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23 .TH TCPDUMP 1 "3 January 2001"
24 .SH NAME
25 tcpdump \- dump traffic on a network
26 .SH SYNOPSIS
27 .na
28 .B tcpdump
29 [
30 .B \-adeflnNOpqRStuvxX
31 ] [
32 .B \-c
33 .I count
34 ]
35 .br
36 .ti +8
37 [
38 .B \-C
39 .I file_size
40 ] [
41 .B \-F
42 .I file
43 ]
44 .br
45 .ti +8
46 [
47 .B \-i
48 .I interface
49 ]
50 [
51 .B \-m
52 .I module
53 ]
54 [
55 .B \-r
56 .I file
57 ]
58 .br
59 .ti +8
60 [
61 .B \-s
62 .I snaplen
63 ]
64 [
65 .B \-T
66 .I type
67 ]
68 [
69 .B \-w
70 .I file
71 ]
72 .br
73 .ti +8
74 [
75 .B \-E
76 .I algo:secret
77 ]
78 [
79 .I expression
80 ]
81 .br
82 .ad
83 .SH DESCRIPTION
84 .LP
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
87 .B \-w
88 flag, which causes it to save the packet data to a file for later
89 analysis, and/or with the
90 .B \-b
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
93 match
94 .I expression
95 will be processed by
96 .IR tcpdump .
97 .LP
98 .I Tcpdump
99 will, if not run with the
100 .B \-c
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
104 .BR kill (1)
105 command); if run with the
106 .B \-c
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.
109 .LP
110 When
111 .I tcpdump
112 finishes capturing packets, it will report counts of:
113 .IP
114 packets ``received by filter'' (the meaning of this depends on the OS on
115 which you're running
116 .IR tcpdump ,
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
121 were processed by
122 .IR tcpdump );
123 .IP
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
126 in the OS on which
127 .I tcpdump
128 is running, if the OS reports that information to applications; if not,
129 it will be reported as 0).
130 .LP
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.
135 .LP
136 Reading packets from a network interface may require that you have
137 special privileges:
138 .TP
139 .B Under SunOS 3.x or 4.x with NIT or BPF:
140 You must have read access to
141 .I /dev/nit
142 or
143 .IR /dev/bpf* .
144 .TP
145 .B Under Solaris with DLPI:
146 You must have read/write access to the network pseudo device, e.g.
147 .IR /dev/le .
148 On at least some versions of Solaris, however, this is not sufficient to
149 allow
150 .I tcpdump
151 to capture in promiscuous mode; on those versions of Solaris, you must
152 be root, or
153 .I tcpdump
154 must be installed setuid to root, in order to capture in promiscuous
155 mode.
156 .TP
157 .B Under HP-UX with DLPI:
158 You must be root or
159 .I tcpdump
160 must be installed setuid to root.
161 .TP
162 .B Under IRIX with snoop:
163 You must be root or
164 .I tcpdump
165 must be installed setuid to root.
166 .TP
167 .B Under Linux:
168 You must be root or
169 .I tcpdump
170 must be installed setuid to root.
171 .TP
172 .B Under Ultrix and Digital UNIX:
173 Once the super-user has enabled promiscuous-mode operation using
174 .IR pfconfig (8),
175 any user may capture network traffic with
176 .IR tcpdump .
177 .TP
178 .B Under BSD:
179 You must have read access to
180 .IR /dev/bpf* .
181 .LP
182 Reading a saved packet file doesn't require special privileges.
183 .SH OPTIONS
184 .TP
185 .B \-a
186 Attempt to convert network and broadcast addresses to names.
187 .TP
188 .B \-c
189 Exit after receiving \fIcount\fP packets.
190 .TP
191 .B \-C
192 Before writing a raw packet to a savefile, check whether the file is
193 currently larger than \fIfile_size\fP and, if so, close the current
194 savefile and open a new one. Savefiles after the first savefile will
195 have the name specified with the
196 .B \-w
197 flag, with a number after it, starting at 2 and continuing upward.
198 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
199 not 1,048,576 bytes).
200 .TP
201 .B \-d
202 Dump the compiled packet-matching code in a human readable form to
203 standard output and stop.
204 .TP
205 .B \-dd
206 Dump packet-matching code as a
207 .B C
208 program fragment.
209 .TP
210 .B \-ddd
211 Dump packet-matching code as decimal numbers (preceded with a count).
212 .TP
213 .B \-e
214 Print the link-level header on each dump line.
215 .TP
216 .B \-E
217 Use \fIalgo:secret\fP for decrypting IPsec ESP packets.
218 Algorithms may be
219 \fBdes-cbc\fP,
220 \fB3des-cbc\fP,
221 \fBblowfish-cbc\fP,
222 \fBrc3-cbc\fP,
223 \fBcast128-cbc\fP, or
224 \fBnone\fP.
225 The default is \fBdes-cbc\fP.
226 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
227 with cryptography enabled.
228 \fIsecret\fP the ascii text for ESP secret key.
229 We cannot take arbitrary binary value at this moment.
230 The option assumes RFC2406 ESP, not RFC1827 ESP.
231 The option is only for debugging purposes, and
232 the use of this option with truly `secret' key is discouraged.
233 By presenting IPsec secret key onto command line
234 you make it visible to others, via
235 .IR ps (1)
236 and other occasions.
237 .TP
238 .B \-f
239 Print `foreign' internet addresses numerically rather than symbolically
240 (this option is intended to get around serious brain damage in
241 Sun's yp server \(em usually it hangs forever translating non-local
242 internet numbers).
243 .TP
244 .B \-F
245 Use \fIfile\fP as input for the filter expression.
246 An additional expression given on the command line is ignored.
247 .TP
248 .B \-i
249 Listen on \fIinterface\fP.
250 If unspecified, \fItcpdump\fP searches the system interface list for the
251 lowest numbered, configured up interface (excluding loopback).
252 Ties are broken by choosing the earliest match.
253 .IP
254 On Linux systems with 2.2 or later kernels, an
255 .I interface
256 argument of ``any'' can be used to capture packets from all interfaces.
257 Note that captures on the ``any'' device will not be done in promiscuous
258 mode.
259 .TP
260 .B \-l
261 Make stdout line buffered.
262 Useful if you want to see the data
263 while capturing it.
264 E.g.,
265 .br
266 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
267 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
268 .TP
269 .B \-m
270 Load SMI MIB module definitions from file \fImodule\fR.
271 This option
272 can be used several times to load several MIB modules into \fItcpdump\fP.
273 .TP
274 .B \-n
275 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
276 .TP
277 .B \-N
278 Don't print domain name qualification of host names.
279 E.g.,
280 if you give this flag then \fItcpdump\fP will print ``nic''
281 instead of ``nic.ddn.mil''.
282 .TP
283 .B \-O
284 Do not run the packet-matching code optimizer.
285 This is useful only
286 if you suspect a bug in the optimizer.
287 .TP
288 .B \-p
289 \fIDon't\fP put the interface
290 into promiscuous mode.
291 Note that the interface might be in promiscuous
292 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
293 `ether host {local-hw-addr} or ether broadcast'.
294 .TP
295 .B \-q
296 Quick (quiet?) output.
297 Print less protocol information so output
298 lines are shorter.
299 .TP
300 .B \-R
301 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
302 If specified, \fItcpdump\fP will not print replay prevention field.
303 Since there is no protocol version field in ESP/AH specification,
304 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
305 .TP
306 .B \-r
307 Read packets from \fIfile\fR (which was created with the -w option).
308 Standard input is used if \fIfile\fR is ``-''.
309 .TP
310 .B \-S
311 Print absolute, rather than relative, TCP sequence numbers.
312 .TP
313 .B \-s
314 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
315 default of 68 (with SunOS's NIT, the minimum is actually 96).
316 68 bytes is adequate for IP, ICMP, TCP
317 and UDP but may truncate protocol information from name server and NFS
318 packets (see below).
