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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 identification, total length and options in an IP packet are printed.
248 Also enables additional packet integrity checks such as verifying the
249 IP and ICMP header checksum.
252 Even more verbose output. For example, additional fields are
253 printed from NFS reply packets.
256 Even more verbose output. For example,
257 telnet \fBSB\fP ... \fBSE\fP options
258 are printed in full. With
260 telnet options are printed in hex as well.
263 Write the raw packets to \fIfile\fR rather than parsing and printing
264 them out. They can later be printed with the \-r option.
265 Standard output is used if \fIfile\fR is ``-''.
268 Print each packet (minus its link level header) in hex.
269 The smaller of the entire packet or
271 bytes will be printed.
274 When printing hex, print ascii too. Thus if
276 is also set, the packet is printed in hex/ascii.
277 This is very handy for analysing new protocols.
280 is not also set, some parts of some packets may be printed
282 .IP "\fI expression\fP"
284 selects which packets will be dumped. If no \fIexpression\fP
285 is given, all packets on the net will be dumped. Otherwise,
286 only packets for which \fIexpression\fP is `true' will be dumped.
288 The \fIexpression\fP consists of one or more
290 Primitives usually consist of an
292 (name or number) preceded by one or more qualifiers. There are three
293 different kinds of qualifier:
295 qualifiers say what kind of thing the id name or number refers to.
301 E.g., `host foo', `net 128.3', `port 20'. If there is no type
306 qualifiers specify a particular transfer direction to and/or from
308 Possible directions are
315 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
316 there is no dir qualifier,
319 For `null' link layers (i.e. point to point protocols such as slip) the
323 qualifiers can be used to specify a desired direction.
325 qualifiers restrict the match to a particular protocol. Possible
344 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
345 no proto qualifier, all protocols consistent with the type are
346 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
347 (except the latter is not legal syntax), `net bar' means `(ip or
348 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
350 [`fddi' is actually an alias for `ether'; the parser treats them
351 identically as meaning ``the data link level used on the specified
352 network interface.'' FDDI headers contain Ethernet-like source
353 and destination addresses, and often contain Ethernet-like packet
354 types, so you can filter on these FDDI fields just as with the
355 analogous Ethernet fields. FDDI headers also contain other fields,
356 but you cannot name them explicitly in a filter expression.
358 Similarly, `tr' is an alias for `ether'; the previous paragraph's
359 statements about FDDI headers also apply to Token Ring headers.]
361 In addition to the above, there are some special `primitive' keywords
362 that don't follow the pattern:
367 and arithmetic expressions. All of these are described below.
369 More complex filter expressions are built up by using the words
374 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
375 To save typing, identical qualifier lists can be omitted. E.g.,
376 `tcp dst port ftp or ftp-data or domain' is exactly the same as
377 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
379 Allowable primitives are:
380 .IP "\fBdst host \fIhost\fR"
381 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
382 which may be either an address or a name.
383 .IP "\fBsrc host \fIhost\fR"
384 True if the IPv4/v6 source field of the packet is \fIhost\fP.
385 .IP "\fBhost \fIhost\fP
386 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
387 Any of the above host expressions can be prepended with the keywords,
388 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
391 \fBip host \fIhost\fR
394 which is equivalent to:
397 \fBether proto \fI\\ip\fB and host \fIhost\fR
400 If \fIhost\fR is a name with multiple IP addresses, each address will
401 be checked for a match.
402 .IP "\fBether dst \fIehost\fP
403 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
404 may be either a name from /etc/ethers or a number (see
407 .IP "\fBether src \fIehost\fP
408 True if the ethernet source address is \fIehost\fP.
409 .IP "\fBether host \fIehost\fP
410 True if either the ethernet source or destination address is \fIehost\fP.
411 .IP "\fBgateway\fP \fIhost\fP
412 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
413 source or destination address was \fIhost\fP but neither the IP source
414 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
415 must be found in both /etc/hosts and /etc/ethers. (An equivalent
419 \fBether host \fIehost \fBand not host \fIhost\fR
422 which can be used with either names or numbers for \fIhost / ehost\fP.)
423 This syntax does not work in IPv6-enabled configuration at this moment.
424 .IP "\fBdst net \fInet\fR"
425 True if the IPv4/v6 destination address of the packet has a network
426 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
427 or a network number (see \fInetworks(4)\fP for details).
