NetBSD/usr.sbin/tcpdump/tcpdump.8

.\" $NetBSD: tcpdump.8,v 1.5 1997/10/03 19:55:59 christos Exp $
.\"
.\" @(#) Header: tcpdump.1,v 1.67 97/06/30 16:31:50 leres Exp  (LBL)
.\"
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.\"	The Regents of the University of California.  All rights reserved.
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that: (1) source code distributions
.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" 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
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.\"
.TH TCPDUMP 1  "30 June 1997"
.SH NAME
tcpdump \- dump traffic on a network
.SH SYNOPSIS
.na
.B tcpdump
[
.B \-adeflnNOpqStvx
] [
.B \-c
.I count
] [
.B \-F
.I file
]
.br
.ti +8
[
.B \-i
.I interface
] [
.B \-r
.I file
]
[
.B \-s
.I snaplen
]
.br
.ti +8
[
.B \-T
.I type
]
[
.B \-w
.I file
]
[
.I expression
]
.br
.ad
.SH DESCRIPTION
.LP
\fITcpdump\fP prints out the headers of packets on a network interface
that match the boolean \fIexpression\fP.
.LP
.B Under SunOS with nit or bpf:
To run
.I tcpdump
you must have read access to
.I /dev/nit
or
.IR /dev/bpf* .
.B Under Solaris with dlpi:
You must have read access to the network pseudo device, e.g.
.IR /dev/le .
.B Under HP-UX with dlpi:
You must be root or it must be installed setuid to root.
.B Under IRIX with snoop:
You must be root or it must be installed setuid to root.
.B Under Linux:
You must be root or it must be installed setuid to root.
.B Under Ultrix and Digital UNIX:
Once the super-user has enabled promiscuous-mode operation using
.IR pfconfig (8),
any user may run
.BR tcpdump .
.B Under BSD:
You must have read access to
.IR /dev/bpf* .
.SH OPTIONS
.TP
.B \-a
Attempt to convert network and broadcast addresses to names.
.TP
.B \-c
Exit after receiving \fIcount\fP packets.
.TP
.B \-d
Dump the compiled packet-matching code in a human readable form to
standard output and stop.
.TP
.B \-dd
Dump packet-matching code as a
.B C
program fragment.
.TP
.B \-ddd
Dump packet-matching code as decimal numbers (preceded with a count).
.TP
.B \-e
Print the link-level header on each dump line.
.TP
.B \-f
Print `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).
.TP
.B \-F
Use \fIfile\fP as input for the filter expression.
An additional expression given on the command line is ignored.
.TP
.B \-i
Listen on \fIinterface\fP.
If unspecified, \fItcpdump\fP searches the system interface list for the
lowest numbered, configured up interface (excluding loopback).
Ties are broken by choosing the earliest match.
.TP
.B \-l
Make stdout line buffered.  Useful if you want to see the data
while capturing it.  E.g.,
.br
``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
.TP
.B \-n
Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
.TP
.B \-N
Don't print domain name qualification of host names.  E.g.,
if you give this flag then \fItcpdump\fP will print ``nic''
instead of ``nic.ddn.mil''.
.TP
.B \-O
Do not run the packet-matching code optimizer.  This is useful only
if you suspect a bug in the optimizer.
.TP
.B \-p
\fIDon't\fP put the interface
into promiscuous mode.  Note that the interface might be in promiscuous
mode for some other reason; hence, `-p' cannot be used as an abbreviation for
`ether host {local-hw-addr} or ether broadcast'.
.TP
.B \-q
Quick (quiet?) output.  Print less protocol information so output
lines are shorter.
.TP
.B \-r
Read packets from \fIfile\fR (which was created with the -w option).
Standard input is used if \fIfile\fR is ``-''.
.TP
.B \-s
Snarf \fIsnaplen\fP bytes of data from each packet rather than the
default of 68 (with SunOS's NIT, the minimum is actually 96).
68 bytes is adequate for IP, ICMP, TCP
and UDP but may truncate protocol information from name server and NFS
packets (see below).  Packets truncated because of a limited snapshot
are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
is the name of the protocol level at which the truncation has occurred.
Note that 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 \fIsnaplen\fP to the smallest number that will
capture the protocol information you're interested in.
