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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 "\fBvlan \fI[vlan_id]\fR"
570 True if the packet is an IEEE 802.1Q VLAN packet.
571 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
573 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
574 changes the decoding offsets for the remainder of \fIexpression\fR
575 on the assumption that the packet is a VLAN packet.
576 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
580 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
583 where \fIp\fR is one of the above protocols.
584 .IP "\fIexpr relop expr\fR"
585 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
586 and \fIexpr\fR is an arithmetic expression composed of integer constants
587 (expressed in standard C syntax), the normal binary operators
588 [+, -, *, /, &, |], a length operator, and special packet data accessors.
590 data inside the packet, use the following syntax:
593 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
596 \fIProto\fR is one of \fBether, fddi, tr,
597 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
598 indicates the protocol layer for the index operation.
599 Note that \fItcp, udp\fR and other upper-layer protocol types only
600 apply to IPv4, not IPv6 (this will be fixed in the future).
601 The byte offset, relative to the indicated protocol layer, is
603 \fISize\fR is optional and indicates the number of bytes in the
604 field of interest; it can be either one, two, or four, and defaults to one.
605 The length operator, indicated by the keyword \fBlen\fP, gives the
606 length of the packet.
608 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
609 The expression `\fBip[0] & 0xf != 5\fP'
610 catches all IP packets with options. The expression
611 `\fBip[6:2] & 0x1fff = 0\fP'
612 catches only unfragmented datagrams and frag zero of fragmented datagrams.
613 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
615 For instance, \fBtcp[0]\fP always means the first
616 byte of the TCP \fIheader\fP, and never means the first byte of an
617 intervening fragment.
619 Primitives may be combined using:
621 A parenthesized group of primitives and operators
622 (parentheses are special to the Shell and must be escaped).
624 Negation (`\fB!\fP' or `\fBnot\fP').
626 Concatenation (`\fB&&\fP' or `\fBand\fP').
628 Alternation (`\fB||\fP' or `\fBor\fP').
630 Negation has highest precedence.
631 Alternation and concatenation have equal precedence and associate
632 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
633 are now required for concatenation.
635 If an identifier is given without a keyword, the most recent keyword
640 \fBnot host vs and ace\fR
646 \fBnot host vs and host ace\fR
649 which should not be confused with
652 \fBnot ( host vs or ace )\fR
656 Expression arguments can be passed to \fItcpdump\fP as either a single
657 argument or as multiple arguments, whichever is more convenient.
658 Generally, if the expression contains Shell metacharacters, it is
659 easier to pass it as a single, quoted argument.
660 Multiple arguments are concatenated with spaces before being parsed.
663 To print all packets arriving at or departing from \fIsundown\fP:
666 \fBtcpdump host sundown\fP
670 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
673 \fBtcpdump host helios and \\( hot or ace \\)\fP
677 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
680 \fBtcpdump ip host ace and not helios\fP
684 To print all traffic between local hosts and hosts at Berkeley:
688 tcpdump net ucb-ether
692 To print all ftp traffic through internet gateway \fIsnup\fP:
693 (note that the expression is quoted to prevent the shell from
694 (mis-)interpreting the parentheses):
698 tcpdump 'gateway snup and (port ftp or ftp-data)'
702 To print traffic neither sourced from nor destined for local hosts
703 (if you gateway to one other net, this stuff should never make it
704 onto your local net).
708 tcpdump ip and not net \fIlocalnet\fP
712 To print the start and end packets (the SYN and FIN packets) of each
713 TCP conversation that involves a non-local host.
717 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
721 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
725 tcpdump 'gateway snup and ip[2:2] > 576'
729 To print IP broadcast or multicast packets that were
731 sent via ethernet broadcast or multicast:
735 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
739 To print all ICMP packets that are not echo requests/replies (i.e., not
744 tcpdump 'icmp[0] != 8 and icmp[0] != 0'
749 The output of \fItcpdump\fP is protocol dependent. The following
750 gives a brief description and examples of most of the formats.
