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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:
175 Once the super-user has enabled promiscuous-mode operation using
177 any user may capture network traffic with
181 You must have read access to
184 Reading a saved packet file doesn't require special privileges.
189 Print each packet (minus its link level header) in ASCII. Handy for
192 Attempt to convert network and broadcast addresses to names.
195 Exit after receiving \fIcount\fP packets.
198 Before writing a raw packet to a savefile, check whether the file is
199 currently larger than \fIfile_size\fP and, if so, close the current
200 savefile and open a new one. Savefiles after the first savefile will
201 have the name specified with the
203 flag, with a number after it, starting at 2 and continuing upward.
204 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
205 not 1,048,576 bytes).
208 Dump the compiled packet-matching code in a human readable form to
209 standard output and stop.
212 Dump packet-matching code as a
217 Dump packet-matching code as decimal numbers (preceded with a count).
220 Print the link-level header on each dump line.
223 Use \fIalgo:secret\fP for decrypting IPsec ESP packets.
229 \fBcast128-cbc\fP, or
231 The default is \fBdes-cbc\fP.
232 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
233 with cryptography enabled.
234 \fIsecret\fP the ascii text for ESP secret key.
235 We cannot take arbitrary binary value at this moment.
236 The option assumes RFC2406 ESP, not RFC1827 ESP.
237 The option is only for debugging purposes, and
238 the use of this option with truly `secret' key is discouraged.
239 By presenting IPsec secret key onto command line
240 you make it visible to others, via
245 Print `foreign' internet addresses numerically rather than symbolically
246 (this option is intended to get around serious brain damage in
247 Sun's yp server \(em usually it hangs forever translating non-local
251 Use \fIfile\fP as input for the filter expression.
252 An additional expression given on the command line is ignored.
255 Listen on \fIinterface\fP.
256 If unspecified, \fItcpdump\fP searches the system interface list for the
257 lowest numbered, configured up interface (excluding loopback).
258 Ties are broken by choosing the earliest match.
260 On Linux systems with 2.2 or later kernels, an
262 argument of ``any'' can be used to capture packets from all interfaces.
263 Note that captures on the ``any'' device will not be done in promiscuous
267 Make stdout line buffered.
268 Useful if you want to see the data
272 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
273 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
276 Load SMI MIB module definitions from file \fImodule\fR.
278 can be used several times to load several MIB modules into \fItcpdump\fP.
281 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
284 Don't print domain name qualification of host names.
286 if you give this flag then \fItcpdump\fP will print ``nic''
287 instead of ``nic.ddn.mil''.
290 Do not run the packet-matching code optimizer.
292 if you suspect a bug in the optimizer.
295 \fIDon't\fP put the interface
296 into promiscuous mode.
297 Note that the interface might be in promiscuous
298 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
299 `ether host {local-hw-addr} or ether broadcast'.
302 Quick (quiet?) output.
303 Print less protocol information so output
307 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
308 If specified, \fItcpdump\fP will not print replay prevention field.
309 Since there is no protocol version field in ESP/AH specification,
310 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
313 Read packets from \fIfile\fR (which was created with the -w option).
314 Standard input is used if \fIfile\fR is ``-''.
317 Print absolute, rather than relative, TCP sequence numbers.
320 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
321 default of 68 (with SunOS's NIT, the minimum is actually 96).
322 68 bytes is adequate for IP, ICMP, TCP
323 and UDP but may truncate protocol information from name server and NFS
325 Packets truncated because of a limited snapshot
326 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
327 is the name of the protocol level at which the truncation has occurred.
328 Note that taking larger snapshots both increases
329 the amount of time it takes to process packets and, effectively,
330 decreases the amount of packet buffering.
331 This may cause packets to be
333 You should limit \fIsnaplen\fP to the smallest number that will
334 capture the protocol information you're interested in.
336 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
339 Force packets selected by "\fIexpression\fP" to be interpreted the
340 specified \fItype\fR.
341 Currently known types are
342 \fBcnfp\fR (Cisco NetFlow protocol),
343 \fBrpc\fR (Remote Procedure Call),
344 \fBrtp\fR (Real-Time Applications protocol),
345 \fBrtcp\fR (Real-Time Applications control protocol),
346 \fBsnmp\fR (Simple Network Management Protocol),
347 \fBvat\fR (Visual Audio Tool),
349 \fBwb\fR (distributed White Board).
352 \fIDon't\fP print a timestamp on each dump line.
355 Print an unformatted timestamp on each dump line.
358 Print a delta (in micro-seconds) between current and previous line
362 Print a timestamp in default format proceeded by date on each dump line.
365 Print undecoded NFS handles.
368 (Slightly more) verbose output.
369 For example, the time to live,
370 identification, total length and options in an IP packet are printed.
371 Also enables additional packet integrity checks such as verifying the
372 IP and ICMP header checksum.
375 Even more verbose output.
376 For example, additional fields are
377 printed from NFS reply packets, and SMB packets are fully decoded.
380 Even more verbose output.
382 telnet \fBSB\fP ... \fBSE\fP options
386 telnet options are printed in hex as well.
389 Write the raw packets to \fIfile\fR rather than parsing and printing
391 They can later be printed with the \-r option.
392 Standard output is used if \fIfile\fR is ``-''.
395 Print each packet (minus its link level header) in hex.
396 The smaller of the entire packet or
398 bytes will be printed.
401 When printing hex, print ascii too.
404 is also set, the packet is printed in hex/ascii.
405 This is very handy for analysing new protocols.
408 is not also set, some parts of some packets may be printed
410 .IP "\fI expression\fP"
412 selects which packets will be dumped.
