1 .\" @(#) $Header: /tcpdump/master/tcpdump/Attic/tcpdump.1,v 1.82 2000-07-25 06:09:32 guy Exp $ (LBL)
3 .\" Copyright (c) 1987, 1988, 1989, 1990, 1991, 1992, 1994, 1995, 1996, 1997
4 .\" The Regents of the University of California. All rights reserved.
5 .\" All rights reserved.
7 .\" Redistribution and use in source and binary forms, with or without
8 .\" modification, are permitted provided that: (1) source code distributions
9 .\" retain the above copyright notice and this paragraph in its entirety, (2)
10 .\" distributions including binary code include the above copyright notice and
11 .\" this paragraph in its entirety in the documentation or other materials
12 .\" provided with the distribution, and (3) all advertising materials mentioning
13 .\" features or use of this software display the following acknowledgement:
14 .\" ``This product includes software developed by the University of California,
15 .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
16 .\" the University nor the names of its contributors may be used to endorse
17 .\" or promote products derived from this software without specific prior
18 .\" written permission.
19 .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
20 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
21 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
79 \fITcpdump\fP prints out the headers of packets on a network interface
80 that match the boolean \fIexpression\fP.
82 .B Under SunOS with nit or bpf:
85 you must have read access to
89 .B Under Solaris with dlpi:
90 You must have read access to the network pseudo device, e.g.
92 .B Under HP-UX with dlpi:
93 You must be root or it must be installed setuid to root.
94 .B Under IRIX with snoop:
95 You must be root or it must be installed setuid to root.
97 You must be root or it must be installed setuid to root.
98 .B Under Ultrix and Digital UNIX:
99 Once the super-user has enabled promiscuous-mode operation using
104 You must have read access to
109 Attempt to convert network and broadcast addresses to names.
112 Exit after receiving \fIcount\fP packets.
115 Dump the compiled packet-matching code in a human readable form to
116 standard output and stop.
119 Dump packet-matching code as a
124 Dump packet-matching code as decimal numbers (preceded with a count).
127 Print the link-level header on each dump line.
130 Use \fIalgo:secret\fP for decrypting IPsec ESP packets. Algorithms may be
135 \fBcast128-cbc\fP, or
137 The default is \fBdes-cbc\fP.
138 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
139 with cryptography enabled.
140 \fIsecret\fP the ascii text for ESP secret key.
141 We cannot take arbitrary binary value at this moment.
142 The option assumes RFC2406 ESP, not RFC1827 ESP.
143 The option is only for debugging purposes, and
144 the use of this option with truly `secret' key is discouraged.
145 By presenting IPsec secret key onto command line
146 you make it visible to others, via
151 Print `foreign' internet addresses numerically rather than symbolically
152 (this option is intended to get around serious brain damage in
153 Sun's yp server \(em usually it hangs forever translating non-local
157 Use \fIfile\fP as input for the filter expression.
158 An additional expression given on the command line is ignored.
161 Listen on \fIinterface\fP.
162 If unspecified, \fItcpdump\fP searches the system interface list for the
163 lowest numbered, configured up interface (excluding loopback).
164 Ties are broken by choosing the earliest match.
167 Make stdout line buffered. Useful if you want to see the data
168 while capturing it. E.g.,
170 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
171 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
174 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
177 Don't print domain name qualification of host names. E.g.,
178 if you give this flag then \fItcpdump\fP will print ``nic''
179 instead of ``nic.ddn.mil''.
182 Load SMI MIB module definitions from file \fImodule\fR. This option
183 can be used several times to load several MIB modules into \fItcpdump\fP.
186 Do not run the packet-matching code optimizer. This is useful only
187 if you suspect a bug in the optimizer.
190 \fIDon't\fP put the interface
191 into promiscuous mode. Note that the interface might be in promiscuous
192 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
193 `ether host {local-hw-addr} or ether broadcast'.
196 Quick (quiet?) output. Print less protocol information so output
200 Read packets from \fIfile\fR (which was created with the -w option).
201 Standard input is used if \fIfile\fR is ``-''.
204 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
205 default of 68 (with SunOS's NIT, the minimum is actually 96).
206 68 bytes is adequate for IP, ICMP, TCP
207 and UDP but may truncate protocol information from name server and NFS
208 packets (see below). Packets truncated because of a limited snapshot
209 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
210 is the name of the protocol level at which the truncation has occurred.
211 Note that taking larger snapshots both increases
212 the amount of time it takes to process packets and, effectively,
213 decreases the amount of packet buffering. This may cause packets to be
214 lost. You should limit \fIsnaplen\fP to the smallest number that will
215 capture the protocol information you're interested in. Setting
216 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
219 Force packets selected by "\fIexpression\fP" to be interpreted the
220 specified \fItype\fR. Currently known types are
221 \fBcnfp\fR (Cisco NetFlow protocol),
222 \fBrpc\fR (Remote Procedure Call),
223 \fBrtp\fR (Real-Time Applications protocol),
224 \fBrtcp\fR (Real-Time Applications control protocol),
225 \fBsnmp\fR (Simple Network Management Protocol),
226 \fBvat\fR (Visual Audio Tool),
228 \fBwb\fR (distributed White Board).