319 Packets truncated because of a limited snapshot
320 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
321 is the name of the protocol level at which the truncation has occurred.
322 Note that taking larger snapshots both increases
323 the amount of time it takes to process packets and, effectively,
324 decreases the amount of packet buffering.
325 This may cause packets to be
326 lost.
327 You should limit \fIsnaplen\fP to the smallest number that will
328 capture the protocol information you're interested in.
329 Setting
330 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
331 .TP
332 .B \-T
333 Force packets selected by "\fIexpression\fP" to be interpreted the
334 specified \fItype\fR.
335 Currently known types are
336 \fBcnfp\fR (Cisco NetFlow protocol),
337 \fBrpc\fR (Remote Procedure Call),
338 \fBrtp\fR (Real-Time Applications protocol),
339 \fBrtcp\fR (Real-Time Applications control protocol),
340 \fBsnmp\fR (Simple Network Management Protocol),
341 \fBvat\fR (Visual Audio Tool),
342 and
343 \fBwb\fR (distributed White Board).
344 .TP
345 .B \-t
346 \fIDon't\fP print a timestamp on each dump line.
347 .TP
348 .B \-tt
349 Print an unformatted timestamp on each dump line.
350 .TP
351 .B \-ttt
352 Print a delta (in micro-seconds) between current and previous line
353 on each dump line.
354 .TP
355 .B \-tttt
356 Print a timestamp in default format proceeded by date on each dump line.
357 .B \-u
358 Print undecoded NFS handles.
359 .TP
360 .B \-v
361 (Slightly more) verbose output.
362 For example, the time to live,
363 identification, total length and options in an IP packet are printed.
364 Also enables additional packet integrity checks such as verifying the
365 IP and ICMP header checksum.
366 .TP
367 .B \-vv
368 Even more verbose output.
369 For example, additional fields are
370 printed from NFS reply packets, and SMB packets are fully decoded.
371 .TP
372 .B \-vvv
373 Even more verbose output.
374 For example,
375 telnet \fBSB\fP ... \fBSE\fP options
376 are printed in full.
377 With
378 .B \-X
379 telnet options are printed in hex as well.
380 .TP
381 .B \-w
382 Write the raw packets to \fIfile\fR rather than parsing and printing
383 them out.
384 They can later be printed with the \-r option.
385 Standard output is used if \fIfile\fR is ``-''.
386 .TP
387 .B \-x
388 Print each packet (minus its link level header) in hex.
389 The smaller of the entire packet or
390 .I snaplen
391 bytes will be printed.
392 .TP
393 .B \-X
394 When printing hex, print ascii too.
395 Thus if
396 .B \-x
397 is also set, the packet is printed in hex/ascii.
398 This is very handy for analysing new protocols.
399 Even if
400 .B \-x
401 is not also set, some parts of some packets may be printed
402 in hex/ascii.
403 .IP "\fI expression\fP"
404 .RS
405 selects which packets will be dumped.
406 If no \fIexpression\fP
407 is given, all packets on the net will be dumped.
408 Otherwise,
409 only packets for which \fIexpression\fP is `true' will be dumped.
410 .LP
411 The \fIexpression\fP consists of one or more
412 .I primitives.
413 Primitives usually consist of an
414 .I id
415 (name or number) preceded by one or more qualifiers.
416 There are three
417 different kinds of qualifier:
418 .IP \fItype\fP
419 qualifiers say what kind of thing the id name or number refers to.
420 Possible types are
421 .BR host ,
422 .B net
423 and
424 .BR port .
425 E.g., `host foo', `net 128.3', `port 20'.
426 If there is no type
427 qualifier,
428 .B host
429 is assumed.
430 .IP \fIdir\fP
431 qualifiers specify a particular transfer direction to and/or from
432 .IR id .
433 Possible directions are
434 .BR src ,
435 .BR dst ,
436 .B "src or dst"
437 and
438 .B "src and"
439 .BR dst .
440 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.
441 If
442 there is no dir qualifier,
443 .B "src or dst"
444 is assumed.
445 For `null' link layers (i.e. point to point protocols such as slip) the
446 .B inbound
447 and
448 .B outbound
449 qualifiers can be used to specify a desired direction.
450 .IP \fIproto\fP
451 qualifiers restrict the match to a particular protocol.
452 Possible
453 protos are:
454 .BR ether ,
455 .BR fddi ,
456 .BR tr ,
457 .BR ip ,
458 .BR ip6 ,
459 .BR arp ,
460 .BR rarp ,
461 .BR decnet ,
462 .B tcp
463 and
464 .BR udp .
465 E.g., `ether src foo', `arp net 128.3', `tcp port 21'.
466 If there is
467 no proto qualifier, all protocols consistent with the type are
468 assumed.
469 E.g., `src foo' means `(ip or arp or rarp) src foo'
470 (except the latter is not legal syntax), `net bar' means `(ip or
471 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
472 .LP
473 [`fddi' is actually an alias for `ether'; the parser treats them
474 identically as meaning ``the data link level used on the specified
475 network interface.'' FDDI headers contain Ethernet-like source
476 and destination addresses, and often contain Ethernet-like packet
477 types, so you can filter on these FDDI fields just as with the
478 analogous Ethernet fields.
479 FDDI headers also contain other fields,
480 but you cannot name them explicitly in a filter expression.
481 .LP
482 Similarly, `tr' is an alias for `ether'; the previous paragraph's
483 statements about FDDI headers also apply to Token Ring headers.]
484 .LP
485 In addition to the above, there are some special `primitive' keywords
486 that don't follow the pattern:
487 .BR gateway ,
488 .BR broadcast ,
489 .BR less ,
490 .B greater
491 and arithmetic expressions.
492 All of these are described below.
493 .LP
494 More complex filter expressions are built up by using the words
495 .BR and ,
496 .B or
497 and
498 .B not
499 to combine primitives.
500 E.g., `host foo and not port ftp and not port ftp-data'.
501 To save typing, identical qualifier lists can be omitted.
502 E.g.,
503 `tcp dst port ftp or ftp-data or domain' is exactly the same as
504 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
505 .LP
506 Allowable primitives are:
507 .IP "\fBdst host \fIhost\fR"
508 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
509 which may be either an address or a name.
510 .IP "\fBsrc host \fIhost\fR"
511 True if the IPv4/v6 source field of the packet is \fIhost\fP.
512 .IP "\fBhost \fIhost\fP
513 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
514 Any of the above host expressions can be prepended with the keywords,
515 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
516 .in +.5i
517 .nf
518 \fBip host \fIhost\fR
519 .fi
520 .in -.5i
521 which is equivalent to:
522 .in +.5i
523 .nf
524 \fBether proto \fI\\ip\fB and host \fIhost\fR
525 .fi
526 .in -.5i
527 If \fIhost\fR is a name with multiple IP addresses, each address will
528 be checked for a match.
529 .IP "\fBether dst \fIehost\fP
530 True if the ethernet destination address is \fIehost\fP.
531 \fIEhost\fP
532 may be either a name from /etc/ethers or a number (see
533 .IR ethers (3N)
534 for numeric format).
535 .IP "\fBether src \fIehost\fP
536 True if the ethernet source address is \fIehost\fP.
537 .IP "\fBether host \fIehost\fP
538 True if either the ethernet source or destination address is \fIehost\fP.
539 .IP "\fBgateway\fP \fIhost\fP
540 True if the packet used \fIhost\fP as a gateway.
541 I.e., the ethernet
542 source or destination address was \fIhost\fP but neither the IP source
543 nor the IP destination was \fIhost\fP.
544 \fIHost\fP must be a name and
545 must be found both by the machine's host-name-to-IP-address resolution
546 mechanisms (host name file, DNS, NIS, etc.) and by the machine's
547 host-name-to-Ethernet-address resolution mechanism (/etc/ethers, etc.).