758 If the '-e' option is given, the link level header is printed out.
759 On ethernets, the source and destination addresses, protocol,
760 and packet length are printed.
762 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
763 the `frame control' field, the source and destination addresses,
764 and the packet length. (The `frame control' field governs the
765 interpretation of the rest of the packet. Normal packets (such
766 as those containing IP datagrams) are `async' packets, with a priority
767 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
768 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
769 the LLC header is printed if it is \fInot\fR an ISO datagram or a
770 so-called SNAP packet.
772 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
773 the `access control' and `frame control' fields, the source and
774 destination addresses, and the packet length. As on FDDI networks,
775 packets are assumed to contain an LLC packet. Regardless of whether
776 the '-e' option is specified or not, the source routing information is
777 printed for source-routed packets.
779 \fI(N.B.: The following description assumes familiarity with
780 the SLIP compression algorithm described in RFC-1144.)\fP
782 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
783 packet type, and compression information are printed out.
784 The packet type is printed first.
785 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
786 No further link information is printed for \fIip\fR packets.
787 For TCP packets, the connection identifier is printed following the type.
788 If the packet is compressed, its encoded header is printed out.
789 The special cases are printed out as
790 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
791 the sequence number (or sequence number and ack) has changed.
792 If it is not a special case,
793 zero or more changes are printed.
794 A change is indicated by U (urgent pointer), W (window), A (ack),
795 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
797 Finally, the amount of data in the packet and compressed header length
800 For example, the following line shows an outbound compressed TCP packet,
801 with an implicit connection identifier; the ack has changed by 6,
802 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
803 data and 6 bytes of compressed header:
806 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
812 Arp/rarp output shows the type of request and its arguments. The
813 format is intended to be self explanatory.
814 Here is a short sample taken from the start of an `rlogin' from
815 host \fIrtsg\fP to host \fIcsam\fP:
819 \f(CWarp who-has csam tell rtsg
820 arp reply csam is-at CSAM\fR
824 The first line says that rtsg sent an arp packet asking
825 for the ethernet address of internet host csam. Csam
826 replies with its ethernet address (in this example, ethernet addresses
827 are in caps and internet addresses in lower case).
829 This would look less redundant if we had done \fBtcpdump \-n\fP:
833 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
834 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
838 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
839 broadcast and the second is point-to-point would be visible:
843 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
844 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
848 For the first packet this says the ethernet source address is RTSG, the
849 destination is the ethernet broadcast address, the type field
850 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
854 \fI(N.B.:The following description assumes familiarity with
855 the TCP protocol described in RFC-793. If you are not familiar
856 with the protocol, neither this description nor \fItcpdump\fP will
857 be of much use to you.)\fP
859 The general format of a tcp protocol line is:
863 \fIsrc > dst: flags data-seqno ack window urgent options\fP
867 \fISrc\fP and \fIdst\fP are the source and destination IP
868 addresses and ports. \fIFlags\fP are some combination of S (SYN),
869 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
870 \fIData-seqno\fP describes the portion of sequence space covered
871 by the data in this packet (see example below).
872 \fIAck\fP is sequence number of the next data expected the other
873 direction on this connection.
874 \fIWindow\fP is the number of bytes of receive buffer space available
875 the other direction on this connection.
876 \fIUrg\fP indicates there is `urgent' data in the packet.
877 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
879 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
880 depend on the contents of the packet's tcp protocol header and
881 are output only if appropriate.
