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25 .TH TCPDUMP 1 "3 January 2001"
27 tcpdump \- dump traffic on a network
81 \fITcpdump\fP prints out the headers of packets on a network interface
82 that match the boolean \fIexpression\fP.
84 .B Under SunOS with nit or bpf:
87 you must have read access to
91 .B Under Solaris with dlpi:
92 You must have read/write access to the network pseudo device, e.g.
94 .B Under HP-UX with dlpi:
95 You must be root or it must be installed setuid to root.
96 .B Under IRIX with snoop:
97 You must be root or it must be installed setuid to root.
99 You must be root or it must be installed setuid to root.
100 .B Under Ultrix and Digital UNIX:
101 Once the super-user has enabled promiscuous-mode operation using
106 You must have read access to
111 Attempt to convert network and broadcast addresses to names.
114 Exit after receiving \fIcount\fP packets.
117 Dump the compiled packet-matching code in a human readable form to
118 standard output and stop.
121 Dump packet-matching code as a
126 Dump packet-matching code as decimal numbers (preceded with a count).
129 Print the link-level header on each dump line.
132 Use \fIalgo:secret\fP for decrypting IPsec ESP packets. Algorithms may be
137 \fBcast128-cbc\fP, or
139 The default is \fBdes-cbc\fP.
140 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
141 with cryptography enabled.
142 \fIsecret\fP the ascii text for ESP secret key.
143 We cannot take arbitrary binary value at this moment.
144 The option assumes RFC2406 ESP, not RFC1827 ESP.
145 The option is only for debugging purposes, and
146 the use of this option with truly `secret' key is discouraged.
147 By presenting IPsec secret key onto command line
148 you make it visible to others, via
153 Print `foreign' internet addresses numerically rather than symbolically
154 (this option is intended to get around serious brain damage in
155 Sun's yp server \(em usually it hangs forever translating non-local
159 Use \fIfile\fP as input for the filter expression.
160 An additional expression given on the command line is ignored.
163 Listen on \fIinterface\fP.
164 If unspecified, \fItcpdump\fP searches the system interface list for the
165 lowest numbered, configured up interface (excluding loopback).
166 Ties are broken by choosing the earliest match.
168 On Linux systems with 2.2 or later kernels, an
170 argument of ``any'' can be used to capture packets from all interfaces.
171 Note that captures on the ``any'' device will not be done in promiscuous
175 Make stdout line buffered. Useful if you want to see the data
176 while capturing it. E.g.,
178 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
179 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
182 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
185 Don't print domain name qualification of host names. E.g.,
186 if you give this flag then \fItcpdump\fP will print ``nic''
187 instead of ``nic.ddn.mil''.
190 Load SMI MIB module definitions from file \fImodule\fR. This option
191 can be used several times to load several MIB modules into \fItcpdump\fP.
194 Do not run the packet-matching code optimizer. This is useful only
195 if you suspect a bug in the optimizer.
198 \fIDon't\fP put the interface
199 into promiscuous mode. Note that the interface might be in promiscuous
200 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
201 `ether host {local-hw-addr} or ether broadcast'.
204 Quick (quiet?) output. Print less protocol information so output
208 Read packets from \fIfile\fR (which was created with the -w option).
209 Standard input is used if \fIfile\fR is ``-''.
212 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
213 default of 68 (with SunOS's NIT, the minimum is actually 96).
214 68 bytes is adequate for IP, ICMP, TCP
215 and UDP but may truncate protocol information from name server and NFS
216 packets (see below). Packets truncated because of a limited snapshot
217 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
218 is the name of the protocol level at which the truncation has occurred.
219 Note that taking larger snapshots both increases
220 the amount of time it takes to process packets and, effectively,
221 decreases the amount of packet buffering. This may cause packets to be
222 lost. You should limit \fIsnaplen\fP to the smallest number that will
223 capture the protocol information you're interested in. Setting
224 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
227 Force packets selected by "\fIexpression\fP" to be interpreted the
228 specified \fItype\fR. Currently known types are
229 \fBcnfp\fR (Cisco NetFlow protocol),
230 \fBrpc\fR (Remote Procedure Call),
231 \fBrtp\fR (Real-Time Applications protocol),
232 \fBrtcp\fR (Real-Time Applications control protocol),
233 \fBsnmp\fR (Simple Network Management Protocol),
234 \fBvat\fR (Visual Audio Tool),
236 \fBwb\fR (distributed White Board).
239 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
240 If specified, \fItcpdump\fP will not print replay prevention field.
241 Since there is no protocol version field in ESP/AH specification,
242 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
245 Print absolute, rather than relative, TCP sequence numbers.
248 \fIDon't\fP print a timestamp on each dump line.
251 Print an unformatted timestamp on each dump line.
254 Print a delta (in micro-seconds) between current and previous line
258 Print a timestamp in default format proceeded by date on each dump line.
261 (Slightly more) verbose output. For example, the time to live,
262 identification, total length and options in an IP packet are printed.
263 Also enables additional packet integrity checks such as verifying the
264 IP and ICMP header checksum.
267 Even more verbose output. For example, additional fields are
268 printed from NFS reply packets.