548 (An equivalent expression is
549 .in +.5i
550 .nf
551 \fBether host \fIehost \fBand not host \fIhost\fR
552 .fi
553 .in -.5i
554 which can be used with either names or numbers for \fIhost / ehost\fP.)
555 This syntax does not work in IPv6-enabled configuration at this moment.
556 .IP "\fBdst net \fInet\fR"
557 True if the IPv4/v6 destination address of the packet has a network
558 number of \fInet\fP.
559 \fINet\fP may be either a name from /etc/networks
560 or a network number (see \fInetworks(4)\fP for details).
561 .IP "\fBsrc net \fInet\fR"
562 True if the IPv4/v6 source address of the packet has a network
563 number of \fInet\fP.
564 .IP "\fBnet \fInet\fR"
565 True if either the IPv4/v6 source or destination address of the packet has a network
566 number of \fInet\fP.
567 .IP "\fBnet \fInet\fR \fBmask \fInetmask\fR"
568 True if the IP address matches \fInet\fR with the specific \fInetmask\fR.
569 May be qualified with \fBsrc\fR or \fBdst\fR.
570 Note that this syntax is not valid for IPv6 \fInet\fR.
571 .IP "\fBnet \fInet\fR/\fIlen\fR"
572 True if the IPv4/v6 address matches \fInet\fR with a netmask \fIlen\fR
573 bits wide.
574 May be qualified with \fBsrc\fR or \fBdst\fR.
575 .IP "\fBdst port \fIport\fR"
576 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
577 destination port value of \fIport\fP.
578 The \fIport\fP can be a number or a name used in /etc/services (see
579 .IR tcp (4P)
580 and
581 .IR udp (4P)).
582 If a name is used, both the port
583 number and protocol are checked.
584 If a number or ambiguous name is used,
585 only the port number is checked (e.g., \fBdst port 513\fR will print both
586 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
587 both tcp/domain and udp/domain traffic).
588 .IP "\fBsrc port \fIport\fR"
589 True if the packet has a source port value of \fIport\fP.
590 .IP "\fBport \fIport\fR"
591 True if either the source or destination port of the packet is \fIport\fP.
592 Any of the above port expressions can be prepended with the keywords,
593 \fBtcp\fP or \fBudp\fP, as in:
594 .in +.5i
595 .nf
596 \fBtcp src port \fIport\fR
597 .fi
598 .in -.5i
599 which matches only tcp packets whose source port is \fIport\fP.
600 .IP "\fBless \fIlength\fR"
601 True if the packet has a length less than or equal to \fIlength\fP.
602 This is equivalent to:
603 .in +.5i
604 .nf
605 \fBlen <= \fIlength\fP.
606 .fi
607 .in -.5i
608 .IP "\fBgreater \fIlength\fR"
609 True if the packet has a length greater than or equal to \fIlength\fP.
610 This is equivalent to:
611 .in +.5i
612 .nf
613 \fBlen >= \fIlength\fP.
614 .fi
615 .in -.5i
616 .IP "\fBip proto \fIprotocol\fR"
617 True if the packet is an IP packet (see
618 .IR ip (4P))
619 of protocol type \fIprotocol\fP.
620 \fIProtocol\fP can be a number or one of the names
621 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
622 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
623 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
624 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
625 Note that this primitive does not chase the protocol header chain.
626 .IP "\fBip6 proto \fIprotocol\fR"
627 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
628 Note that this primitive does not chase the protocol header chain.
629 .IP "\fBip6 protochain \fIprotocol\fR"
630 True if the packet is IPv6 packet,
631 and contains protocol header with type \fIprotocol\fR
632 in its protocol header chain.
633 For example,
634 .in +.5i
635 .nf
636 \fBip6 protochain 6\fR
637 .fi
638 .in -.5i
639 matches any IPv6 packet with TCP protocol header in the protocol header chain.
640 The packet may contain, for example,
641 authentication header, routing header, or hop-by-hop option header,
642 between IPv6 header and TCP header.
643 The BPF code emitted by this primitive is complex and
644 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
645 so this can be somewhat slow.
646 .IP "\fBip protochain \fIprotocol\fR"
647 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
648 .IP "\fBether broadcast\fR"
649 True if the packet is an ethernet broadcast packet.
650 The \fIether\fP
651 keyword is optional.
652 .IP "\fBip broadcast\fR"
653 True if the packet is an IP broadcast packet.
654 It checks for both
655 the all-zeroes and all-ones broadcast conventions, and looks up
656 the local subnet mask.
657 .IP "\fBether multicast\fR"
658 True if the packet is an ethernet multicast packet.
659 The \fIether\fP
660 keyword is optional.
661 This is shorthand for `\fBether[0] & 1 != 0\fP'.
662 .IP "\fBip multicast\fR"
663 True if the packet is an IP multicast packet.
664 .IP "\fBip6 multicast\fR"
665 True if the packet is an IPv6 multicast packet.
666 .IP "\fBether proto \fIprotocol\fR"
667 True if the packet is of ether type \fIprotocol\fR.
668 \fIProtocol\fP can be a number or one of the names
669 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
670 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
671 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
672 Note these identifiers are also keywords
673 and must be escaped via backslash (\\).
674 .IP
675 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR') and Token Ring
676 (e.g., `\fBtr protocol arp\fR'), for most of those protocols, the
677 protocol identification comes from the 802.2 Logical Link Control (LLC)
678 header, which is usually layered on top of the FDDI or Token Ring
679 header.
680 .IP
681 When filtering for most protocol identifiers on FDDI or Token Ring,
682 \fItcpdump\fR checks only the protocol ID field of an LLC header in
683 so-called SNAP format with an Organizational Unit Identifier (OUI) of
684 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
685 is in SNAP format with an OUI of 0x000000.
686 .IP
687 The exceptions are \fIiso\fP, for which it checks the DSAP (Destination
688 Service Access Point) and SSAP (Source Service Access Point) fields of
689 the LLC header, \fIstp\fP and \fInetbeui\fP, where it checks the DSAP of
690 the LLC header, and \fIatalk\fP, where it checks for a SNAP-format
691 packet with an OUI of 0x080007 and the Appletalk etype.
692 .IP
693 In the case of Ethernet, \fItcpdump\fR checks the Ethernet type field
694 for most of those protocols; the exceptions are \fIiso\fP, \fIsap\fP,
695 and \fInetbeui\fP, for which it checks for an 802.3 frame and then
696 checks the LLC header as it does for FDDI and Token Ring, \fIatalk\fP,
697 where it checks both for the Appletalk etype in an Ethernet frame and
698 for a SNAP-format packet as it does for FDDI and Token Ring, \fIaarp\fP,
699 where it checks for the Appletalk ARP etype in either an Ethernet frame
700 or an 802.2 SNAP frame with an OUI of 0x000000, and \fIipx\fP, where it
701 checks for the IPX etype in an Ethernet frame, the IPX DSAP in the LLC
702 header, the 802.3 with no LLC header encapsulation of IPX, and the IPX
703 etype in a SNAP frame.]
704 .IP "\fBdecnet src \fIhost\fR"
705 True if the DECNET source address is
706 .IR host ,
707 which may be an address of the form ``10.123'', or a DECNET host
708 name.
709 [DECNET host name support is only available on Ultrix systems
710 that are configured to run DECNET.]
711 .IP "\fBdecnet dst \fIhost\fR"
712 True if the DECNET destination address is
713 .IR host .
714 .IP "\fBdecnet host \fIhost\fR"
715 True if either the DECNET source or destination address is
716 .IR host .
717 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
718 Abbreviations for:
719 .in +.5i
720 .nf
721 \fBether proto \fIp\fR
722 .fi
723 .in -.5i
724 where \fIp\fR is one of the above protocols.
