tcpdump(8)
- NetBSD Manual Pages
TCPDUMP(8) TCPDUMP(8)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -aAdeflLnNOpqRStuvxX ]
[ -D datalinktype ] [ -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 -r 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 processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing packets
until it is interrupted by a SIGINT signal (generated, for example, by
typing your interrupt character, typically control-C) or a SIGTERM sig-
nal (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 packets 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 tcpdump);
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 SIGINFO signal (generated,
for example, by typing your ``status'' character, typically control-T)
and will continue capturing packets.
Reading packets from a network interface may require that you have spe-
cial privileges: You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privileges.
OPTIONS
-A Print each packet (minus its link level header) in ASCII.
Handy for capturing web pages.
-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 mil-
lions of bytes (1,000,000 bytes, not 1,048,576 bytes).
-d Dump the compiled packet-matching code in a human read-
able form to standard output and stop.
-D Set the data link type to use while capturing packets to
datalinktype.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (preceded
with a count).
-e Print the link-level header on each dump line.
-E Use algo:secret for decrypting IPsec ESP packets. Algo-
rithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
cast128-cbc, or none. The default is des-cbc. The abil-
ity to decrypt packets is only present 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 com-
mand 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 addi-
tional expression given on the command line is ignored.
-i Listen on interface. If unspecified, tcpdump searches
the system interface list for the lowest numbered, con-
figured up interface (excluding loopback). Ties are bro-
ken 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''.
-L List the known data link types and exit.
-m Load SMI MIB module definitions from file module. This
option can be used several times to load several MIB mod-
ules into tcpdump.
-n Don't convert addresses (i.e., host addresses, port num-
bers, 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 information
so output lines are shorter.
-R Assume ESP/AH packets to be based on old specification
(RFC1825 to RFC1829). If specified, tcpdump will not
print replay prevention field. Since there is no proto-
col version field in ESP/AH specification, 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 num-
bers.
-s Snarf snaplen bytes of data from each packet rather than
the default of 68 (with SunOS's NIT, the minimum is actu-
ally 96). 68 bytes is adequate for IP, ICMP, TCP and UDP
but may truncate protocol information from name server
and NFS packets (see below). Packets truncated because
of a limited snapshot are indicated in the output with
``[|proto]'', where proto is the name of the protocol
level at which the truncation has occurred. Note that
taking larger snapshots both increases the amount of time
it takes to process packets and, effectively, decreases
the amount of packet buffering. This may cause packets
to be lost. You should limit snaplen to the smallest
number that will capture the protocol information you're
interested in. Setting snaplen to 0 means use the
required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted
the specified type. Currently known types are cnfp
(Cisco NetFlow protocol), rpc (Remote Procedure Call),
rtp (Real-Time Applications protocol), rtcp (Real-Time
Applications control protocol), snmp (Simple Network Man-
agement Protocol), vat (Visual Audio Tool), and wb (dis-
tributed 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 pre-
vious 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 verifying 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 ``-''.
-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. Other-
wise, only packets for which expression is `true' will be
dumped.
The expression consists of one or more primitives. Prim-
itives usually consist of an id (name or number) 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 proto-
cols such as slip) the inbound and outbound quali-
fiers can be used to specify a desired direction.
proto qualifiers restrict the match to a particular pro-
tocol. 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 consis-
tent 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 source and destination addresses,
and often contain Ethernet-like packet types, so you can
filter on these FDDI fields just as with the analogous
Ethernet fields. FDDI headers also contain other fields,
but you cannot name them explicitly in a filter expres-
sion.
Similarly, `tr' is an alias for `ether'; the previous
paragraph's statements about FDDI headers also apply to
Token Ring headers.]
In addition to the above, there are some special `primi-
tive' 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 primitives. E.g., `host
foo and not port ftp and not port ftp-data'. To save
typing, identical qualifier lists can be omitted. E.g.,
`tcp dst port ftp or ftp-data or domain' is exactly the
same as `tcp dst port ftp or tcp dst port ftp-data or tcp
dst port domain'.