428 .IP "\fBsrc net \fInet\fR"
429 True if the IPv4/v6 source address of the packet has a network
431 .IP "\fBnet \fInet\fR"
432 True if either the IPv4/v6 source or destination address of the packet has a network
434 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
435 True if the IP address matches \fInet\fR with the specific netmask.
436 May be qualified with \fBsrc\fR or \fBdst\fR.
437 Note that this syntax is not valid for IPv6 \fInet\fR.
438 .IP "\fBnet \fInet\fR/\fIlen\fR"
439 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
440 May be qualified with \fBsrc\fR or \fBdst\fR.
441 .IP "\fBdst port \fIport\fR"
442 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
443 destination port value of \fIport\fP.
444 The \fIport\fP can be a number or a name used in /etc/services (see
448 If a name is used, both the port
449 number and protocol are checked. If a number or ambiguous name is used,
450 only the port number is checked (e.g., \fBdst port 513\fR will print both
451 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
452 both tcp/domain and udp/domain traffic).
453 .IP "\fBsrc port \fIport\fR"
454 True if the packet has a source port value of \fIport\fP.
455 .IP "\fBport \fIport\fR"
456 True if either the source or destination port of the packet is \fIport\fP.
457 Any of the above port expressions can be prepended with the keywords,
458 \fBtcp\fP or \fBudp\fP, as in:
461 \fBtcp src port \fIport\fR
464 which matches only tcp packets whose source port is \fIport\fP.
465 .IP "\fBless \fIlength\fR"
466 True if the packet has a length less than or equal to \fIlength\fP.
467 This is equivalent to:
470 \fBlen <= \fIlength\fP.
473 .IP "\fBgreater \fIlength\fR"
474 True if the packet has a length greater than or equal to \fIlength\fP.
475 This is equivalent to:
478 \fBlen >= \fIlength\fP.
481 .IP "\fBip proto \fIprotocol\fR"
482 True if the packet is an ip packet (see
484 of protocol type \fIprotocol\fP.
485 \fIProtocol\fP can be a number or one of the names
486 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
487 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
488 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
489 Note that this primitive does not chase protocol header chain.
490 .IP "\fBip6 proto \fIprotocol\fR"
491 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
492 Note that this primitive does not chase protocol header chain.
493 .IP "\fBip6 protochain \fIprotocol\fR"
494 True if the packet is IPv6 packet,
495 and contains protocol header with type \fIprotocol\fR
496 in its protocol header chain.
500 \fBip6 protochain 6\fR
503 matches any IPv6 packet with TCP protocol header in the protocol header chain.
504 The packet may contain, for example,
505 authentication header, routing header, or hop-by-hop option header,
506 between IPv6 header and TCP header.
507 The BPF code emitted by this primitive is complex and
508 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
509 so this can be somewhat slow.
510 .IP "\fBip protochain \fIprotocol\fR"
511 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
512 .IP "\fBether broadcast\fR"
513 True if the packet is an ethernet broadcast packet. The \fIether\fP
515 .IP "\fBip broadcast\fR"
516 True if the packet is an IP broadcast packet. It checks for both
517 the all-zeroes and all-ones broadcast conventions, and looks up
518 the local subnet mask.
519 .IP "\fBether multicast\fR"
520 True if the packet is an ethernet multicast packet. The \fIether\fP
522 This is shorthand for `\fBether[0] & 1 != 0\fP'.
523 .IP "\fBip multicast\fR"
524 True if the packet is an IP multicast packet.
525 .IP "\fBip6 multicast\fR"
526 True if the packet is an IPv6 multicast packet.
527 .IP "\fBether proto \fIprotocol\fR"
528 True if the packet is of ether type \fIprotocol\fR.
529 \fIProtocol\fP can be a number or a name like
530 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
531 Note these identifiers are also keywords
532 and must be escaped via backslash (\\).
533 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
534 protocol identification comes from the 802.2 Logical Link Control
535 (LLC) header, which is usually layered on top of the FDDI header.
536 \fITcpdump\fP assumes, when filtering on the protocol identifier,
537 that all FDDI packets include an LLC header, and that the LLC header
538 is in so-called SNAP format. The same applies to Token Ring.]
539 .IP "\fBdecnet src \fIhost\fR"
540 True if the DECNET source address is
542 which may be an address of the form ``10.123'', or a DECNET host
543 name. [DECNET host name support is only available on Ultrix systems
544 that are configured to run DECNET.]