271 Even more verbose output. For example,
272 telnet \fBSB\fP ... \fBSE\fP options
273 are printed in full. With
275 telnet options are printed in hex as well.
278 Write the raw packets to \fIfile\fR rather than parsing and printing
279 them out. They can later be printed with the \-r option.
280 Standard output is used if \fIfile\fR is ``-''.
283 Print each packet (minus its link level header) in hex.
284 The smaller of the entire packet or
286 bytes will be printed.
289 When printing hex, print ascii too. Thus if
291 is also set, the packet is printed in hex/ascii.
292 This is very handy for analysing new protocols.
295 is not also set, some parts of some packets may be printed
297 .IP "\fI expression\fP"
299 selects which packets will be dumped. If no \fIexpression\fP
300 is given, all packets on the net will be dumped. Otherwise,
301 only packets for which \fIexpression\fP is `true' will be dumped.
303 The \fIexpression\fP consists of one or more
305 Primitives usually consist of an
307 (name or number) preceded by one or more qualifiers. There are three
308 different kinds of qualifier:
310 qualifiers say what kind of thing the id name or number refers to.
316 E.g., `host foo', `net 128.3', `port 20'. If there is no type
321 qualifiers specify a particular transfer direction to and/or from
323 Possible directions are
330 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
331 there is no dir qualifier,
334 For `null' link layers (i.e. point to point protocols such as slip) the
338 qualifiers can be used to specify a desired direction.
340 qualifiers restrict the match to a particular protocol. Possible
362 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
363 no proto qualifier, all protocols consistent with the type are
364 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
365 (except the latter is not legal syntax), `net bar' means `(ip or
366 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
368 [`fddi' is actually an alias for `ether'; the parser treats them
369 identically as meaning ``the data link level used on the specified
370 network interface.'' FDDI headers contain Ethernet-like source
371 and destination addresses, and often contain Ethernet-like packet
372 types, so you can filter on these FDDI fields just as with the
373 analogous Ethernet fields. FDDI headers also contain other fields,
374 but you cannot name them explicitly in a filter expression.
376 Similarly, `tr' is an alias for `ether'; the previous paragraph's
377 statements about FDDI headers also apply to Token Ring headers.]
379 In addition to the above, there are some special `primitive' keywords
380 that don't follow the pattern:
385 and arithmetic expressions. All of these are described below.
387 More complex filter expressions are built up by using the words
392 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
393 To save typing, identical qualifier lists can be omitted. E.g.,
394 `tcp dst port ftp or ftp-data or domain' is exactly the same as
395 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
397 Allowable primitives are:
398 .IP "\fBdst host \fIhost\fR"
399 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
400 which may be either an address or a name.
401 .IP "\fBsrc host \fIhost\fR"
402 True if the IPv4/v6 source field of the packet is \fIhost\fP.
403 .IP "\fBhost \fIhost\fP
404 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
405 Any of the above host expressions can be prepended with the keywords,
406 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
409 \fBip host \fIhost\fR
412 which is equivalent to:
415 \fBether proto \fI\\ip\fB and host \fIhost\fR
418 If \fIhost\fR is a name with multiple IP addresses, each address will
419 be checked for a match.
420 .IP "\fBether dst \fIehost\fP
421 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
422 may be either a name from /etc/ethers or a number (see
425 .IP "\fBether src \fIehost\fP
426 True if the ethernet source address is \fIehost\fP.
427 .IP "\fBether host \fIehost\fP
428 True if either the ethernet source or destination address is \fIehost\fP.
429 .IP "\fBgateway\fP \fIhost\fP
430 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
431 source or destination address was \fIhost\fP but neither the IP source
432 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
433 must be found in both /etc/hosts and /etc/ethers. (An equivalent
437 \fBether host \fIehost \fBand not host \fIhost\fR
440 which can be used with either names or numbers for \fIhost / ehost\fP.)
441 This syntax does not work in IPv6-enabled configuration at this moment.
442 .IP "\fBdst net \fInet\fR"
443 True if the IPv4/v6 destination address of the packet has a network
444 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
445 or a network number (see \fInetworks(4)\fP for details).
446 .IP "\fBsrc net \fInet\fR"
447 True if the IPv4/v6 source address of the packet has a network
449 .IP "\fBnet \fInet\fR"
450 True if either the IPv4/v6 source or destination address of the packet has a network
452 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
453 True if the IP address matches \fInet\fR with the specific netmask.
454 May be qualified with \fBsrc\fR or \fBdst\fR.
455 Note that this syntax is not valid for IPv6 \fInet\fR.
456 .IP "\fBnet \fInet\fR/\fIlen\fR"
457 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
458 May be qualified with \fBsrc\fR or \fBdst\fR.
459 .IP "\fBdst port \fIport\fR"
460 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
461 destination port value of \fIport\fP.
462 The \fIport\fP can be a number or a name used in /etc/services (see
466 If a name is used, both the port
467 number and protocol are checked. If a number or ambiguous name is used,
468 only the port number is checked (e.g., \fBdst port 513\fR will print both
469 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
470 both tcp/domain and udp/domain traffic).
