tcpdump(8) - NetBSD Manual Pages

Command:

Section:

Arch:

Collection:

>>>

TCPDUMP(8) TCPDUMP(8)

NAME

tcpdump - dump traffic on a network

SYNOPSIS

tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E algo:secret ] [ expression ]

DESCRIPTION

Tcpdump prints out the headers of packets on a network
interface that match the boolean expression. It can also
be run with the -w flag, which causes it to save the
packet data to a file for later analysis, and/or with the
-b flag, which causes it to read from a saved packet file
rather than to read packets from a network interface. In
all cases, only packets that match expression will be pro-
cessed by tcpdump.

Tcpdump will, if not run with the -c flag, continue cap-
turing packets until it is interrupted by a SIGINT signal
(generated, for example, by typing your interrupt charac-
ter, typically control-C) or a SIGTERM signal (typically
generated with the kill(1) command); if run with the -c
flag, it will capture packets until it is interrupted by a
SIGINT or SIGTERM signal or the specified number of pack-
ets have been processed.

When tcpdump finishes capturing packets, it will report
counts of:

packets ``received by filter'' (the meaning of this
depends on the OS on which you're running tcpdump,
and possibly on the way the OS was configured - if
a filter was specified on the command line, on some
OSes it counts packets regardless of whether they
were matched by the filter expression, and on other
OSes it counts only packets that were matched by
the filter expression and were processed by tcp-
dump);

packets ``dropped by kernel'' (this is the number
of packets that were dropped, due to a lack of
buffer space, by the packet capture mechanism in
the OS on which tcpdump is running, if the OS
reports that information to applications; if not,
it will be reported as 0).

On platforms that support the SIGINFO signal, such as most
BSDs, it will report those counts when it receives a SIG-
INFO signal (generated, for example, by typing your ``sta-
tus'' character, typically control-T) and will continue
capturing packets.

3 January 2001 1

TCPDUMP(8) TCPDUMP(8)

Reading packets from a network interface may require that
you have special privileges: You must have read access to
/dev/bpf*.

Reading a saved packet file doesn't require special privi-
leges.

OPTIONS

-a Attempt to convert network and broadcast addresses
to names.

-c Exit after receiving count packets.

-C Before writing a raw packet to a savefile, check
whether the file is currently larger than file_size
and, if so, close the current savefile and open a
new one. Savefiles after the first savefile will
have the name specified with the -w flag, with a
number after it, starting at 2 and continuing
upward. The units of file_size are millions of
bytes (1,000,000 bytes, not 1,048,576 bytes).

-d Dump the compiled packet-matching code in a human
readable form to standard output and stop.

-dd Dump packet-matching code as a C program fragment.

-ddd Dump packet-matching code as decimal numbers (pre-
ceded with a count).

-e Print the link-level header on each dump line.

-E Use algo:secret for decrypting IPsec ESP packets.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc,
rc3-cbc, cast128-cbc, or none. The default is des-
cbc. The ability to decrypt packets is only pre-
sent if tcpdump was compiled with cryptography
enabled. secret the ascii text for ESP secret key.
We cannot take arbitrary binary value at this
moment. The option assumes RFC2406 ESP, not
RFC1827 ESP. The option is only for debugging pur-
poses, and the use of this option with truly
`secret' key is discouraged. By presenting IPsec
secret key onto command line you make it visible to
others, via ps(1) and other occasions.

-f Print `foreign' internet addresses numerically
rather than symbolically (this option is intended
to get around serious brain damage in Sun's yp
server -- usually it hangs forever translating non-
local internet numbers).

-F Use file as input for the filter expression. An
additional expression given on the command line is

3 January 2001 2

TCPDUMP(8) TCPDUMP(8)

ignored.

-i Listen on interface. If unspecified, tcpdump
searches the system interface list for the lowest
numbered, configured up interface (excluding loop-
back). Ties are broken by choosing the earliest
match.

-l Make stdout line buffered. Useful if you want to
see the data while capturing it. E.g.,
``tcpdump -l | tee dat'' or ``tcpdump -l >
dat & tail -f dat''.

-m Load SMI MIB module definitions from file module.
This option can be used several times to load sev-
eral MIB modules into tcpdump.

-n Don't convert addresses (i.e., host addresses, port
numbers, etc.) to names.