545 .IP "\fBdecnet dst \fIhost\fR"
546 True if the DECNET destination address is
548 .IP "\fBdecnet host \fIhost\fR"
549 True if either the DECNET source or destination address is
551 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
555 \fBether proto \fIp\fR
558 where \fIp\fR is one of the above protocols.
559 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
563 \fBether proto \fIp\fR
566 where \fIp\fR is one of the above protocols.
568 \fItcpdump\fP does not currently know how to parse these protocols.
569 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
573 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
576 where \fIp\fR is one of the above protocols.
577 .IP "\fIexpr relop expr\fR"
578 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
579 and \fIexpr\fR is an arithmetic expression composed of integer constants
580 (expressed in standard C syntax), the normal binary operators
581 [+, -, *, /, &, |], a length operator, and special packet data accessors.
583 data inside the packet, use the following syntax:
586 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
589 \fIProto\fR is one of \fBether, fddi, tr,
590 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
591 indicates the protocol layer for the index operation.
592 Note that \fItcp, udp\fR and other upper-layer protocol types only
593 apply to IPv4, not IPv6 (this will be fixed in the future).
594 The byte offset, relative to the indicated protocol layer, is
596 \fISize\fR is optional and indicates the number of bytes in the
597 field of interest; it can be either one, two, or four, and defaults to one.
598 The length operator, indicated by the keyword \fBlen\fP, gives the
599 length of the packet.
601 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
602 The expression `\fBip[0] & 0xf != 5\fP'
603 catches all IP packets with options. The expression
604 `\fBip[6:2] & 0x1fff = 0\fP'
605 catches only unfragmented datagrams and frag zero of fragmented datagrams.
606 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
608 For instance, \fBtcp[0]\fP always means the first
609 byte of the TCP \fIheader\fP, and never means the first byte of an
610 intervening fragment.
612 Primitives may be combined using:
614 A parenthesized group of primitives and operators
615 (parentheses are special to the Shell and must be escaped).
617 Negation (`\fB!\fP' or `\fBnot\fP').
619 Concatenation (`\fB&&\fP' or `\fBand\fP').
621 Alternation (`\fB||\fP' or `\fBor\fP').
623 Negation has highest precedence.
624 Alternation and concatenation have equal precedence and associate
625 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
626 are now required for concatenation.
628 If an identifier is given without a keyword, the most recent keyword
633 \fBnot host vs and ace\fR
639 \fBnot host vs and host ace\fR
642 which should not be confused with
645 \fBnot ( host vs or ace )\fR
649 Expression arguments can be passed to \fItcpdump\fP as either a single
650 argument or as multiple arguments, whichever is more convenient.
651 Generally, if the expression contains Shell metacharacters, it is
652 easier to pass it as a single, quoted argument.
653 Multiple arguments are concatenated with spaces before being parsed.
656 To print all packets arriving at or departing from \fIsundown\fP:
659 \fBtcpdump host sundown\fP
663 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
666 \fBtcpdump host helios and \\( hot or ace \\)\fP
670 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
673 \fBtcpdump ip host ace and not helios\fP
677 To print all traffic between local hosts and hosts at Berkeley:
681 tcpdump net ucb-ether
685 To print all ftp traffic through internet gateway \fIsnup\fP:
686 (note that the expression is quoted to prevent the shell from
687 (mis-)interpreting the parentheses):
691 tcpdump 'gateway snup and (port ftp or ftp-data)'
695 To print traffic neither sourced from nor destined for local hosts
696 (if you gateway to one other net, this stuff should never make it
697 onto your local net).
701 tcpdump ip and not net \fIlocalnet\fP
705 To print the start and end packets (the SYN and FIN packets) of each
706 TCP conversation that involves a non-local host.
710 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
714 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
718 tcpdump 'gateway snup and ip[2:2] > 576'
722 To print IP broadcast or multicast packets that were
724 sent via ethernet broadcast or multicast:
728 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
732 To print all ICMP packets that are not echo requests/replies (i.e., not
737 tcpdump 'icmp[0] != 8 and icmp[0] != 0'
742 The output of \fItcpdump\fP is protocol dependent. The following
743 gives a brief description and examples of most of the formats.
751 If the '-e' option is given, the link level header is printed out.
752 On ethernets, the source and destination addresses, protocol,
753 and packet length are printed.
755 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
756 the `frame control' field, the source and destination addresses,
757 and the packet length. (The `frame control' field governs the
758 interpretation of the rest of the packet. Normal packets (such
759 as those containing IP datagrams) are `async' packets, with a priority
760 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
761 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
762 the LLC header is printed if it is \fInot\fR an ISO datagram or a
763 so-called SNAP packet.
