.\" $OpenBSD: tcpdump.8,v 1.22 2000/04/15 02:15:21 aaron Exp $ .\" .\" Copyright (c) 1987, 1988, 1989, 1990, 1991, 1992, 1994, 1995, 1996 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that: (1) source code distributions .\" retain the above copyright notice and this paragraph in its entirety, (2) .\" distributions including binary code include the above copyright notice and .\" this paragraph in its entirety in the documentation or other materials .\" provided with the distribution, and (3) all advertising materials mentioning .\" features or use of this software display the following acknowledgement: .\" ``This product includes software developed by the University of California, .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of .\" the University nor the names of its contributors may be used to endorse .\" or promote products derived from this software without specific prior .\" written permission. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .Dd May 25, 1999 .Dt TCPDUMP 8 .Os .Sh NAME .Nm tcpdump .Nd dump traffic on a network .Sh SYNOPSIS .Nm tcpdump .Op Fl adeflnNOpqStvxX .Op Fl c Ar count .Op Fl F Ar file .Op Fl i Ar interface .Op Fl r Ar file .Op Fl s Ar snaplen .Op Fl T Ar type .Op Fl w Ar file .Op Ar expression .Sh DESCRIPTION .Nm prints out the headers of packets on a network interface that match the boolean .Ar expression . You must have read access to .Pa /dev/bpf\&* . .Sh OPTIONS .Bl -tag -width "-ddd" .It Fl a Attempt to convert network and broadcast addresses to names. .It Fl c Ar count Exit after receiving .Ar count packets. .It Fl d Dump the compiled packet-matching code in a human readable form to standard output and stop. .It Fl dd Dump packet-matching code as a .Tn C program fragment. .It Fl ddd Dump packet-matching code as decimal numbers preceded with a count. .It Fl e Print the link-level header on each dump line. .It Fl f Print .Dq foreign internet addresses numerically rather than symbolically. This option is intended to get around serious brain damage in Sun's yp server \(em usually it hangs forever translating non-local internet numbers. .It Fl F Ar file Use .Ar file as input for the filter expression. Any additional expressions given on the command line are ignored. .It Fl i Ar interface Listen on .Ar interface . If unspecified, .Nm searches the system interface list for the lowest numbered, configured .Dq up interface (excluding loopback). Ties are broken by choosing the earliest match. .It Fl l Make stdout line buffered. Useful if you want to see the data while capturing it. e.g., .Bd -ragged -offset indent .Nm .Fl l | tee dat .Ed or .br .Bd -ragged -offset indent -compact .Nm .Fl l > dat & tail .Fl f dat .Ed .It Fl n Do not convert addresses (i.e., host addresses, port numbers, etc.) to names. .It Fl N Do not print domain name qualification of host names. For example, if you specify this flag then .Nm will print .Dq nic instead of .Dq nic.ddn.mil . .It Fl O Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer. .It Fl p Do not put the interface into promiscuous mode. The interface might be in promiscuous mode for some other reason; hence, .Fl p cannot be used as an abbreviation for .Dq ether host "{local\&-hw\&-addr}" or .Dq ether broadcast . .It Fl q Quick (quiet?) output. Print less protocol information so output lines are shorter. .It Fl r Ar file Read packets from a .Ar file which was created with the .Fl w option. Standard input is used if .Ar file is .Ql - . .It Fl s Ar snaplen Analyze at most the first .Ar snaplen bytes of data from each packet rather than the default of 68. 68 bytes is adequate for .Tn IP , .Tn ICMP , .Tn TCP , and .Tn UDP but may truncate protocol information from name server and .Tn NFS packets (see below). Packets truncated because of a limited .Ar snaplen are indicated in the output with .Dq Op \*(Ba Ns Em proto , where .Em proto is the name of the protocol level at which the truncation has occurred. Taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. This may cause packets to be lost. You should limit .Ar snaplen to the smallest number that will capture the protocol information you're interested in. .It Fl T Ar type Force packets selected by .Ar expression to be interpreted as the specified .Ar type . Currently known types are .Cm rpc .Pq Remote Procedure Call , .Cm rtp .Pq Real\&-Time Applications protocol , .Cm rtcp .Pq Real\&-Time Applications control protocol , .Cm sack .Po .Tn RFC 2018 No Selective Acknowledgements .Pc , .Cm vat .Pq Visual Audio Tool , and .Cm wb .Pq distributed White Board . .It Fl S Print absolute, rather than relative, .Tn TCP sequence numbers. .It Fl t Do not print a timestamp on each dump line. .It Fl tt