-N Don't print domain name qualification of host
names. E.g., if you give this flag then tcpdump
will print ``nic'' instead of ``nic.ddn.mil''.

-O Do not run the packet-matching code optimizer.
This is useful only if you suspect a bug in the
optimizer.

-p Don't 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'.

-q Quick (quiet?) output. Print less protocol infor-
mation so output lines are shorter.

-R Assume ESP/AH packets to be based on old specifica-
tion (RFC1825 to RFC1829). If specified, tcpdump
will not print replay prevention field. Since
there is no protocol version field in ESP/AH speci-
fication, tcpdump cannot deduce the version of
ESP/AH protocol.

-r Read packets from file (which was created with the
-w option). Standard input is used if file is
``-''.

-S Print absolute, rather than relative, TCP sequence
numbers.

-s Snarf snaplen bytes of data from each packet rather
than the default of 68 (with SunOS's NIT, the mini-
mum is actually 96). 68 bytes is adequate for IP,

3 January 2001 3

TCPDUMP(8) TCPDUMP(8)

ICMP, TCP and UDP but may truncate protocol infor-
mation from name server and NFS packets (see
below). Packets truncated because of a limited
snapshot are indicated in the output with
``[|proto]'', where proto is the name of the proto-
col 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 buffer-
ing. This may cause packets to be lost. You
should limit snaplen to the smallest number that
will capture the protocol information you're inter-
ested in. Setting snaplen to 0 means use the
required length to catch whole packets.

-T Force packets selected by "expression" to be inter-
preted the specified type. Currently known types
are cnfp (Cisco NetFlow protocol), rpc (Remote Pro-
cedure Call), rtp (Real-Time Applications proto-
col), rtcp (Real-Time Applications control proto-
col), snmp (Simple Network Management Protocol),
vat (Visual Audio Tool), and wb (distributed White
Board).

-t Don't print a timestamp on each dump line.

-tt Print an unformatted timestamp on each dump line.

-ttt Print a delta (in micro-seconds) between current
and previous line on each dump line.

-tttt Print a timestamp in default format proceeded by
date on each dump line. -u Print undecoded NFS
handles.

-v (Slightly more) verbose output. For example, the
time to live, identification, total length and
options in an IP packet are printed. Also enables
additional packet integrity checks such as verify-
ing the IP and ICMP header checksum.

-vv Even more verbose output. For example, additional
fields are printed from NFS reply packets, and SMB
packets are fully decoded.

-vvv Even more verbose output. For example, telnet SB
... SE options are printed in full. With -X telnet
options are printed in hex as well.

-w Write the raw packets to file rather than parsing
and printing them out. They can later be printed
with the -r option. Standard output is used if
file is ``-''.

3 January 2001 4

TCPDUMP(8) TCPDUMP(8)

-x Print each packet (minus its link level header) in
hex. The smaller of the entire packet or snaplen
bytes will be printed.

-X When printing hex, print ascii too. Thus if -x is
also set, the packet is printed in hex/ascii. This
is very handy for analysing new protocols. Even if
-x is not also set, some parts of some packets may
be printed in hex/ascii.

expression
selects which packets will be dumped. If no
expression is given, all packets on the net will be
dumped. Otherwise, only packets for which expres-
sion is `true' will be dumped.

The expression consists of one or more primitives.
Primitives usually consist of an id (name or num-
ber) preceded by one or more qualifiers. There are
three different kinds of qualifier:

type qualifiers say what kind of thing the id
name or number refers to. Possible types
are host, net and port. E.g., `host foo',
`net 128.3', `port 20'. If there is no type
qualifier, host is assumed.

dir qualifiers specify a particular transfer
direction to and/or from id. Possible
directions are src, dst, src or dst and src
and dst. E.g., `src foo', `dst net 128.3',
`src or dst port ftp-data'. If there is no
dir qualifier, src or dst is assumed. For
`null' link layers (i.e. point to point pro-
tocols such as slip) the inbound and out-
bound qualifiers can be used to specify a
desired direction.

proto qualifiers restrict the match to a particu-
lar protocol. Possible protos are: ether,
fddi, tr, ip, ip6, arp, rarp, decnet, tcp
and 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'.

