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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 "\fIexpr relop expr\fR"
785 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
786 and \fIexpr\fR is an arithmetic expression composed of integer constants
787 (expressed in standard C syntax), the normal binary operators
788 [+, -, *, /, &, |], a length operator, and special packet data accessors.
790 data inside the packet, use the following syntax:
793 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
796 \fIProto\fR is one of \fBether, fddi, tr, ppp, slip, link,
797 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
798 indicates the protocol layer for the index operation.
799 (\fBether, fddi, tr, ppp, slip\fR and \fBlink\fR all refer to the link
801 Note that \fItcp, udp\fR and other upper-layer protocol types only
802 apply to IPv4, not IPv6 (this will be fixed in the future).
803 The byte offset, relative to the indicated protocol layer, is
805 \fISize\fR is optional and indicates the number of bytes in the
806 field of interest; it can be either one, two, or four, and defaults to one.
807 The length operator, indicated by the keyword \fBlen\fP, gives the
808 length of the packet.
810 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
811 The expression `\fBip[0] & 0xf != 5\fP'
812 catches all IP packets with options.
814 `\fBip[6:2] & 0x1fff = 0\fP'
815 catches only unfragmented datagrams and frag zero of fragmented datagrams.
816 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
818 For instance, \fBtcp[0]\fP always means the first
819 byte of the TCP \fIheader\fP, and never means the first byte of an
820 intervening fragment.
822 Some offsets and field values may be expressed as names rather than
824 The following protocol header field offsets are
825 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
826 code field), and \fBtcpflags\fP (TCP flags field).
828 The following ICMP type field values are available: \fBicmp-echoreply\fP,
829 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
830 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
831 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
832 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
833 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
835 The following TCP flags field values are available: \fBtcp-fin\fP,
836 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
837 \fBtcp-ack\fP, \fBtcp-urg\fP.
839 Primitives may be combined using:
841 A parenthesized group of primitives and operators
842 (parentheses are special to the Shell and must be escaped).
844 Negation (`\fB!\fP' or `\fBnot\fP').
846 Concatenation (`\fB&&\fP' or `\fBand\fP').
848 Alternation (`\fB||\fP' or `\fBor\fP').
850 Negation has highest precedence.
851 Alternation and concatenation have equal precedence and associate
853 Note that explicit \fBand\fR tokens, not juxtaposition,
854 are now required for concatenation.
856 If an identifier is given without a keyword, the most recent keyword
861 \fBnot host vs and ace\fR
867 \fBnot host vs and host ace\fR
870 which should not be confused with
873 \fBnot ( host vs or ace )\fR
877 Expression arguments can be passed to \fItcpdump\fP as either a single
878 argument or as multiple arguments, whichever is more convenient.
879 Generally, if the expression contains Shell metacharacters, it is
880 easier to pass it as a single, quoted argument.
881 Multiple arguments are concatenated with spaces before being parsed.
884 To print all packets arriving at or departing from \fIsundown\fP:
887 \fBtcpdump host sundown\fP
891 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
894 \fBtcpdump host helios and \\( hot or ace \\)\fP
898 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
901 \fBtcpdump ip host ace and not helios\fP
905 To print all traffic between local hosts and hosts at Berkeley:
909 tcpdump net ucb-ether
913 To print all ftp traffic through internet gateway \fIsnup\fP:
914 (note that the expression is quoted to prevent the shell from
915 (mis-)interpreting the parentheses):
919 tcpdump 'gateway snup and (port ftp or ftp-data)'
923 To print traffic neither sourced from nor destined for local hosts
924 (if you gateway to one other net, this stuff should never make it
925 onto your local net).
929 tcpdump ip and not net \fIlocalnet\fP
933 To print the start and end packets (the SYN and FIN packets) of each
934 TCP conversation that involves a non-local host.
938 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
942 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
946 tcpdump 'gateway snup and ip[2:2] > 576'
950 To print IP broadcast or multicast packets that were
952 sent via ethernet broadcast or multicast:
956 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
960 To print all ICMP packets that are not echo requests/replies (i.e., not
965 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
970 The output of \fItcpdump\fP is protocol dependent.
972 gives a brief description and examples of most of the formats.