231 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
232 If specified, \fItcpdump\fP will not print replay prevention field.
233 Since there is no protocol version field in ESP/AH specification,
234 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
237 Print absolute, rather than relative, TCP sequence numbers.
240 \fIDon't\fP print a timestamp on each dump line.
243 Print an unformatted timestamp on each dump line.
246 (Slightly more) verbose output. For example, the time to live
247 and type of service information in an IP packet is printed.
250 Even more verbose output. For example, additional fields are
251 printed from NFS reply packets.
254 Even more verbose output. For example,
255 telnet \fBSB\fP ... \fBSE\fP options
256 are printed in full. With
258 telnet options are printed in hex as well.
261 Write the raw packets to \fIfile\fR rather than parsing and printing
262 them out. They can later be printed with the \-r option.
263 Standard output is used if \fIfile\fR is ``-''.
266 Print each packet (minus its link level header) in hex.
267 The smaller of the entire packet or
269 bytes will be printed.
272 When printing hex, print ascii too. Thus if
274 is also set, the packet is printed in hex/ascii.
275 This is very handy for analysing new protocols.
278 is not also set, some parts of some packets may be printed
280 .IP "\fI expression\fP"
282 selects which packets will be dumped. If no \fIexpression\fP
283 is given, all packets on the net will be dumped. Otherwise,
284 only packets for which \fIexpression\fP is `true' will be dumped.
286 The \fIexpression\fP consists of one or more
288 Primitives usually consist of an
290 (name or number) preceded by one or more qualifiers. There are three
291 different kinds of qualifier:
293 qualifiers say what kind of thing the id name or number refers to.
299 E.g., `host foo', `net 128.3', `port 20'. If there is no type
304 qualifiers specify a particular transfer direction to and/or from
306 Possible directions are
313 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
314 there is no dir qualifier,
317 For `null' link layers (i.e. point to point protocols such as slip) the
321 qualifiers can be used to specify a desired direction.
323 qualifiers restrict the match to a particular protocol. Possible
342 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
343 no proto qualifier, all protocols consistent with the type are
344 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
345 (except the latter is not legal syntax), `net bar' means `(ip or
346 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
348 [`fddi' is actually an alias for `ether'; the parser treats them
349 identically as meaning ``the data link level used on the specified
350 network interface.'' FDDI headers contain Ethernet-like source
351 and destination addresses, and often contain Ethernet-like packet
352 types, so you can filter on these FDDI fields just as with the
353 analogous Ethernet fields. FDDI headers also contain other fields,
354 but you cannot name them explicitly in a filter expression.
356 Similarly, `tr' is an alias for `ether'; the previous paragraph's
357 statements about FDDI headers also apply to Token Ring headers.]
359 In addition to the above, there are some special `primitive' keywords
360 that don't follow the pattern:
365 and arithmetic expressions. All of these are described below.
367 More complex filter expressions are built up by using the words
372 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
373 To save typing, identical qualifier lists can be omitted. E.g.,
374 `tcp dst port ftp or ftp-data or domain' is exactly the same as
375 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
377 Allowable primitives are:
378 .IP "\fBdst host \fIhost\fR"
379 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
380 which may be either an address or a name.
381 .IP "\fBsrc host \fIhost\fR"
382 True if the IPv4/v6 source field of the packet is \fIhost\fP.
383 .IP "\fBhost \fIhost\fP
384 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
385 Any of the above host expressions can be prepended with the keywords,
386 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
389 \fBip host \fIhost\fR
392 which is equivalent to:
395 \fBether proto \fI\\ip\fB and host \fIhost\fR
398 If \fIhost\fR is a name with multiple IP addresses, each address will
399 be checked for a match.
400 .IP "\fBether dst \fIehost\fP
401 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
402 may be either a name from /etc/ethers or a number (see
405 .IP "\fBether src \fIehost\fP
406 True if the ethernet source address is \fIehost\fP.
407 .IP "\fBether host \fIehost\fP
408 True if either the ethernet source or destination address is \fIehost\fP.
409 .IP "\fBgateway\fP \fIhost\fP
410 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
411 source or destination address was \fIhost\fP but neither the IP source
412 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
413 must be found in both /etc/hosts and /etc/ethers. (An equivalent
417 \fBether host \fIehost \fBand not host \fIhost\fR
420 which can be used with either names or numbers for \fIhost / ehost\fP.)
421 This syntax does not work in IPv6-enabled configuration at this moment.
422 .IP "\fBdst net \fInet\fR"
423 True if the IPv4/v6 destination address of the packet has a network
424 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
425 or a network number (see \fInetworks(4)\fP for details).
426 .IP "\fBsrc net \fInet\fR"
427 True if the IPv4/v6 source address of the packet has a network
429 .IP "\fBnet \fInet\fR"
430 True if either the IPv4/v6 source or destination address of the packet has a network
432 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
433 True if the IP address matches \fInet\fR with the specific netmask.
434 May be qualified with \fBsrc\fR or \fBdst\fR.
435 Note that this syntax is not valid for IPv6 \fInet\fR.
436 .IP "\fBnet \fInet\fR/\fIlen\fR"
437 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
438 May be qualified with \fBsrc\fR or \fBdst\fR.
439 .IP "\fBdst port \fIport\fR"
440 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
441 destination port value of \fIport\fP.