[`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

3 January 2001 5

TCPDUMP(8) TCPDUMP(8)

source and destination addresses, and often contain
Ethernet-like packet types, so you can filter on
these FDDI fields just as with the analogous Ether-
net fields. FDDI headers also contain other
fields, but you cannot name them explicitly in a
filter expression.

Similarly, `tr' is an alias for `ether'; the previ-
ous paragraph's statements about FDDI headers also
apply to Token Ring headers.]

In addition to the above, there are some special
`primitive' keywords that don't follow the pattern:
gateway, broadcast, less, greater and arithmetic
expressions. All of these are described below.

More complex filter expressions are built up by
using the words and, or and not to combine primi-
tives. E.g., `host foo and not port ftp and not
port ftp-data'. To save typing, identical quali-
fier 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'.

Allowable primitives are:

dst host host
True if the IPv4/v6 destination field of the
packet is host, which may be either an
address or a name.

src host host
True if the IPv4/v6 source field of the
packet is host.

host host
True if either the IPv4/v6 source or desti-
nation of the packet is host. Any of the
above host expressions can be prepended with
the keywords, ip, arp, rarp, or ip6 as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP
addresses, each address will be checked for
a match.

ether dst ehost
True if the ethernet destination address is
ehost. Ehost may be either a name from
/etc/ethers or a number (see ethers(3N) for
numeric format).

3 January 2001 6

TCPDUMP(8) TCPDUMP(8)

ether src ehost
True if the ethernet source address is
ehost.

ether host ehost
True if either the ethernet source or desti-
nation address is ehost.

gateway host
True if the packet used host as a gateway.
I.e., the ethernet source or destination
address was host but neither the IP source
nor the IP destination was host. Host must
be a name and must be found both by the
machine's host-name-to-IP-address resolution
mechanisms (host name file, DNS, NIS, etc.)
and by the machine's host-name-to-Ethernet-
address resolution mechanism (/etc/ethers,
etc.). (An equivalent expression is
ether host ehost and not host host
which can be used with either names or num-
bers for host / ehost.) This syntax does
not work in IPv6-enabled configuration at
this moment.

dst net net
True if the IPv4/v6 destination address of
the packet has a network number of net. Net
may be either a name from /etc/networks or a
network number (see networks(4) for
details).

src net net
True if the IPv4/v6 source address of the
packet has a network number of net.

net net
True if either the IPv4/v6 source or desti-
nation address of the packet has a network
number of net.

net net mask netmask
True if the IP address matches net with the
specific netmask. May be qualified with src
or dst. Note that this syntax is not valid
for IPv6 net.

net net/len
True if the IPv4/v6 address matches net with
a netmask len bits wide. May be qualified
with src or dst.

dst port port
True if the packet is ip/tcp, ip/udp,

3 January 2001 7

TCPDUMP(8) TCPDUMP(8)

ip6/tcp or ip6/udp and has a destination
port value of port. The port can be a num-
ber or a name used in /etc/services (see
tcp(4P) and 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.,
dst port 513 will print both tcp/login traf-
fic and udp/who traffic, and port domain
will print both tcp/domain and udp/domain
traffic).

src port port
True if the packet has a source port value
of port.

port port
True if either the source or destination
port of the packet is port. Any of the
above port expressions can be prepended with
the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source
port is port.

less length
True if the packet has a length less than or
equal to length. This is equivalent to:
len <= length.

greater length
True if the packet has a length greater than
or equal to length. This is equivalent to:
len >= length.

ip proto protocol
True if the packet is an IP packet (see
ip(4P)) of protocol type protocol. Protocol
can be a number or one of the names icmp,
icmp6, igmp, igrp, pim, ah, esp, vrrp, udp,
or tcp. Note that the identifiers tcp, udp,
and icmp are also keywords and must be
escaped via backslash (\), which is \\ in
the C-shell. Note that this primitive does
not chase the protocol header chain.

ip6 proto protocol
True if the packet is an IPv6 packet of pro-
tocol type protocol. Note that this primi-
tive does not chase the protocol header
chain.