980 If the '-e' option is given, the link level header is printed out.
981 On ethernets, the source and destination addresses, protocol,
982 and packet length are printed.
984 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
985 the `frame control' field, the source and destination addresses,
986 and the packet length.
987 (The `frame control' field governs the
988 interpretation of the rest of the packet.
990 as those containing IP datagrams) are `async' packets, with a priority
991 value between 0 and 7; for example, `\fBasync4\fR'.
993 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
994 the LLC header is printed if it is \fInot\fR an ISO datagram or a
995 so-called SNAP packet.
997 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
998 the `access control' and `frame control' fields, the source and
999 destination addresses, and the packet length.
1000 As on FDDI networks,
1001 packets are assumed to contain an LLC packet.
1002 Regardless of whether
1003 the '-e' option is specified or not, the source routing information is
1004 printed for source-routed packets.
1006 \fI(N.B.: The following description assumes familiarity with
1007 the SLIP compression algorithm described in RFC-1144.)\fP
1009 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
1010 packet type, and compression information are printed out.
1011 The packet type is printed first.
1012 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
1013 No further link information is printed for \fIip\fR packets.
1014 For TCP packets, the connection identifier is printed following the type.
1015 If the packet is compressed, its encoded header is printed out.
1016 The special cases are printed out as
1017 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
1018 the sequence number (or sequence number and ack) has changed.
1019 If it is not a special case,
1020 zero or more changes are printed.
1021 A change is indicated by U (urgent pointer), W (window), A (ack),
1022 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1023 or a new value (=n).
1024 Finally, the amount of data in the packet and compressed header length
1027 For example, the following line shows an outbound compressed TCP packet,
1028 with an implicit connection identifier; the ack has changed by 6,
1029 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1030 data and 6 bytes of compressed header:
1033 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1039 Arp/rarp output shows the type of request and its arguments.
1041 format is intended to be self explanatory.
1042 Here is a short sample taken from the start of an `rlogin' from
1043 host \fIrtsg\fP to host \fIcsam\fP:
1047 \f(CWarp who-has csam tell rtsg
1048 arp reply csam is-at CSAM\fR
1052 The first line says that rtsg sent an arp packet asking
1053 for the ethernet address of internet host csam.
1055 replies with its ethernet address (in this example, ethernet addresses
1056 are in caps and internet addresses in lower case).
1058 This would look less redundant if we had done \fItcpdump \-n\fP:
1062 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1063 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1067 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1068 broadcast and the second is point-to-point would be visible:
1072 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1073 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1077 For the first packet this says the ethernet source address is RTSG, the
1078 destination is the ethernet broadcast address, the type field
1079 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1083 \fI(N.B.:The following description assumes familiarity with
1084 the TCP protocol described in RFC-793.
1085 If you are not familiar
1086 with the protocol, neither this description nor \fItcpdump\fP will
1087 be of much use to you.)\fP
1089 The general format of a tcp protocol line is:
1093 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1097 \fISrc\fP and \fIdst\fP are the source and destination IP
1098 addresses and ports.
1099 \fIFlags\fP are some combination of S (SYN),
1100 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
1101 \fIData-seqno\fP describes the portion of sequence space covered
1102 by the data in this packet (see example below).
1103 \fIAck\fP is sequence number of the next data expected the other
1104 direction on this connection.
1105 \fIWindow\fP is the number of bytes of receive buffer space available
1106 the other direction on this connection.
1107 \fIUrg\fP indicates there is `urgent' data in the packet.
1108 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1110 \fISrc, dst\fP and \fIflags\fP are always present.
1112 depend on the contents of the packet's tcp protocol header and
1113 are output only if appropriate.