725 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
726 Abbreviations for:
727 .in +.5i
728 .nf
729 \fBether proto \fIp\fR
730 .fi
731 .in -.5i
732 where \fIp\fR is one of the above protocols.
733 Note that
734 \fItcpdump\fP does not currently know how to parse these protocols.
735 .IP "\fBvlan \fI[vlan_id]\fR"
736 True if the packet is an IEEE 802.1Q VLAN packet.
737 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
738 \fIvlan_id\fR.
739 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
740 changes the decoding offsets for the remainder of \fIexpression\fR
741 on the assumption that the packet is a VLAN packet.
742 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
743 Abbreviations for:
744 .in +.5i
745 .nf
746 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
747 .fi
748 .in -.5i
749 where \fIp\fR is one of the above protocols.
750 .IP "\fBiso proto \fIprotocol\fR"
751 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
752 \fIProtocol\fP can be a number or one of the names
753 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
754 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
755 Abbreviations for:
756 .in +.5i
757 .nf
758 \fBiso proto \fIp\fR
759 .fi
760 .in -.5i
761 where \fIp\fR is one of the above protocols.
762 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
763 .IP "\fIexpr relop expr\fR"
764 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
765 and \fIexpr\fR is an arithmetic expression composed of integer constants
766 (expressed in standard C syntax), the normal binary operators
767 [+, -, *, /, &, |], a length operator, and special packet data accessors.
768 To access
769 data inside the packet, use the following syntax:
770 .in +.5i
771 .nf
772 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
773 .fi
774 .in -.5i
775 \fIProto\fR is one of \fBether, fddi, tr,
776 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
777 indicates the protocol layer for the index operation.
778 Note that \fItcp, udp\fR and other upper-layer protocol types only
779 apply to IPv4, not IPv6 (this will be fixed in the future).
780 The byte offset, relative to the indicated protocol layer, is
781 given by \fIexpr\fR.
782 \fISize\fR is optional and indicates the number of bytes in the
783 field of interest; it can be either one, two, or four, and defaults to one.
784 The length operator, indicated by the keyword \fBlen\fP, gives the
785 length of the packet.
786
787 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
788 The expression `\fBip[0] & 0xf != 5\fP'
789 catches all IP packets with options.
790 The expression
791 `\fBip[6:2] & 0x1fff = 0\fP'
792 catches only unfragmented datagrams and frag zero of fragmented datagrams.
793 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
794 index operations.
795 For instance, \fBtcp[0]\fP always means the first
796 byte of the TCP \fIheader\fP, and never means the first byte of an
797 intervening fragment.
798
799 Some offsets and field values may be expressed as names rather than
800 as numeric values.
801 The following protocol header field offsets are
802 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
803 code field), and \fBtcpflags\fP (TCP flags field).
804
805 The following ICMP type field values are available: \fBicmp-echoreply\fP,
806 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
807 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
808 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
809 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
810 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
811
812 The following TCP flags field values are available: \fBtcp-fin\fP,
813 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
814 \fBtcp-ack\fP, \fBtcp-urg\fP.
815 .LP
816 Primitives may be combined using:
817 .IP
818 A parenthesized group of primitives and operators
819 (parentheses are special to the Shell and must be escaped).
820 .IP
821 Negation (`\fB!\fP' or `\fBnot\fP').
822 .IP
823 Concatenation (`\fB&&\fP' or `\fBand\fP').
824 .IP
825 Alternation (`\fB||\fP' or `\fBor\fP').
826 .LP
827 Negation has highest precedence.
828 Alternation and concatenation have equal precedence and associate
829 left to right.
830 Note that explicit \fBand\fR tokens, not juxtaposition,
831 are now required for concatenation.
832 .LP
833 If an identifier is given without a keyword, the most recent keyword
834 is assumed.
835 For example,
836 .in +.5i
837 .nf
838 \fBnot host vs and ace\fR
839 .fi
840 .in -.5i
841 is short for
842 .in +.5i
843 .nf
844 \fBnot host vs and host ace\fR
845 .fi
846 .in -.5i
847 which should not be confused with
848 .in +.5i
849 .nf
850 \fBnot ( host vs or ace )\fR
851 .fi
852 .in -.5i
853 .LP
854 Expression arguments can be passed to \fItcpdump\fP as either a single
855 argument or as multiple arguments, whichever is more convenient.
856 Generally, if the expression contains Shell metacharacters, it is
857 easier to pass it as a single, quoted argument.
858 Multiple arguments are concatenated with spaces before being parsed.
859 .SH EXAMPLES
860 .LP
861 To print all packets arriving at or departing from \fIsundown\fP:
862 .RS
863 .nf
864 \fBtcpdump host sundown\fP
865 .fi
866 .RE
867 .LP
868 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
869 .RS
870 .nf
871 \fBtcpdump host helios and \\( hot or ace \\)\fP
872 .fi
873 .RE
874 .LP
875 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
876 .RS
877 .nf
878 \fBtcpdump ip host ace and not helios\fP
879 .fi
880 .RE
881 .LP
882 To print all traffic between local hosts and hosts at Berkeley:
883 .RS
884 .nf
885 .B
886 tcpdump net ucb-ether
887 .fi
888 .RE
889 .LP
890 To print all ftp traffic through internet gateway \fIsnup\fP:
891 (note that the expression is quoted to prevent the shell from
892 (mis-)interpreting the parentheses):
893 .RS
894 .nf
895 .B
896 tcpdump 'gateway snup and (port ftp or ftp-data)'
897 .fi
898 .RE
899 .LP
900 To print traffic neither sourced from nor destined for local hosts
901 (if you gateway to one other net, this stuff should never make it
902 onto your local net).
903 .RS
904 .nf
905 .B
906 tcpdump ip and not net \fIlocalnet\fP
907 .fi
908 .RE
909 .LP
910 To print the start and end packets (the SYN and FIN packets) of each
911 TCP conversation that involves a non-local host.
912 .RS
913 .nf
914 .B
915 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
916 .fi
917 .RE
918 .LP
919 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
920 .RS
921 .nf
922 .B
923 tcpdump 'gateway snup and ip[2:2] > 576'
924 .fi
925 .RE
926 .LP
927 To print IP broadcast or multicast packets that were
928 .I not
929 sent via ethernet broadcast or multicast:
930 .RS
931 .nf
932 .B
933 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
934 .fi
935 .RE
936 .LP
937 To print all ICMP packets that are not echo requests/replies (i.e., not
938 ping packets):
939 .RS
940 .nf
941 .B
942 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
943 .fi
944 .RE
945 .SH OUTPUT FORMAT
946 .LP
947 The output of \fItcpdump\fP is protocol dependent.
948 The following
949 gives a brief description and examples of most of the formats.
950 .de HD
951 .sp 1.5
952 .B
953 ..
954 .HD
955 Link Level Headers
956 .LP
957 If the '-e' option is given, the link level header is printed out.
958 On ethernets, the source and destination addresses, protocol,
959 and packet length are printed.
960 .LP
961 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
962 the `frame control' field, the source and destination addresses,
963 and the packet length.
964 (The `frame control' field governs the
965 interpretation of the rest of the packet.
966 Normal packets (such
967 as those containing IP datagrams) are `async' packets, with a priority
968 value between 0 and 7; for example, `\fBasync4\fR'.
969 Such packets
970 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
971 the LLC header is printed if it is \fInot\fR an ISO datagram or a
972 so-called SNAP packet.
973 .LP
974 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
975 the `access control' and `frame control' fields, the source and
976 destination addresses, and the packet length.
977 As on FDDI networks,
978 packets are assumed to contain an LLC packet.
979 Regardless of whether
980 the '-e' option is specified or not, the source routing information is
981 printed for source-routed packets.
982 .LP
983 \fI(N.B.: The following description assumes familiarity with
984 the SLIP compression algorithm described in RFC-1144.)\fP
985 .LP
986 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
987 packet type, and compression information are printed out.