Print an unformatted timestamp on each dump line. .It Fl v (Slightly more) verbose output. For example, the time to live and type of service information in an .Tn IP packet is printed. .It Fl vv Even more verbose output. For example, additional fields are printed from .Tn NFS reply packets. .It Fl w Ar file Write the raw packets to .Ar file rather than parsing and printing them out. They can be analyzed later with the .Fl r option. Standard output is used if .Ar file is .Ql - . .It Fl x Print each packet (minus its link-level header) in hex. The smaller of the entire packet or .Ar snaplen bytes will be printed. .It Fl X Like .Fl x but dumps the packet in emacs-hexl like format. .It Ar expression selects which packets will be dumped. If no .Ar expression is given, all packets on the net will be dumped. Otherwise, only packets satisfying .Ar expression will be dumped. .Pp The .Ar expression consists of one or more primitives. Primitives usually consist of an .Ar id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifiers: .Bl -tag -width "proto" .It Fa type Specify which kind of address component the .Ar id name or number refers to. Possible types are .Cm host , .Cm net and .Cm port . E.g., .Dq host foo , .Dq net 128.3 , .Dq port 20 . If there is no type qualifier, .Cm host is assumed. .It Ar dir Specify a particular transfer direction to and/or from .Ar id . Possible directions are .Cm src , .Cm dst , .Cm src or dst , and .Cm src and dst . E.g., .Dq src foo , .Dq dst net 128.3 , .Dq src or dst port ftp\&-data . If there is no .Ar dir qualifier, .Cm src or dst is assumed. For null link layers (i.e., point-to-point protocols such as .Tn SLIP ) the .Cm inbound and .Cm outbound qualifiers can be used to specify a desired direction. .It Ar proto Restrict the match to a particular protocol. Possible protocols are: .Cm ether , .Cm fddi , .Cm ip , .Cm arp , .Cm rarp , .Cm decnet , .Cm lat , .Cm moprc , .Cm mopdl , .Cm tcp , and .Cm udp . E.g., .Dq ether src foo , .Dq arp net 128.3 , .Dq tcp port 21 . If there is no protocol qualifier, all protocols consistent with the type are assumed. e.g., .Dq src foo means .Do .Pq ip or arp or rarp src foo .Dc (except the latter is not legal syntax), .Dq net bar means .Do .Pq ip or arp or rarp net bar .Dc and .Dq port 53 means .Do .Pq tcp or udp port 53 .Dc . .Pp .Cm fddi is actually an alias for .Cm ether ; the parser treats them identically as meaning .Qo the data link level used on the specified network interface .Qc . .Tn FDDI headers contain Ethernet-like source and destination addresses, and often contain Ethernet-like packet types, so you can filter on these .Tn FDDI fields just as with the analogous Ethernet fields. .Tn FDDI headers also contain other fields, but you cannot name them explicitly in a filter expression. .El .Pp In addition to the above, there are some special primitive keywords that don't follow the pattern: .Cm gateway , .Cm broadcast , .Cm less , .Cm greater , and arithmetic expressions. All of these are described below. .Pp More complex filter expressions are built up by using the words .Cm and , .Cm or , and .Cm not to combine primitives. e.g., .Do host foo and not port ftp and not port ftp-data .Dc . To save typing, identical qualifier lists can be omitted. e.g., .Dq tcp dst port ftp or ftp-data or domain is exactly the same as .Do tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain .Dc . .Pp Allowable primitives are: .Bl -tag -width "ether proto proto" .It Cm dst host Ar host True if the .Tn IP destination field of the packet is .Ar host , which may be either an address or a name. .It Cm src host Ar host True if the .Tn IP source field of the packet is .Ar host . .It Cm host Ar host True if either the .Tn IP source or destination of the packet is .Ar host . .Pp Any of the above .Ar host expressions can be prepended with the keywords, .Cm ip , .Cm arp , or .Cm rarp as in: .Bd -literal -offset indent .Cm ip host Ar host .Ed .Pp which is equivalent to: .Bd -ragged -offset indent .Cm ether proto .Ar ip .Cm Cm and host .Ar host .Pp .Ed If .Ar host is a name with multiple .Tn IP addresses, each address will be checked for a match. .It Cm ether dst Ar ehost True if the Ethernet destination address is .Ar ehost . .Ar ehost may be either a name from .Pa /etc/ethers or a number (see .Xr ethers 3 for a numeric format). .It Cm ether src Ar ehost True if the Ethernet source address is .Ar ehost . .It Cm ether host Ar ehost True if either the Ethernet source or destination address is .Ar ehost . .It Cm gateway Ar host True if the packet used .Ar host as a gateway; i.e., the Ethernet source or destination address was .Ar host but neither the .Tn IP source nor the .Tn