1115 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1120 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1121 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1122 rtsg.1023 > csam.login: . ack 1 win 4096
1123 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1124 csam.login > rtsg.1023: . ack 2 win 4096
1125 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1126 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1127 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1128 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1132 The first line says that tcp port 1023 on rtsg sent a packet
1135 The \fBS\fP indicates that the \fISYN\fP flag was set.
1136 The packet sequence number was 768512 and it contained no data.
1137 (The notation is `first:last(nbytes)' which means `sequence
1139 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1140 There was no piggy-backed ack, the available receive window was 4096
1141 bytes and there was a max-segment-size option requesting an mss of
1144 Csam replies with a similar packet except it includes a piggy-backed
1146 Rtsg then acks csam's SYN.
1149 The packet contained no data so there is no data sequence number.
1150 Note that the ack sequence
1151 number is a small integer (1).
1152 The first time \fItcpdump\fP sees a
1153 tcp `conversation', it prints the sequence number from the packet.
1154 On subsequent packets of the conversation, the difference between
1155 the current packet's sequence number and this initial sequence number
1157 This means that sequence numbers after the
1158 first can be interpreted
1159 as relative byte positions in the conversation's data stream (with the
1160 first data byte each direction being `1').
1161 `-S' will override this
1162 feature, causing the original sequence numbers to be output.
1164 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1165 in the rtsg \(-> csam side of the conversation).
1166 The PUSH flag is set in the packet.
1167 On the 7th line, csam says it's received data sent by rtsg up to
1168 but not including byte 21.
1169 Most of this data is apparently sitting in the
1170 socket buffer since csam's receive window has gotten 19 bytes smaller.
1171 Csam also sends one byte of data to rtsg in this packet.
1172 On the 8th and 9th lines,
1173 csam sends two bytes of urgent, pushed data to rtsg.
1175 If the snapshot was small enough that \fItcpdump\fP didn't capture
1176 the full TCP header, it interprets as much of the header as it can
1177 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1179 If the header contains a bogus option (one with a length
1180 that's either too small or beyond the end of the header), \fItcpdump\fP
1181 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1182 options (since it's impossible to tell where they start).
1184 length indicates options are present but the IP datagram length is not
1185 long enough for the options to actually be there, \fItcpdump\fP reports
1186 it as ``[\fIbad hdr length\fP]''.
1188 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1190 There are 8 bits in the control bits section of the TCP header:
1192 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1194 Let's assume that we want to watch packets used in establishing
1196 Recall that TCP uses a 3-way handshake protocol
1197 when it initializes a new connection; the connection sequence with
1198 regard to the TCP control bits is
1204 2) Recipient responds with SYN, ACK
1210 Now we're interested in capturing packets that have only the
1211 SYN bit set (Step 1).
1212 Note that we don't want packets from step 2
1213 (SYN-ACK), just a plain initial SYN.
1214 What we need is a correct filter
1215 expression for \fItcpdump\fP.
1217 Recall the structure of a TCP header without options:
1221 -----------------------------------------------------------------
1222 | source port | destination port |
1223 -----------------------------------------------------------------
1225 -----------------------------------------------------------------
1226 | acknowledgment number |
1227 -----------------------------------------------------------------
1228 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1229 -----------------------------------------------------------------
1230 | TCP checksum | urgent pointer |
1231 -----------------------------------------------------------------
1234 A TCP header usually holds 20 octets of data, unless options are
1236 The first line of the graph contains octets 0 - 3, the
1237 second line shows octets 4 - 7 etc.
1239 Starting to count with 0, the relevant TCP control bits are contained
1244 ----------------|---------------|---------------|----------------
1245 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1246 ----------------|---------------|---------------|----------------
1247 | | 13th octet | | |
1250 Let's have a closer look at octet no. 13:
1260 These are the TCP control bits we are interested
1262 We have numbered the bits in this octet from 0 to 7, right to
1263 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1265 Recall that we want to capture packets with only SYN set.
1266 Let's see what happens to octet 13 if a TCP datagram arrives
1267 with the SYN bit set in its header:
1278 control bits section we see that only bit number 1 (SYN) is set.
1280 Assuming that octet number 13 is an 8-bit unsigned integer in
1281 network byte order, the binary value of this octet is
1285 and its decimal representation is
1289 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1292 We're almost done, because now we know that if only SYN is set,
1293 the value of the 13th octet in the TCP header, when interpreted
1294 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1296 This relationship can be expressed as
1302 We can use this expression as the filter for \fItcpdump\fP in order
1303 to watch packets which have only SYN set:
1306 tcpdump -i xl0 tcp[13] == 2
1309 The expression says "let the 13th octet of a TCP datagram have
1310 the decimal value 2", which is exactly what we want.