442 The \fIport\fP can be a number or a name used in /etc/services (see
446 If a name is used, both the port
447 number and protocol are checked. If a number or ambiguous name is used,
448 only the port number is checked (e.g., \fBdst port 513\fR will print both
449 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
450 both tcp/domain and udp/domain traffic).
451 .IP "\fBsrc port \fIport\fR"
452 True if the packet has a source port value of \fIport\fP.
453 .IP "\fBport \fIport\fR"
454 True if either the source or destination port of the packet is \fIport\fP.
455 Any of the above port expressions can be prepended with the keywords,
456 \fBtcp\fP or \fBudp\fP, as in:
459 \fBtcp src port \fIport\fR
462 which matches only tcp packets whose source port is \fIport\fP.
463 .IP "\fBless \fIlength\fR"
464 True if the packet has a length less than or equal to \fIlength\fP.
465 This is equivalent to:
468 \fBlen <= \fIlength\fP.
471 .IP "\fBgreater \fIlength\fR"
472 True if the packet has a length greater than or equal to \fIlength\fP.
473 This is equivalent to:
476 \fBlen >= \fIlength\fP.
479 .IP "\fBip proto \fIprotocol\fR"
480 True if the packet is an ip packet (see
482 of protocol type \fIprotocol\fP.
483 \fIProtocol\fP can be a number or one of the names
484 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
485 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
486 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
487 Note that this primitive does not chase protocol header chain.
488 .IP "\fBip6 proto \fIprotocol\fR"
489 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
490 Note that this primitive does not chase protocol header chain.
491 .IP "\fBip6 protochain \fIprotocol\fR"
492 True if the packet is IPv6 packet,
493 and contains protocol header with type \fIprotocol\fR
494 in its protocol header chain.
498 \fBip6 protochain 6\fR
501 matches any IPv6 packet with TCP protocol header in the protocol header chain.
502 The packet may contain, for example,
503 authentication header, routing header, or hop-by-hop option header,
504 between IPv6 header and TCP header.
505 The BPF code emitted by this primitive is complex and
506 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
507 so this can be somewhat slow.
508 .IP "\fBip protochain \fIprotocol\fR"
509 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
510 .IP "\fBether broadcast\fR"
511 True if the packet is an ethernet broadcast packet. The \fIether\fP
513 .IP "\fBip broadcast\fR"
514 True if the packet is an IP broadcast packet. It checks for both
515 the all-zeroes and all-ones broadcast conventions, and looks up
516 the local subnet mask.
517 .IP "\fBether multicast\fR"
518 True if the packet is an ethernet multicast packet. The \fIether\fP
520 This is shorthand for `\fBether[0] & 1 != 0\fP'.
521 .IP "\fBip multicast\fR"
522 True if the packet is an IP multicast packet.
523 .IP "\fBip6 multicast\fR"
524 True if the packet is an IPv6 multicast packet.
525 .IP "\fBether proto \fIprotocol\fR"
526 True if the packet is of ether type \fIprotocol\fR.
527 \fIProtocol\fP can be a number or a name like
528 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
529 Note these identifiers are also keywords
530 and must be escaped via backslash (\\).
531 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
532 protocol identification comes from the 802.2 Logical Link Control
533 (LLC) header, which is usually layered on top of the FDDI header.
534 \fITcpdump\fP assumes, when filtering on the protocol identifier,
535 that all FDDI packets include an LLC header, and that the LLC header
536 is in so-called SNAP format. The same applies to Token Ring.]
537 .IP "\fBdecnet src \fIhost\fR"
538 True if the DECNET source address is
540 which may be an address of the form ``10.123'', or a DECNET host
541 name. [DECNET host name support is only available on Ultrix systems
542 that are configured to run DECNET.]
543 .IP "\fBdecnet dst \fIhost\fR"
544 True if the DECNET destination address is
546 .IP "\fBdecnet host \fIhost\fR"
547 True if either the DECNET source or destination address is
549 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
553 \fBether proto \fIp\fR
556 where \fIp\fR is one of the above protocols.
557 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
561 \fBether proto \fIp\fR
564 where \fIp\fR is one of the above protocols.
566 \fItcpdump\fP does not currently know how to parse these protocols.
567 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
571 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
574 where \fIp\fR is one of the above protocols.
575 .IP "\fIexpr relop expr\fR"
576 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
577 and \fIexpr\fR is an arithmetic expression composed of integer constants
578 (expressed in standard C syntax), the normal binary operators
579 [+, -, *, /, &, |], a length operator, and special packet data accessors.
581 data inside the packet, use the following syntax:
584 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
587 \fIProto\fR is one of \fBether, fddi, tr,
588 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
589 indicates the protocol layer for the index operation.
590 Note that \fItcp, udp\fR and other upper-layer protocol types only
591 apply to IPv4, not IPv6 (this will be fixed in the future).