.TP
.B \-T
Force packets selected by "\fIexpression\fP" to be interpreted the
specified \fItype\fR. Currently known types are
\fBrpc\fR (Remote Procedure Call),
\fBrtp\fR (Real-Time Applications protocol),
\fBrtcp\fR (Real-Time Applications control protocol),
\fBvat\fR (Visual Audio Tool),
and
\fBwb\fR (distributed White Board).
.TP
.B \-S
Print absolute, rather than relative, TCP sequence numbers.
.TP
.B \-t
\fIDon't\fP print a timestamp on each dump line.
.TP
.B \-tt
Print an unformatted timestamp on each dump line.
.TP
.B \-v
(Slightly more) verbose output.  For example, the time to live
and type of service information in an IP packet is printed.
.TP
.B \-vv
Even more verbose output.  For example, additional fields are
printed from NFS reply packets.
.TP
.B \-w
Write the raw packets to \fIfile\fR rather than parsing and printing
them out.  They can later be printed with the \-r option.
Standard output is used if \fIfile\fR is ``-''.
.TP
.B \-x
Print each packet (minus its link level header) in hex.
The smaller of the entire packet or
.I snaplen
bytes will be printed.
.IP "\fI expression\fP"
.RS
selects which packets will be dumped.  If no \fIexpression\fP
is given, all packets on the net will be dumped.  Otherwise,
only packets for which \fIexpression\fP is `true' will be dumped.
.LP
The \fIexpression\fP consists of one or more
.I primitives.
Primitives usually consist of an
.I id
(name or number) preceded by one or more qualifiers.  There are three
different kinds of qualifier:
.IP \fItype\fP
qualifiers say what kind of thing the id name or number refers to.
Possible types are
.BR host ,
.B net
and
.BR port .
E.g., `host foo', `net 128.3', `port 20'.  If there is no type
qualifier,
.B host
is assumed.
.IP \fIdir\fP
qualifiers specify a particular transfer direction to and/or from
.I id.
Possible directions are
.BR src ,
.BR dst ,
.B "src or dst"
and
.B "src and"
.BR dst .
E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.  If
there is no dir qualifier,
.B "src or dst"
is assumed.
For `null' link layers (i.e. point to point protocols such as slip) the
.B inbound
and
.B outbound
qualifiers can be used to specify a desired direction.
.IP \fIproto\fP
qualifiers restrict the match to a particular protocol.  Possible
protos are:
.BR ether ,
.BR fddi ,
.BR ip ,
.BR arp ,
.BR rarp ,
<<<<<<< tcpdump.8
.BR decnet ,
.BR lat ,
.BR moprc ,
.BR mopdl ,
.BR decnet ,
.BR lat ,
.BR sca ,
.BR moprc ,
.BR mopdl ,
.B tcp
and
.BR udp .
E.g., `ether src foo', `arp net 128.3', `tcp port 21'.  If there is
no proto qualifier, all protocols consistent with the type are
assumed.  E.g., `src foo' means `(ip or arp or rarp) src foo'
(except the latter is not legal syntax), `net bar' means `(ip or
arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
.LP
[`fddi' is actually an alias for `ether'; the parser treats them
identically as meaning ``the data link level used on the specified
network interface.''  FDDI headers contain Ethernet-like source
and destination addresses, and often contain Ethernet-like packet
types, so you can filter on these FDDI fields just as with the
analogous Ethernet fields.  FDDI headers also contain other fields,
but you cannot name them explicitly in a filter expression.]
.LP
In addition to the above, there are some special `primitive' keywords
that don't follow the pattern:
.BR gateway ,
.BR broadcast ,
.BR less ,
.B greater
and arithmetic expressions.  All of these are described below.
.LP
More complex filter expressions are built up by using the words
.BR and ,
.B or
and
.B not
to combine primitives.  E.g., `host foo and not port ftp and not port ftp-data'.
To save typing, identical qualifier lists can be omitted.  E.g.,
`tcp dst port ftp or ftp-data or domain' is exactly the same as
`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
.LP
Allowable primitives are:
.IP "\fBdst host \fIhost\fR"
True if the IP destination field of the packet is \fIhost\fP,
which may be either an address or a name.