1312 Now, let's assume that we need to capture SYN packets, but we
1313 don't care if ACK or any other TCP control bit is set at the
1315 Let's see what happens to octet 13 when a TCP datagram
1316 with SYN-ACK set arrives:
1326 Now bits 1 and 4 are set in the 13th octet.
1332 which translates to decimal
1336 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1339 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1340 expression, because that would select only those packets that have
1341 SYN-ACK set, but not those with only SYN set.
1342 Remember that we don't care
1343 if ACK or any other control bit is set as long as SYN is set.
1345 In order to achieve our goal, we need to logically AND the
1346 binary value of octet 13 with some other value to preserve
1348 We know that we want SYN to be set in any case,
1349 so we'll logically AND the value in the 13th octet with
1350 the binary value of a SYN:
1354 00010010 SYN-ACK 00000010 SYN
1355 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1357 = 00000010 = 00000010
1360 We see that this AND operation delivers the same result
1361 regardless whether ACK or another TCP control bit is set.
1362 The decimal representation of the AND value as well as
1363 the result of this operation is 2 (binary 00000010),
1364 so we know that for packets with SYN set the following
1365 relation must hold true:
1367 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1369 This points us to the \fItcpdump\fP filter expression
1372 tcpdump -i xl0 'tcp[13] & 2 == 2'
1375 Note that you should use single quotes or a backslash
1376 in the expression to hide the AND ('&') special character
1382 UDP format is illustrated by this rwho packet:
1386 \f(CWactinide.who > broadcast.who: udp 84\fP
1390 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1391 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1393 The packet contained 84 bytes of user data.
1395 Some UDP services are recognized (from the source or destination
1396 port number) and the higher level protocol information printed.
1397 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1398 RPC calls (RFC-1050) to NFS.
1400 UDP Name Server Requests
1402 \fI(N.B.:The following description assumes familiarity with
1403 the Domain Service protocol described in RFC-1035.
1404 If you are not familiar
1405 with the protocol, the following description will appear to be written
1408 Name server requests are formatted as
1412 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1414 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1418 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1419 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1420 The query id was `3'.
1421 The `+' indicates the \fIrecursion desired\fP flag
1423 The query length was 37 bytes, not including the UDP and
1424 IP protocol headers.
1425 The query operation was the normal one, \fIQuery\fP,
1426 so the op field was omitted.
1427 If the op had been anything else, it would
1428 have been printed between the `3' and the `+'.
1429 Similarly, the qclass was the normal one,
1430 \fIC_IN\fP, and omitted.
1431 Any other qclass would have been printed
1432 immediately after the `A'.
1434 A few anomalies are checked and may result in extra fields enclosed in
1435 square brackets: If a query contains an answer, authority records or
1436 additional records section,
1441 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1442 is the appropriate count.
1443 If any of the response bits are set (AA, RA or rcode) or any of the
1444 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1445 is printed, where \fIx\fP is the hex value of header bytes two and three.
1447 UDP Name Server Responses
1449 Name server responses are formatted as
1453 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1455 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1456 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1460 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1461 with 3 answer records, 3 name server records and 7 additional records.
1462 The first answer record is type A (address) and its data is internet
1463 address 128.32.137.3.
1464 The total size of the response was 273 bytes,
1465 excluding UDP and IP headers.
1466 The op (Query) and response code
1467 (NoError) were omitted, as was the class (C_IN) of the A record.
1469 In the second example, \fIhelios\fP responds to query 2 with a
1470 response code of non-existent domain (NXDomain) with no answers,
1471 one name server and no authority records.
1472 The `*' indicates that
1473 the \fIauthoritative answer\fP bit was set.
1475 answers, no type, class or data were printed.