IP destination was .Ar host . .Ar host must be a name and must be found in both .Pa /etc/hosts and .Pa /etc/ethers . An equivalent expression is .Bd -ragged -offset indent .Cm ether host .Ar ehost .Cm and not host .Ar host .Ed .Pp which can be used with either names or numbers for .Ar host Ns \&/ Ns Ar ehost . .It Cm dst net Ar net True if the .Tn IP destination address of the packet has a network number of .Ar net . .Ar net may be either a name from .Pa /etc/networks or a network number (see .Xr networks 5 for details). .It Cm src net Ar net True if the .Tn IP source address of the packet has a network number of .Ar net . .It Cm net Ar net True if either the .Tn IP source or destination address of the packet has a network number of .Ar net . .It Cm dst port Ar port True if the packet is ip/tcp or ip/udp and has a destination port value of .Ar port . The .Ar port can be a number or a name used in .Pa /etc/services (see .Xr tcp 4 and .Xr udp 4 ) . If a name is used, both the port number and protocol are checked. If a number or ambiguous name is used only the port number is checked; e.g., .Dq Cm dst port No 513 will print both tcp/login traffic and udp/who traffic, and .Dq Cm dst port No domain will print both tcp/domain and udp/domain traffic. .It Cm src port Ar port True if the packet has a source port value of .Ar port . .It Cm port Ar port True if either the source or destination port of the packet is .Ar port . .Pp Any of the above port expressions can be prepended with the keywords .Cm tcp or .Cm udp , as in: .Bd -literal -offset indent .Cm tcp src port Ar port .Ed .Pp which matches only .Tn TCP packets whose source port is .Ar port . .It Cm less Ar length True if the packet has a length less than or equal to .Ar length . This is equivalent to: .Bd -literal -offset indent .Cm len \*(<= Ar length . .Ed .Pp .It Cm greater Ar length True if the packet has a length greater than or equal to .Ar length . This is equivalent to: .Bd -literal -offset indent .Cm len \*(>= Ar length . .Ed .Pp .It Cm ip proto Ar proto True if the packet is an .Tn IP packet (see .Xr ip 4 ) of protocol type .Ar proto . .Ar proto can be a number or one of the names .Cm icmp , .Cm udp , .Cm nd , or .Cm tcp . The identifiers .Cm tcp , .Cm udp , and .Cm icmp are also shell keywords and must be escaped. .It Cm ether broadcast True if the packet is an Ethernet broadcast packet. The .Cm ether keyword is optional. .It Cm ip broadcast True if the packet is an .Tn IP broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions and looks up the local subnet mask. .It Cm ether multicast True if the packet is an Ethernet multicast packet. The .Cm ether keyword is optional. This is shorthand for .Do .Cm ether Ns [0] \&& 1 !\&= 0 .Dc . .It Cm ip multicast True if the packet is an .Tn IP multicast packet. .It Cm ether proto Ar proto True if the packet is of ether type .Ar proto . .Ar proto can be a number or a name like .Cm ip , .Cm arp , or .Cm rarp . These identifiers are also shell keywords and must be escaped. In the case of .Tn FDDI (e.g., .Dq Cm fddi protocol arp ) , the protocol identification comes from the 802.2 Logical Link Control .Pq Tn LLC header, which is usually layered on top of the .Tn FDDI header. .Nm assumes, when filtering on the protocol identifier, that all .Tn FDDI packets include an .Tn LLC header, and that the .Tn LLC header is in so-called .Tn SNAP format. .It Cm decnet src Ar host True if the .Tn DECNET source address is .Ar host , which may be an address of the form .Dq 10.123 , or a .Tn DECNET host name. .Tn DECNET host name support is only available on systems that are configured to run .Tn DECNET . .It Cm decnet dst Ar host True if the .Tn DECNET destination address is .Ar host . .It Cm decnet host Ar host True if either the .Tn DECNET source or destination address is .Ar host . .It Xo Cm ip , .Cm arp , .Cm rarp , .Cm decnet , .Cm lat , .Cm moprc , .Cm mopdl .Xc Abbreviations for: .Bd -literal -offset indent .Cm ether proto Ar p .Ed .Pp where .Ar p is one of the above protocols. .Nm does not currently know how to parse .Cm lat , .Cm moprc , or .Cm mopdl . .It Cm tcp , udp , icmp Abbreviations for: .Cm ip proto Ar p where .Ar p is one of the above protocols. .It Ar expr relop expr True if the relation holds, where .Ar relop is one of .Ql > , .Ql < , .Ql >= , .Ql <= , .Ql = , .Ql != , and .Ar expr is an arithmetic expression composed of integer constants (expressed in standard .Tn C syntax), the normal binary operators .Pf ( Ns Ql + , .Ql - , .Ql * , .Ql / , .Ql & , .Ql | ) , a length operator, and special packet data accessors. To access data inside the packet, use the following syntax: .Bd -ragged -offset indent .Ar proto Op Ar expr No : Ar size .Ed .Pp .Ar proto is one of .Cm ether , .Cm fddi , .Cm ip , .Cm arp , .Cm rarp , .Cm tcp , .Cm udp , or .Cm icmp , and indicates the protocol layer for the index operation. The byte offset, relative to the indicated protocol layer, is given by .Ar expr . .Ar size is optional and indicates the number of bytes in the field of interest; it can be either one, two, or four, and defaults to one. The length operator, indicated by the keyword .Cm len , gives the length of the packet. .Pp For example, .Dq Cm ether Ns [0] \&& 1 !