471 .IP "\fBsrc port \fIport\fR"
472 True if the packet has a source port value of \fIport\fP.
473 .IP "\fBport \fIport\fR"
474 True if either the source or destination port of the packet is \fIport\fP.
475 Any of the above port expressions can be prepended with the keywords,
476 \fBtcp\fP or \fBudp\fP, as in:
479 \fBtcp src port \fIport\fR
482 which matches only tcp packets whose source port is \fIport\fP.
483 .IP "\fBless \fIlength\fR"
484 True if the packet has a length less than or equal to \fIlength\fP.
485 This is equivalent to:
488 \fBlen <= \fIlength\fP.
491 .IP "\fBgreater \fIlength\fR"
492 True if the packet has a length greater than or equal to \fIlength\fP.
493 This is equivalent to:
496 \fBlen >= \fIlength\fP.
499 .IP "\fBip proto \fIprotocol\fR"
500 True if the packet is an IP packet (see
502 of protocol type \fIprotocol\fP.
503 \fIProtocol\fP can be a number or one of the names
504 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
505 \fIesp\fP, \fIudp\fP, or \fItcp\fP.
506 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
507 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
508 Note that this primitive does not chase protocol header chain.
509 .IP "\fBip6 proto \fIprotocol\fR"
510 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
511 Note that this primitive does not chase protocol header chain.
512 .IP "\fBip6 protochain \fIprotocol\fR"
513 True if the packet is IPv6 packet,
514 and contains protocol header with type \fIprotocol\fR
515 in its protocol header chain.
519 \fBip6 protochain 6\fR
522 matches any IPv6 packet with TCP protocol header in the protocol header chain.
523 The packet may contain, for example,
524 authentication header, routing header, or hop-by-hop option header,
525 between IPv6 header and TCP header.
526 The BPF code emitted by this primitive is complex and
527 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
528 so this can be somewhat slow.
529 .IP "\fBip protochain \fIprotocol\fR"
530 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
531 .IP "\fBether broadcast\fR"
532 True if the packet is an ethernet broadcast packet. The \fIether\fP
534 .IP "\fBip broadcast\fR"
535 True if the packet is an IP broadcast packet. It checks for both
536 the all-zeroes and all-ones broadcast conventions, and looks up
537 the local subnet mask.
538 .IP "\fBether multicast\fR"
539 True if the packet is an ethernet multicast packet. The \fIether\fP
541 This is shorthand for `\fBether[0] & 1 != 0\fP'.
542 .IP "\fBip multicast\fR"
543 True if the packet is an IP multicast packet.
544 .IP "\fBip6 multicast\fR"
545 True if the packet is an IPv6 multicast packet.
546 .IP "\fBether proto \fIprotocol\fR"
547 True if the packet is of ether type \fIprotocol\fR.
548 \fIProtocol\fP can be a number or one of the names
549 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
550 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP, or
552 Note these identifiers are also keywords
553 and must be escaped via backslash (\\).
554 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
555 protocol identification comes from the 802.2 Logical Link Control
556 (LLC) header, which is usually layered on top of the FDDI header.
557 \fITcpdump\fP assumes, when filtering on the protocol identifier,
558 that all FDDI packets include an LLC header, and that the LLC header
559 is in so-called SNAP format. The same applies to Token Ring.]
560 .IP "\fBdecnet src \fIhost\fR"
561 True if the DECNET source address is
563 which may be an address of the form ``10.123'', or a DECNET host
564 name. [DECNET host name support is only available on Ultrix systems
565 that are configured to run DECNET.]
566 .IP "\fBdecnet dst \fIhost\fR"
567 True if the DECNET destination address is
569 .IP "\fBdecnet host \fIhost\fR"
570 True if either the DECNET source or destination address is
572 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR"
576 \fBether proto \fIp\fR
579 where \fIp\fR is one of the above protocols.
580 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
584 \fBether proto \fIp\fR
587 where \fIp\fR is one of the above protocols.
589 \fItcpdump\fP does not currently know how to parse these protocols.
590 .IP "\fBvlan \fI[vlan_id]\fR"
591 True if the packet is an IEEE 802.1Q VLAN packet.
592 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
594 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
595 changes the decoding offsets for the remainder of \fIexpression\fR
596 on the assumption that the packet is a VLAN packet.
597 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
601 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
604 where \fIp\fR is one of the above protocols.
605 .IP "\fBiso proto \fIprotocol\fR"
606 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
607 \fIProtocol\fP can be a number or one of the names
608 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
609 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
616 where \fIp\fR is one of the above protocols.
617 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
618 .IP "\fIexpr relop expr\fR"
619 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
620 and \fIexpr\fR is an arithmetic expression composed of integer constants
621 (expressed in standard C syntax), the normal binary operators
622 [+, -, *, /, &, |], a length operator, and special packet data accessors.
624 data inside the packet, use the following syntax:
627 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
630 \fIProto\fR is one of \fBether, fddi, tr,
631 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
632 indicates the protocol layer for the index operation.
633 Note that \fItcp, udp\fR and other upper-layer protocol types only
634 apply to IPv4, not IPv6 (this will be fixed in the future).
635 The byte offset, relative to the indicated protocol layer, is
637 \fISize\fR is optional and indicates the number of bytes in the
638 field of interest; it can be either one, two, or four, and defaults to one.
