, is
851 \fISize\fR is optional and indicates the number of bytes in the
852 field of interest; it can be either one, two, or four, and defaults to one.
853 The length operator, indicated by the keyword \fBlen\fP, gives the
854 length of the packet.
856 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
857 The expression `\fBip[0] & 0xf != 5\fP'
858 catches all IP packets with options.
860 `\fBip[6:2] & 0x1fff = 0\fP'
861 catches only unfragmented datagrams and frag zero of fragmented datagrams.
862 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
864 For instance, \fBtcp[0]\fP always means the first
865 byte of the TCP \fIheader\fP, and never means the first byte of an
866 intervening fragment.
868 Some offsets and field values may be expressed as names rather than
870 The following protocol header field offsets are
871 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
872 code field), and \fBtcpflags\fP (TCP flags field).
874 The following ICMP type field values are available: \fBicmp-echoreply\fP,
875 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
876 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
877 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
878 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
879 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
881 The following TCP flags field values are available: \fBtcp-fin\fP,
882 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
883 \fBtcp-ack\fP, \fBtcp-urg\fP.
885 Primitives may be combined using:
887 A parenthesized group of primitives and operators
888 (parentheses are special to the Shell and must be escaped).
890 Negation (`\fB!\fP' or `\fBnot\fP').
892 Concatenation (`\fB&&\fP' or `\fBand\fP').
894 Alternation (`\fB||\fP' or `\fBor\fP').
896 Negation has highest precedence.
897 Alternation and concatenation have equal precedence and associate
899 Note that explicit \fBand\fR tokens, not juxtaposition,
900 are now required for concatenation.
902 If an identifier is given without a keyword, the most recent keyword
907 \fBnot host vs and ace\fR
913 \fBnot host vs and host ace\fR
916 which should not be confused with
919 \fBnot ( host vs or ace )\fR
923 Expression arguments can be passed to \fItcpdump\fP as either a single
924 argument or as multiple arguments, whichever is more convenient.
925 Generally, if the expression contains Shell metacharacters, it is
926 easier to pass it as a single, quoted argument.
927 Multiple arguments are concatenated with spaces before being parsed.
930 To print all packets arriving at or departing from \fIsundown\fP:
933 \fBtcpdump host sundown\fP
937 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
940 \fBtcpdump host helios and \\( hot or ace \\)\fP
944 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
947 \fBtcpdump ip host ace and not helios\fP
951 To print all traffic between local hosts and hosts at Berkeley:
955 tcpdump net ucb-ether
959 To print all ftp traffic through internet gateway \fIsnup\fP:
960 (note that the expression is quoted to prevent the shell from
961 (mis-)interpreting the parentheses):
965 tcpdump 'gateway snup and (port ftp or ftp-data)'
969 To print traffic neither sourced from nor destined for local hosts
970 (if you gateway to one other net, this stuff should never make it
971 onto your local net).
975 tcpdump ip and not net \fIlocalnet\fP
979 To print the start and end packets (the SYN and FIN packets) of each
980 TCP conversation that involves a non-local host.
984 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
988 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
992 tcpdump 'gateway snup and ip[2:2] > 576'
996 To print IP broadcast or multicast packets that were
998 sent via ethernet broadcast or multicast:
1002 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
1006 To print all ICMP packets that are not echo requests/replies (i.e., not
1011 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
1016 The output of \fItcpdump\fP is protocol dependent.
1018 gives a brief description and examples of most of the formats.
1026 If the '-e' option is given, the link level header is printed out.
1027 On ethernets, the source and destination addresses, protocol,
1028 and packet length are printed.
1030 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
1031 the `frame control' field, the source and destination addresses,
1032 and the packet length.
1033 (The `frame control' field governs the
1034 interpretation of the rest of the packet.
1035 Normal packets (such
1036 as those containing IP datagrams) are `async' packets, with a priority
1037 value between 0 and 7; for example, `\fBasync4\fR'.
1039 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
1040 the LLC header is printed if it is \fInot\fR an ISO datagram or a
1041 so-called SNAP packet.