592 The byte offset, relative to the indicated protocol layer, is
594 \fISize\fR is optional and indicates the number of bytes in the
595 field of interest; it can be either one, two, or four, and defaults to one.
596 The length operator, indicated by the keyword \fBlen\fP, gives the
597 length of the packet.
599 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
600 The expression `\fBip[0] & 0xf != 5\fP'
601 catches all IP packets with options. The expression
602 `\fBip[6:2] & 0x1fff = 0\fP'
603 catches only unfragmented datagrams and frag zero of fragmented datagrams.
604 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
606 For instance, \fBtcp[0]\fP always means the first
607 byte of the TCP \fIheader\fP, and never means the first byte of an
608 intervening fragment.
610 Primitives may be combined using:
612 A parenthesized group of primitives and operators
613 (parentheses are special to the Shell and must be escaped).
615 Negation (`\fB!\fP' or `\fBnot\fP').
617 Concatenation (`\fB&&\fP' or `\fBand\fP').
619 Alternation (`\fB||\fP' or `\fBor\fP').
621 Negation has highest precedence.
622 Alternation and concatenation have equal precedence and associate
623 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
624 are now required for concatenation.
626 If an identifier is given without a keyword, the most recent keyword
631 \fBnot host vs and ace\fR
637 \fBnot host vs and host ace\fR
640 which should not be confused with
643 \fBnot ( host vs or ace )\fR
647 Expression arguments can be passed to \fItcpdump\fP as either a single
648 argument or as multiple arguments, whichever is more convenient.
649 Generally, if the expression contains Shell metacharacters, it is
650 easier to pass it as a single, quoted argument.
651 Multiple arguments are concatenated with spaces before being parsed.
654 To print all packets arriving at or departing from \fIsundown\fP:
657 \fBtcpdump host sundown\fP
661 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
664 \fBtcpdump host helios and \\( hot or ace \\)\fP
668 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
671 \fBtcpdump ip host ace and not helios\fP
675 To print all traffic between local hosts and hosts at Berkeley:
679 tcpdump net ucb-ether
683 To print all ftp traffic through internet gateway \fIsnup\fP:
684 (note that the expression is quoted to prevent the shell from
685 (mis-)interpreting the parentheses):
689 tcpdump 'gateway snup and (port ftp or ftp-data)'
693 To print traffic neither sourced from nor destined for local hosts
694 (if you gateway to one other net, this stuff should never make it
695 onto your local net).
699 tcpdump ip and not net \fIlocalnet\fP
703 To print the start and end packets (the SYN and FIN packets) of each
704 TCP conversation that involves a non-local host.
708 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
712 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
716 tcpdump 'gateway snup and ip[2:2] > 576'
720 To print IP broadcast or multicast packets that were
722 sent via ethernet broadcast or multicast:
726 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
730 To print all ICMP packets that are not echo requests/replies (i.e., not
735 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
740 The output of \fItcpdump\fP is protocol dependent. The following
741 gives a brief description and examples of most of the formats.
749 If the '-e' option is given, the link level header is printed out.
750 On ethernets, the source and destination addresses, protocol,
751 and packet length are printed.
753 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
754 the `frame control' field, the source and destination addresses,
755 and the packet length. (The `frame control' field governs the
756 interpretation of the rest of the packet. Normal packets (such
757 as those containing IP datagrams) are `async' packets, with a priority
758 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
759 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
760 the LLC header is printed if it is \fInot\fR an ISO datagram or a
761 so-called SNAP packet.
763 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
764 the `access control' and `frame control' fields, the source and
765 destination addresses, and the packet length. As on FDDI networks,
766 packets are assumed to contain an LLC packet. Regardless of whether
767 the '-e' option is specified or not, the source routing information is
768 printed for source-routed packets.
770 \fI(N.B.: The following description assumes familiarity with
771 the SLIP compression algorithm described in RFC-1144.)\fP
773 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
774 packet type, and compression information are printed out.
775 The packet type is printed first.
776 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
777 No further link information is printed for \fIip\fR packets.
778 For TCP packets, the connection identifier is printed following the type.
779 If the packet is compressed, its encoded header is printed out.
780 The special cases are printed out as
781 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
782 the sequence number (or sequence number and ack) has changed.
783 If it is not a special case,
784 zero or more changes are printed.
785 A change is indicated by U (urgent pointer), W (window), A (ack),
786 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
788 Finally, the amount of data in the packet and compressed header length
791 For example, the following line shows an outbound compressed TCP packet,
792 with an implicit connection identifier; the ack has changed by 6,
793 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
794 data and 6 bytes of compressed header:
797 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
803 Arp/rarp output shows the type of request and its arguments. The
804 format is intended to be self explanatory.
805 Here is a short sample taken from the start of an `rlogin' from
806 host \fIrtsg\fP to host \fIcsam\fP:
810 \f(CWarp who-has csam tell rtsg
811 arp reply csam is-at CSAM\fR
815 The first line says that rtsg sent an arp packet asking
816 for the ethernet address of internet host csam. Csam
817 replies with its ethernet address (in this example, ethernet addresses
818 are in caps and internet addresses in lower case).