.IP "\fBsrc host \fIhost\fR"
True if the IP source field of the packet is \fIhost\fP.
.IP "\fBhost \fIhost\fP
True if either the IP source or destination of the packet is \fIhost\fP.
Any of the above host expressions can be prepended with the keywords,
\fBip\fP, \fBarp\fP, or \fBrarp\fP as in:
.in +.5i
.nf
\fBip host \fIhost\fR
.fi
.in -.5i
which is equivalent to:
.in +.5i
.nf
\fBether proto \fI\\ip\fB and host \fIhost\fR
.fi
.in -.5i
If \fIhost\fR is a name with multiple IP addresses, each address will
be checked for a match.
.IP "\fBether dst \fIehost\fP
True if the ethernet destination address is \fIehost\fP.  \fIEhost\fP
may be either a name from /etc/ethers or a number (see
.IR ethers (3N)
for numeric format).
.IP "\fBether src \fIehost\fP
True if the ethernet source address is \fIehost\fP.
.IP "\fBether host \fIehost\fP
True if either the ethernet source or destination address is \fIehost\fP.
.IP "\fBgateway\fP \fIhost\fP
True if the packet used \fIhost\fP as a gateway.  I.e., the ethernet
source or destination address was \fIhost\fP but neither the IP source
nor the IP destination was \fIhost\fP.  \fIHost\fP must be a name and
must be found in both /etc/hosts and /etc/ethers.  (An equivalent
expression is
.in +.5i
.nf
\fBether host \fIehost \fBand not host \fIhost\fR
.fi
.in -.5i
which can be used with either names or numbers for \fIhost / ehost\fP.)
.IP "\fBdst net \fInet\fR"
True if the IP destination address of the packet has a network
number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
or a network number (see \fInetworks(4)\fP for details).
.IP "\fBsrc net \fInet\fR"
True if the IP source address of the packet has a network
number of \fInet\fP.
.IP "\fBnet \fInet\fR"
True if either the IP source or destination address of the packet has a network
number of \fInet\fP.
.IP "\fBnet \fInet\fR \fBmask \fImask\fR"
True if the IP address matches \fInet\fR with the specific netmask.
May be qualified with \fBsrc\fR or \fBdst\fR.
.IP "\fBnet \fInet\fR/\fIlen\fR"
True if the IP address matches \fInet\fR a netmask \fIlen\fR bits wide.
May be qualified with \fBsrc\fR or \fBdst\fR.
.IP "\fBdst port \fIport\fR"
True if the packet is ip/tcp or ip/udp and has a
destination port value of \fIport\fP.
The \fIport\fP can be a number or a name used in /etc/services (see
.IR tcp (4P)
and
.IR udp (4P)).
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., \fBdst port 513\fR will print both
tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
both tcp/domain and udp/domain traffic).
.IP "\fBsrc port \fIport\fR"
True if the packet has a source port value of \fIport\fP.
.IP "\fBport \fIport\fR"
True if either the source or destination port of the packet is \fIport\fP.
Any of the above port expressions can be prepended with the keywords,
\fBtcp\fP or \fBudp\fP, as in:
.in +.5i
.nf
\fBtcp src port \fIport\fR
.fi
.in -.5i
which matches only tcp packets whose source port is \fIport\fP.
.IP "\fBless \fIlength\fR"
True if the packet has a length less than or equal to \fIlength\fP.
This is equivalent to:
.in +.5i
.nf
\fBlen <= \fIlength\fP.
.fi
.in -.5i
.IP "\fBgreater \fIlength\fR"
True if the packet has a length greater than or equal to \fIlength\fP.
This is equivalent to:
.in +.5i
.nf
\fBlen >= \fIlength\fP.
.fi
.in -.5i
.IP "\fBip proto \fIprotocol\fR"
True if the packet is an ip packet (see
.IR ip (4P))
of protocol type \fIprotocol\fP.
\fIProtocol\fP can be a number or one of the names
\fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
.IP "\fBether broadcast\fR"
True if the packet is an ethernet broadcast packet.  The \fIether\fP
keyword is optional.