413 If no \fIexpression\fP
414 is given, all packets on the net will be dumped.
416 only packets for which \fIexpression\fP is `true' will be dumped.
418 The \fIexpression\fP consists of one or more
420 Primitives usually consist of an
422 (name or number) preceded by one or more qualifiers.
424 different kinds of qualifier:
426 qualifiers say what kind of thing the id name or number refers to.
432 E.g., `host foo', `net 128.3', `port 20'.
438 qualifiers specify a particular transfer direction to and/or from
440 Possible directions are
447 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.
449 there is no dir qualifier,
452 For `null' link layers (i.e. point to point protocols such as slip) the
456 qualifiers can be used to specify a desired direction.
458 qualifiers restrict the match to a particular protocol.
472 E.g., `ether src foo', `arp net 128.3', `tcp port 21'.
474 no proto qualifier, all protocols consistent with the type are
476 E.g., `src foo' means `(ip or arp or rarp) src foo'
477 (except the latter is not legal syntax), `net bar' means `(ip or
478 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
480 [`fddi' is actually an alias for `ether'; the parser treats them
481 identically as meaning ``the data link level used on the specified
482 network interface.'' FDDI headers contain Ethernet-like source
483 and destination addresses, and often contain Ethernet-like packet
484 types, so you can filter on these FDDI fields just as with the
485 analogous Ethernet fields.
486 FDDI headers also contain other fields,
487 but you cannot name them explicitly in a filter expression.
489 Similarly, `tr' is an alias for `ether'; the previous paragraph's
490 statements about FDDI headers also apply to Token Ring headers.]
492 In addition to the above, there are some special `primitive' keywords
493 that don't follow the pattern:
498 and arithmetic expressions.
499 All of these are described below.
501 More complex filter expressions are built up by using the words
506 to combine primitives.
507 E.g., `host foo and not port ftp and not port ftp-data'.
508 To save typing, identical qualifier lists can be omitted.
510 `tcp dst port ftp or ftp-data or domain' is exactly the same as
511 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
513 Allowable primitives are:
514 .IP "\fBdst host \fIhost\fR"
515 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
516 which may be either an address or a name.
517 .IP "\fBsrc host \fIhost\fR"
518 True if the IPv4/v6 source field of the packet is \fIhost\fP.
519 .IP "\fBhost \fIhost\fP
520 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
521 Any of the above host expressions can be prepended with the keywords,
522 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
525 \fBip host \fIhost\fR
528 which is equivalent to:
531 \fBether proto \fI\\ip\fB and host \fIhost\fR
534 If \fIhost\fR is a name with multiple IP addresses, each address will
535 be checked for a match.
536 .IP "\fBether dst \fIehost\fP
537 True if the ethernet destination address is \fIehost\fP.
539 may be either a name from /etc/ethers or a number (see
542 .IP "\fBether src \fIehost\fP
543 True if the ethernet source address is \fIehost\fP.
544 .IP "\fBether host \fIehost\fP
545 True if either the ethernet source or destination address is \fIehost\fP.
546 .IP "\fBgateway\fP \fIhost\fP
547 True if the packet used \fIhost\fP as a gateway.
549 source or destination address was \fIhost\fP but neither the IP source
550 nor the IP destination was \fIhost\fP.
551 \fIHost\fP must be a name and
552 must be found both by the machine's host-name-to-IP-address resolution
553 mechanisms (host name file, DNS, NIS, etc.) and by the machine's
554 host-name-to-Ethernet-address resolution mechanism (/etc/ethers, etc.).
555 (An equivalent expression is
558 \fBether host \fIehost \fBand not host \fIhost\fR
561 which can be used with either names or numbers for \fIhost / ehost\fP.)
562 This syntax does not work in IPv6-enabled configuration at this moment.
563 .IP "\fBdst net \fInet\fR"
564 True if the IPv4/v6 destination address of the packet has a network
566 \fINet\fP may be either a name from /etc/networks
567 or a network number (see \fInetworks(4)\fP for details).
568 .IP "\fBsrc net \fInet\fR"
569 True if the IPv4/v6 source address of the packet has a network
571 .IP "\fBnet \fInet\fR"
572 True if either the IPv4/v6 source or destination address of the packet has a network
574 .IP "\fBnet \fInet\fR \fBmask \fInetmask\fR"
575 True if the IP address matches \fInet\fR with the specific \fInetmask\fR.
576 May be qualified with \fBsrc\fR or \fBdst\fR.
577 Note that this syntax is not valid for IPv6 \fInet\fR.
578 .IP "\fBnet \fInet\fR/\fIlen\fR"
579 True if the IPv4/v6 address matches \fInet\fR with a netmask \fIlen\fR
581 May be qualified with \fBsrc\fR or \fBdst\fR.
582 .IP "\fBdst port \fIport\fR"
583 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
584 destination port value of \fIport\fP.
585 The \fIport\fP can be a number or a name used in /etc/services (see
589 If a name is used, both the port
590 number and protocol are checked.
591 If a number or ambiguous name is used,
592 only the port number is checked (e.g., \fBdst port 513\fR will print both
593 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
594 both tcp/domain and udp/domain traffic).
595 .IP "\fBsrc port \fIport\fR"
596 True if the packet has a source port value of \fIport\fP.
597 .IP "\fBport \fIport\fR"
598 True if either the source or destination port of the packet is \fIport\fP.
599 Any of the above port expressions can be prepended with the keywords,
600 \fBtcp\fP or \fBudp\fP, as in:
603 \fBtcp src port \fIport\fR
606 which matches only tcp packets whose source port is \fIport\fP.