ip6 protochain protocol
True if the packet is IPv6 packet, and

3 January 2001 8

TCPDUMP(8) TCPDUMP(8)

contains protocol header with type protocol
in its protocol header chain. For example,
ip6 protochain 6
matches any IPv6 packet with TCP protocol
header in the protocol header chain. The
packet may contain, for example, authentica-
tion header, routing header, or hop-by-hop
option header, between IPv6 header and TCP
header. The BPF code emitted by this primi-
tive is complex and cannot be optimized by
BPF optimizer code in tcpdump, so this can
be somewhat slow.

ip protochain protocol
Equivalent to ip6 protochain protocol, but
this is for IPv4.

ether broadcast
True if the packet is an ethernet broadcast
packet. The ether keyword is optional.

ip broadcast
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.

ether multicast
True if the packet is an ethernet multicast
packet. The ether keyword is optional.
This is shorthand for `ether[0] & 1 != 0'.

ip multicast
True if the packet is an IP multicast
packet.

ip6 multicast
True if the packet is an IPv6 multicast
packet.

ether proto protocol
True if the packet is of ether type proto-
col. Protocol can be a number or one of the
names ip, ip6, arp, rarp, atalk, aarp, dec-
net, sca, lat, mopdl, moprc, iso, stp, ipx,
or netbeui. Note these identifiers are also
keywords and must be escaped via backslash
(\).

[In the case of FDDI (e.g., `fddi protocol
arp') and Token Ring (e.g., `tr protocol
arp'), for most of those protocols, the pro-
tocol identification comes from the 802.2
Logical Link Control (LLC) header, which is

3 January 2001 9

TCPDUMP(8) TCPDUMP(8)

usually layered on top of the FDDI or Token
Ring header.

When filtering for most protocol identifiers
on FDDI or Token Ring, tcpdump checks only
the protocol ID field of an LLC header in
so-called SNAP format with an Organizational
Unit Identifier (OUI) of 0x000000, for
encapsulated Ethernet; it doesn't check
whether the packet is in SNAP format with an
OUI of 0x000000.

The exceptions are iso, for which it checks
the DSAP (Destination Service Access Point)
and SSAP (Source Service Access Point)
fields of the LLC header, stp and netbeui,
where it checks the DSAP of the LLC header,
and atalk, where it checks for a SNAP-format
packet with an OUI of 0x080007 and the
Appletalk etype.

In the case of Ethernet, tcpdump checks the
Ethernet type field for most of those proto-
cols; the exceptions are iso, sap, and net-
beui, for which it checks for an 802.3 frame
and then checks the LLC header as it does
for FDDI and Token Ring, atalk, where it
checks both for the Appletalk etype in an
Ethernet frame and for a SNAP-format packet
as it does for FDDI and Token Ring, aarp,
where it checks for the Appletalk ARP etype
in either an Ethernet frame or an 802.2 SNAP
frame with an OUI of 0x000000, and ipx,
where it checks for the IPX etype in an Eth-
ernet frame, the IPX DSAP in the LLC header,
the 802.3 with no LLC header encapsulation
of IPX, and the IPX etype in a SNAP frame.]

decnet src host
True if the DECNET source address is 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.]

decnet dst host
True if the DECNET destination address is
host.

decnet host host
True if either the DECNET source or destina-
tion address is host.

3 January 2001 10

TCPDUMP(8) TCPDUMP(8)

ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp,
ipx, netbeui
Abbreviations for:
ether proto p
where p is one of the above protocols.

lat, moprc, mopdl
Abbreviations for:
ether proto p
where p is one of the above protocols. Note
that tcpdump does not currently know how to
parse these protocols.

vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN
packet. If [vlan_id] is specified, only
true is the packet has the specified
vlan_id. Note that the first vlan keyword
encountered in expression changes the decod-
ing offsets for the remainder of expression
on the assumption that the packet is a VLAN
packet.

tcp, udp, icmp
Abbreviations for:
ip proto p or ip6 proto p
where p is one of the above protocols.

iso proto protocol
True if the packet is an OSI packet of pro-
tocol type protocol. Protocol can be a num-
ber or one of the names clnp, esis, or isis.

clnp, esis, isis
Abbreviations for:
iso proto p
where p is one of the above protocols. Note
that tcpdump does an incomplete job of pars-
ing these protocols.

expr relop expr
True if the relation holds, where relop is
one of >, <, >=, <=, =, !=, and expr 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:
proto [ expr : size ]
Proto is one of ether, fddi, tr, ip, arp,
rarp, tcp, udp, icmp or ip6, and indicates
the protocol layer for the index operation.
Note that tcp, udp and other upper-layer

3 January 2001 11

TCPDUMP(8) TCPDUMP(8)

protocol types only apply to IPv4, not IPv6
(this will be fixed in the future). The
byte offset, relative to the indicated pro-
tocol layer, is given by expr. 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
len, gives the length of the packet.