639 The length operator, indicated by the keyword \fBlen\fP, gives the
640 length of the packet.
642 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
643 The expression `\fBip[0] & 0xf != 5\fP'
644 catches all IP packets with options. The expression
645 `\fBip[6:2] & 0x1fff = 0\fP'
646 catches only unfragmented datagrams and frag zero of fragmented datagrams.
647 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
649 For instance, \fBtcp[0]\fP always means the first
650 byte of the TCP \fIheader\fP, and never means the first byte of an
651 intervening fragment.
653 Primitives may be combined using:
655 A parenthesized group of primitives and operators
656 (parentheses are special to the Shell and must be escaped).
658 Negation (`\fB!\fP' or `\fBnot\fP').
660 Concatenation (`\fB&&\fP' or `\fBand\fP').
662 Alternation (`\fB||\fP' or `\fBor\fP').
664 Negation has highest precedence.
665 Alternation and concatenation have equal precedence and associate
666 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
667 are now required for concatenation.
669 If an identifier is given without a keyword, the most recent keyword
674 \fBnot host vs and ace\fR
680 \fBnot host vs and host ace\fR
683 which should not be confused with
686 \fBnot ( host vs or ace )\fR
690 Expression arguments can be passed to \fItcpdump\fP as either a single
691 argument or as multiple arguments, whichever is more convenient.
692 Generally, if the expression contains Shell metacharacters, it is
693 easier to pass it as a single, quoted argument.
694 Multiple arguments are concatenated with spaces before being parsed.
697 To print all packets arriving at or departing from \fIsundown\fP:
700 \fBtcpdump host sundown\fP
704 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
707 \fBtcpdump host helios and \\( hot or ace \\)\fP
711 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
714 \fBtcpdump ip host ace and not helios\fP
718 To print all traffic between local hosts and hosts at Berkeley:
722 tcpdump net ucb-ether
726 To print all ftp traffic through internet gateway \fIsnup\fP:
727 (note that the expression is quoted to prevent the shell from
728 (mis-)interpreting the parentheses):
732 tcpdump 'gateway snup and (port ftp or ftp-data)'
736 To print traffic neither sourced from nor destined for local hosts
737 (if you gateway to one other net, this stuff should never make it
738 onto your local net).
742 tcpdump ip and not net \fIlocalnet\fP
746 To print the start and end packets (the SYN and FIN packets) of each
747 TCP conversation that involves a non-local host.
751 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
755 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
759 tcpdump 'gateway snup and ip[2:2] > 576'
763 To print IP broadcast or multicast packets that were
765 sent via ethernet broadcast or multicast:
769 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
773 To print all ICMP packets that are not echo requests/replies (i.e., not
778 tcpdump 'icmp[0] != 8 and icmp[0] != 0'
783 The output of \fItcpdump\fP is protocol dependent. The following
784 gives a brief description and examples of most of the formats.
792 If the '-e' option is given, the link level header is printed out.
793 On ethernets, the source and destination addresses, protocol,
794 and packet length are printed.
796 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
797 the `frame control' field, the source and destination addresses,
798 and the packet length. (The `frame control' field governs the
799 interpretation of the rest of the packet. Normal packets (such
800 as those containing IP datagrams) are `async' packets, with a priority
801 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
802 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
803 the LLC header is printed if it is \fInot\fR an ISO datagram or a
804 so-called SNAP packet.
806 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
807 the `access control' and `frame control' fields, the source and
808 destination addresses, and the packet length. As on FDDI networks,
809 packets are assumed to contain an LLC packet. Regardless of whether
810 the '-e' option is specified or not, the source routing information is
811 printed for source-routed packets.
813 \fI(N.B.: The following description assumes familiarity with
814 the SLIP compression algorithm described in RFC-1144.)\fP
816 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
817 packet type, and compression information are printed out.
818 The packet type is printed first.
819 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
820 No further link information is printed for \fIip\fR packets.
821 For TCP packets, the connection identifier is printed following the type.
822 If the packet is compressed, its encoded header is printed out.
823 The special cases are printed out as
824 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
825 the sequence number (or sequence number and ack) has changed.
826 If it is not a special case,
827 zero or more changes are printed.
828 A change is indicated by U (urgent pointer), W (window), A (ack),
829 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
831 Finally, the amount of data in the packet and compressed header length
834 For example, the following line shows an outbound compressed TCP packet,
835 with an implicit connection identifier; the ack has changed by 6,
836 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
837 data and 6 bytes of compressed header:
840 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
846 Arp/rarp output shows the type of request and its arguments. The
847 format is intended to be self explanatory.
848 Here is a short sample taken from the start of an `rlogin' from
849 host \fIrtsg\fP to host \fIcsam\fP:
853 \f(CWarp who-has csam tell rtsg
854 arp reply csam is-at CSAM\fR
858 The first line says that rtsg sent an arp packet asking
859 for the ethernet address of internet host csam. Csam
860 replies with its ethernet address (in this example, ethernet addresses
861 are in caps and internet addresses in lower case).