1043 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
1044 the `access control' and `frame control' fields, the source and
1045 destination addresses, and the packet length.
1046 As on FDDI networks,
1047 packets are assumed to contain an LLC packet.
1048 Regardless of whether
1049 the '-e' option is specified or not, the source routing information is
1050 printed for source-routed packets.
1052 \fI(N.B.: The following description assumes familiarity with
1053 the SLIP compression algorithm described in RFC-1144.)\fP
1055 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
1056 packet type, and compression information are printed out.
1057 The packet type is printed first.
1058 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
1059 No further link information is printed for \fIip\fR packets.
1060 For TCP packets, the connection identifier is printed following the type.
1061 If the packet is compressed, its encoded header is printed out.
1062 The special cases are printed out as
1063 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
1064 the sequence number (or sequence number and ack) has changed.
1065 If it is not a special case,
1066 zero or more changes are printed.
1067 A change is indicated by U (urgent pointer), W (window), A (ack),
1068 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1069 or a new value (=n).
1070 Finally, the amount of data in the packet and compressed header length
1073 For example, the following line shows an outbound compressed TCP packet,
1074 with an implicit connection identifier; the ack has changed by 6,
1075 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1076 data and 6 bytes of compressed header:
1079 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1085 Arp/rarp output shows the type of request and its arguments.
1087 format is intended to be self explanatory.
1088 Here is a short sample taken from the start of an `rlogin' from
1089 host \fIrtsg\fP to host \fIcsam\fP:
1093 \f(CWarp who-has csam tell rtsg
1094 arp reply csam is-at CSAM\fR
1098 The first line says that rtsg sent an arp packet asking
1099 for the ethernet address of internet host csam.
1101 replies with its ethernet address (in this example, ethernet addresses
1102 are in caps and internet addresses in lower case).
1104 This would look less redundant if we had done \fItcpdump \-n\fP:
1108 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1109 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1113 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1114 broadcast and the second is point-to-point would be visible:
1118 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1119 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1123 For the first packet this says the ethernet source address is RTSG, the
1124 destination is the ethernet broadcast address, the type field
1125 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1129 \fI(N.B.:The following description assumes familiarity with
1130 the TCP protocol described in RFC-793.
1131 If you are not familiar
1132 with the protocol, neither this description nor \fItcpdump\fP will
1133 be of much use to you.)\fP
1135 The general format of a tcp protocol line is:
1139 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1143 \fISrc\fP and \fIdst\fP are the source and destination IP
1144 addresses and ports.
1145 \fIFlags\fP are some combination of S (SYN),
1146 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
1147 \fIData-seqno\fP describes the portion of sequence space covered
1148 by the data in this packet (see example below).
1149 \fIAck\fP is sequence number of the next data expected the other
1150 direction on this connection.
1151 \fIWindow\fP is the number of bytes of receive buffer space available
1152 the other direction on this connection.
1153 \fIUrg\fP indicates there is `urgent' data in the packet.
1154 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1156 \fISrc, dst\fP and \fIflags\fP are always present.
1158 depend on the contents of the packet's tcp protocol header and
1159 are output only if appropriate.
1161 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1166 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1167 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1168 rtsg.1023 > csam.login: . ack 1 win 4096
1169 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1170 csam.login > rtsg.1023: . ack 2 win 4096
1171 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1172 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1173 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1174 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1178 The first line says that tcp port 1023 on rtsg sent a packet
1181 The \fBS\fP indicates that the \fISYN\fP flag was set.
1182 The packet sequence number was 768512 and it contained no data.
1183 (The notation is `first:last(nbytes)' which means `sequence
1185 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1186 There was no piggy-backed ack, the available receive window was 4096
1187 bytes and there was a max-segment-size option requesting an mss of
1190 Csam replies with a similar packet except it includes a piggy-backed
1192 Rtsg then acks csam's SYN.
1195 The packet contained no data so there is no data sequence number.
1196 Note that the ack sequence
1197 number is a small integer (1).