883 Here is the opening portion of an rlogin from host \fIrtsg\fP to
888 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
889 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
890 rtsg.1023 > csam.login: . ack 1 win 4096
891 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
892 csam.login > rtsg.1023: . ack 2 win 4096
893 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
894 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
895 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
896 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
900 The first line says that tcp port 1023 on rtsg sent a packet
902 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
903 The packet sequence number was 768512 and it contained no data.
904 (The notation is `first:last(nbytes)' which means `sequence
906 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
907 There was no piggy-backed ack, the available receive window was 4096
908 bytes and there was a max-segment-size option requesting an mss of
911 Csam replies with a similar packet except it includes a piggy-backed
912 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
914 The packet contained no data so there is no data sequence number.
915 Note that the ack sequence
916 number is a small integer (1). The first time \fItcpdump\fP sees a
917 tcp `conversation', it prints the sequence number from the packet.
918 On subsequent packets of the conversation, the difference between
919 the current packet's sequence number and this initial sequence number
920 is printed. This means that sequence numbers after the
921 first can be interpreted
922 as relative byte positions in the conversation's data stream (with the
923 first data byte each direction being `1'). `-S' will override this
924 feature, causing the original sequence numbers to be output.
926 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
927 in the rtsg \(-> csam side of the conversation).
928 The PUSH flag is set in the packet.
929 On the 7th line, csam says it's received data sent by rtsg up to
930 but not including byte 21. Most of this data is apparently sitting in the
931 socket buffer since csam's receive window has gotten 19 bytes smaller.
932 Csam also sends one byte of data to rtsg in this packet.
933 On the 8th and 9th lines,
934 csam sends two bytes of urgent, pushed data to rtsg.
936 If the snapshot was small enough that \fItcpdump\fP didn't capture
937 the full TCP header, it interprets as much of the header as it can
938 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
939 be interpreted. If the header contains a bogus option (one with a length
940 that's either too small or beyond the end of the header), \fItcpdump\fP
941 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
942 options (since it's impossible to tell where they start). If the header
943 length indicates options are present but the IP datagram length is not
944 long enough for the options to actually be there, \fItcpdump\fP reports
945 it as ``[\fIbad hdr length\fP]''.
947 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
949 There are 6 bits in the control bits section of the TCP header:
951 .I URG | ACK | PSH | RST | SYN | FIN
953 Let's assume that we want to watch packets used in establishing
954 a TCP connection. Recall that TCP uses a 3-way handshake protocol
955 when it initializes a new connection; the connection sequence with
956 regard to the TCP control bits is
962 2) Recipient responds with SYN, ACK
968 Now we're interested in capturing packets that have only the
969 SYN bit set (Step 1). Note that we don't want packets from step 2
970 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
971 expression for \fItcpdump\fP.
973 Recall the structure of a TCP header without options:
977 -----------------------------------------------------------------
978 | source port | destination port |
979 -----------------------------------------------------------------
981 -----------------------------------------------------------------
982 | acknowledgment number |
983 -----------------------------------------------------------------
984 | HL | reserved |U|A|P|R|S|F| window size |
985 -----------------------------------------------------------------
986 | TCP checksum | urgent pointer |
987 -----------------------------------------------------------------
990 A TCP header usually holds 20 octets of data, unless options are
991 present. The fist line of the graph contains octets 0 - 3, the
992 second line shows octets 4 - 7 etc.
994 Starting to count with 0, the relevant TCP control bits are contained
999 ----------------|---------------|---------------|----------------
1000 | HL | reserved |U|A|P|R|S|F| window size |
1001 ----------------|---------------|---------------|----------------
1002 | | 13th octet | | |
1005 Let's have a closer look at octet no. 13:
1015 We see that this octet contains 2 bytes from the reserved field.
1016 According to RFC 793 this field is reserved for future use and must
1017 be 0. The remaining 6 bits are the TCP control bits we are interested
1018 in. We have numbered the bits in this octet from 0 to 7, right to
1019 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1021 Recall that we want to capture packets with only SYN set.
1022 Let's see what happens to octet 13 if a TCP datagram arrives
1023 with the SYN bit set in its header:
1033 We already mentioned that bits number 7 and 6 belong to the
1034 reserved field, so they must must be 0. Looking at the
1035 control bits section we see that only bit number 1 (SYN) is set.