863 This would look less redundant if we had done \fBtcpdump \-n\fP:
867 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
868 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
872 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
873 broadcast and the second is point-to-point would be visible:
877 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
878 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
882 For the first packet this says the ethernet source address is RTSG, the
883 destination is the ethernet broadcast address, the type field
884 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
888 \fI(N.B.:The following description assumes familiarity with
889 the TCP protocol described in RFC-793. If you are not familiar
890 with the protocol, neither this description nor \fItcpdump\fP will
891 be of much use to you.)\fP
893 The general format of a tcp protocol line is:
897 \fIsrc > dst: flags data-seqno ack window urgent options\fP
901 \fISrc\fP and \fIdst\fP are the source and destination IP
902 addresses and ports. \fIFlags\fP are some combination of S (SYN),
903 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
904 \fIData-seqno\fP describes the portion of sequence space covered
905 by the data in this packet (see example below).
906 \fIAck\fP is sequence number of the next data expected the other
907 direction on this connection.
908 \fIWindow\fP is the number of bytes of receive buffer space available
909 the other direction on this connection.
910 \fIUrg\fP indicates there is `urgent' data in the packet.
911 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
913 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
914 depend on the contents of the packet's tcp protocol header and
915 are output only if appropriate.
917 Here is the opening portion of an rlogin from host \fIrtsg\fP to
922 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
923 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
924 rtsg.1023 > csam.login: . ack 1 win 4096
925 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
926 csam.login > rtsg.1023: . ack 2 win 4096
927 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
928 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
929 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
930 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
934 The first line says that tcp port 1023 on rtsg sent a packet
936 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
937 The packet sequence number was 768512 and it contained no data.
938 (The notation is `first:last(nbytes)' which means `sequence
940 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
941 There was no piggy-backed ack, the available receive window was 4096
942 bytes and there was a max-segment-size option requesting an mss of
945 Csam replies with a similar packet except it includes a piggy-backed
946 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
948 The packet contained no data so there is no data sequence number.
949 Note that the ack sequence
950 number is a small integer (1). The first time \fItcpdump\fP sees a
951 tcp `conversation', it prints the sequence number from the packet.
952 On subsequent packets of the conversation, the difference between
953 the current packet's sequence number and this initial sequence number
954 is printed. This means that sequence numbers after the
955 first can be interpreted
956 as relative byte positions in the conversation's data stream (with the
957 first data byte each direction being `1'). `-S' will override this
958 feature, causing the original sequence numbers to be output.
960 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
961 in the rtsg \(-> csam side of the conversation).
962 The PUSH flag is set in the packet.
963 On the 7th line, csam says it's received data sent by rtsg up to
964 but not including byte 21. Most of this data is apparently sitting in the
965 socket buffer since csam's receive window has gotten 19 bytes smaller.
966 Csam also sends one byte of data to rtsg in this packet.
967 On the 8th and 9th lines,
968 csam sends two bytes of urgent, pushed data to rtsg.
970 If the snapshot was small enough that \fItcpdump\fP didn't capture
971 the full TCP header, it interprets as much of the header as it can
972 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
973 be interpreted. If the header contains a bogus option (one with a length
974 that's either too small or beyond the end of the header), \fItcpdump\fP
975 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
976 options (since it's impossible to tell where they start). If the header
977 length indicates options are present but the IP datagram length is not
978 long enough for the options to actually be there, \fItcpdump\fP reports
979 it as ``[\fIbad hdr length\fP]''.
981 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
983 There are 6 bits in the control bits section of the TCP header:
985 .I URG | ACK | PSH | RST | SYN | FIN
987 Let's assume that we want to watch packets used in establishing
988 a TCP connection. Recall that TCP uses a 3-way handshake protocol
989 when it initializes a new connection; the connection sequence with
990 regard to the TCP control bits is
996 2) Recipient responds with SYN, ACK
1002 Now we're interested in capturing packets that have only the
1003 SYN bit set (Step 1). Note that we don't want packets from step 2
1004 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
1005 expression for \fItcpdump\fP.
1007 Recall the structure of a TCP header without options:
1011 -----------------------------------------------------------------
1012 | source port | destination port |
1013 -----------------------------------------------------------------
1015 -----------------------------------------------------------------
1016 | acknowledgment number |
1017 -----------------------------------------------------------------
1018 | HL | reserved |U|A|P|R|S|F| window size |
1019 -----------------------------------------------------------------
1020 | TCP checksum | urgent pointer |
1021 -----------------------------------------------------------------
1024 A TCP header usually holds 20 octets of data, unless options are
1025 present. The fist line of the graph contains octets 0 - 3, the
1026 second line shows octets 4 - 7 etc.
1028 Starting to count with 0, the relevant TCP control bits are contained
1033 ----------------|---------------|---------------|----------------
1034 | HL | reserved |U|A|P|R|S|F| window size |
1035 ----------------|---------------|---------------|----------------
1036 | | 13th octet | | |
1039 Let's have a closer look at octet no. 13:
1049 We see that this octet contains 2 bytes from the reserved field.
1050 According to RFC 793 this field is reserved for future use and must
1051 be 0. The remaining 6 bits are the TCP control bits we are interested
1052 in. We have numbered the bits in this octet from 0 to 7, right to
1053 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1055 Recall that we want to capture packets with only SYN set.