1198 The first time \fItcpdump\fP sees a
1199 tcp `conversation', it prints the sequence number from the packet.
1200 On subsequent packets of the conversation, the difference between
1201 the current packet's sequence number and this initial sequence number
1203 This means that sequence numbers after the
1204 first can be interpreted
1205 as relative byte positions in the conversation's data stream (with the
1206 first data byte each direction being `1').
1207 `-S' will override this
1208 feature, causing the original sequence numbers to be output.
1210 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1211 in the rtsg \(-> csam side of the conversation).
1212 The PUSH flag is set in the packet.
1213 On the 7th line, csam says it's received data sent by rtsg up to
1214 but not including byte 21.
1215 Most of this data is apparently sitting in the
1216 socket buffer since csam's receive window has gotten 19 bytes smaller.
1217 Csam also sends one byte of data to rtsg in this packet.
1218 On the 8th and 9th lines,
1219 csam sends two bytes of urgent, pushed data to rtsg.
1221 If the snapshot was small enough that \fItcpdump\fP didn't capture
1222 the full TCP header, it interprets as much of the header as it can
1223 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1225 If the header contains a bogus option (one with a length
1226 that's either too small or beyond the end of the header), \fItcpdump\fP
1227 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1228 options (since it's impossible to tell where they start).
1230 length indicates options are present but the IP datagram length is not
1231 long enough for the options to actually be there, \fItcpdump\fP reports
1232 it as ``[\fIbad hdr length\fP]''.
1234 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1236 There are 8 bits in the control bits section of the TCP header:
1238 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1240 Let's assume that we want to watch packets used in establishing
1242 Recall that TCP uses a 3-way handshake protocol
1243 when it initializes a new connection; the connection sequence with
1244 regard to the TCP control bits is
1250 2) Recipient responds with SYN, ACK
1256 Now we're interested in capturing packets that have only the
1257 SYN bit set (Step 1).
1258 Note that we don't want packets from step 2
1259 (SYN-ACK), just a plain initial SYN.
1260 What we need is a correct filter
1261 expression for \fItcpdump\fP.
1263 Recall the structure of a TCP header without options:
1267 -----------------------------------------------------------------
1268 | source port | destination port |
1269 -----------------------------------------------------------------
1271 -----------------------------------------------------------------
1272 | acknowledgment number |
1273 -----------------------------------------------------------------
1274 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1275 -----------------------------------------------------------------
1276 | TCP checksum | urgent pointer |
1277 -----------------------------------------------------------------
1280 A TCP header usually holds 20 octets of data, unless options are
1282 The first line of the graph contains octets 0 - 3, the
1283 second line shows octets 4 - 7 etc.
1285 Starting to count with 0, the relevant TCP control bits are contained
1290 ----------------|---------------|---------------|----------------
1291 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1292 ----------------|---------------|---------------|----------------
1293 | | 13th octet | | |
1296 Let's have a closer look at octet no. 13:
1306 These are the TCP control bits we are interested
1308 We have numbered the bits in this octet from 0 to 7, right to
1309 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1311 Recall that we want to capture packets with only SYN set.
1312 Let's see what happens to octet 13 if a TCP datagram arrives
1313 with the SYN bit set in its header:
1324 control bits section we see that only bit number 1 (SYN) is set.
1326 Assuming that octet number 13 is an 8-bit unsigned integer in
1327 network byte order, the binary value of this octet is
1331 and its decimal representation is
1335 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1338 We're almost done, because now we know that if only SYN is set,
1339 the value of the 13th octet in the TCP header, when interpreted
1340 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1342 This relationship can be expressed as
1348 We can use this expression as the filter for \fItcpdump\fP in order
1349 to watch packets which have only SYN set:
1352 tcpdump -i xl0 tcp[13] == 2
1355 The expression says "let the 13th octet of a TCP datagram have
1356 the decimal value 2", which is exactly what we want.