.IP "\fBip broadcast\fR"
True if the packet is an IP broadcast packet.  It checks for both
the all-zeroes and all-ones broadcast conventions, and looks up
the local subnet mask.
.IP "\fBether multicast\fR"
True if the packet is an ethernet multicast packet.  The \fIether\fP
keyword is optional.
This is shorthand for `\fBether[0] & 1 != 0\fP'.
.IP "\fBip multicast\fR"
True if the packet is an IP multicast packet.
.IP  "\fBether proto \fIprotocol\fR"
True if the packet is of ether type \fIprotocol\fR.
\fIProtocol\fP can be a number or a name like
\fIip\fP, \fIarp\fP, or \fIrarp\fP.
Note these identifiers are also keywords
and must be escaped via backslash (\\).
[In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
protocol identification comes from the 802.2 Logical Link Control
(LLC) header, which is usually layered on top of the FDDI header.
\fITcpdump\fP assumes, when filtering on the protocol identifier,
that all FDDI packets include an LLC header, and that the LLC header
is in so-called SNAP format.]
.IP "\fBdecnet src \fIhost\fR"
True if the DECNET source address is
.IR host ,
which may be an address of the form ``10.123'', or a DECNET host
name.  [DECNET host name support is only available on Ultrix systems
that are configured to run DECNET.]
.IP "\fBdecnet dst \fIhost\fR"
True if the DECNET destination address is
.IR host .
.IP "\fBdecnet host \fIhost\fR"
True if either the DECNET source or destination address is
.IR host .
.IP "\fBip\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
Abbreviations for:
.in +.5i
.nf
\fBether proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
.IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
Abbreviations for:
.in +.5i
.nf
\fBether proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
Note that
\fItcpdump\fP does not currently know how to parse these protocols.
.IP  "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
Abbreviations for:
.in +.5i
.nf
\fBip proto \fIp\fR
.fi
.in -.5i
where \fIp\fR is one of the above protocols.
.IP  "\fIexpr relop expr\fR"
True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
and \fIexpr\fR is an arithmetic expression composed of integer constants
(expressed in standard C syntax), the normal binary operators
[+, -, *, /, &, |], a length operator, and special packet data accessors.
To access
data inside the packet, use the following syntax:
.in +.5i
.nf
\fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
.fi
.in -.5i
\fIProto\fR is one of \fBether, fddi,
ip, arp, rarp, tcp, udp, \fRor \fBicmp\fR, and
indicates the protocol layer for the index operation.
The byte offset, relative to the indicated protocol layer, is
given by \fIexpr\fR.
\fISize\fR 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 \fBlen\fP, gives the
length of the packet.

For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
The expression `\fBip[0] & 0xf != 5\fP'
catches all IP packets with options. The expression
`\fBip[6:2] & 0x1fff = 0\fP'
catches only unfragmented datagrams and frag zero of fragmented datagrams.
This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
index operations.
For instance, \fBtcp[0]\fP always means the first
byte of the TCP \fIheader\fP, and never means the first byte of an
intervening fragment.
.LP
Primitives may be combined using:
.IP
A parenthesized group of primitives and operators
(parentheses are special to the Shell and must be escaped).
.IP
Negation (`\fB!\fP' or `\fBnot\fP').
.IP
Concatenation (`\fB&&\fP' or `\fBand\fP').
.IP
Alternation (`\fB||\fP' or `\fBor\fP').
.LP
Negation has highest precedence.
Alternation and concatenation have equal precedence and associate
left to right.  Note that explicit \fBand\fR tokens, not juxtaposition,
are now required for concatenation.
.LP
If an identifier is given without a keyword, the most recent keyword
is assumed.