1056 Let's see what happens to octet 13 if a TCP datagram arrives
1057 with the SYN bit set in its header:
1067 We already mentioned that bits number 7 and 6 belong to the
1068 reserved field, so they must must be 0. Looking at the
1069 control bits section we see that only bit number 1 (SYN) is set.
1071 Assuming that octet number 13 is an 8-bit unsigned integer in
1072 network byte order, the binary value of this octet is
1076 and its decimal representation is
1080 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1083 We're almost done, because now we know that if only SYN is set,
1084 the value of the 13th octet in the TCP header, when interpreted
1085 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1087 This relationship can be expressed as
1093 We can use this expression as the filter for \fItcpdump\fP in order
1094 to watch packets which have only SYN set:
1097 tcpdump -i xl0 tcp[13] == 2
1100 The expression says "let the 13th octet of a TCP datagram have
1101 the decimal value 2", which is exactly what we want.
1103 Now, let's assume that we need to capture SYN packets, but we
1104 don't care if ACK or any other TCP control bit is set at the
1105 same time. Let's see what happens to octet 13 when a TCP datagram
1106 with SYN-ACK set arrives:
1116 Now bits 1 and 4 are set in the 13th octet. The binary value of
1121 which translates to decimal
1125 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1128 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1129 expression, because that would select only those packets that have
1130 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1131 if ACK or any other control bit is set as long as SYN is set.
1133 In order to achieve our goal, we need to logically AND the
1134 binary value of octet 13 with some other value to preserve
1135 the SYN bit. We know that we want SYN to be set in any case,
1136 so we'll logically AND the value in the 13th octet with
1137 the binary value of a SYN:
1141 00010010 SYN-ACK 00000010 SYN
1142 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1144 = 00000010 = 00000010
1147 We see that this AND operation delivers the same result
1148 regardless whether ACK or another TCP control bit is set.
1149 The decimal representation of the AND value as well as
1150 the result of this operation is 2 (binary 00000010),
1151 so we know that for packets with SYN set the following
1152 relation must hold true:
1154 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1156 This points us to the \fItcpdump\fP filter expression
1159 tcpdump -i xl0 'tcp[13] & 2 == 2'
1162 Note that you should use single quotes or a backslash
1163 in the expression to hide the AND ('&') special character
1169 UDP format is illustrated by this rwho packet:
1173 \f(CWactinide.who > broadcast.who: udp 84\fP
1177 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1178 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1179 broadcast address. The packet contained 84 bytes of user data.
1181 Some UDP services are recognized (from the source or destination
1182 port number) and the higher level protocol information printed.
1183 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1184 RPC calls (RFC-1050) to NFS.
1186 UDP Name Server Requests
1188 \fI(N.B.:The following description assumes familiarity with
1189 the Domain Service protocol described in RFC-1035. If you are not familiar
1190 with the protocol, the following description will appear to be written
1193 Name server requests are formatted as
1197 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1199 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1203 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1204 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1205 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1206 was set. The query length was 37 bytes, not including the UDP and
1207 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1208 so the op field was omitted. If the op had been anything else, it would
1209 have been printed between the `3' and the `+'.
1210 Similarly, the qclass was the normal one,
1211 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1212 immediately after the `A'.
1214 A few anomalies are checked and may result in extra fields enclosed in
1215 square brackets: If a query contains an answer, name server or
1221 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1222 is the appropriate count.
1223 If any of the response bits are set (AA, RA or rcode) or any of the
1224 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1225 is printed, where \fIx\fP is the hex value of header bytes two and three.
1227 UDP Name Server Responses
1229 Name server responses are formatted as
1233 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1235 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1236 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1240 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1241 with 3 answer records, 3 name server records and 7 authority records.
1242 The first answer record is type A (address) and its data is internet
1243 address 128.32.137.3. The total size of the response was 273 bytes,
1244 excluding UDP and IP headers. The op (Query) and response code
1245 (NoError) were omitted, as was the class (C_IN) of the A record.
1247 In the second example, \fIhelios\fP responds to query 2 with a
1248 response code of non-existent domain (NXDomain) with no answers,
1249 one name server and no authority records. The `*' indicates that
1250 the \fIauthoritative answer\fP bit was set. Since there were no
1251 answers, no type, class or data were printed.
1253 Other flag characters that might appear are `\-' (recursion available,
1254 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1255 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1258 Note that name server requests and responses tend to be large and the
1259 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1260 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1261 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1262 has worked well for me.
1267 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1268 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1269 NetBEUI SMB data is also done.
1271 By default a fairly minimal decode is done, with a much more detailed
1272 decode done if -v is used. Be warned that with -v a single SMB packet
1273 may take up a page or more, so only use -v if you really want all the
1276 If you are decoding SMB sessions containing unicode strings then you
1277 may wish to set the environment variable USE_UNICODE to 1. A patch to
1278 auto-detect unicode srings would be welcome.