1358 Now, let's assume that we need to capture SYN packets, but we
1359 don't care if ACK or any other TCP control bit is set at the
1361 Let's see what happens to octet 13 when a TCP datagram
1362 with SYN-ACK set arrives:
1372 Now bits 1 and 4 are set in the 13th octet.
1378 which translates to decimal
1382 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1385 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1386 expression, because that would select only those packets that have
1387 SYN-ACK set, but not those with only SYN set.
1388 Remember that we don't care
1389 if ACK or any other control bit is set as long as SYN is set.
1391 In order to achieve our goal, we need to logically AND the
1392 binary value of octet 13 with some other value to preserve
1394 We know that we want SYN to be set in any case,
1395 so we'll logically AND the value in the 13th octet with
1396 the binary value of a SYN:
1400 00010010 SYN-ACK 00000010 SYN
1401 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1403 = 00000010 = 00000010
1406 We see that this AND operation delivers the same result
1407 regardless whether ACK or another TCP control bit is set.
1408 The decimal representation of the AND value as well as
1409 the result of this operation is 2 (binary 00000010),
1410 so we know that for packets with SYN set the following
1411 relation must hold true:
1413 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1415 This points us to the \fItcpdump\fP filter expression
1418 tcpdump -i xl0 'tcp[13] & 2 == 2'
1421 Note that you should use single quotes or a backslash
1422 in the expression to hide the AND ('&') special character
1428 UDP format is illustrated by this rwho packet:
1432 \f(CWactinide.who > broadcast.who: udp 84\fP
1436 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1437 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1439 The packet contained 84 bytes of user data.
1441 Some UDP services are recognized (from the source or destination
1442 port number) and the higher level protocol information printed.
1443 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1444 RPC calls (RFC-1050) to NFS.
1446 UDP Name Server Requests
1448 \fI(N.B.:The following description assumes familiarity with
1449 the Domain Service protocol described in RFC-1035.
1450 If you are not familiar
1451 with the protocol, the following description will appear to be written
1454 Name server requests are formatted as
1458 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1460 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1464 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1465 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1466 The query id was `3'.
1467 The `+' indicates the \fIrecursion desired\fP flag
1469 The query length was 37 bytes, not including the UDP and
1470 IP protocol headers.
1471 The query operation was the normal one, \fIQuery\fP,
1472 so the op field was omitted.
1473 If the op had been anything else, it would
1474 have been printed between the `3' and the `+'.
1475 Similarly, the qclass was the normal one,
1476 \fIC_IN\fP, and omitted.
1477 Any other qclass would have been printed
1478 immediately after the `A'.
1480 A few anomalies are checked and may result in extra fields enclosed in
1481 square brackets: If a query contains an answer, authority records or
1482 additional records section,
1487 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1488 is the appropriate count.
1489 If any of the response bits are set (AA, RA or rcode) or any of the
1490 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1491 is printed, where \fIx\fP is the hex value of header bytes two and three.
1493 UDP Name Server Responses
1495 Name server responses are formatted as
1499 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1501 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1502 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1506 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1507 with 3 answer records, 3 name server records and 7 additional records.
1508 The first answer record is type A (address) and its data is internet
1509 address 128.32.137.3.
1510 The total size of the response was 273 bytes,
1511 excluding UDP and IP headers.
1512 The op (Query) and response code
1513 (NoError) were omitted, as was the class (C_IN) of the A record.
1515 In the second example, \fIhelios\fP responds to query 2 with a
1516 response code of non-existent domain (NXDomain) with no answers,
1517 one name server and no authority records.
1518 The `*' indicates that
1519 the \fIauthoritative answer\fP bit was set.
1521 answers, no type, class or data were printed.
1523 Other flag characters that might appear are `\-' (recursion available,
1524 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1526 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1529 Note that name server requests and responses tend to be large and the
1530 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1532 Use the \fB\-s\fP flag to increase the snaplen if you
1533 need to seriously investigate name server traffic.
1535 has worked well for me.
1540 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1541 on UDP/137, UDP/138 and TCP/139.