607 .IP "\fBless \fIlength\fR"
608 True if the packet has a length less than or equal to \fIlength\fP.
609 This is equivalent to:
612 \fBlen <= \fIlength\fP.
615 .IP "\fBgreater \fIlength\fR"
616 True if the packet has a length greater than or equal to \fIlength\fP.
617 This is equivalent to:
620 \fBlen >= \fIlength\fP.
623 .IP "\fBip proto \fIprotocol\fR"
624 True if the packet is an IP packet (see
626 of protocol type \fIprotocol\fP.
627 \fIProtocol\fP can be a number or one of the names
628 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
629 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
630 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
631 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
632 Note that this primitive does not chase the protocol header chain.
633 .IP "\fBip6 proto \fIprotocol\fR"
634 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
635 Note that this primitive does not chase the protocol header chain.
636 .IP "\fBip6 protochain \fIprotocol\fR"
637 True if the packet is IPv6 packet,
638 and contains protocol header with type \fIprotocol\fR
639 in its protocol header chain.
643 \fBip6 protochain 6\fR
646 matches any IPv6 packet with TCP protocol header in the protocol header chain.
647 The packet may contain, for example,
648 authentication header, routing header, or hop-by-hop option header,
649 between IPv6 header and TCP header.
650 The BPF code emitted by this primitive is complex and
651 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
652 so this can be somewhat slow.
653 .IP "\fBip protochain \fIprotocol\fR"
654 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
655 .IP "\fBether broadcast\fR"
656 True if the packet is an ethernet broadcast packet.
659 .IP "\fBip broadcast\fR"
660 True if the packet is an IP broadcast packet.
662 the all-zeroes and all-ones broadcast conventions, and looks up
663 the local subnet mask.
664 .IP "\fBether multicast\fR"
665 True if the packet is an ethernet multicast packet.
668 This is shorthand for `\fBether[0] & 1 != 0\fP'.
669 .IP "\fBip multicast\fR"
670 True if the packet is an IP multicast packet.
671 .IP "\fBip6 multicast\fR"
672 True if the packet is an IPv6 multicast packet.
673 .IP "\fBether proto \fIprotocol\fR"
674 True if the packet is of ether type \fIprotocol\fR.
675 \fIProtocol\fP can be a number or one of the names
676 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
677 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
678 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
679 Note these identifiers are also keywords
680 and must be escaped via backslash (\\).
682 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR') and Token Ring
683 (e.g., `\fBtr protocol arp\fR'), for most of those protocols, the
684 protocol identification comes from the 802.2 Logical Link Control (LLC)
685 header, which is usually layered on top of the FDDI or Token Ring
688 When filtering for most protocol identifiers on FDDI or Token Ring,
689 \fItcpdump\fR checks only the protocol ID field of an LLC header in
690 so-called SNAP format with an Organizational Unit Identifier (OUI) of
691 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
692 is in SNAP format with an OUI of 0x000000.
694 The exceptions are \fIiso\fP, for which it checks the DSAP (Destination
695 Service Access Point) and SSAP (Source Service Access Point) fields of
696 the LLC header, \fIstp\fP and \fInetbeui\fP, where it checks the DSAP of
697 the LLC header, and \fIatalk\fP, where it checks for a SNAP-format
698 packet with an OUI of 0x080007 and the Appletalk etype.
700 In the case of Ethernet, \fItcpdump\fR checks the Ethernet type field
701 for most of those protocols; the exceptions are \fIiso\fP, \fIsap\fP,
702 and \fInetbeui\fP, for which it checks for an 802.3 frame and then
703 checks the LLC header as it does for FDDI and Token Ring, \fIatalk\fP,
704 where it checks both for the Appletalk etype in an Ethernet frame and
705 for a SNAP-format packet as it does for FDDI and Token Ring, \fIaarp\fP,
706 where it checks for the Appletalk ARP etype in either an Ethernet frame
707 or an 802.2 SNAP frame with an OUI of 0x000000, and \fIipx\fP, where it
708 checks for the IPX etype in an Ethernet frame, the IPX DSAP in the LLC
709 header, the 802.3 with no LLC header encapsulation of IPX, and the IPX
710 etype in a SNAP frame.]
711 .IP "\fBdecnet src \fIhost\fR"
712 True if the DECNET source address is
714 which may be an address of the form ``10.123'', or a DECNET host
716 [DECNET host name support is only available on Ultrix systems
717 that are configured to run DECNET.]
718 .IP "\fBdecnet dst \fIhost\fR"
719 True if the DECNET destination address is
721 .IP "\fBdecnet host \fIhost\fR"
722 True if either the DECNET source or destination address is
724 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
728 \fBether proto \fIp\fR
731 where \fIp\fR is one of the above protocols.
732 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
736 \fBether proto \fIp\fR
739 where \fIp\fR is one of the above protocols.
741 \fItcpdump\fP does not currently know how to parse these protocols.
742 .IP "\fBvlan \fI[vlan_id]\fR"
743 True if the packet is an IEEE 802.1Q VLAN packet.
744 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
746 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
747 changes the decoding offsets for the remainder of \fIexpression\fR
748 on the assumption that the packet is a VLAN packet.
749 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
753 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
756 where \fIp\fR is one of the above protocols.
757 .IP "\fBiso proto \fIprotocol\fR"
758 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
759 \fIProtocol\fP can be a number or one of the names
760 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
761 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
768 where \fIp\fR is one of the above protocols.
769 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
770 .IP "\fIexpr relop expr\fR"
771 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
772 and \fIexpr\fR is an arithmetic expression composed of integer constants
773 (expressed in standard C syntax), the normal binary operators
774 [+, -, *, /, &, |], a length operator, and special packet data accessors.