765 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
766 the `access control' and `frame control' fields, the source and
767 destination addresses, and the packet length. As on FDDI networks,
768 packets are assumed to contain an LLC packet. Regardless of whether
769 the '-e' option is specified or not, the source routing information is
770 printed for source-routed packets.
772 \fI(N.B.: The following description assumes familiarity with
773 the SLIP compression algorithm described in RFC-1144.)\fP
775 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
776 packet type, and compression information are printed out.
777 The packet type is printed first.
778 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
779 No further link information is printed for \fIip\fR packets.
780 For TCP packets, the connection identifier is printed following the type.
781 If the packet is compressed, its encoded header is printed out.
782 The special cases are printed out as
783 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
784 the sequence number (or sequence number and ack) has changed.
785 If it is not a special case,
786 zero or more changes are printed.
787 A change is indicated by U (urgent pointer), W (window), A (ack),
788 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
790 Finally, the amount of data in the packet and compressed header length
793 For example, the following line shows an outbound compressed TCP packet,
794 with an implicit connection identifier; the ack has changed by 6,
795 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
796 data and 6 bytes of compressed header:
799 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
805 Arp/rarp output shows the type of request and its arguments. The
806 format is intended to be self explanatory.
807 Here is a short sample taken from the start of an `rlogin' from
808 host \fIrtsg\fP to host \fIcsam\fP:
812 \f(CWarp who-has csam tell rtsg
813 arp reply csam is-at CSAM\fR
817 The first line says that rtsg sent an arp packet asking
818 for the ethernet address of internet host csam. Csam
819 replies with its ethernet address (in this example, ethernet addresses
820 are in caps and internet addresses in lower case).
822 This would look less redundant if we had done \fBtcpdump \-n\fP:
826 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
827 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
831 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
832 broadcast and the second is point-to-point would be visible:
836 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
837 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
841 For the first packet this says the ethernet source address is RTSG, the
842 destination is the ethernet broadcast address, the type field
843 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
847 \fI(N.B.:The following description assumes familiarity with
848 the TCP protocol described in RFC-793. If you are not familiar
849 with the protocol, neither this description nor \fItcpdump\fP will
850 be of much use to you.)\fP
852 The general format of a tcp protocol line is:
856 \fIsrc > dst: flags data-seqno ack window urgent options\fP
860 \fISrc\fP and \fIdst\fP are the source and destination IP
861 addresses and ports. \fIFlags\fP are some combination of S (SYN),
862 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
863 \fIData-seqno\fP describes the portion of sequence space covered
864 by the data in this packet (see example below).
865 \fIAck\fP is sequence number of the next data expected the other
866 direction on this connection.
867 \fIWindow\fP is the number of bytes of receive buffer space available
868 the other direction on this connection.
869 \fIUrg\fP indicates there is `urgent' data in the packet.
870 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
872 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
873 depend on the contents of the packet's tcp protocol header and
874 are output only if appropriate.
876 Here is the opening portion of an rlogin from host \fIrtsg\fP to
881 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
882 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
883 rtsg.1023 > csam.login: . ack 1 win 4096
884 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
885 csam.login > rtsg.1023: . ack 2 win 4096
886 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
887 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
888 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
889 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
893 The first line says that tcp port 1023 on rtsg sent a packet
895 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
896 The packet sequence number was 768512 and it contained no data.
897 (The notation is `first:last(nbytes)' which means `sequence
899 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
900 There was no piggy-backed ack, the available receive window was 4096
901 bytes and there was a max-segment-size option requesting an mss of
904 Csam replies with a similar packet except it includes a piggy-backed
905 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
907 The packet contained no data so there is no data sequence number.
908 Note that the ack sequence
909 number is a small integer (1). The first time \fItcpdump\fP sees a
910 tcp `conversation', it prints the sequence number from the packet.
911 On subsequent packets of the conversation, the difference between
912 the current packet's sequence number and this initial sequence number
913 is printed. This means that sequence numbers after the
914 first can be interpreted
915 as relative byte positions in the conversation's data stream (with the
916 first data byte each direction being `1'). `-S' will override this
917 feature, causing the original sequence numbers to be output.
919 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
920 in the rtsg \(-> csam side of the conversation).