1477 Other flag characters that might appear are `\-' (recursion available,
1478 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1480 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1483 Note that name server requests and responses tend to be large and the
1484 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1486 Use the \fB\-s\fP flag to increase the snaplen if you
1487 need to seriously investigate name server traffic.
1489 has worked well for me.
1494 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1495 on UDP/137, UDP/138 and TCP/139.
1496 Some primitive decoding of IPX and
1497 NetBEUI SMB data is also done.
1499 By default a fairly minimal decode is done, with a much more detailed
1500 decode done if -v is used.
1501 Be warned that with -v a single SMB packet
1502 may take up a page or more, so only use -v if you really want all the
1505 If you are decoding SMB sessions containing unicode strings then you
1506 may wish to set the environment variable USE_UNICODE to 1.
1508 auto-detect unicode srings would be welcome.
1510 For information on SMB packet formats and what all te fields mean see
1511 www.cifs.org or the pub/samba/specs/ directory on your favourite
1512 samba.org mirror site.
1513 The SMB patches were written by Andrew Tridgell
1517 NFS Requests and Replies
1519 Sun NFS (Network File System) requests and replies are printed as:
1523 \fIsrc.xid > dst.nfs: len op args\fP
1524 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1527 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1528 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1529 sushi.201b > wrl.nfs:
1530 144 lookup fh 9,74/4096.6878 "xcolors"
1531 wrl.nfs > sushi.201b:
1532 reply ok 128 lookup fh 9,74/4134.3150
1537 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1538 to \fIwrl\fP (note that the number following the src host is a
1539 transaction id, \fInot\fP the source port).
1540 The request was 112 bytes,
1541 excluding the UDP and IP headers.
1542 The operation was a \fIreadlink\fP
1543 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1544 (If one is lucky, as in this case, the file handle can be interpreted
1545 as a major,minor device number pair, followed by the inode number and
1547 \fIWrl\fP replies `ok' with the contents of the link.
1549 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1550 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1551 Note that the data printed
1552 depends on the operation type.
1553 The format is intended to be self
1554 explanatory if read in conjunction with
1555 an NFS protocol spec.
1557 If the \-v (verbose) flag is given, additional information is printed.
1563 sushi.1372a > wrl.nfs:
1564 148 read fh 21,11/12.195 8192 bytes @ 24576
1565 wrl.nfs > sushi.1372a:
1566 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1571 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1572 which have been omitted from this example.) In the first line,
1573 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1574 at byte offset 24576.
1575 \fIWrl\fP replies `ok'; the packet shown on the
1576 second line is the first fragment of the reply, and hence is only 1472
1577 bytes long (the other bytes will follow in subsequent fragments, but
1578 these fragments do not have NFS or even UDP headers and so might not be
1579 printed, depending on the filter expression used).
1580 Because the \-v flag
1581 is given, some of the file attributes (which are returned in addition
1582 to the file data) are printed: the file type (``REG'', for regular file),
1583 the file mode (in octal), the uid and gid, and the file size.
1585 If the \-v flag is given more than once, even more details are printed.
1587 Note that NFS requests are very large and much of the detail won't be printed
1588 unless \fIsnaplen\fP is increased.
1589 Try using `\fB\-s 192\fP' to watch
1592 NFS reply packets do not explicitly identify the RPC operation.
1594 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1595 replies using the transaction ID.
1596 If a reply does not closely follow the
1597 corresponding request, it might not be parsable.
1599 AFS Requests and Replies
1601 Transarc AFS (Andrew File System) requests and replies are printed
1607 \fIsrc.sport > dst.dport: rx packet-type\fP
1608 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1609 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1612 elvis.7001 > pike.afsfs:
1613 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1614 new fid 536876964/1/1 ".newsrc"
1615 pike.afsfs > elvis.7001: rx data fs reply rename
1620 In the first line, host elvis sends a RX packet to pike.
1622 a RX data packet to the fs (fileserver) service, and is the start of
1624 The RPC call was a rename, with the old directory file id
1625 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1626 file id of 536876964/1/1 and a new filename of `.newsrc'.
1628 responds with a RPC reply to the rename call (which was successful, because
1629 it was a data packet and not an abort packet).