For example, `ether[0] & 1 != 0' catches all
multicast traffic. The expression `ip[0] &
0xf != 5' catches all IP packets with
options. The expression `ip[6:2] & 0x1fff =
0' catches only unfragmented datagrams and
frag zero of fragmented datagrams. This
check is implicitly applied to the tcp and
udp index operations. For instance, tcp[0]
always means the first byte of the TCP
header, and never means the first byte of an
intervening fragment.

Some offsets and field values may be
expressed as names rather than as numeric
values. The following protocol header field
offsets are available: icmptype (ICMP type
field), icmpcode (ICMP code field), and
tcpflags (TCP flags field).

The following ICMP type field values are
available: icmp-echoreply, icmp-unreach,
icmp-sourcequench, icmp-redirect, icmp-echo,
icmp-routeradvert, icmp-routersolicit, icmp-
timxceed, icmp-paramprob, icmp-tstamp, icmp-
tstampreply, icmp-ireq, icmp-ireqreply,
icmp-maskreq, icmp-maskreply.

The following TCP flags field values are
available: tcp-fin, tcp-syn, tcp-rst, tcp-
push, tcp-push, tcp-ack, tcp-urg.

Primitives may be combined using:

A parenthesized group of primitives and
operators (parentheses are special to the
Shell and must be escaped).

Negation (`!' or `not').

Concatenation (`&&' or `and').

Alternation (`||' or `or').

Negation has highest precedence. Alternation and

3 January 2001 12

TCPDUMP(8) TCPDUMP(8)

concatenation have equal precedence and associate
left to right. Note that explicit and tokens, not
juxtaposition, are now required for concatenation.

If an identifier is given without a keyword, the
most recent keyword is assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )

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.

EXAMPLES

To print all packets arriving at or departing from sun-
down:
tcpdump host sundown

To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and any host except
helios:
tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at
Berkeley:
tcpdump net ucb-ether

To print all ftp traffic through internet gateway snup:
(note that the expression is quoted to prevent the shell
from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'

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).
tcpdump ip and not net localnet

To print the start and end packets (the SYN and FIN pack-
ets) of each TCP conversation that involves a non-local
host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

To print IP packets longer than 576 bytes sent through
gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'

3 January 2001 13

TCPDUMP(8) TCPDUMP(8)

To print IP broadcast or multicast packets that were not
sent via ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

To print all ICMP packets that are not echo
requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

OUTPUT FORMAT

The output of tcpdump is protocol dependent. The follow-
ing gives a brief description and examples of most of the
formats.

Link Level Headers

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.

On FDDI networks, the '-e' option causes tcpdump 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 data-
grams) are `async' packets, with a priority value between
0 and 7; for example, `async4'. Such packets are assumed
to contain an 802.2 Logical Link Control (LLC) packet; the
LLC header is printed if it is not an ISO datagram or a
so-called SNAP packet.

On Token Ring networks, the '-e' option causes tcpdump to
print the `access control' and `frame control' fields, the
source and destination addresses, and the packet length.
As on FDDI networks, packets are assumed to contain an LLC
packet. Regardless of whether the '-e' option is speci-
fied or not, the source routing information is printed for
source-routed packets.

(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC-1144.)

On SLIP links, a direction indicator (``I'' for inbound,
``O'' for outbound), packet type, and compression informa-
tion are printed out. The packet type is printed first.
The three types are ip, utcp, and ctcp. No further link
information is printed for ip 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 *S+n and *SA+n,
where n is the amount by which the sequence number (or
sequence number and ack) has changed. If it is not a spe-
cial 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

3 January 2001 14

TCPDUMP(8) TCPDUMP(8)

(+n or -n), or a new value (=n). Finally, the amount of
data in the packet and compressed header length are
printed.