921 The PUSH flag is set in the packet.
922 On the 7th line, csam says it's received data sent by rtsg up to
923 but not including byte 21. Most of this data is apparently sitting in the
924 socket buffer since csam's receive window has gotten 19 bytes smaller.
925 Csam also sends one byte of data to rtsg in this packet.
926 On the 8th and 9th lines,
927 csam sends two bytes of urgent, pushed data to rtsg.
929 If the snapshot was small enough that \fItcpdump\fP didn't capture
930 the full TCP header, it interprets as much of the header as it can
931 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
932 be interpreted. If the header contains a bogus option (one with a length
933 that's either too small or beyond the end of the header), \fItcpdump\fP
934 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
935 options (since it's impossible to tell where they start). If the header
936 length indicates options are present but the IP datagram length is not
937 long enough for the options to actually be there, \fItcpdump\fP reports
938 it as ``[\fIbad hdr length\fP]''.
940 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
942 There are 6 bits in the control bits section of the TCP header:
944 .I URG | ACK | PSH | RST | SYN | FIN
946 Let's assume that we want to watch packets used in establishing
947 a TCP connection. Recall that TCP uses a 3-way handshake protocol
948 when it initializes a new connection; the connection sequence with
949 regard to the TCP control bits is
955 2) Recipient responds with SYN, ACK
961 Now we're interested in capturing packets that have only the
962 SYN bit set (Step 1). Note that we don't want packets from step 2
963 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
964 expression for \fItcpdump\fP.
966 Recall the structure of a TCP header without options:
970 -----------------------------------------------------------------
971 | source port | destination port |
972 -----------------------------------------------------------------
974 -----------------------------------------------------------------
975 | acknowledgment number |
976 -----------------------------------------------------------------
977 | HL | reserved |U|A|P|R|S|F| window size |
978 -----------------------------------------------------------------
979 | TCP checksum | urgent pointer |
980 -----------------------------------------------------------------
983 A TCP header usually holds 20 octets of data, unless options are
984 present. The fist line of the graph contains octets 0 - 3, the
985 second line shows octets 4 - 7 etc.
987 Starting to count with 0, the relevant TCP control bits are contained
992 ----------------|---------------|---------------|----------------
993 | HL | reserved |U|A|P|R|S|F| window size |
994 ----------------|---------------|---------------|----------------
998 Let's have a closer look at octet no. 13:
1008 We see that this octet contains 2 bytes from the reserved field.
1009 According to RFC 793 this field is reserved for future use and must
1010 be 0. The remaining 6 bits are the TCP control bits we are interested
1011 in. We have numbered the bits in this octet from 0 to 7, right to
1012 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1014 Recall that we want to capture packets with only SYN set.
1015 Let's see what happens to octet 13 if a TCP datagram arrives
1016 with the SYN bit set in its header:
1026 We already mentioned that bits number 7 and 6 belong to the
1027 reserved field, so they must must be 0. Looking at the
1028 control bits section we see that only bit number 1 (SYN) is set.
1030 Assuming that octet number 13 is an 8-bit unsigned integer in
1031 network byte order, the binary value of this octet is
1035 and its decimal representation is
1039 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1042 We're almost done, because now we know that if only SYN is set,
1043 the value of the 13th octet in the TCP header, when interpreted
1044 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1046 This relationship can be expressed as
1052 We can use this expression as the filter for \fItcpdump\fP in order
1053 to watch packets which have only SYN set:
1056 tcpdump -i xl0 tcp[13] == 2
1059 The expression says "let the 13th octet of a TCP datagram have
1060 the decimal value 2", which is exactly what we want.
1062 Now, let's assume that we need to capture SYN packets, but we
1063 don't care if ACK or any other TCP control bit is set at the
1064 same time. Let's see what happens to octet 13 when a TCP datagram
1065 with SYN-ACK set arrives:
1075 Now bits 1 and 4 are set in the 13th octet. The binary value of
1080 which translates to decimal
1084 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1087 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1088 expression, because that would select only those packets that have
1089 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1090 if ACK or any other control bit is set as long as SYN is set.
1092 In order to achieve our goal, we need to logically AND the
1093 binary value of octet 13 with some other value to preserve
1094 the SYN bit. We know that we want SYN to be set in any case,
1095 so we'll logically AND the value in the 13th octet with
1096 the binary value of a SYN:
1100 00010010 SYN-ACK 00000010 SYN
1101 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1103 = 00000010 = 00000010
1106 We see that this AND operation delivers the same result
1107 regardless whether ACK or another TCP control bit is set.