1037 Assuming that octet number 13 is an 8-bit unsigned integer in
1038 network byte order, the binary value of this octet is
1042 and its decimal representation is
1046 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1049 We're almost done, because now we know that if only SYN is set,
1050 the value of the 13th octet in the TCP header, when interpreted
1051 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1053 This relationship can be expressed as
1059 We can use this expression as the filter for \fItcpdump\fP in order
1060 to watch packets which have only SYN set:
1063 tcpdump -i xl0 tcp[13] == 2
1066 The expression says "let the 13th octet of a TCP datagram have
1067 the decimal value 2", which is exactly what we want.
1069 Now, let's assume that we need to capture SYN packets, but we
1070 don't care if ACK or any other TCP control bit is set at the
1071 same time. Let's see what happens to octet 13 when a TCP datagram
1072 with SYN-ACK set arrives:
1082 Now bits 1 and 4 are set in the 13th octet. The binary value of
1087 which translates to decimal
1091 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1094 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1095 expression, because that would select only those packets that have
1096 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1097 if ACK or any other control bit is set as long as SYN is set.
1099 In order to achieve our goal, we need to logically AND the
1100 binary value of octet 13 with some other value to preserve
1101 the SYN bit. We know that we want SYN to be set in any case,
1102 so we'll logically AND the value in the 13th octet with
1103 the binary value of a SYN:
1107 00010010 SYN-ACK 00000010 SYN
1108 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1110 = 00000010 = 00000010
1113 We see that this AND operation delivers the same result
1114 regardless whether ACK or another TCP control bit is set.
1115 The decimal representation of the AND value as well as
1116 the result of this operation is 2 (binary 00000010),
1117 so we know that for packets with SYN set the following
1118 relation must hold true:
1120 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1122 This points us to the \fItcpdump\fP filter expression
1125 tcpdump -i xl0 'tcp[13] & 2 == 2'
1128 Note that you should use single quotes or a backslash
1129 in the expression to hide the AND ('&') special character
1135 UDP format is illustrated by this rwho packet:
1139 \f(CWactinide.who > broadcast.who: udp 84\fP
1143 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1144 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1145 broadcast address. The packet contained 84 bytes of user data.
1147 Some UDP services are recognized (from the source or destination
1148 port number) and the higher level protocol information printed.
1149 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1150 RPC calls (RFC-1050) to NFS.
1152 UDP Name Server Requests
1154 \fI(N.B.:The following description assumes familiarity with
1155 the Domain Service protocol described in RFC-1035. If you are not familiar
1156 with the protocol, the following description will appear to be written
1159 Name server requests are formatted as
1163 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1165 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1169 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1170 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1171 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1172 was set. The query length was 37 bytes, not including the UDP and
1173 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1174 so the op field was omitted. If the op had been anything else, it would
1175 have been printed between the `3' and the `+'.
1176 Similarly, the qclass was the normal one,
1177 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1178 immediately after the `A'.
1180 A few anomalies are checked and may result in extra fields enclosed in
1181 square brackets: If a query contains an answer, name server or
1187 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1188 is the appropriate count.
1189 If any of the response bits are set (AA, RA or rcode) or any of the
1190 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1191 is printed, where \fIx\fP is the hex value of header bytes two and three.
1193 UDP Name Server Responses
1195 Name server responses are formatted as
1199 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1201 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1202 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1206 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1207 with 3 answer records, 3 name server records and 7 authority records.
1208 The first answer record is type A (address) and its data is internet
1209 address 128.32.137.3. The total size of the response was 273 bytes,
1210 excluding UDP and IP headers. The op (Query) and response code
1211 (NoError) were omitted, as was the class (C_IN) of the A record.