1542 Some primitive decoding of IPX and
1543 NetBEUI SMB data is also done.
1545 By default a fairly minimal decode is done, with a much more detailed
1546 decode done if -v is used.
1547 Be warned that with -v a single SMB packet
1548 may take up a page or more, so only use -v if you really want all the
1551 If you are decoding SMB sessions containing unicode strings then you
1552 may wish to set the environment variable USE_UNICODE to 1.
1554 auto-detect unicode srings would be welcome.
1556 For information on SMB packet formats and what all te fields mean see
1557 www.cifs.org or the pub/samba/specs/ directory on your favourite
1558 samba.org mirror site.
1559 The SMB patches were written by Andrew Tridgell
1563 NFS Requests and Replies
1565 Sun NFS (Network File System) requests and replies are printed as:
1569 \fIsrc.xid > dst.nfs: len op args\fP
1570 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1573 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1574 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1575 sushi.201b > wrl.nfs:
1576 144 lookup fh 9,74/4096.6878 "xcolors"
1577 wrl.nfs > sushi.201b:
1578 reply ok 128 lookup fh 9,74/4134.3150
1583 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1584 to \fIwrl\fP (note that the number following the src host is a
1585 transaction id, \fInot\fP the source port).
1586 The request was 112 bytes,
1587 excluding the UDP and IP headers.
1588 The operation was a \fIreadlink\fP
1589 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1590 (If one is lucky, as in this case, the file handle can be interpreted
1591 as a major,minor device number pair, followed by the inode number and
1593 \fIWrl\fP replies `ok' with the contents of the link.
1595 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1596 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1597 Note that the data printed
1598 depends on the operation type.
1599 The format is intended to be self
1600 explanatory if read in conjunction with
1601 an NFS protocol spec.
1603 If the \-v (verbose) flag is given, additional information is printed.
1609 sushi.1372a > wrl.nfs:
1610 148 read fh 21,11/12.195 8192 bytes @ 24576
1611 wrl.nfs > sushi.1372a:
1612 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1617 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1618 which have been omitted from this example.) In the first line,
1619 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1620 at byte offset 24576.
1621 \fIWrl\fP replies `ok'; the packet shown on the
1622 second line is the first fragment of the reply, and hence is only 1472
1623 bytes long (the other bytes will follow in subsequent fragments, but
1624 these fragments do not have NFS or even UDP headers and so might not be
1625 printed, depending on the filter expression used).
1626 Because the \-v flag
1627 is given, some of the file attributes (which are returned in addition
1628 to the file data) are printed: the file type (``REG'', for regular file),
1629 the file mode (in octal), the uid and gid, and the file size.
1631 If the \-v flag is given more than once, even more details are printed.
1633 Note that NFS requests are very large and much of the detail won't be printed
1634 unless \fIsnaplen\fP is increased.
1635 Try using `\fB\-s 192\fP' to watch
1638 NFS reply packets do not explicitly identify the RPC operation.
1640 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1641 replies using the transaction ID.
1642 If a reply does not closely follow the
1643 corresponding request, it might not be parsable.
1645 AFS Requests and Replies
1647 Transarc AFS (Andrew File System) requests and replies are printed
1653 \fIsrc.sport > dst.dport: rx packet-type\fP
1654 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1655 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1658 elvis.7001 > pike.afsfs:
1659 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1660 new fid 536876964/1/1 ".newsrc"
1661 pike.afsfs > elvis.7001: rx data fs reply rename
1666 In the first line, host elvis sends a RX packet to pike.
1668 a RX data packet to the fs (fileserver) service, and is the start of
1670 The RPC call was a rename, with the old directory file id
1671 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1672 file id of 536876964/1/1 and a new filename of `.newsrc'.
1674 responds with a RPC reply to the rename call (which was successful, because
1675 it was a data packet and not an abort packet).
1677 In general, all AFS RPCs are decoded at least by RPC call name.
1679 AFS RPCs have at least some of the arguments decoded (generally only
1680 the `interesting' arguments, for some definition of interesting).