776 data inside the packet, use the following syntax:
779 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
782 \fIProto\fR is one of \fBether, fddi, tr, ppp, slip, link,
783 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
784 indicates the protocol layer for the index operation.
785 (\fBether, fddi, tr, ppp, slip\fR and \fBlink\fR all refer to the link
787 Note that \fItcp, udp\fR and other upper-layer protocol types only
788 apply to IPv4, not IPv6 (this will be fixed in the future).
789 The byte offset, relative to the indicated protocol layer, is
791 \fISize\fR is optional and indicates the number of bytes in the
792 field of interest; it can be either one, two, or four, and defaults to one.
793 The length operator, indicated by the keyword \fBlen\fP, gives the
794 length of the packet.
796 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
797 The expression `\fBip[0] & 0xf != 5\fP'
798 catches all IP packets with options.
800 `\fBip[6:2] & 0x1fff = 0\fP'
801 catches only unfragmented datagrams and frag zero of fragmented datagrams.
802 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
804 For instance, \fBtcp[0]\fP always means the first
805 byte of the TCP \fIheader\fP, and never means the first byte of an
806 intervening fragment.
808 Some offsets and field values may be expressed as names rather than
810 The following protocol header field offsets are
811 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
812 code field), and \fBtcpflags\fP (TCP flags field).
814 The following ICMP type field values are available: \fBicmp-echoreply\fP,
815 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
816 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
817 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
818 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
819 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
821 The following TCP flags field values are available: \fBtcp-fin\fP,
822 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
823 \fBtcp-ack\fP, \fBtcp-urg\fP.
825 Primitives may be combined using:
827 A parenthesized group of primitives and operators
828 (parentheses are special to the Shell and must be escaped).
830 Negation (`\fB!\fP' or `\fBnot\fP').
832 Concatenation (`\fB&&\fP' or `\fBand\fP').
834 Alternation (`\fB||\fP' or `\fBor\fP').
836 Negation has highest precedence.
837 Alternation and concatenation have equal precedence and associate
839 Note that explicit \fBand\fR tokens, not juxtaposition,
840 are now required for concatenation.
842 If an identifier is given without a keyword, the most recent keyword
847 \fBnot host vs and ace\fR
853 \fBnot host vs and host ace\fR
856 which should not be confused with
859 \fBnot ( host vs or ace )\fR
863 Expression arguments can be passed to \fItcpdump\fP as either a single
864 argument or as multiple arguments, whichever is more convenient.
865 Generally, if the expression contains Shell metacharacters, it is
866 easier to pass it as a single, quoted argument.
867 Multiple arguments are concatenated with spaces before being parsed.
870 To print all packets arriving at or departing from \fIsundown\fP:
873 \fBtcpdump host sundown\fP
877 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
880 \fBtcpdump host helios and \\( hot or ace \\)\fP
884 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
887 \fBtcpdump ip host ace and not helios\fP
891 To print all traffic between local hosts and hosts at Berkeley:
895 tcpdump net ucb-ether
899 To print all ftp traffic through internet gateway \fIsnup\fP:
900 (note that the expression is quoted to prevent the shell from
901 (mis-)interpreting the parentheses):
905 tcpdump 'gateway snup and (port ftp or ftp-data)'
909 To print traffic neither sourced from nor destined for local hosts
910 (if you gateway to one other net, this stuff should never make it
911 onto your local net).
915 tcpdump ip and not net \fIlocalnet\fP
919 To print the start and end packets (the SYN and FIN packets) of each
920 TCP conversation that involves a non-local host.
924 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
928 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
932 tcpdump 'gateway snup and ip[2:2] > 576'
936 To print IP broadcast or multicast packets that were
938 sent via ethernet broadcast or multicast:
942 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
946 To print all ICMP packets that are not echo requests/replies (i.e., not
951 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
956 The output of \fItcpdump\fP is protocol dependent.
958 gives a brief description and examples of most of the formats.
966 If the '-e' option is given, the link level header is printed out.
967 On ethernets, the source and destination addresses, protocol,
968 and packet length are printed.
970 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
971 the `frame control' field, the source and destination addresses,
972 and the packet length.
973 (The `frame control' field governs the
974 interpretation of the rest of the packet.
976 as those containing IP datagrams) are `async' packets, with a priority
977 value between 0 and 7; for example, `\fBasync4\fR'.
979 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
980 the LLC header is printed if it is \fInot\fR an ISO datagram or a
981 so-called SNAP packet.
983 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
984 the `access control' and `frame control' fields, the source and
985 destination addresses, and the packet length.
987 packets are assumed to contain an LLC packet.
988 Regardless of whether
989 the '-e' option is specified or not, the source routing information is
990 printed for source-routed packets.
992 \fI(N.B.: The following description assumes familiarity with
993 the SLIP compression algorithm described in RFC-1144.)\fP
995 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
996 packet type, and compression information are printed out.
997 The packet type is printed first.
998 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
999 No further link information is printed for \fIip\fR packets.
1000 For TCP packets, the connection identifier is printed following the type.
1001 If the packet is compressed, its encoded header is printed out.
1002 The special cases are printed out as
1003 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
1004 the sequence number (or sequence number and ack) has changed.
1005 If it is not a special case,
1006 zero or more changes are printed.
1007 A change is indicated by U (urgent pointer), W (window), A (ack),
1008 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1009 or a new value (=n).
1010 Finally, the amount of data in the packet and compressed header length
1013 For example, the following line shows an outbound compressed TCP packet,
1014 with an implicit connection identifier; the ack has changed by 6,
1015 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1016 data and 6 bytes of compressed header:
1019 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1025 Arp/rarp output shows the type of request and its arguments.
