usr.sbin/tcpdump/tcpdump.8

.\"	$OpenBSD: tcpdump.8,v 1.29 2001/09/03 13:27:14 jakob Exp $
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.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\&* .
.Pp
The options are as follows:
.Bl -tag -width Ds
.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 cnfp
.Pq Cisco NetFlow protocol ,
.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 ttt
Print day and month in timestamp.
.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
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
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 the packet filter logging interface
.Pa pflog ,
logging reason (rule match, bad-offset, fragment, short,
normalize, memory), action taken (pass/block), direction (in/out) and interface
information is printed out for each packet.
.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 <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
.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 connection, 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.