\&= 0 catches all multicast traffic. The expression .Dq Cm ip Ns [0] \&& 0xf !\&= 5 catches all .Tn IP packets with options. The expression .Dq Cm ip Ns [6:2] \&& 0x1fff \&= 0 catches only unfragmented datagrams and frag zero of fragmented datagrams. This check is implicitly applied to the .Cm tcp and .C, udp index operations. For instance, .Dq Cm tcp Ns [0] always means the first byte of the .Tn TCP header, and never means the first byte of an intervening fragment. .El .Pp Primitives may be combined using a parenthesized group of primitives and operators. Parentheses are special to the shell and must be escaped. Allowed primitives and operators are: .Bd -ragged -offset indent Negation .Po .Dq Cm ! or .Dq Cm not .Pc .br Concatenation .Po .Dq Cm \&&\&& or .Dq Cm and .Pc .br Alternation .Po .Dq Cm || or .Dq Cm or .Pc .Ed .Pp Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Explicit .Cm and tokens, not juxtaposition, are now required for concatenation. .Pp If an identifier is given without a keyword, the most recent keyword is assumed. For example, .Bd -ragged -offset indent .Cm not host vs .Cm and ace .Ed .Pp is short for .Bd -ragged -offset indent .Cm not host vs .Cm and host ace .Ed .Pp which should not be confused with .Bd -ragged -offset indent .Cm not .Pq Cm host No vs Cm or No ace .Ed .Pp Expression arguments can be passed to .Nm as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains shell metacharacters, it is easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed. .Sh EXAMPLES .Pp To print all packets arriving at or departing from sundown: .Bd -ragged -offset indent .Nm .Cm host No sundown .Ed .Pp To print traffic between helios and either hot or ace: .Bd -ragged -offset indent .Nm .Cm host helios .Cm and .Pq hot Cm or No ace .Ed .Pp To print all .Tn IP packets between ace and any host except helios: .Bd -ragged -offset indent .Nm .Cm ip host ace .Cm and not helios .Ed .Pp To print all traffic between local hosts and hosts at Berkeley: .Bd -ragged -offset indent .Nm .Cm net ucb\(enether .Ed .Pp To print all .Tn FTP traffic through internet gateway snup: .Bd -ragged -offset indent .Nm \&' .Cm gateway snup .Cm and .Pq Cm port No ftp Cm or No ftp\&-data \&' .Pp The expression is quoted to prevent the shell from mis\(eninterpreting the parentheses. .Ed .Pp To print traffic neither sourced from nor destined for local hosts .Po if you gateway to one other net, this stuff should never make it onto your local net .Pc : .Bd -ragged -offset indent .Nm .Cm ip and not net localnet .Ed .Pp To print the start and end packets (the .Tn SYN and .Tn FIN packets) of each .Tn TCP connection that involves a non-local host: .Bd -ragged -offset indent .Nm \&' .Cm tcp Ns [13] \&& 3 !\&= 0 .Cm and not src and dst net localnet \&' .Ed .Pp To print .Tn IP packets longer than 576 bytes sent through gateway snup: .Bd -ragged -offset indent .Nm \&' .Cm gateway snup and ip Ns [2:2] \&> 576 \&' .Ed .Pp To print .Tn IP broadcast or multicast packets that were .Em not sent via Ethernet broadcast or multicast: .Bd -ragged -offset indent .Nm \&' .Cm ether Ns [0] \&& 1 = 0 .Cm and ip Ns [16] \&>\&= 224 \&' .Ed .Pp To print all .Tn ICMP packets that are not echo requests/replies (i.e., not ping packets): .Bd -ragged -offset indent .Nm \&' .Cm icmp Ns [0] != 8 .Cm and icmp Ns [0] !\&= 0 \&' .Ed .El .Sh OUTPUT FORMAT .Pp The output of .Nm is protocol dependent. The following gives a brief description and examples of most of the formats. .Pp .Em Link Level Headers .Pp If the .Fl e option is given, the link level header is printed out. On Ethernets, the source and destination addresses, protocol, and packet length are printed. .Pp On .Tn FDDI networks, the .Fl e option causes .Nm to print the frame control field, the source and destination addresses, and the packet length. The frame control field governs the interpretation of the rest of the packet. Normal packets (such as those containing .Tn IP datagrams) are .Dq async packets, with a priority value between 0 and 7; for example, .Sy async4 . Such packets are assumed to contain an 802.2 Logical Link Control .Pq Tn LLC packet; the .Tn LLC header is printed