1027 format is intended to be self explanatory.
1028 Here is a short sample taken from the start of an `rlogin' from
1029 host \fIrtsg\fP to host \fIcsam\fP:
1033 \f(CWarp who-has csam tell rtsg
1034 arp reply csam is-at CSAM\fR
1038 The first line says that rtsg sent an arp packet asking
1039 for the ethernet address of internet host csam.
1041 replies with its ethernet address (in this example, ethernet addresses
1042 are in caps and internet addresses in lower case).
1044 This would look less redundant if we had done \fItcpdump \-n\fP:
1048 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1049 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1053 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1054 broadcast and the second is point-to-point would be visible:
1058 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1059 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1063 For the first packet this says the ethernet source address is RTSG, the
1064 destination is the ethernet broadcast address, the type field
1065 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1069 \fI(N.B.:The following description assumes familiarity with
1070 the TCP protocol described in RFC-793.
1071 If you are not familiar
1072 with the protocol, neither this description nor \fItcpdump\fP will
1073 be of much use to you.)\fP
1075 The general format of a tcp protocol line is:
1079 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1083 \fISrc\fP and \fIdst\fP are the source and destination IP
1084 addresses and ports.
1085 \fIFlags\fP are some combination of S (SYN),
1086 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
1087 \fIData-seqno\fP describes the portion of sequence space covered
1088 by the data in this packet (see example below).
1089 \fIAck\fP is sequence number of the next data expected the other
1090 direction on this connection.
1091 \fIWindow\fP is the number of bytes of receive buffer space available
1092 the other direction on this connection.
1093 \fIUrg\fP indicates there is `urgent' data in the packet.
1094 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1096 \fISrc, dst\fP and \fIflags\fP are always present.
1098 depend on the contents of the packet's tcp protocol header and
1099 are output only if appropriate.
1101 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1106 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1107 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1108 rtsg.1023 > csam.login: . ack 1 win 4096
1109 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1110 csam.login > rtsg.1023: . ack 2 win 4096
1111 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1112 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1113 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1114 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1118 The first line says that tcp port 1023 on rtsg sent a packet
1121 The \fBS\fP indicates that the \fISYN\fP flag was set.
1122 The packet sequence number was 768512 and it contained no data.
1123 (The notation is `first:last(nbytes)' which means `sequence
1125 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1126 There was no piggy-backed ack, the available receive window was 4096
1127 bytes and there was a max-segment-size option requesting an mss of
1130 Csam replies with a similar packet except it includes a piggy-backed
1132 Rtsg then acks csam's SYN.
1135 The packet contained no data so there is no data sequence number.
1136 Note that the ack sequence
1137 number is a small integer (1).
1138 The first time \fItcpdump\fP sees a
1139 tcp `conversation', it prints the sequence number from the packet.
1140 On subsequent packets of the conversation, the difference between
1141 the current packet's sequence number and this initial sequence number
1143 This means that sequence numbers after the
1144 first can be interpreted
1145 as relative byte positions in the conversation's data stream (with the
1146 first data byte each direction being `1').
1147 `-S' will override this
1148 feature, causing the original sequence numbers to be output.
1150 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1151 in the rtsg \(-> csam side of the conversation).
1152 The PUSH flag is set in the packet.
1153 On the 7th line, csam says it's received data sent by rtsg up to
1154 but not including byte 21.
1155 Most of this data is apparently sitting in the
1156 socket buffer since csam's receive window has gotten 19 bytes smaller.
1157 Csam also sends one byte of data to rtsg in this packet.
1158 On the 8th and 9th lines,
1159 csam sends two bytes of urgent, pushed data to rtsg.
1161 If the snapshot was small enough that \fItcpdump\fP didn't capture
1162 the full TCP header, it interprets as much of the header as it can
1163 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1165 If the header contains a bogus option (one with a length
1166 that's either too small or beyond the end of the header), \fItcpdump\fP
1167 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1168 options (since it's impossible to tell where they start).
1170 length indicates options are present but the IP datagram length is not
1171 long enough for the options to actually be there, \fItcpdump\fP reports
1172 it as ``[\fIbad hdr length\fP]''.
1174 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1176 There are 8 bits in the control bits section of the TCP header:
1178 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1180 Let's assume that we want to watch packets used in establishing
1182 Recall that TCP uses a 3-way handshake protocol
1183 when it initializes a new connection; the connection sequence with
1184 regard to the TCP control bits is
1190 2) Recipient responds with SYN, ACK
1196 Now we're interested in capturing packets that have only the
1197 SYN bit set (Step 1).
1198 Note that we don't want packets from step 2
1199 (SYN-ACK), just a plain initial SYN.
1200 What we need is a correct filter
1201 expression for \fItcpdump\fP.
1203 Recall the structure of a TCP header without options:
1207 -----------------------------------------------------------------
1208 | source port | destination port |
1209 -----------------------------------------------------------------
1211 -----------------------------------------------------------------
1212 | acknowledgment number |
1213 -----------------------------------------------------------------
1214 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1215 -----------------------------------------------------------------
1216 | TCP checksum | urgent pointer |
1217 -----------------------------------------------------------------
1220 A TCP header usually holds 20 octets of data, unless options are
1222 The first line of the graph contains octets 0 - 3, the
1223 second line shows octets 4 - 7 etc.
1225 Starting to count with 0, the relevant TCP control bits are contained
1230 ----------------|---------------|---------------|----------------
1231 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1232 ----------------|---------------|---------------|----------------
1233 | | 13th octet | | |
1236 Let's have a closer look at octet no. 13:
1246 These are the TCP control bits we are interested
1248 We have numbered the bits in this octet from 0 to 7, right to
1249 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1251 Recall that we want to capture packets with only SYN set.