For example, the following line shows an outbound com-
pressed TCP packet, with an implicit connection identi-
fier; 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:
O ctcp * A+6 S+49 I+6 3 (6)

ARP/RARP Packets

Arp/rarp output shows the type of request and its argu-
ments. 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:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
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, ether-
net addresses are in caps and internet addresses in lower
case).

This would look less redundant if we had done tcpdump -n:
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

If we had done tcpdump -e, the fact that the first packet
is broadcast and the second is point-to-point would be
visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
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.

TCP Packets

(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.)

The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses
and ports. Flags are some combination of S (SYN), F
(FIN), P (PUSH) or R (RST) or a single `.' (no flags).
Data-seqno describes the portion of sequence space covered
by the data in this packet (see example below). Ack is
sequence number of the next data expected the other

3 January 2001 15

TCPDUMP(8) TCPDUMP(8)

direction on this connection. Window is the number of
bytes of receive buffer space available the other direc-
tion on this connection. Urg indicates there is `urgent'
data in the packet. Options are tcp options enclosed in
angle brackets (e.g., <mss 1024>).

Src, dst and flags are always present. The other fields
depend on the contents of the packet's tcp protocol header
and are output only if appropriate.

Here is the opening portion of an rlogin from host rtsg to
host csam.
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
The first line says that tcp port 1023 on rtsg sent a
packet to port login on csam. The S indicates that the
SYN flag was set. The packet sequence number was 768512
and it contained no data. (The notation is
`first:last(nbytes)' which means `sequence numbers first
up to but not including last which is 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.

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 con-
tained no data so there is no data sequence number. Note
that the ack sequence number is a small integer (1). The
first time tcpdump sees a tcp `conversation', it prints
the sequence number from the packet. On subsequent pack-
ets of the conversation, the difference between the cur-
rent 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 fea-
ture, causing the original sequence numbers to be output.

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 includ-
ing 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

3 January 2001 16

TCPDUMP(8) TCPDUMP(8)

two bytes of urgent, pushed data to rtsg.

If the snapshot was small enough that tcpdump didn't cap-
ture the full TCP header, it interprets as much of the
header as it can and then reports ``[|tcp]'' 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 ``[bad opt]'' and does not interpret any
further options (since it's impossible to tell where they
start). If the header length indicates options are pre-
sent but the IP datagram length is not long enough for the
options to actually be there, tcpdump reports it as ``[bad
hdr length]''.

Capturing TCP packets with particular flag combinations
(SYN-ACK, URG-ACK, etc.)

There are 8 bits in the control bits section of the TCP
header:

CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

Let's assume that we want to watch packets used in estab-
lishing a TCP connection. Recall that TCP uses a 3-way
handshake protocol when it initializes a new connection;
the connection sequence with regard to the TCP control
bits is

1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK

Now we're interested in capturing packets that have only
the SYN bit set (Step 1). Note that we don't want packets
from step 2 (SYN-ACK), just a plain initial SYN. What we
need is a correct filter expression for tcpdump.

Recall the structure of a TCP header without options:

0 15 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless

3 January 2001 17

TCPDUMP(8) TCPDUMP(8)

options are present. The first line of the graph contains
octets 0 - 3, the second line shows octets 4 - 7 etc.

Starting to count with 0, the relevant TCP control bits
are contained in octet 13:

0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |

Let's have a closer look at octet no. 13:

| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|

These are the TCP control bits we are interested in. We
have numbered the bits in this octet from 0 to 7, right to
left, so the PSH bit is bit number 3, while the URG bit is
number 5.

Recall that we want to capture packets with only SYN set.
Let's see what happens to octet 13 if a TCP datagram
arrives with the SYN bit set in its header:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Looking at the control bits section we see that only bit
number 1 (SYN) is set.

Assuming that octet number 13 is an 8-bit unsigned integer
in network byte order, the binary value of this octet is

00000010

and its decimal representation is

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2

We're almost done, because now we know that if only SYN is
set, the value of the 13th octet in the TCP header, when
interpreted as a 8-bit unsigned integer in network byte
order, must be exactly 2.

This relationship can be expressed as

3 January 2001 18

TCPDUMP(8) TCPDUMP(8)

tcp[13] == 2

We can use this expression as the filter for tcpdump in
order to watch packets which have only SYN set:
tcpdump -i xl0 tcp[13] == 2

The expression says "let the 13th octet of a TCP datagram
have the decimal value 2", which is exactly what we want.