820 This would look less redundant if we had done \fBtcpdump \-n\fP:
824 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
825 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
829 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
830 broadcast and the second is point-to-point would be visible:
834 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
835 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
839 For the first packet this says the ethernet source address is RTSG, the
840 destination is the ethernet broadcast address, the type field
841 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
845 \fI(N.B.:The following description assumes familiarity with
846 the TCP protocol described in RFC-793. If you are not familiar
847 with the protocol, neither this description nor \fItcpdump\fP will
848 be of much use to you.)\fP
850 The general format of a tcp protocol line is:
854 \fIsrc > dst: flags data-seqno ack window urgent options\fP
858 \fISrc\fP and \fIdst\fP are the source and destination IP
859 addresses and ports. \fIFlags\fP are some combination of S (SYN),
860 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
861 \fIData-seqno\fP describes the portion of sequence space covered
862 by the data in this packet (see example below).
863 \fIAck\fP is sequence number of the next data expected the other
864 direction on this connection.
865 \fIWindow\fP is the number of bytes of receive buffer space available
866 the other direction on this connection.
867 \fIUrg\fP indicates there is `urgent' data in the packet.
868 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
870 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
871 depend on the contents of the packet's tcp protocol header and
872 are output only if appropriate.
874 Here is the opening portion of an rlogin from host \fIrtsg\fP to
879 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
880 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
881 rtsg.1023 > csam.login: . ack 1 win 4096
882 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
883 csam.login > rtsg.1023: . ack 2 win 4096
884 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
885 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
886 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
887 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
891 The first line says that tcp port 1023 on rtsg sent a packet
893 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
894 The packet sequence number was 768512 and it contained no data.
895 (The notation is `first:last(nbytes)' which means `sequence
897 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
898 There was no piggy-backed ack, the available receive window was 4096
899 bytes and there was a max-segment-size option requesting an mss of
902 Csam replies with a similar packet except it includes a piggy-backed
903 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
905 The packet contained no data so there is no data sequence number.
906 Note that the ack sequence
907 number is a small integer (1). The first time \fItcpdump\fP sees a
908 tcp `conversation', it prints the sequence number from the packet.
909 On subsequent packets of the conversation, the difference between
910 the current packet's sequence number and this initial sequence number
911 is printed. This means that sequence numbers after the
912 first can be interpreted
913 as relative byte positions in the conversation's data stream (with the
914 first data byte each direction being `1'). `-S' will override this
915 feature, causing the original sequence numbers to be output.
917 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
918 in the rtsg \(-> csam side of the conversation).
919 The PUSH flag is set in the packet.
920 On the 7th line, csam says it's received data sent by rtsg up to
921 but not including byte 21. Most of this data is apparently sitting in the
922 socket buffer since csam's receive window has gotten 19 bytes smaller.
923 Csam also sends one byte of data to rtsg in this packet.
924 On the 8th and 9th lines,
925 csam sends two bytes of urgent, pushed data to rtsg.
927 If the snapshot was small enough that \fItcpdump\fP didn't capture
928 the full TCP header, it interprets as much of the header as it can
929 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
930 be interpreted. If the header contains a bogus option (one with a length
931 that's either too small or beyond the end of the header), \fItcpdump\fP
932 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
933 options (since it's impossible to tell where they start). If the header
934 length indicates options are present but the IP datagram length is not
935 long enough for the options to actually be there, \fItcpdump\fP reports
936 it as ``[\fIbad hdr length\fP]''.
938 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
940 There are 6 bits in the control bits section of the TCP header:
942 .I URG | ACK | PSH | RST | SYN | FIN
944 Let's assume that we want to watch packets used in establishing
945 a TCP connection. Recall that TCP uses a 3-way handshake protocol
946 when it initializes a new connection; the connection sequence with
947 regard to the TCP control bits is
953 2) Recipient responds with SYN, ACK
959 Now we're interested in capturing packets that have only the
960 SYN bit set (Step 1). Note that we don't want packets from step 2
961 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
962 expression for \fItcpdump\fP.
964 Recall the structure of a TCP header without options:
968 -----------------------------------------------------------------
969 | source port | destination port |
970 -----------------------------------------------------------------
972 -----------------------------------------------------------------
973 | acknowledgment number |
974 -----------------------------------------------------------------
975 | HL | reserved |U|A|P|R|S|F| window size |
976 -----------------------------------------------------------------
977 | TCP checksum | urgent pointer |
978 -----------------------------------------------------------------
981 A TCP header usually holds 20 octets of data, unless options are
982 present. The fist line of the graph contains octets 0 - 3, the
983 second line shows octets 4 - 7 etc.
985 Starting to count with 0, the relevant TCP control bits are contained
990 ----------------|---------------|---------------|----------------
991 | HL | reserved |U|A|P|R|S|F| window size |
992 ----------------|---------------|---------------|----------------
996 Let's have a closer look at octet no. 13:
1006 We see that this octet contains 2 bytes from the reserved field.
1007 According to RFC 793 this field is reserved for future use and must
1008 be 0. The remaining 6 bits are the TCP control bits we are interested
1009 in. We have numbered the bits in this octet from 0 to 7, right to
1010 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1012 Recall that we want to capture packets with only SYN set.