1252 Let's see what happens to octet 13 if a TCP datagram arrives
1253 with the SYN bit set in its header:
1264 control bits section we see that only bit number 1 (SYN) is set.
1266 Assuming that octet number 13 is an 8-bit unsigned integer in
1267 network byte order, the binary value of this octet is
1271 and its decimal representation is
1275 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1278 We're almost done, because now we know that if only SYN is set,
1279 the value of the 13th octet in the TCP header, when interpreted
1280 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1282 This relationship can be expressed as
1288 We can use this expression as the filter for \fItcpdump\fP in order
1289 to watch packets which have only SYN set:
1292 tcpdump -i xl0 tcp[13] == 2
1295 The expression says "let the 13th octet of a TCP datagram have
1296 the decimal value 2", which is exactly what we want.
1298 Now, let's assume that we need to capture SYN packets, but we
1299 don't care if ACK or any other TCP control bit is set at the
1301 Let's see what happens to octet 13 when a TCP datagram
1302 with SYN-ACK set arrives:
1312 Now bits 1 and 4 are set in the 13th octet.
1318 which translates to decimal
1322 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1325 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1326 expression, because that would select only those packets that have
1327 SYN-ACK set, but not those with only SYN set.
1328 Remember that we don't care
1329 if ACK or any other control bit is set as long as SYN is set.
1331 In order to achieve our goal, we need to logically AND the
1332 binary value of octet 13 with some other value to preserve
1334 We know that we want SYN to be set in any case,
1335 so we'll logically AND the value in the 13th octet with
1336 the binary value of a SYN:
1340 00010010 SYN-ACK 00000010 SYN
1341 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1343 = 00000010 = 00000010
1346 We see that this AND operation delivers the same result
1347 regardless whether ACK or another TCP control bit is set.
1348 The decimal representation of the AND value as well as
1349 the result of this operation is 2 (binary 00000010),
1350 so we know that for packets with SYN set the following
1351 relation must hold true:
1353 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1355 This points us to the \fItcpdump\fP filter expression
1358 tcpdump -i xl0 'tcp[13] & 2 == 2'
1361 Note that you should use single quotes or a backslash
1362 in the expression to hide the AND ('&') special character
1368 UDP format is illustrated by this rwho packet:
1372 \f(CWactinide.who > broadcast.who: udp 84\fP
1376 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1377 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1379 The packet contained 84 bytes of user data.
1381 Some UDP services are recognized (from the source or destination
1382 port number) and the higher level protocol information printed.
1383 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1384 RPC calls (RFC-1050) to NFS.
1386 UDP Name Server Requests
1388 \fI(N.B.:The following description assumes familiarity with
1389 the Domain Service protocol described in RFC-1035.
1390 If you are not familiar
1391 with the protocol, the following description will appear to be written
1394 Name server requests are formatted as
1398 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1400 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1404 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1405 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1406 The query id was `3'.
1407 The `+' indicates the \fIrecursion desired\fP flag
1409 The query length was 37 bytes, not including the UDP and
1410 IP protocol headers.
1411 The query operation was the normal one, \fIQuery\fP,
1412 so the op field was omitted.
1413 If the op had been anything else, it would
1414 have been printed between the `3' and the `+'.
1415 Similarly, the qclass was the normal one,
1416 \fIC_IN\fP, and omitted.
1417 Any other qclass would have been printed
1418 immediately after the `A'.
1420 A few anomalies are checked and may result in extra fields enclosed in
1421 square brackets: If a query contains an answer, authority records or
1422 additional records section,
1427 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1428 is the appropriate count.
1429 If any of the response bits are set (AA, RA or rcode) or any of the
1430 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1431 is printed, where \fIx\fP is the hex value of header bytes two and three.
1433 UDP Name Server Responses
1435 Name server responses are formatted as
1439 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1441 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1442 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1446 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1447 with 3 answer records, 3 name server records and 7 additional records.
1448 The first answer record is type A (address) and its data is internet
1449 address 128.32.137.3.
1450 The total size of the response was 273 bytes,
1451 excluding UDP and IP headers.
1452 The op (Query) and response code
1453 (NoError) were omitted, as was the class (C_IN) of the A record.
1455 In the second example, \fIhelios\fP responds to query 2 with a
1456 response code of non-existent domain (NXDomain) with no answers,
1457 one name server and no authority records.
1458 The `*' indicates that
1459 the \fIauthoritative answer\fP bit was set.
1461 answers, no type, class or data were printed.
1463 Other flag characters that might appear are `\-' (recursion available,
1464 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1466 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1469 Note that name server requests and responses tend to be large and the
1470 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1472 Use the \fB\-s\fP flag to increase the snaplen if you
1473 need to seriously investigate name server traffic.
1475 has worked well for me.
1480 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1481 on UDP/137, UDP/138 and TCP/139.
1482 Some primitive decoding of IPX and
1483 NetBEUI SMB data is also done.
1485 By default a fairly minimal decode is done, with a much more detailed
1486 decode done if -v is used.
1487 Be warned that with -v a single SMB packet
1488 may take up a page or more, so only use -v if you really want all the
1491 If you are decoding SMB sessions containing unicode strings then you
1492 may wish to set the environment variable USE_UNICODE to 1.
1494 auto-detect unicode srings would be welcome.
1496 For information on SMB packet formats and what all te fields mean see
1497 www.cifs.org or the pub/samba/specs/ directory on your favourite
1498 samba.org mirror site.