1213 In the second example, \fIhelios\fP responds to query 2 with a
1214 response code of non-existent domain (NXDomain) with no answers,
1215 one name server and no authority records. The `*' indicates that
1216 the \fIauthoritative answer\fP bit was set. Since there were no
1217 answers, no type, class or data were printed.
1219 Other flag characters that might appear are `\-' (recursion available,
1220 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1221 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1224 Note that name server requests and responses tend to be large and the
1225 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1226 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1227 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1228 has worked well for me.
1233 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1234 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1235 NetBEUI SMB data is also done.
1237 By default a fairly minimal decode is done, with a much more detailed
1238 decode done if -v is used. Be warned that with -v a single SMB packet
1239 may take up a page or more, so only use -v if you really want all the
1242 If you are decoding SMB sessions containing unicode strings then you
1243 may wish to set the environment variable USE_UNICODE to 1. A patch to
1244 auto-detect unicode srings would be welcome.
1246 For information on SMB packet formats and what all te fields mean see
1247 www.cifs.org or the pub/samba/specs/ directory on your favourite
1248 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1252 NFS Requests and Replies
1254 Sun NFS (Network File System) requests and replies are printed as:
1258 \fIsrc.xid > dst.nfs: len op args\fP
1259 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1262 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1263 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1264 sushi.201b > wrl.nfs:
1265 144 lookup fh 9,74/4096.6878 "xcolors"
1266 wrl.nfs > sushi.201b:
1267 reply ok 128 lookup fh 9,74/4134.3150
1272 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1273 to \fIwrl\fP (note that the number following the src host is a
1274 transaction id, \fInot\fP the source port). The request was 112 bytes,
1275 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1276 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1277 (If one is lucky, as in this case, the file handle can be interpreted
1278 as a major,minor device number pair, followed by the inode number and
1280 \fIWrl\fP replies `ok' with the contents of the link.
1282 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1283 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1284 depends on the operation type. The format is intended to be self
1285 explanatory if read in conjunction with
1286 an NFS protocol spec.
1288 If the \-v (verbose) flag is given, additional information is printed.
1294 sushi.1372a > wrl.nfs:
1295 148 read fh 21,11/12.195 8192 bytes @ 24576
1296 wrl.nfs > sushi.1372a:
1297 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1302 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1303 which have been omitted from this example.) In the first line,
1304 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1305 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1306 second line is the first fragment of the reply, and hence is only 1472
1307 bytes long (the other bytes will follow in subsequent fragments, but
1308 these fragments do not have NFS or even UDP headers and so might not be
1309 printed, depending on the filter expression used). Because the \-v flag
1310 is given, some of the file attributes (which are returned in addition
1311 to the file data) are printed: the file type (``REG'', for regular file),
1312 the file mode (in octal), the uid and gid, and the file size.
1314 If the \-v flag is given more than once, even more details are printed.
1316 Note that NFS requests are very large and much of the detail won't be printed
1317 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1320 NFS reply packets do not explicitly identify the RPC operation. Instead,
1321 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1322 replies using the transaction ID. If a reply does not closely follow the
1323 corresponding request, it might not be parsable.
1325 AFS Requests and Replies
1327 Transarc AFS (Andrew File System) requests and replies are printed
1333 \fIsrc.sport > dst.dport: rx packet-type\fP
1334 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1335 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1338 elvis.7001 > pike.afsfs:
1339 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1340 new fid 536876964/1/1 ".newsrc"
1341 pike.afsfs > elvis.7001: rx data fs reply rename
1346 In the first line, host elvis sends a RX packet to pike. This was
1347 a RX data packet to the fs (fileserver) service, and is the start of
1348 an RPC call. The RPC call was a rename, with the old directory file id
1349 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1350 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1351 responds with a RPC reply to the rename call (which was successful, because
1352 it was a data packet and not an abort packet).
1354 In general, all AFS RPCs are decoded at least by RPC call name. Most
1355 AFS RPCs have at least some of the arguments decoded (generally only
1356 the `interesting' arguments, for some definition of interesting).