if it is .Em not an .Tn ISO datagram or a so-called .Tn SNAP packet. .Pp The following description assumes familiarity with the .Tn SLIP compression algorithm described in .Tn RFC 1144 . .Pp On .Tn SLIP links, a direction indicator .Po .Ql I for inbound , .Ql O for outbound .Pc , packet type, and compression information are printed out. The packet type is printed first. The three types are .Cm ip , .Cm utcp , and .Cm ctcp . No further link information is printed for .Cm ip packets. For .Tn TCP packets, the connection identifier is printed following the type. If the packet is compressed, its encoded header is printed out. The special cases are printed out as .Cm \&*S\&+ Ns Ar n and .Cm \&*SA\&+ Ns Ar n , where .Ar n is the amount by which the sequence number (or sequence number and ack) has changed. If it is not a special case, zero or more changes are printed. A change is indicated by .Sq U .Pq urgent pointer , .Sq W .Pq window , .Sq A .Pq ack , .Sq S .Pq sequence number , and .Sq I .Pq packet ID , followed by a delta .Pq \&+n or \&-n , or a new value .Pq \&=n . Finally, the amount of data in the packet and compressed header length are printed. .Pp For example, the following line shows an outbound compressed .Tn TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header: .Bd -ragged -offset indent O .Cm ctcp No \&* .Cm A No \&+6 .Cm S No \&+49 .Cm I No \&+6 3 .Pq 6 .Ed .Pp .Tn Em ARP\&/ Ns Tn Em RARP Packets .Pp arp/rarp output shows the type of request and its arguments. The format is intended to be self-explanatory. Here is a short sample taken from the start of an rlogin from host rtsg to host csam: .Bd -literal -offset indent arp who\&-has csam tell rtsg arp reply csam is\&-at CSAM .Ed .Pp In this example, Ethernet addresses are in caps and internet addresses in lower case. The first line says that rtsg sent an arp packet asking for the Ethernet address of internet host csam. csam replies with its Ethernet address CSAM. .Pp This would look less redundant if we had done .Nm .Fl n : .Bd -literal -offset indent arp who\&-has 128.3.254.6 tell 128.3.254.68 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4 .Ed .Pp If we had done .Nm .Fl e , the fact that the first packet is broadcast and the second is point-to-point would be visible: .Bd -literal -offset indent RTSG Broadcast 0806 64: arp who-has csam tell rtsg CSAM RTSG 0806 64: arp reply csam is-at CSAM .Ed .Pp For the first packet this says the Ethernet source address is RTSG, the destination is the Ethernet broadcast address, the type field contained hex 0806 (type .Dv ETHER_ARP ) and the total length was 64 bytes. .Pp .Tn Em TCP Packets .Pp The following description assumes familiarity with the .Tn TCP protocol described in .Tn RFC 793 . If you are not familiar with the protocol, neither this description nor .Nm will be of much use to you. .Pp The general format of a tcp protocol line is: .Bd -ragged -offset indent .Ar src No \&> Ar dst : .Ar flags data\&-seqno ack window urgent options .Ed .Pp .Ar src and .Ar dst are the source and destination .Tn IP addresses and ports. .Ar flags is some combination of .Sq S .Pq Tn SYN , .Sq F .Pq Tn FIN , .Sq P .Pq Tn PUSH , or .Sq R .Pq Tn RST , .Sq W .Pq Tn congestion Window reduced , .Sq E .Pq Tn ecn ECHO or a single .Ql \&. .Pq no flags . .Ar data\&-seqno describes the portion of sequence space covered by the data in this packet (see example below). .Ar ack is the sequence number of the next data expected by the other end of this connection. .Ar window is the number of bytes of receive buffer space available at the other end of this connection. .Ar urg indicates there is urgent data in the packet. .Ar options are tcp options enclosed in angle brackets (e.g., .Aq mss 1024 ) . .Pp .Ar src , Ar dst and .Ar flags are always present. The other fields depend on the contents of the packet's tcp protocol header and are output only if appropriate. .Pp Here is the opening portion of an rlogin from host rtsg to host csam. .Bd -literal -offset indent rtsg.1023 > csam.login: S 768512:768512(0) win 4096 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 rtsg.1023 > csam.login: . ack 1 win 4096 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096 csam.login > rtsg.1023: . ack 2 win 4096 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1 .Ed .Pp The first line says that tcp port 1023 on rtsg sent a packet to port login on host csam. The .Ql S indicates that the .Tn SYN flag was set. The packet sequence number was 768512 and it contained no data. The notation is .Sm off .So .Ar first : Ns Ar last .Ns Po Ns Ar nbytes .Pc .Sc .Sm on which means sequence numbers .Ar first up to but not including .Ar last which is .Ar nbytes