1108 The decimal representation of the AND value as well as
1109 the result of this operation is 2 (binary 00000010),
1110 so we know that for packets with SYN set the following
1111 relation must hold true:
1113 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1115 This points us to the \fItcpdump\fP filter expression
1118 tcpdump -i xl0 'tcp[13] & 2 == 2'
1121 Note that you should use single quotes or a backslash
1122 in the expression to hide the AND ('&') special character
1128 UDP format is illustrated by this rwho packet:
1132 \f(CWactinide.who > broadcast.who: udp 84\fP
1136 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1137 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1138 broadcast address. The packet contained 84 bytes of user data.
1140 Some UDP services are recognized (from the source or destination
1141 port number) and the higher level protocol information printed.
1142 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1143 RPC calls (RFC-1050) to NFS.
1145 UDP Name Server Requests
1147 \fI(N.B.:The following description assumes familiarity with
1148 the Domain Service protocol described in RFC-1035. If you are not familiar
1149 with the protocol, the following description will appear to be written
1152 Name server requests are formatted as
1156 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1158 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1162 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1163 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1164 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1165 was set. The query length was 37 bytes, not including the UDP and
1166 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1167 so the op field was omitted. If the op had been anything else, it would
1168 have been printed between the `3' and the `+'.
1169 Similarly, the qclass was the normal one,
1170 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1171 immediately after the `A'.
1173 A few anomalies are checked and may result in extra fields enclosed in
1174 square brackets: If a query contains an answer, name server or
1180 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1181 is the appropriate count.
1182 If any of the response bits are set (AA, RA or rcode) or any of the
1183 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1184 is printed, where \fIx\fP is the hex value of header bytes two and three.
1186 UDP Name Server Responses
1188 Name server responses are formatted as
1192 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1194 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1195 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1199 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1200 with 3 answer records, 3 name server records and 7 authority records.
1201 The first answer record is type A (address) and its data is internet
1202 address 128.32.137.3. The total size of the response was 273 bytes,
1203 excluding UDP and IP headers. The op (Query) and response code
1204 (NoError) were omitted, as was the class (C_IN) of the A record.
1206 In the second example, \fIhelios\fP responds to query 2 with a
1207 response code of non-existent domain (NXDomain) with no answers,
1208 one name server and no authority records. The `*' indicates that
1209 the \fIauthoritative answer\fP bit was set. Since there were no
1210 answers, no type, class or data were printed.
1212 Other flag characters that might appear are `\-' (recursion available,
1213 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1214 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1217 Note that name server requests and responses tend to be large and the
1218 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1219 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1220 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1221 has worked well for me.
1226 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1227 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1228 NetBEUI SMB data is also done.
1230 By default a fairly minimal decode is done, with a much more detailed
1231 decode done if -v is used. Be warned that with -v a single SMB packet
1232 may take up a page or more, so only use -v if you really want all the
1235 If you are decoding SMB sessions containing unicode strings then you
1236 may wish to set the environment variable USE_UNICODE to 1. A patch to
1237 auto-detect unicode srings would be welcome.
1239 For information on SMB packet formats and what all te fields mean see
1240 www.cifs.org or the pub/samba/specs/ directory on your favourite
1241 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1245 NFS Requests and Replies
1247 Sun NFS (Network File System) requests and replies are printed as:
1251 \fIsrc.xid > dst.nfs: len op args\fP
1252 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1255 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1256 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1257 sushi.201b > wrl.nfs:
1258 144 lookup fh 9,74/4096.6878 "xcolors"
1259 wrl.nfs > sushi.201b:
1260 reply ok 128 lookup fh 9,74/4134.3150
1265 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1266 to \fIwrl\fP (note that the number following the src host is a
1267 transaction id, \fInot\fP the source port). The request was 112 bytes,
1268 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1269 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1270 (If one is lucky, as in this case, the file handle can be interpreted
1271 as a major,minor device number pair, followed by the inode number and
1273 \fIWrl\fP replies `ok' with the contents of the link.
1275 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1276 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1277 depends on the operation type. The format is intended to be self
1278 explanatory if read in conjunction with
1279 an NFS protocol spec.