1280 For information on SMB packet formats and what all te fields mean see
1281 www.cifs.org or the pub/samba/specs/ directory on your favourite
1282 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1286 NFS Requests and Replies
1288 Sun NFS (Network File System) requests and replies are printed as:
1292 \fIsrc.xid > dst.nfs: len op args\fP
1293 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1296 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1297 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1298 sushi.201b > wrl.nfs:
1299 144 lookup fh 9,74/4096.6878 "xcolors"
1300 wrl.nfs > sushi.201b:
1301 reply ok 128 lookup fh 9,74/4134.3150
1306 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1307 to \fIwrl\fP (note that the number following the src host is a
1308 transaction id, \fInot\fP the source port). The request was 112 bytes,
1309 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1310 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1311 (If one is lucky, as in this case, the file handle can be interpreted
1312 as a major,minor device number pair, followed by the inode number and
1314 \fIWrl\fP replies `ok' with the contents of the link.
1316 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1317 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1318 depends on the operation type. The format is intended to be self
1319 explanatory if read in conjunction with
1320 an NFS protocol spec.
1322 If the \-v (verbose) flag is given, additional information is printed.
1328 sushi.1372a > wrl.nfs:
1329 148 read fh 21,11/12.195 8192 bytes @ 24576
1330 wrl.nfs > sushi.1372a:
1331 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1336 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1337 which have been omitted from this example.) In the first line,
1338 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1339 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1340 second line is the first fragment of the reply, and hence is only 1472
1341 bytes long (the other bytes will follow in subsequent fragments, but
1342 these fragments do not have NFS or even UDP headers and so might not be
1343 printed, depending on the filter expression used). Because the \-v flag
1344 is given, some of the file attributes (which are returned in addition
1345 to the file data) are printed: the file type (``REG'', for regular file),
1346 the file mode (in octal), the uid and gid, and the file size.
1348 If the \-v flag is given more than once, even more details are printed.
1350 Note that NFS requests are very large and much of the detail won't be printed
1351 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1354 NFS reply packets do not explicitly identify the RPC operation. Instead,
1355 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1356 replies using the transaction ID. If a reply does not closely follow the
1357 corresponding request, it might not be parsable.
1359 AFS Requests and Replies
1361 Transarc AFS (Andrew File System) requests and replies are printed
1367 \fIsrc.sport > dst.dport: rx packet-type\fP
1368 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1369 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1372 elvis.7001 > pike.afsfs:
1373 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1374 new fid 536876964/1/1 ".newsrc"
1375 pike.afsfs > elvis.7001: rx data fs reply rename
1380 In the first line, host elvis sends a RX packet to pike. This was
1381 a RX data packet to the fs (fileserver) service, and is the start of
1382 an RPC call. The RPC call was a rename, with the old directory file id
1383 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1384 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1385 responds with a RPC reply to the rename call (which was successful, because
1386 it was a data packet and not an abort packet).
1388 In general, all AFS RPCs are decoded at least by RPC call name. Most
1389 AFS RPCs have at least some of the arguments decoded (generally only
1390 the `interesting' arguments, for some definition of interesting).
1392 The format is intended to be self-describing, but it will probably
1393 not be useful to people who are not familiar with the workings of
1396 If the -v (verbose) flag is given twice, acknowledgement packets and
1397 additional header information is printed, such as the the RX call ID,
1398 call number, sequence number, serial number, and the RX packet flags.
1400 If the -v flag is given twice, additional information is printed,
1401 such as the the RX call ID, serial number, and the RX packet flags.
1402 The MTU negotiation information is also printed from RX ack packets.
1404 If the -v flag is given three times, the security index and service id
1407 Error codes are printed for abort packets, with the exception of Ubik
1408 beacon packets (because abort packets are used to signify a yes vote
1409 for the Ubik protocol).
1411 Note that AFS requests are very large and many of the arguments won't
1412 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1413 to watch AFS traffic.
1415 AFS reply packets do not explicitly identify the RPC operation. Instead,
1416 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1417 replies using the call number and service ID. If a reply does not closely
1419 corresponding request, it might not be parsable.
1422 KIP Appletalk (DDP in UDP)
1424 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1425 and dumped as DDP packets (i.e., all the UDP header information is
1426 discarded). The file
1428 is used to translate appletalk net and node numbers to names.
1429 Lines in this file have the form
1441 The first two lines give the names of appletalk networks. The third
1442 line gives the name of a particular host (a host is distinguished
1443 from a net by the 3rd octet in the number \-
1444 a net number \fImust\fP have two octets and a host number \fImust\fP
1445 have three octets.) The number and name should be separated by
1446 whitespace (blanks or tabs).
1449 file may contain blank lines or comment lines (lines starting with
1452 Appletalk addresses are printed in the form
1458 \f(CW144.1.209.2 > icsd-net.112.220
1459 office.2 > icsd-net.112.220
1460 jssmag.149.235 > icsd-net.2\fR
1466 doesn't exist or doesn't contain an entry for some appletalk
1467 host/net number, addresses are printed in numeric form.)
1468 In the first example, NBP (DDP port 2) on net 144.1 node 209
1469 is sending to whatever is listening on port 220 of net icsd node 112.
1470 The second line is the same except the full name of the source node
1471 is known (`office'). The third line is a send from port 235 on
1472 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1473 the broadcast address (255) is indicated by a net name with no host
1474 number \- for this reason it's a good idea to keep node names and
1475 net names distinct in /etc/atalk.names).