1499 The SMB patches were written by Andrew Tridgell
1503 NFS Requests and Replies
1505 Sun NFS (Network File System) requests and replies are printed as:
1509 \fIsrc.xid > dst.nfs: len op args\fP
1510 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1513 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1514 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1515 sushi.201b > wrl.nfs:
1516 144 lookup fh 9,74/4096.6878 "xcolors"
1517 wrl.nfs > sushi.201b:
1518 reply ok 128 lookup fh 9,74/4134.3150
1523 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1524 to \fIwrl\fP (note that the number following the src host is a
1525 transaction id, \fInot\fP the source port).
1526 The request was 112 bytes,
1527 excluding the UDP and IP headers.
1528 The operation was a \fIreadlink\fP
1529 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1530 (If one is lucky, as in this case, the file handle can be interpreted
1531 as a major,minor device number pair, followed by the inode number and
1533 \fIWrl\fP replies `ok' with the contents of the link.
1535 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1536 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1537 Note that the data printed
1538 depends on the operation type.
1539 The format is intended to be self
1540 explanatory if read in conjunction with
1541 an NFS protocol spec.
1543 If the \-v (verbose) flag is given, additional information is printed.
1549 sushi.1372a > wrl.nfs:
1550 148 read fh 21,11/12.195 8192 bytes @ 24576
1551 wrl.nfs > sushi.1372a:
1552 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1557 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1558 which have been omitted from this example.) In the first line,
1559 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1560 at byte offset 24576.
1561 \fIWrl\fP replies `ok'; the packet shown on the
1562 second line is the first fragment of the reply, and hence is only 1472
1563 bytes long (the other bytes will follow in subsequent fragments, but
1564 these fragments do not have NFS or even UDP headers and so might not be
1565 printed, depending on the filter expression used).
1566 Because the \-v flag
1567 is given, some of the file attributes (which are returned in addition
1568 to the file data) are printed: the file type (``REG'', for regular file),
1569 the file mode (in octal), the uid and gid, and the file size.
1571 If the \-v flag is given more than once, even more details are printed.
1573 Note that NFS requests are very large and much of the detail won't be printed
1574 unless \fIsnaplen\fP is increased.
1575 Try using `\fB\-s 192\fP' to watch
1578 NFS reply packets do not explicitly identify the RPC operation.
1580 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1581 replies using the transaction ID.
1582 If a reply does not closely follow the
1583 corresponding request, it might not be parsable.
1585 AFS Requests and Replies
1587 Transarc AFS (Andrew File System) requests and replies are printed
1593 \fIsrc.sport > dst.dport: rx packet-type\fP
1594 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1595 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1598 elvis.7001 > pike.afsfs:
1599 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1600 new fid 536876964/1/1 ".newsrc"
1601 pike.afsfs > elvis.7001: rx data fs reply rename
1606 In the first line, host elvis sends a RX packet to pike.
1608 a RX data packet to the fs (fileserver) service, and is the start of
1610 The RPC call was a rename, with the old directory file id
1611 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1612 file id of 536876964/1/1 and a new filename of `.newsrc'.
1614 responds with a RPC reply to the rename call (which was successful, because
1615 it was a data packet and not an abort packet).
1617 In general, all AFS RPCs are decoded at least by RPC call name.
1619 AFS RPCs have at least some of the arguments decoded (generally only
1620 the `interesting' arguments, for some definition of interesting).
1622 The format is intended to be self-describing, but it will probably
1623 not be useful to people who are not familiar with the workings of
1626 If the -v (verbose) flag is given twice, acknowledgement packets and
1627 additional header information is printed, such as the the RX call ID,
1628 call number, sequence number, serial number, and the RX packet flags.
1630 If the -v flag is given twice, additional information is printed,
1631 such as the the RX call ID, serial number, and the RX packet flags.
1632 The MTU negotiation information is also printed from RX ack packets.
1634 If the -v flag is given three times, the security index and service id
1637 Error codes are printed for abort packets, with the exception of Ubik
1638 beacon packets (because abort packets are used to signify a yes vote
1639 for the Ubik protocol).
1641 Note that AFS requests are very large and many of the arguments won't
1642 be printed unless \fIsnaplen\fP is increased.
1643 Try using `\fB-s 256\fP'
1644 to watch AFS traffic.
1646 AFS reply packets do not explicitly identify the RPC operation.
1648 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1649 replies using the call number and service ID.
1650 If a reply does not closely
1652 corresponding request, it might not be parsable.
1655 KIP Appletalk (DDP in UDP)
1657 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1658 and dumped as DDP packets (i.e., all the UDP header information is
1662 is used to translate appletalk net and node numbers to names.
1663 Lines in this file have the form
1675 The first two lines give the names of appletalk networks.
1677 line gives the name of a particular host (a host is distinguished
1678 from a net by the 3rd octet in the number \-
1679 a net number \fImust\fP have two octets and a host number \fImust\fP
1680 have three octets.) The number and name should be separated by
1681 whitespace (blanks or tabs).
1684 file may contain blank lines or comment lines (lines starting with
1687 Appletalk addresses are printed in the form
1693 \f(CW144.1.209.2 > icsd-net.112.220
1694 office.2 > icsd-net.112.220
1695 jssmag.149.235 > icsd-net.2\fR
1701 doesn't exist or doesn't contain an entry for some appletalk
1702 host/net number, addresses are printed in numeric form.)
1703 In the first example, NBP (DDP port 2) on net 144.1 node 209
1704 is sending to whatever is listening on port 220 of net icsd node 112.
1705 The second line is the same except the full name of the source node
1706 is known (`office').
1707 The third line is a send from port 235 on
1708 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1709 the broadcast address (255) is indicated by a net name with no host
1710 number \- for this reason it's a good idea to keep node names and
1711 net names distinct in /etc/atalk.names).