1013 Let's see what happens to octet 13 if a TCP datagram arrives
1014 with the SYN bit set in its header:
1024 We already mentioned that bits number 7 and 6 belong to the
1025 reserved field, so they must must be 0. Looking at the
1026 control bits section we see that only bit number 1 (SYN) is set.
1028 Assuming that octet number 13 is an 8-bit unsigned integer in
1029 network byte order, the binary value of this octet is
1033 and its decimal representation is
1037 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1040 We're almost done, because now we know that if only SYN is set,
1041 the value of the 13th octet in the TCP header, when interpreted
1042 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1044 This relationship can be expressed as
1050 We can use this expression as the filter for \fItcpdump\fP in order
1051 to watch packets which have only SYN set:
1054 tcpdump -i xl0 tcp[13] == 2
1057 The expression says "let the 13th octet of a TCP datagram have
1058 the decimal value 2", which is exactly what we want.
1060 Now, let's assume that we need to capture SYN packets, but we
1061 don't care if ACK or any other TCP control bit is set at the
1062 same time. Let's see what happens to octet 13 when a TCP datagram
1063 with SYN-ACK set arrives:
1073 Now bits 1 and 4 are set in the 13th octet. The binary value of
1078 which translates to decimal
1082 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1085 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1086 expression, because that would select only those packets that have
1087 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1088 if ACK or any other control bit is set as long as SYN is set.
1090 In order to achieve our goal, we need to logically AND the
1091 binary value of octet 13 with some other value to preserve
1092 the SYN bit. We know that we want SYN to be set in any case,
1093 so we'll logically AND the value in the 13th octet with
1094 the binary value of a SYN:
1098 00010010 SYN-ACK 00000010 SYN
1099 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1101 = 00000010 = 00000010
1104 We see that this AND operation delivers the same result
1105 regardless whether ACK or another TCP control bit is set.
1106 The decimal representation of the AND value as well as
1107 the result of this operation is 2 (binary 00000010),
1108 so we know that for packets with SYN set the following
1109 relation must hold true:
1111 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1113 This points us to the \fItcpdump\fP filter expression
1116 tcpdump -i xl0 'tcp[13] & 2 == 2'
1119 Note that you should use single quotes or a backslash
1120 in the expression to hide the AND ('&') special character
1126 UDP format is illustrated by this rwho packet:
1130 \f(CWactinide.who > broadcast.who: udp 84\fP
1134 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1135 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1136 broadcast address. The packet contained 84 bytes of user data.
1138 Some UDP services are recognized (from the source or destination
1139 port number) and the higher level protocol information printed.
1140 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1141 RPC calls (RFC-1050) to NFS.
1143 UDP Name Server Requests
1145 \fI(N.B.:The following description assumes familiarity with
1146 the Domain Service protocol described in RFC-1035. If you are not familiar
1147 with the protocol, the following description will appear to be written
1150 Name server requests are formatted as
1154 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1156 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1160 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1161 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1162 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1163 was set. The query length was 37 bytes, not including the UDP and
1164 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1165 so the op field was omitted. If the op had been anything else, it would
1166 have been printed between the `3' and the `+'.
1167 Similarly, the qclass was the normal one,
1168 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1169 immediately after the `A'.
1171 A few anomalies are checked and may result in extra fields enclosed in
1172 square brackets: If a query contains an answer, name server or
1178 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1179 is the appropriate count.
1180 If any of the response bits are set (AA, RA or rcode) or any of the
1181 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1182 is printed, where \fIx\fP is the hex value of header bytes two and three.
1184 UDP Name Server Responses
1186 Name server responses are formatted as
1190 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1192 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1193 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1197 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1198 with 3 answer records, 3 name server records and 7 authority records.
1199 The first answer record is type A (address) and its data is internet
1200 address 128.32.137.3. The total size of the response was 273 bytes,
1201 excluding UDP and IP headers. The op (Query) and response code
1202 (NoError) were omitted, as was the class (C_IN) of the A record.
1204 In the second example, \fIhelios\fP responds to query 2 with a
1205 response code of non-existent domain (NXDomain) with no answers,
1206 one name server and no authority records. The `*' indicates that
1207 the \fIauthoritative answer\fP bit was set. Since there were no
1208 answers, no type, class or data were printed.
1210 Other flag characters that might appear are `\-' (recursion available,
1211 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1212 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1215 Note that name server requests and responses tend to be large and the
1216 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1217 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1218 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1219 has worked well for me.
1224 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1225 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1226 NetBEUI SMB data is also done.
1228 By default a fairly minimal decode is done, with a much more detailed
1229 decode done if -v is used. Be warned that with -v a single SMB packet
1230 may take up a page or more, so only use -v if you really want all the
1233 If you are decoding SMB sessions containing unicode strings then you
1234 may wish to set the environment variable USE_UNICODE to 1. A patch to
1235 auto-detect unicode srings would be welcome.