988 The packet type is printed first.
989 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
990 No further link information is printed for \fIip\fR packets.
991 For TCP packets, the connection identifier is printed following the type.
992 If the packet is compressed, its encoded header is printed out.
993 The special cases are printed out as
994 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
995 the sequence number (or sequence number and ack) has changed.
996 If it is not a special case,
997 zero or more changes are printed.
998 A change is indicated by U (urgent pointer), W (window), A (ack),
999 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1000 or a new value (=n).
1001 Finally, the amount of data in the packet and compressed header length
1002 are printed.
1003 .LP
1004 For example, the following line shows an outbound compressed TCP packet,
1005 with an implicit connection identifier; the ack has changed by 6,
1006 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1007 data and 6 bytes of compressed header:
1008 .RS
1009 .nf
1010 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1011 .fi
1012 .RE
1013 .HD
1014 ARP/RARP Packets
1015 .LP
1016 Arp/rarp output shows the type of request and its arguments.
1017 The
1018 format is intended to be self explanatory.
1019 Here is a short sample taken from the start of an `rlogin' from
1020 host \fIrtsg\fP to host \fIcsam\fP:
1021 .RS
1022 .nf
1023 .sp .5
1024 \f(CWarp who-has csam tell rtsg
1025 arp reply csam is-at CSAM\fR
1026 .sp .5
1027 .fi
1028 .RE
1029 The first line says that rtsg sent an arp packet asking
1030 for the ethernet address of internet host csam.
1031 Csam
1032 replies with its ethernet address (in this example, ethernet addresses
1033 are in caps and internet addresses in lower case).
1034 .LP
1035 This would look less redundant if we had done \fItcpdump \-n\fP:
1036 .RS
1037 .nf
1038 .sp .5
1039 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1040 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1041 .fi
1042 .RE
1043 .LP
1044 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1045 broadcast and the second is point-to-point would be visible:
1046 .RS
1047 .nf
1048 .sp .5
1049 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1050 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1051 .sp .5
1052 .fi
1053 .RE
1054 For the first packet this says the ethernet source address is RTSG, the
1055 destination is the ethernet broadcast address, the type field
1056 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1057 .HD
1058 TCP Packets
1059 .LP
1060 \fI(N.B.:The following description assumes familiarity with
1061 the TCP protocol described in RFC-793.
1062 If you are not familiar
1063 with the protocol, neither this description nor \fItcpdump\fP will
1064 be of much use to you.)\fP
1065 .LP
1066 The general format of a tcp protocol line is:
1067 .RS
1068 .nf
1069 .sp .5
1070 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1071 .sp .5
1072 .fi
1073 .RE
1074 \fISrc\fP and \fIdst\fP are the source and destination IP
1075 addresses and ports.
1076 \fIFlags\fP are some combination of S (SYN),
1077 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
1078 \fIData-seqno\fP describes the portion of sequence space covered
1079 by the data in this packet (see example below).
1080 \fIAck\fP is sequence number of the next data expected the other
1081 direction on this connection.
1082 \fIWindow\fP is the number of bytes of receive buffer space available
1083 the other direction on this connection.
1084 \fIUrg\fP indicates there is `urgent' data in the packet.
1085 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1086 .LP
1087 \fISrc, dst\fP and \fIflags\fP are always present.
1088 The other fields
1089 depend on the contents of the packet's tcp protocol header and
1090 are output only if appropriate.
1091 .LP
1092 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1093 host \fIcsam\fP.
1094 .RS
1095 .nf
1096 .sp .5
1097 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1098 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1099 rtsg.1023 > csam.login: . ack 1 win 4096
1100 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1101 csam.login > rtsg.1023: . ack 2 win 4096
1102 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1103 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1104 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1105 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1106 .sp .5
1107 .fi
1108 .RE
1109 The first line says that tcp port 1023 on rtsg sent a packet
1110 to port \fIlogin\fP
1111 on csam.
1112 The \fBS\fP indicates that the \fISYN\fP flag was set.
1113 The packet sequence number was 768512 and it contained no data.
1114 (The notation is `first:last(nbytes)' which means `sequence
1115 numbers \fIfirst\fP
1116 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1117 There was no piggy-backed ack, the available receive window was 4096
1118 bytes and there was a max-segment-size option requesting an mss of
1119 1024 bytes.
1120 .LP
1121 Csam replies with a similar packet except it includes a piggy-backed
1122 ack for rtsg's SYN.
1123 Rtsg then acks csam's SYN.
1124 The `.' means no
1125 flags were set.
1126 The packet contained no data so there is no data sequence number.
1127 Note that the ack sequence
1128 number is a small integer (1).
1129 The first time \fItcpdump\fP sees a
1130 tcp `conversation', it prints the sequence number from the packet.
1131 On subsequent packets of the conversation, the difference between
1132 the current packet's sequence number and this initial sequence number
1133 is printed.
1134 This means that sequence numbers after the
1135 first can be interpreted
1136 as relative byte positions in the conversation's data stream (with the
1137 first data byte each direction being `1').
1138 `-S' will override this
1139 feature, causing the original sequence numbers to be output.
1140 .LP
1141 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1142 in the rtsg \(-> csam side of the conversation).
1143 The PUSH flag is set in the packet.
1144 On the 7th line, csam says it's received data sent by rtsg up to
1145 but not including byte 21.
1146 Most of this data is apparently sitting in the
1147 socket buffer since csam's receive window has gotten 19 bytes smaller.
1148 Csam also sends one byte of data to rtsg in this packet.
1149 On the 8th and 9th lines,
1150 csam sends two bytes of urgent, pushed data to rtsg.
1151 .LP
1152 If the snapshot was small enough that \fItcpdump\fP didn't capture
1153 the full TCP header, it interprets as much of the header as it can
1154 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1155 be interpreted.
1156 If the header contains a bogus option (one with a length
1157 that's either too small or beyond the end of the header), \fItcpdump\fP
1158 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1159 options (since it's impossible to tell where they start).
1160 If the header
1161 length indicates options are present but the IP datagram length is not
1162 long enough for the options to actually be there, \fItcpdump\fP reports
1163 it as ``[\fIbad hdr length\fP]''.
1164 .HD
1165 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1166 .PP
1167 There are 8 bits in the control bits section of the TCP header:
1168 .IP
1169 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1170 .PP
1171 Let's assume that we want to watch packets used in establishing
1172 a TCP connection.
1173 Recall that TCP uses a 3-way handshake protocol
1174 when it initializes a new connection; the connection sequence with
1175 regard to the TCP control bits is
1176 .PP
1177 .RS
1178 1) Caller sends SYN
1179 .RE
1180 .RS
1181 2) Recipient responds with SYN, ACK
1182 .RE
1183 .RS
1184 3) Caller sends ACK
1185 .RE
1186 .PP
1187 Now we're interested in capturing packets that have only the
1188 SYN bit set (Step 1).
1189 Note that we don't want packets from step 2
1190 (SYN-ACK), just a plain initial SYN.
1191 What we need is a correct filter
1192 expression for \fItcpdump\fP.
1193 .PP
1194 Recall the structure of a TCP header without options:
1195 .PP
1196 .nf
1197 0 15 31
1198 -----------------------------------------------------------------
1199 | source port | destination port |
1200 -----------------------------------------------------------------
1201 | sequence number |
1202 -----------------------------------------------------------------
1203 | acknowledgment number |
1204 -----------------------------------------------------------------
1205 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1206 -----------------------------------------------------------------
1207 | TCP checksum | urgent pointer |
1208 -----------------------------------------------------------------
1209 .fi
1210 .PP
1211 A TCP header usually holds 20 octets of data, unless options are
1212 present.
1213 The first line of the graph contains octets 0 - 3, the
1214 second line shows octets 4 - 7 etc.