1713 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1714 packets have their contents interpreted.
1715 Other protocols just dump
1716 the protocol name (or number if no name is registered for the
1717 protocol) and packet size.
1719 \fBNBP packets\fP are formatted like the following examples:
1723 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1724 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1725 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1729 The first line is a name lookup request for laserwriters sent by net icsd host
1730 112 and broadcast on net jssmag.
1731 The nbp id for the lookup is 190.
1732 The second line shows a reply for this request (note that it has the
1733 same id) from host jssmag.209 saying that it has a laserwriter
1734 resource named "RM1140" registered on port 250.
1736 another reply to the same request saying host techpit has laserwriter
1737 "techpit" registered on port 186.
1739 \fBATP packet\fP formatting is demonstrated by the following example:
1743 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1744 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1745 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1746 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1747 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1748 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1749 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1750 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1751 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1752 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1753 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1754 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1755 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1756 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1760 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1761 up to 8 packets (the `<0-7>').
1762 The hex number at the end of the line
1763 is the value of the `userdata' field in the request.
1765 Helios responds with 8 512-byte packets.
1766 The `:digit' following the
1767 transaction id gives the packet sequence number in the transaction
1768 and the number in parens is the amount of data in the packet,
1769 excluding the atp header.
1770 The `*' on packet 7 indicates that the
1773 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1775 resends them then jssmag.209 releases the transaction.
1777 jssmag.209 initiates the next request.
1778 The `*' on the request
1779 indicates that XO (`exactly once') was \fInot\fP set.
1784 Fragmented Internet datagrams are printed as
1788 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1789 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1793 (The first form indicates there are more fragments.
1795 indicates this is the last fragment.)
1797 \fIId\fP is the fragment id.
1798 \fISize\fP is the fragment
1799 size (in bytes) excluding the IP header.
1800 \fIOffset\fP is this
1801 fragment's offset (in bytes) in the original datagram.
1803 The fragment information is output for each fragment.
1805 fragment contains the higher level protocol header and the frag
1806 info is printed after the protocol info.
1808 after the first contain no higher level protocol header and the
1809 frag info is printed after the source and destination addresses.
1810 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1811 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1815 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1816 arizona > rtsg: (frag 595a:204@328)
1817 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1821 There are a couple of things to note here: First, addresses in the
1822 2nd line don't include port numbers.
1823 This is because the TCP
1824 protocol information is all in the first fragment and we have no idea
1825 what the port or sequence numbers are when we print the later fragments.
1826 Second, the tcp sequence information in the first line is printed as if there
1827 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1828 the first frag and 204 in the second).
1829 If you are looking for holes
1830 in the sequence space or trying to match up acks
1831 with packets, this can fool you.
1833 A packet with the IP \fIdon't fragment\fP flag is marked with a
1834 trailing \fB(DF)\fP.
1838 By default, all output lines are preceded by a timestamp.
1840 is the current clock time in the form
1846 and is as accurate as the kernel's clock.
1847 The timestamp reflects the time the kernel first saw the packet.
1849 is made to account for the time lag between when the
1850 ethernet interface removed the packet from the wire and when the kernel
1851 serviced the `new packet' interrupt.
1853 traffic(1C), nit(4P), bpf(4), pcap(3)
1855 The original authors are:
1859 Steven McCanne, all of the
1860 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1862 It is currently being maintained by tcpdump.org.
1864 The current version is available via http:
1867 .I http://www.tcpdump.org/
1870 The original distribution is available via anonymous ftp:
1873 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1876 IPv6/IPsec support is added by WIDE/KAME project.
1877 This program uses Eric Young's SSLeay library, under specific configuration.
1879 Please send problems, bugs, questions, desirable enhancements, etc. to:
1882 tcpdump-workers@tcpdump.org
1885 Please send source code contributions, etc. to:
1891 NIT doesn't let you watch your own outbound traffic, BPF will.
1892 We recommend that you use the latter.
1894 On Linux systems with 2.0[.x] kernels:
1896 packets on the loopback device will be seen twice;
1898 packet filtering cannot be done in the kernel, so that all packets must
1899 be copied from the kernel in order to be filtered in user mode;
1901 all of a packet, not just the part that's within the snapshot length,
1902 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1903 asked to copy only part of a packet to userland, will not report the
1904 true length of the packet; this would cause most IP packets to get an
1908 capturing on some PPP devices won't work correctly.
1910 We recommend that you upgrade to a 2.2 or later kernel.
1912 Some attempt should be made to reassemble IP fragments or, at least
1913 to compute the right length for the higher level protocol.
1915 Name server inverse queries are not dumped correctly: the (empty)
1916 question section is printed rather than real query in the answer
1918 Some believe that inverse queries are themselves a bug and
1919 prefer to fix the program generating them rather than \fItcpdump\fP.
1921 A packet trace that crosses a daylight savings time change will give
1922 skewed time stamps (the time change is ignored).
1924 Filter expressions that manipulate FDDI or Token Ring headers assume
1925 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1927 This is true for IP, ARP, and DECNET Phase IV, but is not true
1928 for protocols such as ISO CLNS.
1929 Therefore, the filter may inadvertently
1930 accept certain packets that do not properly match the filter expression.
1932 Filter expressions on fields other than those that manipulate Token Ring
1933 headers will not correctly handle source-routed Token Ring packets.
1936 should chase header chain, but at this moment it does not.
1937 .BR "ip6 protochain"
1938 is supplied for this behavior.
1940 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1941 does not work against IPv6 packets.
1942 It only looks at IPv4 packets.
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