1237 For information on SMB packet formats and what all te fields mean see
1238 www.cifs.org or the pub/samba/specs/ directory on your favourite
1239 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1243 NFS Requests and Replies
1245 Sun NFS (Network File System) requests and replies are printed as:
1249 \fIsrc.xid > dst.nfs: len op args\fP
1250 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1253 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1254 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1255 sushi.201b > wrl.nfs:
1256 144 lookup fh 9,74/4096.6878 "xcolors"
1257 wrl.nfs > sushi.201b:
1258 reply ok 128 lookup fh 9,74/4134.3150
1263 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1264 to \fIwrl\fP (note that the number following the src host is a
1265 transaction id, \fInot\fP the source port). The request was 112 bytes,
1266 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1267 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1268 (If one is lucky, as in this case, the file handle can be interpreted
1269 as a major,minor device number pair, followed by the inode number and
1271 \fIWrl\fP replies `ok' with the contents of the link.
1273 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1274 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1275 depends on the operation type. The format is intended to be self
1276 explanatory if read in conjunction with
1277 an NFS protocol spec.
1279 If the \-v (verbose) flag is given, additional information is printed.
1285 sushi.1372a > wrl.nfs:
1286 148 read fh 21,11/12.195 8192 bytes @ 24576
1287 wrl.nfs > sushi.1372a:
1288 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1293 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1294 which have been omitted from this example.) In the first line,
1295 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1296 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1297 second line is the first fragment of the reply, and hence is only 1472
1298 bytes long (the other bytes will follow in subsequent fragments, but
1299 these fragments do not have NFS or even UDP headers and so might not be
1300 printed, depending on the filter expression used). Because the \-v flag
1301 is given, some of the file attributes (which are returned in addition
1302 to the file data) are printed: the file type (``REG'', for regular file),
1303 the file mode (in octal), the uid and gid, and the file size.
1305 If the \-v flag is given more than once, even more details are printed.
1307 Note that NFS requests are very large and much of the detail won't be printed
1308 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1311 NFS reply packets do not explicitly identify the RPC operation. Instead,
1312 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1313 replies using the transaction ID. If a reply does not closely follow the
1314 corresponding request, it might not be parsable.
1316 AFS Requests and Replies
1318 Transarc AFS (Andrew File System) requests and replies are printed
1324 \fIsrc.sport > dst.dport: rx packet-type\fP
1325 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1326 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1329 elvis.7001 > pike.afsfs:
1330 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1331 new fid 536876964/1/1 ".newsrc"
1332 pike.afsfs > elvis.7001: rx data fs reply rename
1337 In the first line, host elvis sends a RX packet to pike. This was
1338 a RX data packet to the fs (fileserver) service, and is the start of
1339 an RPC call. The RPC call was a rename, with the old directory file id
1340 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1341 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1342 responds with a RPC reply to the rename call (which was successful, because
1343 it was a data packet and not an abort packet).
1345 In general, all AFS RPCs are decoded at least by RPC call name. Most
1346 AFS RPCs have at least some of the arguments decoded (generally only
1347 the `interesting' arguments, for some definition of interesting).
1349 The format is intended to be self-describing, but it will probably
1350 not be useful to people who are not familiar with the workings of
1353 If the -v (verbose) flag is given, the RX call number and sequence number
1356 If the -v flag is given twice, additional information is printed,
1357 such as the the RX call ID, serial number, and the RX packet flags.
1358 The MTU negotiation information is also printed from RX ack packets.
1360 If the -v flag is given three times, the security index and service id
1363 Error codes are printed for abort packets, with the exception of Ubik
1364 beacon packets (because abort packets are used to signify a yes vote
1365 for the Ubik protocol).
1367 Note that AFS requests are very large and many of the arguments won't
1368 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1369 to watch AFS traffic.
1371 AFS reply packets do not explicitly identify the RPC operation. Instead,
1372 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1373 replies using the call number and service ID. If a reply does not closely
1375 corresponding request, it might not be parsable.
1378 KIP Appletalk (DDP in UDP)
1380 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1381 and dumped as DDP packets (i.e., all the UDP header information is
1382 discarded). The file
1384 is used to translate appletalk net and node numbers to names.
1385 Lines in this file have the form
1397 The first two lines give the names of appletalk networks. The third
1398 line gives the name of a particular host (a host is distinguished
1399 from a net by the 3rd octet in the number \-
1400 a net number \fImust\fP have two octets and a host number \fImust\fP
1401 have three octets.) The number and name should be separated by
1402 whitespace (blanks or tabs).
1405 file may contain blank lines or comment lines (lines starting with
1408 Appletalk addresses are printed in the form
1414 \f(CW144.1.209.2 > icsd-net.112.220
1415 office.2 > icsd-net.112.220
1416 jssmag.149.235 > icsd-net.2\fR
1422 doesn't exist or doesn't contain an entry for some appletalk
1423 host/net number, addresses are printed in numeric form.)
1424 In the first example, NBP (DDP port 2) on net 144.1 node 209
1425 is sending to whatever is listening on port 220 of net icsd node 112.