bytes of user data. There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment-size option requesting an mss of 1024 bytes. .Pp Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's .Tn SYN . Rtsg then acks csam's .Tn SYN . The .Ql \&. means no flags were set. The packet contained no data so there is no data sequence number. The ack sequence number is a 32-bit integer. The first time .Nm sees a tcp connection, it prints the sequence number from the packet. On subsequent packets of the connnection, the difference between the current packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the connection's data stream .Po with the first data byte each direction being 1 .Pc . .Fl S will override this feature, causing the original sequence numbers to be output. .Pp On the 6th line, rtsg sends csam 19 bytes of data .Po bytes 2 through 20 in the rtsg -> csam side of the connection .Pc . The .Tn PUSH flag is set in the packet. On the 7th line, csam says it's received data sent by rtsg up to but not including byte 21. Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg. .Pp .Tn Em UDP Packets .Pp .Tn UDP format is illustrated by this rwho packet: .Bd -literal -offset indent actinide.who \&> broadcast.who: udp 84 .Ed .Pp This says that port who on host actinide sent a udp datagram to port who on host broadcast, the Internet broadcast address. The packet contained 84 bytes of user data. .Pp Some .Tn UDP services are recognized (from the source or destination port number) and the higher level protocol information printed. In particular, Domain Name service requests .Pq Tn RFC 1034/1035 and .Tn Sun RPC calls .Pq Tn RFC 1050 to .Tn NFS . .Pp .Tn Em UDP Name Server Requests .Pp The following description assumes familiarity with the Domain Service protocol described in .Tn RFC 1035 . If you are not familiar with the protocol, the following description will appear to be written in greek. .Pp Name server requests are formatted as .Bd -ragged -offset indent .Ar src > .Ar dst : .Ar id op Ns ? .Ar flags qtype qclass name .Pq Ar len .Pp e.g., .Pp h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37) .Ed .Pp Host h2opolo asked the domain server on helios for an address record .Pq Ar qtype Ns \&=A associated with the name ucbvax.berkeley.edu. The query .Ar id was 3. The .Ql + indicates the recursion desired flag was set. The query length was 37 bytes, not including the .Tn UDP and .Tn IP protocol headers. The query operation was the normal one .Pq Query so the .Ar op field was omitted. If .Ar op had been anything else, it would have been printed between the 3 and the .Ql + . Similarly, the .Ar qclass was the normal one .Pq Tn C_IN and was omitted. Any other .Ar qclass would have been printed immediately after the A. .Pp A few anomalies are checked and may result in extra fields enclosed in square brackets: if a query contains an answer, name server or authority section, .Ar ancount , .Ar nscount , or .Ar arcount are printed as .Dq Bq Ar n Ns a , .Dq Bq Ar n Ns n , or .Dq Bq Ar n Ns au where .Ar n is the appropriate count. If any of the response bits are set .Po .Tn AA , RA or rcode .Pc or any of the .Dq must be zero bits are set in bytes two and three, .Dq Bq b2\&&3\&= Ns Ar x is printed, where .Ar x is the hex value of header bytes two and three. .Pp .Tn Em UDP Name Server Responses .Pp Name server responses are formatted as .Bd -ragged -offset indent .Ar src No > Ar dst : .Ar id op rcode flags .Ar a / .Ar n / .Ar au .Ar type class data .Pq Ar len .Pp e.g., .Pp helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273) .br helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97) .Ed .Pp In the first example, helios responds to query .Ar id 3 from h2opolo with 3 answer records, 3 name server records and 7 authority records. The first answer record is type A .Pq address and its data is internet address 128.32.137.3. The total size of the response was 273 bytes, excluding .Tn UDP and .Tn IP headers. The .Ar op .Pq Query and .Ar rcode .Pq NoError were omitted, as was the .Ar class .Pq Tn C_IN of the A record. .Pp In the second example, helios responds to query .Ar op 2 with a .Ar rcode of non-existent domain .Pq NXDomain with no answers, one name server and no authority records. The .Ql * indicates that the authoritative answer bit was set. Since there were no answers, no .Ar type , .Ar class or .Ar data were printed. .Pp Other flag characters that might appear are .Ql - (recursion available, .Tn RA , .Em not set) and .Dq \*(Ba (truncated message, .Tn TC , set). If the question section doesn't contain exactly one entry, .Dq Bq Ar n Ns q is printed. .Pp Name server requests and responses tend to be large and the default .Ar