1682 The format is intended to be self-describing, but it will probably
1683 not be useful to people who are not familiar with the workings of
1686 If the -v (verbose) flag is given twice, acknowledgement packets and
1687 additional header information is printed, such as the the RX call ID,
1688 call number, sequence number, serial number, and the RX packet flags.
1690 If the -v flag is given twice, additional information is printed,
1691 such as the the RX call ID, serial number, and the RX packet flags.
1692 The MTU negotiation information is also printed from RX ack packets.
1694 If the -v flag is given three times, the security index and service id
1697 Error codes are printed for abort packets, with the exception of Ubik
1698 beacon packets (because abort packets are used to signify a yes vote
1699 for the Ubik protocol).
1701 Note that AFS requests are very large and many of the arguments won't
1702 be printed unless \fIsnaplen\fP is increased.
1703 Try using `\fB-s 256\fP'
1704 to watch AFS traffic.
1706 AFS reply packets do not explicitly identify the RPC operation.
1708 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1709 replies using the call number and service ID.
1710 If a reply does not closely
1712 corresponding request, it might not be parsable.
1715 KIP Appletalk (DDP in UDP)
1717 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1718 and dumped as DDP packets (i.e., all the UDP header information is
1722 is used to translate appletalk net and node numbers to names.
1723 Lines in this file have the form
1735 The first two lines give the names of appletalk networks.
1737 line gives the name of a particular host (a host is distinguished
1738 from a net by the 3rd octet in the number \-
1739 a net number \fImust\fP have two octets and a host number \fImust\fP
1740 have three octets.) The number and name should be separated by
1741 whitespace (blanks or tabs).
1744 file may contain blank lines or comment lines (lines starting with
1747 Appletalk addresses are printed in the form
1753 \f(CW144.1.209.2 > icsd-net.112.220
1754 office.2 > icsd-net.112.220
1755 jssmag.149.235 > icsd-net.2\fR
1761 doesn't exist or doesn't contain an entry for some appletalk
1762 host/net number, addresses are printed in numeric form.)
1763 In the first example, NBP (DDP port 2) on net 144.1 node 209
1764 is sending to whatever is listening on port 220 of net icsd node 112.
1765 The second line is the same except the full name of the source node
1766 is known (`office').
1767 The third line is a send from port 235 on
1768 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1769 the broadcast address (255) is indicated by a net name with no host
1770 number \- for this reason it's a good idea to keep node names and
1771 net names distinct in /etc/atalk.names).
1773 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1774 packets have their contents interpreted.
1775 Other protocols just dump
1776 the protocol name (or number if no name is registered for the
1777 protocol) and packet size.
1779 \fBNBP packets\fP are formatted like the following examples:
1783 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1784 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1785 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1789 The first line is a name lookup request for laserwriters sent by net icsd host
1790 112 and broadcast on net jssmag.
1791 The nbp id for the lookup is 190.
1792 The second line shows a reply for this request (note that it has the
1793 same id) from host jssmag.209 saying that it has a laserwriter
1794 resource named "RM1140" registered on port 250.
1796 another reply to the same request saying host techpit has laserwriter
1797 "techpit" registered on port 186.
1799 \fBATP packet\fP formatting is demonstrated by the following example:
1803 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1804 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1805 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1806 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1807 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1808 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1809 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1810 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1811 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1812 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1813 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1814 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1815 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1816 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1820 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1821 up to 8 packets (the `<0-7>').
1822 The hex number at the end of the line
1823 is the value of the `userdata' field in the request.
1825 Helios responds with 8 512-byte packets.
1826 The `:digit' following the
1827 transaction id gives the packet sequence number in the transaction
1828 and the number in parens is the amount of data in the packet,
1829 excluding the atp header.
1830 The `*' on packet 7 indicates that the
1833 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1835 resends them then jssmag.209 releases the transaction.
1837 jssmag.209 initiates the next request.
1838 The `*' on the request
1839 indicates that XO (`exactly once') was \fInot\fP set.
1844 Fragmented Internet datagrams are printed as
1848 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1849 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1853 (The first form indicates there are more fragments.