Now, let's assume that we need to capture SYN packets, but
we don't care if ACK or any other TCP control bit is set
at the same time. Let's see what happens to octet 13 when
a TCP datagram with SYN-ACK set arrives:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Now bits 1 and 4 are set in the 13th octet. The binary
value of octet 13 is

00010010

which translates to decimal

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump fil-
ter expression, because that would select only those pack-
ets that have SYN-ACK set, but not those with only SYN
set. Remember that we don't care if ACK or any other con-
trol bit is set as long as SYN is set.

In order to achieve our goal, we need to logically AND the
binary value of octet 13 with some other value to preserve
the SYN bit. We know that we want SYN to be set in any
case, so we'll logically AND the value in the 13th octet
with the binary value of a SYN:

00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010

We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as the
result of this operation is 2 (binary 00000010), so we
know that for packets with SYN set the following relation
must hold true:

3 January 2001 19

TCPDUMP(8) TCPDUMP(8)

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'

Note that you should use single quotes or a backslash in
the expression to hide the AND ('&') special character
from the shell.

UDP Packets

UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp data-
gram to port who on host broadcast, the Internet broadcast
address. The packet contained 84 bytes of user data.

Some UDP services are recognized (from the source or des-
tination port number) and the higher level protocol infor-
mation printed. In particular, Domain Name service
requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to
NFS.

UDP Name Server Requests

(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 descrip-
tion will appear to be written in greek.)

Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an
address record (qtype=A) associated with the name ucb-
vax.berkeley.edu. The query id was `3'. The `+' indi-
cates the recursion desired flag was set. The query
length was 37 bytes, not including the UDP and IP protocol
headers. The query operation was the normal one, Query,
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, C_IN, and
omitted. Any other qclass would have been printed immedi-
ately after the `A'.

A few anomalies are checked and may result in extra fields
enclosed in square brackets: If a query contains an
answer, authority records or additional records section,
ancount, nscount, or arcount are printed as `[na]', `[nn]'
or `[nau]' where n 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=x]' is printed, where x is the hex value of header
bytes two and three.

3 January 2001 20

TCPDUMP(8) TCPDUMP(8)

UDP Name Server Responses

Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from
h2opolo with 3 answer records, 3 name server records and 7
additional 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.

In the second example, helios 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 authoritative answer bit was set.
Since there were no answers, no type, class or data were
printed.

Other flag characters that might appear are `-' (recursion
available, RA, not set) and `|' (truncated message, TC,
set). If the `question' section doesn't contain exactly
one entry, `[nq]' is printed.

Note that name server requests and responses tend to be
large and the default snaplen of 68 bytes may not capture
enough of the packet to print. Use the -s flag to
increase the snaplen if you need to seriously investigate
name server traffic. `-s 128' has worked well for me.

SMB/CIFS decoding

tcpdump now includes fairly extensive SMB/CIFS/NBT decod-
ing for data on UDP/137, UDP/138 and TCP/139. Some primi-
tive decoding of IPX and NetBEUI SMB data is also done.

By default a fairly minimal decode is done, with a much
more detailed decode done if -v is used. Be warned that
with -v a single SMB packet may take up a page or more, so
only use -v if you really want all the gory details.

If you are decoding SMB sessions containing unicode
strings then you may wish to set the environment variable
USE_UNICODE to 1. A patch to auto-detect unicode srings
would be welcome.

For information on SMB packet formats and what all te
fields mean see www.cifs.org or the pub/samba/specs/
directory on your favourite samba.org mirror site. The
SMB patches were written by Andrew Tridgell

3 January 2001 21

TCPDUMP(8) TCPDUMP(8)

(tridge@samba.org).

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are
printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results

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

In the first line, host sushi sends a transaction with id
6709 to wrl (note that the number following the src host
is a transaction id, not the source port). The request
was 112 bytes, excluding the UDP and IP headers. The
operation was a readlink (read symbolic link) on file han-
dle (fh) 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 gen-
eration number.) Wrl replies `ok' with the contents of
the link.

In the third line, sushi asks wrl to lookup the name
`xcolors' 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.