snaplen of 68 bytes may not capture enough of the packet to print. Use the .Fl s flag to increase the .Ar snaplen if you need to seriously investigate name server traffic. .Dq Fl s No 128 has worked well for me. .Pp .Tn Em NFS Requests and Replies .Pp .Tn Sun NFS .Pq Network File System requests and replies are printed as: .Bd -ragged -offset indent .Ar src Ns . Ns Ar xid > .Ar dst Ns . Ns Ar nfs : .Ns Ar len .Ns Ar op args .br .Ar src Ns . Ns Ar nfs > .Ar dst Ns . Ns Ar xid : .Ns Ar reply stat len op results .Ed .Pp .Bd -literal -offset indent sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.6709: reply ok 40 readlink "../var" sushi.201b > wrl.nfs: 144 lookup fh 9,74/4096.6878 "xcolors" wrl.nfs > sushi.201b: reply ok 128 lookup fh 9,74/4134.3150 .Ed .Pp In the first line, host sushi sends a transaction with ID 6709 to wrl. The number following the src host is a transaction ID, .Em not the source port. The request was 112 bytes, excluding the .Tn UDP and .Tn IP headers. The .Ar op was a readlink (read symbolic link) on fh .Pq Dq file handle 21,24/10.731657119. If one is lucky, as in this case, the file handle can be interpreted as a major,minor device number pair, followed by the inode number and generation number. Wrl replies with a .Ar stat of ok and the contents of the link. .Pp In the third line, sushi asks wrl to lookup the name .Dq xcolors in directory file 9,74/4096.6878. The data printed depends on the operation type. The format is intended to be self-explanatory if read in conjunction with an .Tn NFS protocol spec. .Pp If the .Fl v .Pq verbose flag is given, additional information is printed. For example: .Bd -literal -offset indent sushi.1372a > wrl.nfs: 148 read fh 21,11/12.195 8192 bytes @ 24576 wrl.nfs > sushi.1372a: reply ok 1472 read REG 100664 ids 417/0 sz 29388 .Ed .Pp .Fl v also prints the .Tn IP No header Tn TTL , ID , and fragmentation fields, which have been omitted from this example. In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl replies with a .Ar stat of ok; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long. The other bytes will follow in subsequent fragments, but these fragments do not have .Tn NFS or even .Tn UDP headers and so might not be printed, depending on the filter expression used. Because the .Fl v flag is given, some of the file attributes .Po which are returned in addition to the file data .Pc are printed: the file type .Pq So REG Sc , No for regular file , the file mode .Pq in octal , the UID and GID, and the file size. .Pp If the .Fl v flag is given more than once, even more details are printed. .Pp .Tn NFS requests are very large and much of the detail won't be printed unless .Ar snaplen is increased. Try using .Dq Fl s No 192 to watch .Tn NFS traffic. .Pp .Tn NFS reply packets do not explicitly identify the .Tn RPC operation. Instead, .Nm keeps track of .Dq recent requests, and matches them to the replies using the .Ar xid .Pq transaction ID . If a reply does not closely follow the corresponding request, it might not be parsable. .Pp .Tn Em KIP AppleTalk .Em Pq Tn DDP No in Tn UDP .Pp AppleTalk .Tn DDP packets encapsulated in .Tn UDP datagrams are de-encapsulated and dumped as .Tn DDP packets .Po i.e., all the .Tn UDP header information is discarded .Pc . The file .Pa /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file have the form .Bd -literal -offset indent .Ar number name 1.254 ether 16.1 icsd-net 1.254.110 ace .Ed .Pp The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host (a host is distinguished from a net by the 3rd octet in the number; a net number .Em must have two octets and a host number .Em must have three octets). The number and name should be separated by whitespace (blanks or tabs). The .Pa /etc/atalk.names file may contain blank lines or comment lines (lines starting with a .Ql # ) . .Pp AppleTalk addresses are printed in the form .Bd -ragged -offset indent .Ar net Ns . Ns Ar host Ns . .Ns Ar port .Pp e.g., .Pp 144.1.209.2 > icsd-net.112.220 .br office.2 > icsd-net.112.220 .br jssmag.149.235 > icsd-net.2 .Ed .Pp If .Pa /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses are printed in numeric form. In the first example, .Tn NBP .Pq Tn DDP No port 2 on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd-net node 112. The second line is the same except the full name of the source node is known .Pq Dq office . The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net .Tn NBP port. The broadcast address (255) is indicated by a net name with no host number; for this reason it is a good idea to keep node names and net names distinct in .Pa /etc/atalk.names . .Pp .Tn NBP .Pq name binding protocol and .Tn ATP .Pq AppleTalk