For example,
.in +.5i
.nf
\fBnot host vs and ace\fR
.fi
.in -.5i
is short for
.in +.5i
.nf
\fBnot host vs and host ace\fR
.fi
.in -.5i
which should not be confused with
.in +.5i
.nf
\fBnot ( host vs or ace )\fR
.fi
.in -.5i
.LP
Expression arguments can be passed to tcpdump 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
.LP
To print all packets arriving at or departing from \fIsundown\fP:
.RS
.nf
\fBtcpdump host sundown\fP
.fi
.RE
.LP
To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
.RS
.nf
\fBtcpdump host helios and \\( hot or ace \\)\fP
.fi
.RE
.LP
To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
.RS
.nf
\fBtcpdump ip host ace and not helios\fP
.fi
.RE
.LP
To print all traffic between local hosts and hosts at Berkeley:
.RS
.nf
.B
tcpdump net ucb-ether
.fi
.RE
.LP
To print all ftp traffic through internet gateway \fIsnup\fP:
(note that the expression is quoted to prevent the shell from
(mis-)interpreting the parentheses):
.RS
.nf
.B
tcpdump 'gateway snup and (port ftp or ftp-data)'
.fi
.RE
.LP
To print traffic neither sourced from nor destined for local hosts
(if you gateway to one other net, this stuff should never make it
onto your local net).
.RS
.nf
.B
tcpdump ip and not net \fIlocalnet\fP
.fi
.RE
.LP
To print the start and end packets (the SYN and FIN packets) of each
TCP conversation that involves a non-local host.
.RS
.nf
.B
tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
.fi
.RE
.LP
To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
.RS
.nf
.B
tcpdump 'gateway snup and ip[2:2] > 576'
.fi
.RE
.LP
To print IP broadcast or multicast packets that were
.I not
sent via ethernet broadcast or multicast:
.RS
.nf
.B
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
.fi
.RE
.LP
To print all ICMP packets that are not echo requests/replies (i.e., not
ping packets):
.RS
.nf
.B
tcpdump 'icmp[0] != 8 and icmp[0] != 0"
.fi
.RE
.SH OUTPUT FORMAT
.LP
The output of \fItcpdump\fP is protocol dependent.  The following
gives a brief description and examples of most of the formats.
.de HD
.sp 1.5
.B
..
.HD
Link Level Headers
.LP
If the '-e' option is given, the link level header is printed out.
On ethernets, the source and destination addresses, protocol,
and packet length are printed.
.LP
On FDDI networks, the  '-e' option causes \fItcpdump\fP 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 IP datagrams) are `async' packets, with a priority
value between 0 and 7; for example, `\fBasync4\fR'.  Such packets
are assumed to contain an 802.2 Logical Link Control (LLC) packet;
the LLC header is printed if it is \fInot\fR an ISO datagram or a
so-called SNAP packet.
.LP
\fI(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC-1144.)\fP
.LP
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
packet type, and compression information are printed out.
The packet type is printed first.
The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
No further link information is printed for \fIip\fR packets.
For 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
\fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR 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 U (urgent pointer), W (window), A (ack),
S (sequence number), and I (packet ID), followed by a delta (+n or -n),
or a new value (=n).
Finally, the amount of data in the packet and compressed header length
are printed.
.LP
For example, the following line shows an outbound compressed 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:
.RS
.nf
\fBO ctcp * A+6 S+49 I+6 3 (6)\fP
.fi
.RE
.HD
ARP/RARP Packets
.LP
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 \fIrtsg\fP to host \fIcsam\fP:
.RS
.nf
.sp .5
\f(CWarp who-has csam tell rtsg
arp reply csam is-at CSAM\fP
.sp .5
.fi
.RE
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 (in this example, ethernet addresses
are in caps and internet addresses in lower case).
.LP
This would look less redundant if we had done \fBtcpdump \-n\fP:
.RS
.nf
.sp .5
\f(CWarp 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\fP
.fi
.RE
.LP
If we had done \fBtcpdump \-e\fP, the fact that the first packet is
broadcast and the second is point-to-point would be visible:
.RS
.nf
.sp .5
\f(CWRTSG Broadcast 0806  64: arp who-has csam tell rtsg
CSAM RTSG 0806  64: arp reply csam is-at CSAM\fP
.sp .5
.fi
.RE
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 ETHER_ARP) and the total length was 64 bytes.
.HD
TCP Packets
.LP
\fI(N.B.:The following description assumes familiarity with
the TCP protocol described in RFC-793.  If you are not familiar
with the protocol, neither this description nor tcpdump will
be of much use to you.)\fP
.LP
The general format of a tcp protocol line is:
.RS
.nf
.sp .5
\fIsrc > dst: flags data-seqno ack window urgent options\fP
.sp .5
.fi
.RE
\fISrc\fP and \fIdst\fP are the source and destination IP
addresses and ports.  \fIFlags\fP are some combination of S (SYN),
F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
\fIData-seqno\fP describes the portion of sequence space covered
by the data in this packet (see example below).