1631 In general, all AFS RPCs are decoded at least by RPC call name.
1633 AFS RPCs have at least some of the arguments decoded (generally only
1634 the `interesting' arguments, for some definition of interesting).
1636 The format is intended to be self-describing, but it will probably
1637 not be useful to people who are not familiar with the workings of
1640 If the -v (verbose) flag is given twice, acknowledgement packets and
1641 additional header information is printed, such as the the RX call ID,
1642 call number, sequence number, serial number, and the RX packet flags.
1644 If the -v flag is given twice, additional information is printed,
1645 such as the the RX call ID, serial number, and the RX packet flags.
1646 The MTU negotiation information is also printed from RX ack packets.
1648 If the -v flag is given three times, the security index and service id
1651 Error codes are printed for abort packets, with the exception of Ubik
1652 beacon packets (because abort packets are used to signify a yes vote
1653 for the Ubik protocol).
1655 Note that AFS requests are very large and many of the arguments won't
1656 be printed unless \fIsnaplen\fP is increased.
1657 Try using `\fB-s 256\fP'
1658 to watch AFS traffic.
1660 AFS reply packets do not explicitly identify the RPC operation.
1662 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1663 replies using the call number and service ID.
1664 If a reply does not closely
1666 corresponding request, it might not be parsable.
1669 KIP Appletalk (DDP in UDP)
1671 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1672 and dumped as DDP packets (i.e., all the UDP header information is
1676 is used to translate appletalk net and node numbers to names.
1677 Lines in this file have the form
1689 The first two lines give the names of appletalk networks.
1691 line gives the name of a particular host (a host is distinguished
1692 from a net by the 3rd octet in the number \-
1693 a net number \fImust\fP have two octets and a host number \fImust\fP
1694 have three octets.) The number and name should be separated by
1695 whitespace (blanks or tabs).
1698 file may contain blank lines or comment lines (lines starting with
1701 Appletalk addresses are printed in the form
1707 \f(CW144.1.209.2 > icsd-net.112.220
1708 office.2 > icsd-net.112.220
1709 jssmag.149.235 > icsd-net.2\fR
1715 doesn't exist or doesn't contain an entry for some appletalk
1716 host/net number, addresses are printed in numeric form.)
1717 In the first example, NBP (DDP port 2) on net 144.1 node 209
1718 is sending to whatever is listening on port 220 of net icsd node 112.
1719 The second line is the same except the full name of the source node
1720 is known (`office').
1721 The third line is a send from port 235 on
1722 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1723 the broadcast address (255) is indicated by a net name with no host
1724 number \- for this reason it's a good idea to keep node names and
1725 net names distinct in /etc/atalk.names).
1727 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1728 packets have their contents interpreted.
1729 Other protocols just dump
1730 the protocol name (or number if no name is registered for the
1731 protocol) and packet size.
1733 \fBNBP packets\fP are formatted like the following examples:
1737 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1738 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1739 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1743 The first line is a name lookup request for laserwriters sent by net icsd host
1744 112 and broadcast on net jssmag.
1745 The nbp id for the lookup is 190.
1746 The second line shows a reply for this request (note that it has the
1747 same id) from host jssmag.209 saying that it has a laserwriter
1748 resource named "RM1140" registered on port 250.
1750 another reply to the same request saying host techpit has laserwriter
1751 "techpit" registered on port 186.
1753 \fBATP packet\fP formatting is demonstrated by the following example:
1757 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1758 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1759 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1760 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1761 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1762 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1763 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1764 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1765 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1766 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1767 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1768 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1769 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1770 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1774 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1775 up to 8 packets (the `<0-7>').
1776 The hex number at the end of the line
1777 is the value of the `userdata' field in the request.
1779 Helios responds with 8 512-byte packets.
1780 The `:digit' following the
1781 transaction id gives the packet sequence number in the transaction
1782 and the number in parens is the amount of data in the packet,
1783 excluding the atp header.
1784 The `*' on packet 7 indicates that the
1787 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1789 resends them then jssmag.209 releases the transaction.
1791 jssmag.209 initiates the next request.
1792 The `*' on the request
1793 indicates that XO (`exactly once') was \fInot\fP set.