1426 The second line is the same except the full name of the source node
1427 is known (`office'). The third line is a send from port 235 on
1428 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1429 the broadcast address (255) is indicated by a net name with no host
1430 number \- for this reason it's a good idea to keep node names and
1431 net names distinct in /etc/atalk.names).
1433 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1434 packets have their contents interpreted. Other protocols just dump
1435 the protocol name (or number if no name is registered for the
1436 protocol) and packet size.
1438 \fBNBP packets\fP are formatted like the following examples:
1442 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1443 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1444 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1448 The first line is a name lookup request for laserwriters sent by net icsd host
1449 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1450 The second line shows a reply for this request (note that it has the
1451 same id) from host jssmag.209 saying that it has a laserwriter
1452 resource named "RM1140" registered on port 250. The third line is
1453 another reply to the same request saying host techpit has laserwriter
1454 "techpit" registered on port 186.
1456 \fBATP packet\fP formatting is demonstrated by the following example:
1460 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1461 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1462 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1463 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1464 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1465 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1466 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1467 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1468 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1469 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1470 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1471 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1472 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1473 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1477 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1478 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1479 is the value of the `userdata' field in the request.
1481 Helios responds with 8 512-byte packets. The `:digit' following the
1482 transaction id gives the packet sequence number in the transaction
1483 and the number in parens is the amount of data in the packet,
1484 excluding the atp header. The `*' on packet 7 indicates that the
1487 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1488 resends them then jssmag.209 releases the transaction. Finally,
1489 jssmag.209 initiates the next request. The `*' on the request
1490 indicates that XO (`exactly once') was \fInot\fP set.
1495 Fragmented Internet datagrams are printed as
1499 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1500 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1504 (The first form indicates there are more fragments. The second
1505 indicates this is the last fragment.)
1507 \fIId\fP is the fragment id. \fISize\fP is the fragment
1508 size (in bytes) excluding the IP header. \fIOffset\fP is this
1509 fragment's offset (in bytes) in the original datagram.
1511 The fragment information is output for each fragment. The first
1512 fragment contains the higher level protocol header and the frag
1513 info is printed after the protocol info. Fragments
1514 after the first contain no higher level protocol header and the
1515 frag info is printed after the source and destination addresses.
1516 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1517 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1521 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1522 arizona > rtsg: (frag 595a:204@328)
1523 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1527 There are a couple of things to note here: First, addresses in the
1528 2nd line don't include port numbers. This is because the TCP
1529 protocol information is all in the first fragment and we have no idea
1530 what the port or sequence numbers are when we print the later fragments.
1531 Second, the tcp sequence information in the first line is printed as if there
1532 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1533 the first frag and 204 in the second). If you are looking for holes
1534 in the sequence space or trying to match up acks
1535 with packets, this can fool you.
1537 A packet with the IP \fIdon't fragment\fP flag is marked with a
1538 trailing \fB(DF)\fP.
1542 By default, all output lines are preceded by a timestamp. The timestamp
1543 is the current clock time in the form
1549 and is as accurate as the kernel's clock.
1550 The timestamp reflects the time the kernel first saw the packet. No attempt
1551 is made to account for the time lag between when the
1552 ethernet interface removed the packet from the wire and when the kernel
1553 serviced the `new packet' interrupt.
1555 traffic(1C), nit(4P), bpf(4), pcap(3)
1557 The original authors are:
1561 Steven McCanne, all of the
1562 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1564 It is currently being maintained by tcpdump.org.
1566 The current version is available via http:
1569 .I http://www.tcpdump.org/
1572 The original distribution is available via anonymous ftp:
1575 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1578 IPv6/IPsec support is added by WIDE/KAME project.
1579 This program uses Eric Young's SSLeay library, under specific configuration.
1581 Please send problems, bugs, questions, desirable enhancements, etc. to:
1584 tcpdump-workers@tcpdump.org
1587 Please send source code contributions, etc. to:
1593 NIT doesn't let you watch your own outbound traffic, BPF will.
1594 We recommend that you use the latter.
1596 Some attempt should be made to reassemble IP fragments or, at least
1597 to compute the right length for the higher level protocol.
1599 Name server inverse queries are not dumped correctly: the (empty)
1600 question section is printed rather than real query in the answer
1601 section. Some believe that inverse queries are themselves a bug and
1602 prefer to fix the program generating them rather than \fItcpdump\fP.
1604 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1606 Even if we were inclined to do anything to promote the use of
1607 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1608 networks so we'd would have no way of testing this code.
1610 A packet trace that crosses a daylight savings time change will give
1611 skewed time stamps (the time change is ignored).
1613 Filter expressions that manipulate FDDI or Token Ring headers assume
1614 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1615 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1616 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1617 accept certain packets that do not properly match the filter expression.
1619 Filter expressions on fields other than those that manipulate Token Ring
1620 headers will not correctly handle source-routed Token Ring packets.
1623 should chase header chain, but at this moment it does not.
1624 .BR "ip6 protochain"
1625 is supplied for this behavior.
1627 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1628 does not work against IPv6 packets.
1629 It only looks at IPv4 packets.
projects / tcpdump / blob