1855 indicates this is the last fragment.)
1857 \fIId\fP is the fragment id.
1858 \fISize\fP is the fragment
1859 size (in bytes) excluding the IP header.
1860 \fIOffset\fP is this
1861 fragment's offset (in bytes) in the original datagram.
1863 The fragment information is output for each fragment.
1865 fragment contains the higher level protocol header and the frag
1866 info is printed after the protocol info.
1868 after the first contain no higher level protocol header and the
1869 frag info is printed after the source and destination addresses.
1870 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1871 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1875 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1876 arizona > rtsg: (frag 595a:204@328)
1877 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1881 There are a couple of things to note here: First, addresses in the
1882 2nd line don't include port numbers.
1883 This is because the TCP
1884 protocol information is all in the first fragment and we have no idea
1885 what the port or sequence numbers are when we print the later fragments.
1886 Second, the tcp sequence information in the first line is printed as if there
1887 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1888 the first frag and 204 in the second).
1889 If you are looking for holes
1890 in the sequence space or trying to match up acks
1891 with packets, this can fool you.
1893 A packet with the IP \fIdon't fragment\fP flag is marked with a
1894 trailing \fB(DF)\fP.
1898 By default, all output lines are preceded by a timestamp.
1900 is the current clock time in the form
1906 and is as accurate as the kernel's clock.
1907 The timestamp reflects the time the kernel first saw the packet.
1909 is made to account for the time lag between when the
1910 ethernet interface removed the packet from the wire and when the kernel
1911 serviced the `new packet' interrupt.
1913 traffic(1C), nit(4P), bpf(4), pcap(3)
1915 The original authors are:
1919 Steven McCanne, all of the
1920 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1922 It is currently being maintained by tcpdump.org.
1924 The current version is available via http:
1927 .I http://www.tcpdump.org/
1930 The original distribution is available via anonymous ftp:
1933 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1936 IPv6/IPsec support is added by WIDE/KAME project.
1937 This program uses Eric Young's SSLeay library, under specific configuration.
1939 Please send problems, bugs, questions, desirable enhancements, etc. to:
1942 tcpdump-workers@tcpdump.org
1945 Please send source code contributions, etc. to:
1951 NIT doesn't let you watch your own outbound traffic, BPF will.
1952 We recommend that you use the latter.
1954 On Linux systems with 2.0[.x] kernels:
1956 packets on the loopback device will be seen twice;
1958 packet filtering cannot be done in the kernel, so that all packets must
1959 be copied from the kernel in order to be filtered in user mode;
1961 all of a packet, not just the part that's within the snapshot length,
1962 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1963 asked to copy only part of a packet to userland, will not report the
1964 true length of the packet; this would cause most IP packets to get an
1968 capturing on some PPP devices won't work correctly.
1970 We recommend that you upgrade to a 2.2 or later kernel.
1972 Some attempt should be made to reassemble IP fragments or, at least
1973 to compute the right length for the higher level protocol.
1975 Name server inverse queries are not dumped correctly: the (empty)
1976 question section is printed rather than real query in the answer
1978 Some believe that inverse queries are themselves a bug and
1979 prefer to fix the program generating them rather than \fItcpdump\fP.
1981 A packet trace that crosses a daylight savings time change will give
1982 skewed time stamps (the time change is ignored).
1984 Filter expressions that manipulate FDDI or Token Ring headers assume
1985 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1987 This is true for IP, ARP, and DECNET Phase IV, but is not true
1988 for protocols such as ISO CLNS.
1989 Therefore, the filter may inadvertently
1990 accept certain packets that do not properly match the filter expression.
1992 Filter expressions on fields other than those that manipulate Token Ring
1993 headers will not correctly handle source-routed Token Ring packets.
1996 should chase header chain, but at this moment it does not.
1997 .BR "ip6 protochain"
1998 is supplied for this behavior.
2000 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
2001 does not work against IPv6 packets.
2002 It only looks at IPv4 packets.
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