If the -v (verbose) flag is given, additional information
is printed. For example:

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

(-v also prints the IP header TTL, ID, length, and frag-
mentation fields, which have been omitted from this exam-
ple.) In the first line, sushi asks wrl to read 8192
bytes from file 21,11/12.195, at byte offset 24576. Wrl
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

3 January 2001 22

TCPDUMP(8) TCPDUMP(8)

regular file), the file mode (in octal), the uid and gid,
and the file size.

If the -v flag is given more than once, even more details
are printed.

Note that NFS requests are very large and much of the
detail won't be printed unless snaplen is increased. Try
using `-s 192' to watch NFS traffic.

NFS reply packets do not explicitly identify the RPC oper-
ation. Instead, tcpdump keeps track of ``recent''
requests, and matches them to the replies using the trans-
action ID. If a reply does not closely follow the corre-
sponding request, it might not be parsable.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are
printed as:

src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args

elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename

In the first line, host elvis sends a RX packet to pike.
This was a RX data packet to the fs (fileserver) service,
and is the start of an RPC call. The RPC call was a
rename, with the old directory file id of 536876964/1/1
and an old filename of `.newsrc.new', and a new directory
file id of 536876964/1/1 and a new filename of `.newsrc'.
The host pike responds with a RPC reply to the rename call
(which was successful, because it was a data packet and
not an abort packet).

In general, all AFS RPCs are decoded at least by RPC call
name. Most AFS RPCs have at least some of the arguments
decoded (generally only the `interesting' arguments, for
some definition of interesting).

The format is intended to be self-describing, but it will
probably not be useful to people who are not familiar with
the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgement
packets and additional header information is printed, such
as the the RX call ID, call number, sequence number,
serial number, and the RX packet flags.

3 January 2001 23

TCPDUMP(8) TCPDUMP(8)

If the -v flag is given twice, additional information is
printed, such as the the RX call ID, serial number, and
the RX packet flags. The MTU negotiation information is
also printed from RX ack packets.

If the -v flag is given three times, the security index
and service id are printed.

Error codes are printed for abort packets, with the excep-
tion of Ubik beacon packets (because abort packets are
used to signify a yes vote for the Ubik protocol).

Note that AFS requests are very large and many of the
arguments won't be printed unless snaplen is increased.
Try using `-s 256' to watch AFS traffic.

AFS reply packets do not explicitly identify the RPC oper-
ation. Instead, tcpdump keeps track of ``recent''
requests, and matches them to the replies using the call
number and service ID. If a reply does not closely follow
the corresponding request, it might not be parsable.

KIP Appletalk (DDP in UDP)

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
/etc/atalk.names is used to translate appletalk net and
node numbers to names. Lines in this file have the form
number name

1.254 ether
16.1 icsd-net
1.254.110 ace
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 must have two octets and a host number must
have three octets.) The number and name should be sepa-
rated by whitespace (blanks or tabs). The
/etc/atalk.names file may contain blank lines or comment
lines (lines starting with a `#').

Appletalk addresses are printed in the form
net.host.port

144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /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

3 January 2001 24

TCPDUMP(8) TCPDUMP(8)

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 indi-
cated 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).

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.

NBP packets are formatted like the following examples:
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
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.

ATP packet formatting is demonstrated by the following
example:
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
Jssmag.209 initiates transaction id 12266 with host helios
by requesting up to 8 packets (the `<0-7>'). The hex num-
ber at the end of the line is the value of the `userdata'
field in the request.

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.

3 January 2001 25

TCPDUMP(8) TCPDUMP(8)

Jssmag.209 then requests that packets 3 & 5 be retransmit-
ted. 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 not set.

IP Fragmentation

Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments. The
second indicates this is the last fragment.)

Id is the fragment id. Size is the fragment size (in
bytes) excluding the IP header. Offset is this fragment's
offset (in bytes) in the original datagram.

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 connec-
tion that doesn't appear to handle 576 byte datagrams:
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
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.

A packet with the IP don't fragment flag is marked with a
trailing (DF).

Timestamps

By default, all output lines are preceded by a timestamp.
The timestamp is the current clock time in the form
hh:mm:ss.frac
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

3 January 2001 26

TCPDUMP(8) TCPDUMP(8)

and when the kernel serviced the `new packet' interrupt.