1358 The format is intended to be self-describing, but it will probably
1359 not be useful to people who are not familiar with the workings of
1362 If the -v (verbose) flag is given twice, acknowledgement packets and
1363 additional header information is printed, such as the the RX call ID,
1364 call number, sequence number, serial number, and the RX packet flags.
1366 If the -v flag is given twice, additional information is printed,
1367 such as the the RX call ID, serial number, and the RX packet flags.
1368 The MTU negotiation information is also printed from RX ack packets.
1370 If the -v flag is given three times, the security index and service id
1373 Error codes are printed for abort packets, with the exception of Ubik
1374 beacon packets (because abort packets are used to signify a yes vote
1375 for the Ubik protocol).
1377 Note that AFS requests are very large and many of the arguments won't
1378 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1379 to watch AFS traffic.
1381 AFS reply packets do not explicitly identify the RPC operation. Instead,
1382 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1383 replies using the call number and service ID. If a reply does not closely
1385 corresponding request, it might not be parsable.
1388 KIP Appletalk (DDP in UDP)
1390 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1391 and dumped as DDP packets (i.e., all the UDP header information is
1392 discarded). The file
1394 is used to translate appletalk net and node numbers to names.
1395 Lines in this file have the form
1407 The first two lines give the names of appletalk networks. The third
1408 line gives the name of a particular host (a host is distinguished
1409 from a net by the 3rd octet in the number \-
1410 a net number \fImust\fP have two octets and a host number \fImust\fP
1411 have three octets.) The number and name should be separated by
1412 whitespace (blanks or tabs).
1415 file may contain blank lines or comment lines (lines starting with
1418 Appletalk addresses are printed in the form
1424 \f(CW144.1.209.2 > icsd-net.112.220
1425 office.2 > icsd-net.112.220
1426 jssmag.149.235 > icsd-net.2\fR
1432 doesn't exist or doesn't contain an entry for some appletalk
1433 host/net number, addresses are printed in numeric form.)
1434 In the first example, NBP (DDP port 2) on net 144.1 node 209
1435 is sending to whatever is listening on port 220 of net icsd node 112.
1436 The second line is the same except the full name of the source node
1437 is known (`office'). The third line is a send from port 235 on
1438 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1439 the broadcast address (255) is indicated by a net name with no host
1440 number \- for this reason it's a good idea to keep node names and
1441 net names distinct in /etc/atalk.names).
1443 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1444 packets have their contents interpreted. Other protocols just dump
1445 the protocol name (or number if no name is registered for the
1446 protocol) and packet size.
1448 \fBNBP packets\fP are formatted like the following examples:
1452 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1453 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1454 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1458 The first line is a name lookup request for laserwriters sent by net icsd host
1459 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1460 The second line shows a reply for this request (note that it has the
1461 same id) from host jssmag.209 saying that it has a laserwriter
1462 resource named "RM1140" registered on port 250. The third line is
1463 another reply to the same request saying host techpit has laserwriter
1464 "techpit" registered on port 186.
1466 \fBATP packet\fP formatting is demonstrated by the following example:
1470 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1471 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1472 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1473 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1474 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1475 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1476 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1477 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1478 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1479 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1480 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1481 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1482 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1483 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1487 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1488 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1489 is the value of the `userdata' field in the request.
1491 Helios responds with 8 512-byte packets. The `:digit' following the
1492 transaction id gives the packet sequence number in the transaction
1493 and the number in parens is the amount of data in the packet,
1494 excluding the atp header. The `*' on packet 7 indicates that the
1497 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1498 resends them then jssmag.209 releases the transaction. Finally,
1499 jssmag.209 initiates the next request. The `*' on the request
1500 indicates that XO (`exactly once') was \fInot\fP set.
1505 Fragmented Internet datagrams are printed as
1509 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1510 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1514 (The first form indicates there are more fragments. The second
1515 indicates this is the last fragment.)