1215 .PP
1216 Starting to count with 0, the relevant TCP control bits are contained
1217 in octet 13:
1218 .PP
1219 .nf
1220 0 7| 15| 23| 31
1221 ----------------|---------------|---------------|----------------
1222 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1223 ----------------|---------------|---------------|----------------
1224 | | 13th octet | | |
1225 .fi
1226 .PP
1227 Let's have a closer look at octet no. 13:
1228 .PP
1229 .nf
1230 | |
1231 |---------------|
1232 |C|E|U|A|P|R|S|F|
1233 |---------------|
1234 |7 5 3 0|
1235 .fi
1236 .PP
1237 These are the TCP control bits we are interested
1238 in.
1239 We have numbered the bits in this octet from 0 to 7, right to
1240 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1241 .PP
1242 Recall that we want to capture packets with only SYN set.
1243 Let's see what happens to octet 13 if a TCP datagram arrives
1244 with the SYN bit set in its header:
1245 .PP
1246 .nf
1247 |C|E|U|A|P|R|S|F|
1248 |---------------|
1249 |0 0 0 0 0 0 1 0|
1250 |---------------|
1251 |7 6 5 4 3 2 1 0|
1252 .fi
1253 .PP
1254 Looking at the
1255 control bits section we see that only bit number 1 (SYN) is set.
1256 .PP
1257 Assuming that octet number 13 is an 8-bit unsigned integer in
1258 network byte order, the binary value of this octet is
1259 .IP
1260 00000010
1261 .PP
1262 and its decimal representation is
1263 .PP
1264 .nf
1265 7 6 5 4 3 2 1 0
1266 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1267 .fi
1268 .PP
1269 We're almost done, because now we know that if only SYN is set,
1270 the value of the 13th octet in the TCP header, when interpreted
1271 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1272 .PP
1273 This relationship can be expressed as
1274 .RS
1275 .B
1276 tcp[13] == 2
1277 .RE
1278 .PP
1279 We can use this expression as the filter for \fItcpdump\fP in order
1280 to watch packets which have only SYN set:
1281 .RS
1282 .B
1283 tcpdump -i xl0 tcp[13] == 2
1284 .RE
1285 .PP
1286 The expression says "let the 13th octet of a TCP datagram have
1287 the decimal value 2", which is exactly what we want.
1288 .PP
1289 Now, let's assume that we need to capture SYN packets, but we
1290 don't care if ACK or any other TCP control bit is set at the
1291 same time.
1292 Let's see what happens to octet 13 when a TCP datagram
1293 with SYN-ACK set arrives:
1294 .PP
1295 .nf
1296 |C|E|U|A|P|R|S|F|
1297 |---------------|
1298 |0 0 0 1 0 0 1 0|
1299 |---------------|
1300 |7 6 5 4 3 2 1 0|
1301 .fi
1302 .PP
1303 Now bits 1 and 4 are set in the 13th octet.
1304 The binary value of
1305 octet 13 is
1306 .IP
1307 00010010
1308 .PP
1309 which translates to decimal
1310 .PP
1311 .nf
1312 7 6 5 4 3 2 1 0
1313 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1314 .fi
1315 .PP
1316 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1317 expression, because that would select only those packets that have
1318 SYN-ACK set, but not those with only SYN set.
1319 Remember that we don't care
1320 if ACK or any other control bit is set as long as SYN is set.
1321 .PP
1322 In order to achieve our goal, we need to logically AND the
1323 binary value of octet 13 with some other value to preserve
1324 the SYN bit.
1325 We know that we want SYN to be set in any case,
1326 so we'll logically AND the value in the 13th octet with
1327 the binary value of a SYN:
1328 .PP
1329 .nf
1330
1331 00010010 SYN-ACK 00000010 SYN
1332 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1333 -------- --------
1334 = 00000010 = 00000010
1335 .fi
1336 .PP
1337 We see that this AND operation delivers the same result
1338 regardless whether ACK or another TCP control bit is set.
1339 The decimal representation of the AND value as well as
1340 the result of this operation is 2 (binary 00000010),
1341 so we know that for packets with SYN set the following
1342 relation must hold true:
1343 .IP
1344 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1345 .PP
1346 This points us to the \fItcpdump\fP filter expression
1347 .RS
1348 .B
1349 tcpdump -i xl0 'tcp[13] & 2 == 2'
1350 .RE
1351 .PP
1352 Note that you should use single quotes or a backslash
1353 in the expression to hide the AND ('&') special character
1354 from the shell.
1355 .HD
1356 .B
1357 UDP Packets
1358 .LP
1359 UDP format is illustrated by this rwho packet:
1360 .RS
1361 .nf
1362 .sp .5
1363 \f(CWactinide.who > broadcast.who: udp 84\fP
1364 .sp .5
1365 .fi
1366 .RE
1367 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1368 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1369 broadcast address.
1370 The packet contained 84 bytes of user data.
1371 .LP
1372 Some UDP services are recognized (from the source or destination
1373 port number) and the higher level protocol information printed.
1374 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1375 RPC calls (RFC-1050) to NFS.
1376 .HD
1377 UDP Name Server Requests
1378 .LP
1379 \fI(N.B.:The following description assumes familiarity with
1380 the Domain Service protocol described in RFC-1035.
1381 If you are not familiar
1382 with the protocol, the following description will appear to be written
1383 in greek.)\fP
1384 .LP
1385 Name server requests are formatted as
1386 .RS
1387 .nf
1388 .sp .5
1389 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1390 .sp .5
1391 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1392 .sp .5
1393 .fi
1394 .RE
1395 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1396 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1397 The query id was `3'.
1398 The `+' indicates the \fIrecursion desired\fP flag
1399 was set.
1400 The query length was 37 bytes, not including the UDP and
1401 IP protocol headers.
1402 The query operation was the normal one, \fIQuery\fP,
1403 so the op field was omitted.
1404 If the op had been anything else, it would
1405 have been printed between the `3' and the `+'.
1406 Similarly, the qclass was the normal one,
1407 \fIC_IN\fP, and omitted.
1408 Any other qclass would have been printed
1409 immediately after the `A'.
1410 .LP
1411 A few anomalies are checked and may result in extra fields enclosed in
1412 square brackets: If a query contains an answer, authority records or
1413 additional records section,
1414 .IR ancount ,
1415 .IR nscount ,
1416 or
1417 .I arcount
1418 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1419 is the appropriate count.
1420 If any of the response bits are set (AA, RA or rcode) or any of the
1421 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1422 is printed, where \fIx\fP is the hex value of header bytes two and three.
1423 .HD
1424 UDP Name Server Responses
1425 .LP
1426 Name server responses are formatted as
1427 .RS
1428 .nf
1429 .sp .5
1430 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1431 .sp .5
1432 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1433 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1434 .sp .5
1435 .fi
1436 .RE
1437 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1438 with 3 answer records, 3 name server records and 7 additional records.
1439 The first answer record is type A (address) and its data is internet
1440 address 128.32.137.3.
1441 The total size of the response was 273 bytes,
1442 excluding UDP and IP headers.
1443 The op (Query) and response code
1444 (NoError) were omitted, as was the class (C_IN) of the A record.
1445 .LP
1446 In the second example, \fIhelios\fP responds to query 2 with a
1447 response code of non-existent domain (NXDomain) with no answers,
1448 one name server and no authority records.
1449 The `*' indicates that
1450 the \fIauthoritative answer\fP bit was set.
1451 Since there were no
1452 answers, no type, class or data were printed.
1453 .LP
1454 Other flag characters that might appear are `\-' (recursion available,
1455 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1456 If the
1457 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1458 is printed.
1459 .LP
1460 Note that name server requests and responses tend to be large and the
1461 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1462 to print.
1463 Use the \fB\-s\fP flag to increase the snaplen if you
1464 need to seriously investigate name server traffic.
1465 `\fB\-s 128\fP'
1466 has worked well for me.