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 destination
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).
ether src ehost
True if the ethernet source address is ehost.
ether host ehost
True if either the ethernet source or destination
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 destina-
tion 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 numbers 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 num-
ber (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 destination
address of the packet has a network number of net.
net net mask netmask
True if the IP address matches net with the spe-
cific 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, ip6/tcp or
ip6/udp and has a destination port value of port.
The port can be a number 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 traffic 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 expres-
sions 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 identi-
fiers 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 protocol
type protocol. Note that this primitive does not
chase the protocol header chain.
ip6 protochain protocol
True if the packet is IPv6 packet, and 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 con-
tain, for example, authentication header, routing
header, or hop-by-hop option header, between IPv6
header and TCP header. The BPF code emitted by
this primitive 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 broad-
cast 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 protocol.
Protocol can be a number or one of the names ip,
ip6, arp, rarp, atalk, aarp, decnet, 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 protocol identification
comes from the 802.2 Logical Link Control (LLC)
header, which is 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 proto-
col 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 Ether-
net type field for most of those protocols; the
exceptions are iso, sap, and netbeui, 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 Ethernet frame, the IPX DSAP in the
LLC header, the 802.3 with no LLC header encapsu-
lation 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 config-
ured to run DECNET.]
decnet dst host
True if the DECNET destination address is host.
decnet host host
True if either the DECNET source or destination
address is host.
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
decoding 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 protocol
type protocol. Protocol can be a number 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 parsing 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, ppp, slip, link,
ip, arp, rarp, tcp, udp, icmp or ip6, and indi-
cates the protocol layer for the index operation.
(ether, fddi, tr, ppp, slip and link all refer to
the link layer.) Note that tcp, udp and other
upper-layer protocol types only apply to IPv4, not
IPv6 (this will be fixed in the future). The byte
offset, relative to the indicated protocol 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 key-
word len, gives the length of the packet.
For example, `ether[0] & 1 != 0' catches all mul-
ticast traffic. The expression `ip[0] & 0xf != 5'
catches all IP packets with options. The expres-
sion `ip[6:2] & 0x1fff = 0' catches only unfrag-
mented datagrams and frag zero of fragmented data-
grams. 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 follow-
ing 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 avail-
able: icmp-echoreply, icmp-unreach, icmp-source-
quench, icmp-redirect, icmp-echo, icmp-routerad-
vert, icmp-routersolicit, icmp-timxceed, icmp-
paramprob, icmp-tstamp, icmp-tstampreply, icmp-
ireq, icmp-ireqreply, icmp-maskreq, icmp-maskre-
ply.
The following TCP flags field values are avail-
able: 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 con-
catenation 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 sin-
gle, quoted argument. Multiple arguments are concate-
nated with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
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 packets) 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'
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 following
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, proto-
col, 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 datagrams) 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 net-
works, packets are assumed to contain an LLC packet. Regardless
of whether the '-e' option is specified or not, the source rout-
ing 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 information 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 special case, zero or more changes are printed. A
change is indicated by U (urgent pointer), W (window), A (ack),
S (sequence number), and I (packet ID), followed by a delta (+n
or -n), or a new value (=n). Finally, the amount of data in the
packet and compressed header length are printed.
For example, the following line shows an outbound compressed TCP
packet, with an implicit connection identifier; the ack has
changed by 6, the sequence number by 49, and the packet ID by 6;
there are 3 bytes of data and 6 bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
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:
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, ethernet 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 direction on this connection. Window is the
number of bytes of receive buffer space available the other
direction 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 out-
put 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 num-
bers 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 contained 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 `con-
versation', it prints the sequence number from the packet. On
subsequent packets of the conversation, the difference between
the current packet's sequence number and this initial sequence
number is printed. This means that sequence numbers after the
first can be interpreted as relative byte positions in the con-
versation's data stream (with the first data byte each direction
being `1'). `-S' will override this feature, causing the origi-
nal 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 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.
If the snapshot was small enough that tcpdump didn't capture 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 inter-
pret any further options (since it's impossible to tell where
they start). If the header length indicates options are present
but the IP datagram length is not long enough for the options to
actually be there, tcpdump reports it as ``[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 establishing
a TCP connection. Recall that TCP uses a 3-way handshake proto-
col 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 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 con-
tained 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
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 data-
gram 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 filter
expression, because that would select only those packets that
have SYN-ACK set, but not those with only SYN set. Remember
that we don't care if ACK or any other control 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 regard-
less 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:
( ( 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 datagram 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 destination
port number) and the higher level protocol information printed.