1798 Fragmented Internet datagrams are printed as
1802 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1803 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1807 (The first form indicates there are more fragments.
1809 indicates this is the last fragment.)
1811 \fIId\fP is the fragment id.
1812 \fISize\fP is the fragment
1813 size (in bytes) excluding the IP header.
1814 \fIOffset\fP is this
1815 fragment's offset (in bytes) in the original datagram.
1817 The fragment information is output for each fragment.
1819 fragment contains the higher level protocol header and the frag
1820 info is printed after the protocol info.
1822 after the first contain no higher level protocol header and the
1823 frag info is printed after the source and destination addresses.
1824 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1825 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1829 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1830 arizona > rtsg: (frag 595a:204@328)
1831 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1835 There are a couple of things to note here: First, addresses in the
1836 2nd line don't include port numbers.
1837 This is because the TCP
1838 protocol information is all in the first fragment and we have no idea
1839 what the port or sequence numbers are when we print the later fragments.
1840 Second, the tcp sequence information in the first line is printed as if there
1841 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1842 the first frag and 204 in the second).
1843 If you are looking for holes
1844 in the sequence space or trying to match up acks
1845 with packets, this can fool you.
1847 A packet with the IP \fIdon't fragment\fP flag is marked with a
1848 trailing \fB(DF)\fP.
1852 By default, all output lines are preceded by a timestamp.
1854 is the current clock time in the form
1860 and is as accurate as the kernel's clock.
1861 The timestamp reflects the time the kernel first saw the packet.
1863 is made to account for the time lag between when the
1864 ethernet interface removed the packet from the wire and when the kernel
1865 serviced the `new packet' interrupt.
1867 traffic(1C), nit(4P), bpf(4), pcap(3)
1869 The original authors are:
1873 Steven McCanne, all of the
1874 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1876 It is currently being maintained by tcpdump.org.
1878 The current version is available via http:
1881 .I http://www.tcpdump.org/
1884 The original distribution is available via anonymous ftp:
1887 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1890 IPv6/IPsec support is added by WIDE/KAME project.
1891 This program uses Eric Young's SSLeay library, under specific configuration.
1893 Please send problems, bugs, questions, desirable enhancements, etc. to:
1896 tcpdump-workers@tcpdump.org
1899 Please send source code contributions, etc. to:
1905 NIT doesn't let you watch your own outbound traffic, BPF will.
1906 We recommend that you use the latter.
1908 On Linux systems with 2.0[.x] kernels:
1910 packets on the loopback device will be seen twice;
1912 packet filtering cannot be done in the kernel, so that all packets must
1913 be copied from the kernel in order to be filtered in user mode;
1915 all of a packet, not just the part that's within the snapshot length,
1916 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1917 asked to copy only part of a packet to userland, will not report the
1918 true length of the packet; this would cause most IP packets to get an
1922 capturing on some PPP devices won't work correctly.
1924 We recommend that you upgrade to a 2.2 or later kernel.
1926 Some attempt should be made to reassemble IP fragments or, at least
1927 to compute the right length for the higher level protocol.
1929 Name server inverse queries are not dumped correctly: the (empty)
1930 question section is printed rather than real query in the answer
1932 Some believe that inverse queries are themselves a bug and
1933 prefer to fix the program generating them rather than \fItcpdump\fP.
1935 A packet trace that crosses a daylight savings time change will give
1936 skewed time stamps (the time change is ignored).
1938 Filter expressions that manipulate FDDI or Token Ring headers assume
1939 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1941 This is true for IP, ARP, and DECNET Phase IV, but is not true
1942 for protocols such as ISO CLNS.
1943 Therefore, the filter may inadvertently
1944 accept certain packets that do not properly match the filter expression.
1946 Filter expressions on fields other than those that manipulate Token Ring
1947 headers will not correctly handle source-routed Token Ring packets.
1950 should chase header chain, but at this moment it does not.
1951 .BR "ip6 protochain"
1952 is supplied for this behavior.
1954 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1955 does not work against IPv6 packets.
1956 It only looks at IPv4 packets.
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