1281 If the \-v (verbose) flag is given, additional information is printed.
1287 sushi.1372a > wrl.nfs:
1288 148 read fh 21,11/12.195 8192 bytes @ 24576
1289 wrl.nfs > sushi.1372a:
1290 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1295 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1296 which have been omitted from this example.) In the first line,
1297 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1298 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1299 second line is the first fragment of the reply, and hence is only 1472
1300 bytes long (the other bytes will follow in subsequent fragments, but
1301 these fragments do not have NFS or even UDP headers and so might not be
1302 printed, depending on the filter expression used). Because the \-v flag
1303 is given, some of the file attributes (which are returned in addition
1304 to the file data) are printed: the file type (``REG'', for regular file),
1305 the file mode (in octal), the uid and gid, and the file size.
1307 If the \-v flag is given more than once, even more details are printed.
1309 Note that NFS requests are very large and much of the detail won't be printed
1310 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1313 NFS reply packets do not explicitly identify the RPC operation. Instead,
1314 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1315 replies using the transaction ID. If a reply does not closely follow the
1316 corresponding request, it might not be parsable.
1318 AFS Requests and Replies
1320 Transarc AFS (Andrew File System) requests and replies are printed
1326 \fIsrc.sport > dst.dport: rx packet-type\fP
1327 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1328 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1331 elvis.7001 > pike.afsfs:
1332 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1333 new fid 536876964/1/1 ".newsrc"
1334 pike.afsfs > elvis.7001: rx data fs reply rename
1339 In the first line, host elvis sends a RX packet to pike. This was
1340 a RX data packet to the fs (fileserver) service, and is the start of
1341 an RPC call. The RPC call was a rename, with the old directory file id
1342 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1343 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1344 responds with a RPC reply to the rename call (which was successful, because
1345 it was a data packet and not an abort packet).
1347 In general, all AFS RPCs are decoded at least by RPC call name. Most
1348 AFS RPCs have at least some of the arguments decoded (generally only
1349 the `interesting' arguments, for some definition of interesting).
1351 The format is intended to be self-describing, but it will probably
1352 not be useful to people who are not familiar with the workings of
1355 If the -v (verbose) flag is given, the RX call number and sequence number
1358 If the -v flag is given twice, additional information is printed,
1359 such as the the RX call ID, serial number, and the RX packet flags.
1360 The MTU negotiation information is also printed from RX ack packets.
1362 If the -v flag is given three times, the security index and service id
1365 Error codes are printed for abort packets, with the exception of Ubik
1366 beacon packets (because abort packets are used to signify a yes vote
1367 for the Ubik protocol).
1369 Note that AFS requests are very large and many of the arguments won't
1370 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1371 to watch AFS traffic.
1373 AFS reply packets do not explicitly identify the RPC operation. Instead,
1374 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1375 replies using the call number and service ID. If a reply does not closely
1377 corresponding request, it might not be parsable.
1380 KIP Appletalk (DDP in UDP)
1382 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1383 and dumped as DDP packets (i.e., all the UDP header information is
1384 discarded). The file
1386 is used to translate appletalk net and node numbers to names.
1387 Lines in this file have the form
1399 The first two lines give the names of appletalk networks. The third
1400 line gives the name of a particular host (a host is distinguished
1401 from a net by the 3rd octet in the number \-
1402 a net number \fImust\fP have two octets and a host number \fImust\fP
1403 have three octets.) The number and name should be separated by
1404 whitespace (blanks or tabs).
1407 file may contain blank lines or comment lines (lines starting with
1410 Appletalk addresses are printed in the form
1416 \f(CW144.1.209.2 > icsd-net.112.220
1417 office.2 > icsd-net.112.220
1418 jssmag.149.235 > icsd-net.2\fR
1424 doesn't exist or doesn't contain an entry for some appletalk
1425 host/net number, addresses are printed in numeric form.)
1426 In the first example, NBP (DDP port 2) on net 144.1 node 209
1427 is sending to whatever is listening on port 220 of net icsd node 112.
1428 The second line is the same except the full name of the source node
1429 is known (`office'). The third line is a send from port 235 on
1430 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1431 the broadcast address (255) is indicated by a net name with no host
1432 number \- for this reason it's a good idea to keep node names and
1433 net names distinct in /etc/atalk.names).
1435 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1436 packets have their contents interpreted. Other protocols just dump
1437 the protocol name (or number if no name is registered for the
1438 protocol) and packet size.
1440 \fBNBP packets\fP are formatted like the following examples:
1444 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1445 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1446 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1450 The first line is a name lookup request for laserwriters sent by net icsd host
1451 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1452 The second line shows a reply for this request (note that it has the
1453 same id) from host jssmag.209 saying that it has a laserwriter
1454 resource named "RM1140" registered on port 250. The third line is
1455 another reply to the same request saying host techpit has laserwriter
1456 "techpit" registered on port 186.