In particular, Domain Name service requests (RFC-1034/1035) and
Sun RPC calls (RFC-1050) to NFS.
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 description 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 ucbvax.berkeley.edu.
The query id was `3'. The `+' indicates 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 immediately
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.
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 decoding for
data on UDP/137, UDP/138 and TCP/139. Some primitive 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 strings 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
favorite samba.org mirror site. The SMB patches were written by
Andrew Tridgell (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 trans-
action 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 handle (fh) 21,24/10.731657119.
(If one is lucky, as in this case, the file handle can be inter-
preted as a major,minor device number pair, followed by the
inode number and generation 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 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 `ok'; the
packet shown on the second line is the first fragment of the
reply, and hence is only 1472 bytes long (the other bytes will
follow in subsequent fragments, but these fragments do not have
NFS or even UDP headers and so might not be printed, depending
on the filter expression used). Because the -v flag is given,
some of the file attributes (which are returned in addition to
the file data) are printed: the file type (``REG'', for regular
file), the file mode (in octal), the uid and gid, and the file
size.
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 operation.
Instead, tcpdump keeps track of ``recent'' requests, and matches
them to the replies using the transaction ID. If a reply does
not closely follow the corresponding request, it might not be
parsable.
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 (gen-
erally only the `interesting' arguments, for some definition of
interesting).
The format is intended to be self-describing, but it will proba-
bly not be useful to people who are not familiar with the work-
ings 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.
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 ser-
vice id are printed.
Error codes are printed for abort packets, with the exception 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 operation.
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 dis-
tinguished 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 separated 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 listening on port 220
of net icsd node 112. The second line is the same except the
full name of the source node is known (`office'). The third
line is a send from port 235 on net jssmag node 149 to broadcast
on the icsd-net NBP port (note that the broadcast address (255)
is indicated by a net name with no host number - for this reason
it's a good idea to keep node names and net names distinct in
/etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction pro-
tocol) 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 say-
ing 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 number 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 indi-
cates that the EOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmitted.
Helios resends them then jssmag.209 releases the transaction.
Finally, jssmag.209 initiates the next request. The `*' on the
request indicates that XO (`exactly once') was 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 connection 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 trail-
ing (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 and when the kernel
serviced the `new packet' interrupt.
SEE ALSO
bpf(4), pcap(3)
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California,
Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available via http:
http://www.tcpdump.org/
The original distribution is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project. This program
uses Eric Young's SSLeay library, under specific configuration.
BUGS
Please send problems, bugs, questions, desirable enhancements,
etc. to:
tcpdump-workers@tcpdump.org
Please send source code contributions, etc. to:
patches@tcpdump.org
NIT doesn't let you watch your own outbound traffic, BPF will.
We recommend that you use the latter.
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at
least to compute the right length for the higher level protocol.
Name server inverse queries are not dumped correctly: the
(empty) question section is printed rather than real query in
the answer section. Some believe that inverse queries are them-
selves a bug and prefer to fix the program generating them
rather than tcpdump.
A packet trace that crosses a daylight savings time change will
give skewed time stamps (the time change is ignored).
Filter expressions that manipulate FDDI or Token Ring headers
assume that all FDDI and Token Ring packets are SNAP-encapsu-
lated Ethernet packets. This is true for IP, ARP, and DECNET
Phase IV, but is not true for protocols such as ISO CLNS.
Therefore, the filter may inadvertently accept certain packets
that do not properly match the filter expression.
Filter expressions on fields other than those that manipulate
Token Ring headers will not correctly handle source-routed Token
Ring packets.
ip6 proto should chase header chain, but at this moment it does
not. ip6 protochain is supplied for this behavior.
Arithmetic expression against transport layer headers, like
tcp[0], does not work against IPv6 packets. It only looks at
IPv4 packets.
22 September 2002 TCPDUMP(8)
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