\fIAck\fP is sequence number of the next data expected the other
direction on this connection.
\fIWindow\fP is the number of bytes of receive buffer space available
the other direction on this connection.
\fIUrg\fP indicates there is `urgent' data in the packet.
\fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
.LP
\fISrc, dst\fP and \fIflags\fP are always present.  The other fields
depend on the contents of the packet's tcp protocol header and
are output only if appropriate.
.LP
Here is the opening portion of an rlogin from host \fIrtsg\fP to
host \fIcsam\fP.
.RS
.nf
.sp .5
\s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
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\fP\s+2
.sp .5
.fi
.RE
The first line says that tcp port 1023 on rtsg sent a packet
to port \fIlogin\fP
on csam.  The \fBS\fP indicates that the \fISYN\fP flag was set.
The packet sequence number was 768512 and it contained no data.
(The notation is `first:last(nbytes)' which means `sequence
numbers \fIfirst\fP
up to but not including \fIlast\fP which is \fInbytes\fP 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.
.LP
Csam replies with a similar packet except it includes a piggy-backed
ack for rtsg's SYN.  Rtsg then acks csam's SYN.  The `.' means no
flags were set.
The packet contained no data so there is no data sequence number.
Note that the ack sequence
number is a small integer (1).  The first time \fBtcpdump\fP sees a
tcp `conversation', it prints the sequence number from the packet.
On subsequent packets of the conversation, 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 conversation's data stream (with the
first data byte each direction being `1').  `-S' will override this
feature, causing the original sequence numbers to be output.
.LP
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
in the rtsg \(-> csam side of the conversation).
The 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.
.LP
If the snapshot was small enough that \fBtcpdump\fP didn't capture
the full TCP header, it interprets as much of the header as it can
and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
be interpreted.  If the header contains a bogus option (one with a length
that's either too small or beyond the end of the header), tcpdump reports
it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
it's impossible to tell where they start).  If the header length indicates
options are present but the IP datagram length is not long enough for the
options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
.HD
.B
UDP Packets
.LP
UDP format is illustrated by this rwho packet:
.RS
.nf
.sp .5
\f(CWactinide.who > broadcast.who: udp 84\fP
.sp .5
.fi
.RE
This says that port \fIwho\fP on host \fIactinide\fP sent a udp
datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
broadcast address.  The packet contained 84 bytes of user data.
.LP
Some UDP services are recognized (from the source or destination
port number) and the higher level protocol information printed.
In particular, Domain Name service requests (RFC-1034/1035) and Sun
RPC calls (RFC-1050) to NFS.
.HD
UDP Name Server Requests
.LP
\fI(N.B.:The following description assumes familiarity with
the Domain Service protocol described in RFC-1035.  If you are not familiar
with the protocol, the following description will appear to be written
in greek.)\fP
.LP
Name server requests are formatted as
.RS
.nf
.sp .5
\fIsrc > dst: id op? flags qtype qclass name (len)\fP
.sp .5
\f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
.sp .5
.fi
.RE
Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
The query id was `3'.  The `+' indicates the \fIrecursion desired\fP flag
was set.  The query length was 37 bytes, not including the UDP and
IP protocol headers.  The query operation was the normal one, \fIQuery\fP,
so the op field was omitted.  If the op had been anything else, it would
have been printed between the `3' and the `+'.
Similarly, the qclass was the normal one,
\fIC_IN\fP, and omitted.  Any other qclass would have been printed
immediately after the `A'.
.LP
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,
.IR ancount ,
.IR nscount ,
or
.I arcount
are printed as `[\fIn\fPa]', `[\fIn\fPn]' or  `[\fIn\fPau]' where \fIn\fP
is the appropriate count.
If any of the response bits are set (AA, RA or rcode) or any of the
`must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
is printed, where \fIx\fP is the hex value of header bytes two and three.