transaction protocol packets have their contents interpreted. Other protocols just dump the protocol name .Po or number if no name is registered for the protocol .Pc and packet size. .Pp .Tn NBP packets are formatted like the following examples: .Bd -literal -offet indent icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*" jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186 .Ed .Pp The first line is a name lookup request for laserwriters sent by net icsdi-net host 112 and broadcast on net jssmag. The nbp ID for the lookup is 190. The second line shows a reply for this request .Pq note that it has the same id from host jssmag.209 saying that it has a laserwriter resource named RM1140 registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter techpit registered on port 186. .Pp .Tn ATP packet formatting is demonstrated by the following example: .Bd -literal -offset indent jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002 .Ed .Pp Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets .Sm off .Pq the Dq Aq 0 \&- 7 . .Sm on The hex number at the end of the line is the value of the .Ar userdata field in the request. .Pp Helios responds with 8 512\(enbyte packets. The .Dq : Ns Ar n following the transaction id gives the packet sequence number in the transaction and the number in parentheses is the amount of data in the packet, excluding the atp header. The .Ql * on packet 7 indicates that the .Tn EOM bit was set. .Pp Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios resends them then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The .Ql * on the request indicates that XO .Pq exactly once was .Em not set. .Pp .Tn Em IP Fragmentation .Pp Fragmented Internet datagrams are printed as .Bd -ragged -offset indent .Po .Cm frag Ar id : .Ar size @ .Ar offset .Op \&+ .Pc .Ed .Pp A .Ql + indicates there are more fragments. The last fragment will have no .Ql + . .Pp .Ar id is the fragment ID. .Ar size is the fragment size .Pq in bytes excluding the .Tn IP header. .Ar offset is this fragment's offset .Pq in bytes in the original datagram. .Pp The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the fragment info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the fragment info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl\(enrtsg.arpa over a .Tn CSNET connection that doesn't appear to handle 576 byte datagrams: .Bd -literal -offset indent arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+) arizona > rtsg: (frag 595a:204@328) rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560 .Ed .Pp There are a couple of things to note here: first, addresses in the 2nd line don't include port numbers. This is because the .Tn TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes .Po 308 in the first frag and 204 in the second .Pc . If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you. .Pp A packet with the .Tn IP .Sy don\&'t fragment flag is marked with a trailing .Dq Pq Tn DF . .Pp .Em Timestamps .Pp By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form .Sm off .Ar hh : mm : ss . frac .Sm on and is as accurate as the kernel's clock. The timestamp reflects the time the kernel first saw the packet. No attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire and when the kernel serviced the .Dq new packet interrupt. .Sh SEE ALSO .\" traffic(1C), nit(4P), .Xr bpf 4 , .Xr pcap 3 .Sh AUTHORS Van Jacobson .Pq van@ee.lbl.gov , Craig Leres .Pq leres@ee.lbl.gov and Steven McCanne .Pq mccanne@ee.lbl.gov , all of the Lawrence Berkeley Laboratory, University of California, Berkeley, CA. .Sh BUGS Please send bug reports to tcpdump@ee.lbl.gov or libpcap@ee.lbl.gov. .Pp Some attempt should be made to reassemble .Tn IP fragments or, at least to compute the right length for the higher level protocol. .Pp Name server inverse queries are not dumped correctly: The .Pq empty question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than .Nm tcpdump . .Pp Apple Ethertalk .Tn DDP packets could be dumped as easily as .Tn KIP DDP packets but aren't. Even if we were inclined to do anything to promote the use of Ethertalk (we aren't, .Tn LBL doesn't allow Ethertalk on any of its networks so we'd would have no way of testing this code). .Pp A packet trace that crosses a daylight saving time change will give skewed time stamps (the time change is ignored). .Pp Filter expressions that manipulate .Tn FDDI headers assume that all .Tn FDDI packets are encapsulated Ethernet packets. This is true for .Tn IP , .Tn ARP , and .Tn DECNET Phase IV, but is not true for protocols such as .Tn ISO CLNS . Therefore, the filter may inadvertently accept certain packets that do not properly match the filter expression.