1467
1468 .HD
1469 SMB/CIFS decoding
1470 .LP
1471 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1472 on UDP/137, UDP/138 and TCP/139.
1473 Some primitive decoding of IPX and
1474 NetBEUI SMB data is also done.
1475
1476 By default a fairly minimal decode is done, with a much more detailed
1477 decode done if -v is used.
1478 Be warned that with -v a single SMB packet
1479 may take up a page or more, so only use -v if you really want all the
1480 gory details.
1481
1482 If you are decoding SMB sessions containing unicode strings then you
1483 may wish to set the environment variable USE_UNICODE to 1.
1484 A patch to
1485 auto-detect unicode srings would be welcome.
1486
1487 For information on SMB packet formats and what all te fields mean see
1488 www.cifs.org or the pub/samba/specs/ directory on your favourite
1489 samba.org mirror site.
1490 The SMB patches were written by Andrew Tridgell
1491 (tridge@samba.org).
1492
1493 .HD
1494 NFS Requests and Replies
1495 .LP
1496 Sun NFS (Network File System) requests and replies are printed as:
1497 .RS
1498 .nf
1499 .sp .5
1500 \fIsrc.xid > dst.nfs: len op args\fP
1501 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1502 .sp .5
1503 \f(CW
1504 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1505 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1506 sushi.201b > wrl.nfs:
1507 144 lookup fh 9,74/4096.6878 "xcolors"
1508 wrl.nfs > sushi.201b:
1509 reply ok 128 lookup fh 9,74/4134.3150
1510 \fR
1511 .sp .5
1512 .fi
1513 .RE
1514 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1515 to \fIwrl\fP (note that the number following the src host is a
1516 transaction id, \fInot\fP the source port).
1517 The request was 112 bytes,
1518 excluding the UDP and IP headers.
1519 The operation was a \fIreadlink\fP
1520 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1521 (If one is lucky, as in this case, the file handle can be interpreted
1522 as a major,minor device number pair, followed by the inode number and
1523 generation number.)
1524 \fIWrl\fP replies `ok' with the contents of the link.
1525 .LP
1526 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1527 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1528 Note that the data printed
1529 depends on the operation type.
1530 The format is intended to be self
1531 explanatory if read in conjunction with
1532 an NFS protocol spec.
1533 .LP
1534 If the \-v (verbose) flag is given, additional information is printed.
1535 For example:
1536 .RS
1537 .nf
1538 .sp .5
1539 \f(CW
1540 sushi.1372a > wrl.nfs:
1541 148 read fh 21,11/12.195 8192 bytes @ 24576
1542 wrl.nfs > sushi.1372a:
1543 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1544 \fP
1545 .sp .5
1546 .fi
1547 .RE
1548 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1549 which have been omitted from this example.) In the first line,
1550 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1551 at byte offset 24576.
1552 \fIWrl\fP replies `ok'; the packet shown on the
1553 second line is the first fragment of the reply, and hence is only 1472
1554 bytes long (the other bytes will follow in subsequent fragments, but
1555 these fragments do not have NFS or even UDP headers and so might not be
1556 printed, depending on the filter expression used).
1557 Because the \-v flag
1558 is given, some of the file attributes (which are returned in addition
1559 to the file data) are printed: the file type (``REG'', for regular file),
1560 the file mode (in octal), the uid and gid, and the file size.
1561 .LP
1562 If the \-v flag is given more than once, even more details are printed.
1563 .LP
1564 Note that NFS requests are very large and much of the detail won't be printed
1565 unless \fIsnaplen\fP is increased.
1566 Try using `\fB\-s 192\fP' to watch
1567 NFS traffic.
1568 .LP
1569 NFS reply packets do not explicitly identify the RPC operation.
1570 Instead,
1571 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1572 replies using the transaction ID.
1573 If a reply does not closely follow the
1574 corresponding request, it might not be parsable.
1575 .HD
1576 AFS Requests and Replies
1577 .LP
1578 Transarc AFS (Andrew File System) requests and replies are printed
1579 as:
1580 .HD
1581 .RS
1582 .nf
1583 .sp .5
1584 \fIsrc.sport > dst.dport: rx packet-type\fP
1585 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1586 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1587 .sp .5
1588 \f(CW
1589 elvis.7001 > pike.afsfs:
1590 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1591 new fid 536876964/1/1 ".newsrc"
1592 pike.afsfs > elvis.7001: rx data fs reply rename
1593 \fR
1594 .sp .5
1595 .fi
1596 .RE
1597 In the first line, host elvis sends a RX packet to pike.
1598 This was
1599 a RX data packet to the fs (fileserver) service, and is the start of
1600 an RPC call.
1601 The RPC call was a rename, with the old directory file id
1602 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1603 file id of 536876964/1/1 and a new filename of `.newsrc'.
1604 The host pike
1605 responds with a RPC reply to the rename call (which was successful, because
1606 it was a data packet and not an abort packet).
1607 .LP
1608 In general, all AFS RPCs are decoded at least by RPC call name.
1609 Most
1610 AFS RPCs have at least some of the arguments decoded (generally only
1611 the `interesting' arguments, for some definition of interesting).
1612 .LP
1613 The format is intended to be self-describing, but it will probably
1614 not be useful to people who are not familiar with the workings of
1615 AFS and RX.
1616 .LP
1617 If the -v (verbose) flag is given twice, acknowledgement packets and
1618 additional header information is printed, such as the the RX call ID,
1619 call number, sequence number, serial number, and the RX packet flags.
1620 .LP
1621 If the -v flag is given twice, additional information is printed,
1622 such as the the RX call ID, serial number, and the RX packet flags.
1623 The MTU negotiation information is also printed from RX ack packets.
1624 .LP
1625 If the -v flag is given three times, the security index and service id
1626 are printed.
1627 .LP
1628 Error codes are printed for abort packets, with the exception of Ubik
1629 beacon packets (because abort packets are used to signify a yes vote
1630 for the Ubik protocol).
1631 .LP
1632 Note that AFS requests are very large and many of the arguments won't
1633 be printed unless \fIsnaplen\fP is increased.
1634 Try using `\fB-s 256\fP'
1635 to watch AFS traffic.
1636 .LP
1637 AFS reply packets do not explicitly identify the RPC operation.
1638 Instead,
1639 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1640 replies using the call number and service ID.
1641 If a reply does not closely
1642 follow the
1643 corresponding request, it might not be parsable.
1644
1645 .HD
1646 KIP Appletalk (DDP in UDP)
1647 .LP
1648 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1649 and dumped as DDP packets (i.e., all the UDP header information is
1650 discarded).
1651 The file
1652 .I /etc/atalk.names
1653 is used to translate appletalk net and node numbers to names.
1654 Lines in this file have the form
1655 .RS
1656 .nf
1657 .sp .5
1658 \fInumber name\fP
1659
1660 \f(CW1.254 ether
1661 16.1 icsd-net
1662 1.254.110 ace\fR
1663 .sp .5
1664 .fi
1665 .RE
1666 The first two lines give the names of appletalk networks.
1667 The third
1668 line gives the name of a particular host (a host is distinguished
1669 from a net by the 3rd octet in the number \-
1670 a net number \fImust\fP have two octets and a host number \fImust\fP
1671 have three octets.) The number and name should be separated by
1672 whitespace (blanks or tabs).
1673 The
1674 .I /etc/atalk.names
1675 file may contain blank lines or comment lines (lines starting with
1676 a `#').
1677 .LP
1678 Appletalk addresses are printed in the form
1679 .RS
1680 .nf
1681 .sp .5
1682 \fInet.host.port\fP
1683
1684 \f(CW144.1.209.2 > icsd-net.112.220
1685 office.2 > icsd-net.112.220
1686 jssmag.149.235 > icsd-net.2\fR
1687 .sp .5
1688 .fi
1689 .RE
1690 (If the
1691 .I /etc/atalk.names
1692 doesn't exist or doesn't contain an entry for some appletalk
1693 host/net number, addresses are printed in numeric form.)