1458 \fBATP packet\fP formatting is demonstrated by the following example:
1462 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1463 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1464 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1465 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1466 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1467 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1468 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1469 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1470 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1471 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1472 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1473 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1474 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1475 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1479 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1480 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1481 is the value of the `userdata' field in the request.
1483 Helios responds with 8 512-byte packets. The `:digit' following the
1484 transaction id gives the packet sequence number in the transaction
1485 and the number in parens is the amount of data in the packet,
1486 excluding the atp header. The `*' on packet 7 indicates that the
1489 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1490 resends them then jssmag.209 releases the transaction. Finally,
1491 jssmag.209 initiates the next request. The `*' on the request
1492 indicates that XO (`exactly once') was \fInot\fP set.
1497 Fragmented Internet datagrams are printed as
1501 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1502 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1506 (The first form indicates there are more fragments. The second
1507 indicates this is the last fragment.)
1509 \fIId\fP is the fragment id. \fISize\fP is the fragment
1510 size (in bytes) excluding the IP header. \fIOffset\fP is this
1511 fragment's offset (in bytes) in the original datagram.
1513 The fragment information is output for each fragment. The first
1514 fragment contains the higher level protocol header and the frag
1515 info is printed after the protocol info. Fragments
1516 after the first contain no higher level protocol header and the
1517 frag info is printed after the source and destination addresses.
1518 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1519 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1523 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1524 arizona > rtsg: (frag 595a:204@328)
1525 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1529 There are a couple of things to note here: First, addresses in the
1530 2nd line don't include port numbers. This is because the TCP
1531 protocol information is all in the first fragment and we have no idea
1532 what the port or sequence numbers are when we print the later fragments.
1533 Second, the tcp sequence information in the first line is printed as if there
1534 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1535 the first frag and 204 in the second). If you are looking for holes
1536 in the sequence space or trying to match up acks
1537 with packets, this can fool you.
1539 A packet with the IP \fIdon't fragment\fP flag is marked with a
1540 trailing \fB(DF)\fP.
1544 By default, all output lines are preceded by a timestamp. The timestamp
1545 is the current clock time in the form
1551 and is as accurate as the kernel's clock.
1552 The timestamp reflects the time the kernel first saw the packet. No attempt
1553 is made to account for the time lag between when the
1554 ethernet interface removed the packet from the wire and when the kernel
1555 serviced the `new packet' interrupt.
1557 traffic(1C), nit(4P), bpf(4), pcap(3)
1559 The original authors are:
1563 Steven McCanne, all of the
1564 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1566 It is currently being maintained by tcpdump.org.
1568 The current version is available via http:
1571 .I http://www.tcpdump.org/
1574 The original distribution is available via anonymous ftp:
1577 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1580 IPv6/IPsec support is added by WIDE/KAME project.
1581 This program uses Eric Young's SSLeay library, under specific configuration.
1583 Please send problems, bugs, questions, desirable enhancements, etc. to:
1586 tcpdump-workers@tcpdump.org
1589 Please send source code contributions, etc. to:
1595 NIT doesn't let you watch your own outbound traffic, BPF will.
1596 We recommend that you use the latter.
1598 Some attempt should be made to reassemble IP fragments or, at least
1599 to compute the right length for the higher level protocol.
1601 Name server inverse queries are not dumped correctly: the (empty)
1602 question section is printed rather than real query in the answer
1603 section. Some believe that inverse queries are themselves a bug and
1604 prefer to fix the program generating them rather than \fItcpdump\fP.
1606 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1608 Even if we were inclined to do anything to promote the use of
1609 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1610 networks so we'd would have no way of testing this code.
1612 A packet trace that crosses a daylight savings time change will give
1613 skewed time stamps (the time change is ignored).
1615 Filter expressions that manipulate FDDI or Token Ring headers assume
1616 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1617 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1618 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1619 accept certain packets that do not properly match the filter expression.
1621 Filter expressions on fields other than those that manipulate Token Ring
1622 headers will not correctly handle source-routed Token Ring packets.
1625 should chase header chain, but at this moment it does not.
1626 .BR "ip6 protochain"
1627 is supplied for this behavior.
1629 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1630 does not work against IPv6 packets.
1631 It only looks at IPv4 packets.
projects / tcpdump / blob