.HD
UDP Name Server Responses
.LP
Name server responses are formatted as
.RS
.nf
.sp .5
\fIsrc > dst:  id op rcode flags a/n/au type class data (len)\fP
.sp .5
\f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
.sp .5
.fi
.RE
In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
with 3 answer records, 3 name server records and 7 authority records.
The first answer record is type A (address) and its data is internet
address 128.32.137.3.  The total size of the response was 273 bytes,
excluding UDP and IP headers.  The op (Query) and response code
(NoError) were omitted, as was the class (C_IN) of the A record.
.LP
In the second example, \fIhelios\fP responds to query 2 with a
response code of non-existent domain (NXDomain) with no answers,
one name server and no authority records.  The `*' indicates that
the \fIauthoritative answer\fP bit was set.  Since there were no
answers, no type, class or data were printed.
.LP
Other flag characters that might appear are `\-' (recursion available,
RA, \fInot\fP set) and `|' (truncated message, TC, set).  If the
`question' section doesn't contain exactly one entry, `[\fIn\fPq]'
is printed.
.LP
Note that name server requests and responses tend to be large and the
default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
to print.  Use the \fB\-s\fP flag to increase the snaplen if you
need to seriously investigate name server traffic.  `\fB\-s 128\fP'
has worked well for me.

.HD
NFS Requests and Replies
.LP
Sun NFS (Network File System) requests and replies are printed as:
.RS
.nf
.sp .5
\fIsrc.xid > dst.nfs: len op args\fP
\fIsrc.nfs > dst.xid: reply stat len op results\fP
.sp .5
\f(CW
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
\fP
.sp .5
.fi
.RE
In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
to \fIwrl\fP (note that the number following the src host is a
transaction id, \fInot\fP the source port).  The request was 112 bytes,
excluding the UDP and IP headers.  The operation was a \fIreadlink\fP
(read symbolic link) on file handle (\fIfh\fP) 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.)
\fIWrl\fP replies `ok' with the contents of the link.
.LP
In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
`\fIxcolors\fP' in directory file 9,74/4096.6878.  Note that the data printed
depends on the operation type.  The format is intended to be self
explanatory if read in conjunction with
an NFS protocol spec.
.LP
If the \-v (verbose) flag is given, additional information is printed.
For example:
.RS
.nf
.sp .5
\f(CW
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
\fP
.sp .5
.fi
.RE
(\-v also prints the IP header TTL, ID, and fragmentation fields,
which have been omitted from this example.)  In the first line,
\fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
at byte offset 24576.  \fIWrl\fP replies `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 NFS or even UDP headers and so might not be
printed, depending on the filter expression used).  Because the \-v flag
is given, some of the file attributes (which are returned in addition
to the file data) are printed: the file type (``REG'', for regular file),
the file mode (in octal), the uid and gid, and the file size.
.LP
If the \-v flag is given more than once, even more details are printed.
.LP
Note that NFS requests are very large and much of the detail won't be printed
unless \fIsnaplen\fP is increased.  Try using `\fB\-s 192\fP' to watch
NFS traffic.
.LP
NFS reply packets do not explicitly identify the RPC operation.  Instead,
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
replies using the transaction ID.  If a reply does not closely follow the
corresponding request, it might not be parsable.
.HD
KIP Appletalk (DDP in UDP)
.LP
Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
and dumped as DDP packets (i.e., all the UDP header information is
discarded).  The file
.I /etc/atalk.names
is used to translate appletalk net and node numbers to names.
Lines in this file have the form
.RS
.nf
.sp .5
\fInumber	name\fP

\f(CW1.254		ether
16.1		icsd-net
1.254.110	ace\fP
.sp .5
.fi
.RE
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 \fImust\fP have two octets and a host number \fImust\fP
have three octets.)  The number and name should be separated by
whitespace (blanks or tabs).
The
.I /etc/atalk.names
file may contain blank lines or comment lines (lines starting with
a `#').
.LP
Appletalk addresses are printed in the form
.RS
.nf
.sp .5
\fInet.host.port\fP

\f(CW144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2\fP
.sp .5
.fi
.RE
(If the
.I /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, NBP (DDP port 2) on net 144.1 node 209
is sending to whatever is listening on port 220 of net icsd node 112.