1694 In the first example, NBP (DDP port 2) on net 144.1 node 209
1695 is sending to whatever is listening on port 220 of net icsd node 112.
1696 The second line is the same except the full name of the source node
1697 is known (`office').
1698 The third line is a send from port 235 on
1699 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1700 the broadcast address (255) is indicated by a net name with no host
1701 number \- for this reason it's a good idea to keep node names and
1702 net names distinct in /etc/atalk.names).
1703 .LP
1704 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1705 packets have their contents interpreted.
1706 Other protocols just dump
1707 the protocol name (or number if no name is registered for the
1708 protocol) and packet size.
1709
1710 \fBNBP packets\fP are formatted like the following examples:
1711 .RS
1712 .nf
1713 .sp .5
1714 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1715 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1716 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1717 .sp .5
1718 .fi
1719 .RE
1720 The first line is a name lookup request for laserwriters sent by net icsd host
1721 112 and broadcast on net jssmag.
1722 The nbp id for the lookup is 190.
1723 The second line shows a reply for this request (note that it has the
1724 same id) from host jssmag.209 saying that it has a laserwriter
1725 resource named "RM1140" registered on port 250.
1726 The third line is
1727 another reply to the same request saying host techpit has laserwriter
1728 "techpit" registered on port 186.
1729
1730 \fBATP packet\fP formatting is demonstrated by the following example:
1731 .RS
1732 .nf
1733 .sp .5
1734 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1735 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1736 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1737 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1738 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1739 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1740 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1741 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1742 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1743 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1744 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1745 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1746 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1747 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1748 .sp .5
1749 .fi
1750 .RE
1751 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1752 up to 8 packets (the `<0-7>').
1753 The hex number at the end of the line
1754 is the value of the `userdata' field in the request.
1755 .LP
1756 Helios responds with 8 512-byte packets.
1757 The `:digit' following the
1758 transaction id gives the packet sequence number in the transaction
1759 and the number in parens is the amount of data in the packet,
1760 excluding the atp header.
1761 The `*' on packet 7 indicates that the
1762 EOM bit was set.
1763 .LP
1764 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1765 Helios
1766 resends them then jssmag.209 releases the transaction.
1767 Finally,
1768 jssmag.209 initiates the next request.
1769 The `*' on the request
1770 indicates that XO (`exactly once') was \fInot\fP set.
1771
1772 .HD
1773 IP Fragmentation
1774 .LP
1775 Fragmented Internet datagrams are printed as
1776 .RS
1777 .nf
1778 .sp .5
1779 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1780 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1781 .sp .5
1782 .fi
1783 .RE
1784 (The first form indicates there are more fragments.
1785 The second
1786 indicates this is the last fragment.)
1787 .LP
1788 \fIId\fP is the fragment id.
1789 \fISize\fP is the fragment
1790 size (in bytes) excluding the IP header.
1791 \fIOffset\fP is this
1792 fragment's offset (in bytes) in the original datagram.
1793 .LP
1794 The fragment information is output for each fragment.
1795 The first
1796 fragment contains the higher level protocol header and the frag
1797 info is printed after the protocol info.
1798 Fragments
1799 after the first contain no higher level protocol header and the
1800 frag info is printed after the source and destination addresses.
1801 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1802 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1803 .RS
1804 .nf
1805 .sp .5
1806 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1807 arizona > rtsg: (frag 595a:204@328)
1808 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1809 .sp .5
1810 .fi
1811 .RE
1812 There are a couple of things to note here: First, addresses in the
1813 2nd line don't include port numbers.
1814 This is because the TCP
1815 protocol information is all in the first fragment and we have no idea
1816 what the port or sequence numbers are when we print the later fragments.
1817 Second, the tcp sequence information in the first line is printed as if there
1818 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1819 the first frag and 204 in the second).
1820 If you are looking for holes
1821 in the sequence space or trying to match up acks
1822 with packets, this can fool you.
1823 .LP
1824 A packet with the IP \fIdon't fragment\fP flag is marked with a
1825 trailing \fB(DF)\fP.
1826 .HD
1827 Timestamps
1828 .LP
1829 By default, all output lines are preceded by a timestamp.
1830 The timestamp
1831 is the current clock time in the form
1832 .RS
1833 .nf
1834 \fIhh:mm:ss.frac\fP
1835 .fi
1836 .RE
1837 and is as accurate as the kernel's clock.
1838 The timestamp reflects the time the kernel first saw the packet.
1839 No attempt
1840 is made to account for the time lag between when the
1841 ethernet interface removed the packet from the wire and when the kernel
1842 serviced the `new packet' interrupt.
1843 .SH "SEE ALSO"
1844 traffic(1C), nit(4P), bpf(4), pcap(3)
1845 .SH AUTHORS
1846 The original authors are:
1847 .LP
1848 Van Jacobson,
1849 Craig Leres and
1850 Steven McCanne, all of the
1851 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1852 .LP
1853 It is currently being maintained by tcpdump.org.
1854 .LP
1855 The current version is available via http:
1856 .LP
1857 .RS
1858 .I http://www.tcpdump.org/
1859 .RE
1860 .LP
1861 The original distribution is available via anonymous ftp:
1862 .LP
1863 .RS
1864 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1865 .RE
1866 .LP
1867 IPv6/IPsec support is added by WIDE/KAME project.
1868 This program uses Eric Young's SSLeay library, under specific configuration.
1869 .SH BUGS
1870 Please send problems, bugs, questions, desirable enhancements, etc. to:
1871 .LP
1872 .RS
1873 tcpdump-workers@tcpdump.org
1874 .RE
1875 .LP
1876 Please send source code contributions, etc. to:
1877 .LP
1878 .RS
1879 patches@tcpdump.org
1880 .RE
1881 .LP
1882 NIT doesn't let you watch your own outbound traffic, BPF will.
1883 We recommend that you use the latter.
1884 .LP
1885 On Linux systems with 2.0[.x] kernels:
1886 .IP
1887 packets on the loopback device will be seen twice;
1888 .IP
1889 packet filtering cannot be done in the kernel, so that all packets must
1890 be copied from the kernel in order to be filtered in user mode;
1891 .IP
1892 all of a packet, not just the part that's within the snapshot length,
1893 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1894 asked to copy only part of a packet to userland, will not report the
1895 true length of the packet; this would cause most IP packets to get an
1896 error from
1897 .BR tcpdump ).
1898 .LP
1899 We recommend that you upgrade to a 2.2 or later kernel.
1900 .LP
1901 Some attempt should be made to reassemble IP fragments or, at least
1902 to compute the right length for the higher level protocol.
1903 .LP
1904 Name server inverse queries are not dumped correctly: the (empty)
1905 question section is printed rather than real query in the answer
1906 section.
1907 Some believe that inverse queries are themselves a bug and
1908 prefer to fix the program generating them rather than \fItcpdump\fP.
1909 .LP
1910 A packet trace that crosses a daylight savings time change will give
1911 skewed time stamps (the time change is ignored).
1912 .LP
1913 Filter expressions that manipulate FDDI or Token Ring headers assume
1914 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1915 packets.
1916 This is true for IP, ARP, and DECNET Phase IV, but is not true
1917 for protocols such as ISO CLNS.
1918 Therefore, the filter may inadvertently
1919 accept certain packets that do not properly match the filter expression.
1920 .LP
1921 Filter expressions on fields other than those that manipulate Token Ring
1922 headers will not correctly handle source-routed Token Ring packets.
1923 .LP
1924 .BR "ip6 proto"
1925 should chase header chain, but at this moment it does not.
1926 .BR "ip6 protochain"
1927 is supplied for this behavior.
1928 .LP
1929 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1930 does not work against IPv6 packets.
1931 It only looks at IPv4 packets.