1517 \fIId\fP is the fragment id. \fISize\fP is the fragment
1518 size (in bytes) excluding the IP header. \fIOffset\fP is this
1519 fragment's offset (in bytes) in the original datagram.
1521 The fragment information is output for each fragment. The first
1522 fragment contains the higher level protocol header and the frag
1523 info is printed after the protocol info. Fragments
1524 after the first contain no higher level protocol header and the
1525 frag info is printed after the source and destination addresses.
1526 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1527 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1531 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1532 arizona > rtsg: (frag 595a:204@328)
1533 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1537 There are a couple of things to note here: First, addresses in the
1538 2nd line don't include port numbers. This is because the TCP
1539 protocol information is all in the first fragment and we have no idea
1540 what the port or sequence numbers are when we print the later fragments.
1541 Second, the tcp sequence information in the first line is printed as if there
1542 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1543 the first frag and 204 in the second). If you are looking for holes
1544 in the sequence space or trying to match up acks
1545 with packets, this can fool you.
1547 A packet with the IP \fIdon't fragment\fP flag is marked with a
1548 trailing \fB(DF)\fP.
1552 By default, all output lines are preceded by a timestamp. The timestamp
1553 is the current clock time in the form
1559 and is as accurate as the kernel's clock.
1560 The timestamp reflects the time the kernel first saw the packet. No attempt
1561 is made to account for the time lag between when the
1562 ethernet interface removed the packet from the wire and when the kernel
1563 serviced the `new packet' interrupt.
1565 traffic(1C), nit(4P), bpf(4), pcap(3)
1567 The original authors are:
1571 Steven McCanne, all of the
1572 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1574 It is currently being maintained by tcpdump.org.
1576 The current version is available via http:
1579 .I http://www.tcpdump.org/
1582 The original distribution is available via anonymous ftp:
1585 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1588 IPv6/IPsec support is added by WIDE/KAME project.
1589 This program uses Eric Young's SSLeay library, under specific configuration.
1591 Please send problems, bugs, questions, desirable enhancements, etc. to:
1594 tcpdump-workers@tcpdump.org
1597 Please send source code contributions, etc. to:
1603 NIT doesn't let you watch your own outbound traffic, BPF will.
1604 We recommend that you use the latter.
1606 On Linux systems with 2.0[.x] kernels, packets on the loopback device
1607 will be seen twice. In addition, if the interface is put into
1608 promiscuous mode, it will not be taken out of promiscuous mode when
1610 exits; you will have to take it out of promiscuous mode, if appropriate,
1613 command. Also, packet filtering cannot be done in the kernel, so that
1614 all packets must be copied from the kernel in order to be filtered in
1615 user mode. We recommend that you upgrade to a 2.2 or later kernel.
1617 Some attempt should be made to reassemble IP fragments or, at least
1618 to compute the right length for the higher level protocol.
1620 Name server inverse queries are not dumped correctly: the (empty)
1621 question section is printed rather than real query in the answer
1622 section. Some believe that inverse queries are themselves a bug and
1623 prefer to fix the program generating them rather than \fItcpdump\fP.
1625 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1627 Even if we were inclined to do anything to promote the use of
1628 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1629 networks so we'd would have no way of testing this code.
1631 A packet trace that crosses a daylight savings time change will give
1632 skewed time stamps (the time change is ignored).
1634 Filter expressions that manipulate FDDI or Token Ring headers assume
1635 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1636 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1637 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1638 accept certain packets that do not properly match the filter expression.
1640 Filter expressions on fields other than those that manipulate Token Ring
1641 headers will not correctly handle source-routed Token Ring packets.
1644 should chase header chain, but at this moment it does not.
1645 .BR "ip6 protochain"
1646 is supplied for this behavior.
1648 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1649 does not work against IPv6 packets.
1650 It only looks at IPv4 packets.
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