The second line is the same except the full name of the source node
is known (`office').  The third line is a send from port 235 on
net jssmag node 149 to broadcast on the icsd-net NBP port (note that
the broadcast address (255) is indicated by a net name with no host
number \- for this reason it's a good idea to keep node names and
net names distinct in /etc/atalk.names).
.LP
NBP (name binding protocol) and ATP (Appletalk transaction protocol)
packets have their contents interpreted.  Other protocols just dump
the protocol name (or number if no name is registered for the
protocol) and packet size.

\fBNBP packets\fP are formatted like the following examples:
.RS
.nf
.sp .5
\s-2\f(CWicsd-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\fP\s+2
.sp .5
.fi
.RE
The first line is a name lookup request for laserwriters sent by net icsd host
112 and broadcast on net jssmag.  The nbp id for the lookup is 190.
The second line shows a reply for this request (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.

\fBATP packet\fP formatting is demonstrated by the following example:
.RS
.nf
.sp .5
\s-2\f(CWjssmag.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\fP\s+2
.sp .5
.fi
.RE
Jssmag.209 initiates transaction id 12266 with host helios by requesting
up to 8 packets (the `<0-7>').  The hex number at the end of the line
is the value of the `userdata' field in the request.
.LP
Helios responds with 8 512-byte packets.  The `:digit' following the
transaction id gives the packet sequence number in the transaction
and the number in parens is the amount of data in the packet,
excluding the atp header.  The `*' on packet 7 indicates that the
EOM bit was set.
.LP
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 `*' on the request
indicates that XO (`exactly once') was \fInot\fP set.

.HD
IP Fragmentation
.LP
Fragmented Internet datagrams are printed as
.RS
.nf
.sp .5
\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
.sp .5
.fi
.RE
(The first form indicates there are more fragments.  The second
indicates this is the last fragment.)
.LP
\fIId\fP is the fragment id.  \fISize\fP is the fragment
size (in bytes) excluding the IP header.  \fIOffset\fP is this
fragment's offset (in bytes) in the original datagram.
.LP
The fragment information is output for each fragment.  The first
fragment contains the higher level protocol header and the frag
info is printed after the protocol info.  Fragments
after the first contain no higher level protocol header and the
frag info is printed after the source and destination addresses.
For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
over a CSNET connection that doesn't appear to handle 576 byte datagrams:
.RS
.nf
.sp .5
\s-2\f(CWarizona.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\fP\s+2
.sp .5
.fi
.RE
There are a couple of things to note here:  First, addresses in the
2nd line don't include port numbers.  This is because the 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 (308 in
the first frag and 204 in the second).  If you are looking for holes
in the sequence space or trying to match up acks
with packets, this can fool you.
.LP
A packet with the IP \fIdon't fragment\fP flag is marked with a
trailing \fB(DF)\fP.
.HD
Timestamps
.LP
By default, all output lines are preceded by a timestamp.  The timestamp
is the current clock time in the form
.RS
.nf
\fIhh:mm:ss.frac\fP
.fi
.RE
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 `new packet' interrupt.
.SH "SEE ALSO"
traffic(1C), nit(4P), bpf(4), pcap(3)
.SH AUTHORS
Van Jacobson,
Craig Leres and
Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.LP
The current version is available via anonymous ftp:
.LP
.RS
.I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
.RE
.SH BUGS
Please send bug reports to tcpdump@ee.lbl.gov.
.LP
NIT doesn't let you watch your own outbound traffic, BPF will.
We recommend that you use the latter.
.LP
Some attempt should be made to reassemble IP fragments or, at least
to compute the right length for the higher level protocol.
.LP
Name server inverse queries are not dumped correctly: The (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 tcpdump.
.LP
Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
packets but aren't.
Even if we were inclined to do anything to promote the use of
Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
networks so we'd would have no way of testing this code.
.LP
A packet trace that crosses a daylight savings time change will give
skewed time stamps (the time change is ignored).
.LP
Filters expressions that manipulate FDDI headers assume that all FDDI
packets are encapsulated Ethernet packets.  This is true for IP, ARP,
and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
Therefore, the filter may inadvertently accept certain packets that
do not properly match the filter expression.