1477 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1478 packets have their contents interpreted. Other protocols just dump
1479 the protocol name (or number if no name is registered for the
1480 protocol) and packet size.
1482 \fBNBP packets\fP are formatted like the following examples:
1486 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1487 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1488 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1492 The first line is a name lookup request for laserwriters sent by net icsd host
1493 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1494 The second line shows a reply for this request (note that it has the
1495 same id) from host jssmag.209 saying that it has a laserwriter
1496 resource named "RM1140" registered on port 250. The third line is
1497 another reply to the same request saying host techpit has laserwriter
1498 "techpit" registered on port 186.
1500 \fBATP packet\fP formatting is demonstrated by the following example:
1504 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1505 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1506 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1507 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1508 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1509 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1510 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1511 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1512 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1513 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1514 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1515 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1516 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1517 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1521 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1522 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1523 is the value of the `userdata' field in the request.
1525 Helios responds with 8 512-byte packets. The `:digit' following the
1526 transaction id gives the packet sequence number in the transaction
1527 and the number in parens is the amount of data in the packet,
1528 excluding the atp header. The `*' on packet 7 indicates that the
1531 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1532 resends them then jssmag.209 releases the transaction. Finally,
1533 jssmag.209 initiates the next request. The `*' on the request
1534 indicates that XO (`exactly once') was \fInot\fP set.
1539 Fragmented Internet datagrams are printed as
1543 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1544 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1548 (The first form indicates there are more fragments. The second
1549 indicates this is the last fragment.)
1551 \fIId\fP is the fragment id. \fISize\fP is the fragment
1552 size (in bytes) excluding the IP header. \fIOffset\fP is this
1553 fragment's offset (in bytes) in the original datagram.
1555 The fragment information is output for each fragment. The first
1556 fragment contains the higher level protocol header and the frag
1557 info is printed after the protocol info. Fragments
1558 after the first contain no higher level protocol header and the
1559 frag info is printed after the source and destination addresses.
1560 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1561 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1565 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1566 arizona > rtsg: (frag 595a:204@328)
1567 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1571 There are a couple of things to note here: First, addresses in the
1572 2nd line don't include port numbers. This is because the TCP
1573 protocol information is all in the first fragment and we have no idea
1574 what the port or sequence numbers are when we print the later fragments.
1575 Second, the tcp sequence information in the first line is printed as if there
1576 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1577 the first frag and 204 in the second). If you are looking for holes
1578 in the sequence space or trying to match up acks
1579 with packets, this can fool you.
1581 A packet with the IP \fIdon't fragment\fP flag is marked with a
1582 trailing \fB(DF)\fP.
1586 By default, all output lines are preceded by a timestamp. The timestamp
1587 is the current clock time in the form
1593 and is as accurate as the kernel's clock.
1594 The timestamp reflects the time the kernel first saw the packet. No attempt
1595 is made to account for the time lag between when the
1596 ethernet interface removed the packet from the wire and when the kernel
1597 serviced the `new packet' interrupt.
1601 The original authors are:
1605 Steven McCanne, all of the
1606 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1608 It is currently being maintained by tcpdump.org.
1610 The current version is available via http:
1613 .I http://www.tcpdump.org/
1616 The original distribution is available via anonymous ftp:
1619 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1622 IPv6/IPsec support is added by WIDE/KAME project.
1623 This program uses Eric Young's SSLeay library, under specific configuration.
1625 Please send problems, bugs, questions, desirable enhancements, etc. to:
1628 tcpdump-workers@tcpdump.org
1631 Please send source code contributions, etc. to:
1637 NIT doesn't let you watch your own outbound traffic, BPF will.
1638 We recommend that you use the latter.
1640 On Linux systems with 2.0[.x] kernels:
1642 packets on the loopback device will be seen twice;
1644 packet filtering cannot be done in the kernel, so that all packets must
1645 be copied from the kernel in order to be filtered in user mode;
1647 all of a packet, not just the part that's within the snapshot length,
1648 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1649 asked to copy only part of a packet to userland, will not report the
1650 true length of the packet; this would cause most IP packets to get an
1654 We recommend that you upgrade to a 2.2 or later kernel.
1656 Some attempt should be made to reassemble IP fragments or, at least
1657 to compute the right length for the higher level protocol.
1659 Name server inverse queries are not dumped correctly: the (empty)
1660 question section is printed rather than real query in the answer
1661 section. Some believe that inverse queries are themselves a bug and
1662 prefer to fix the program generating them rather than \fItcpdump\fP.
1664 A packet trace that crosses a daylight savings time change will give
1665 skewed time stamps (the time change is ignored).
1667 Filter expressions that manipulate FDDI or Token Ring headers assume
1668 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1669 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1670 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1671 accept certain packets that do not properly match the filter expression.
1673 Filter expressions on fields other than those that manipulate Token Ring
1674 headers will not correctly handle source-routed Token Ring packets.
1677 should chase header chain, but at this moment it does not.
1678 .BR "ip6 protochain"
1679 is supplied for this behavior.
1681 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1682 does not work against IPv6 packets.
1683 It only looks at IPv4 packets.
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