TCPDUMP(8) OpenBSD System Manager's Manual TCPDUMP(8)NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [-adefILlNnOopqStvXx] [-c count] [-D direction]
[-E [espalg:]espkey] [-F file] [-i interface] [-r file]
[-s snaplen] [-T type] [-w file] [-y datalinktype] [expression]
DESCRIPTION
tcpdump prints out the headers of packets on a network interface that
match the boolean expression. You must have read access to /dev/bpf*.
The options are as follows:
-a Attempt to convert network and broadcast addresses to names.
-c count Exit after receiving count packets.
-D direction
Select packets flowing in the specified direction. Valid
directions are: in and out. The default is to accept packets
flowing in any direction.
-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 preceded with a
count.
-E [espalg:]espkey
Try to decrypt RFC 2406 ESP (Encapsulating Security Payload)
traffic using the specified hex key espkey. Supported
algorithms for espalg are: aes128, aes128-hmac96, blowfish,
blowfish-hmac96, cast, cast-hmac96, des3, des3-hmac96, des and
des-hmac96. The algorithm defaults to aes128-hmac96. This
option should be used for debugging only, since the key will
show up in ps(1) output.
-e Print the link-level header on each dump line.
-F file Use file as input for the filter expression. Any additional
expressions given on the command line are ignored.
-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.
-I Print the interface on each dump line.
-i interface
Listen on interface. If unspecified, tcpdump searches the
system interface list for the lowest numbered, configured
``up'' interface (excluding loopback). Ties are broken by
choosing the earliest match.
-L List the supported data link types for the interface and exit.
-l Make stdout line buffered. Useful if you want to see the data
while capturing it. For example:
# tcpdump -l | tee dat
or
# tcpdump -l > dat & tail -f dat
-N Do not print domain name qualification of host names. For
example, if you specify this flag then tcpdump will print
``nic'' instead of ``nic.ddn.mil''.
-n Do not convert addresses (host addresses, port numbers, etc.)
to names.
-O Do not run the packet-matching code optimizer. This is useful
only if you suspect a bug in the optimizer.
-o Print a guess of the possible operating system(s) of hosts that
sent TCP SYN packets. See pf.os(5) for a description of the
passive operating system fingerprints.
-p Do not put the interface into promiscuous mode. 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 file Read packets from a 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 snaplen
Analyze at most the first snaplen bytes of data from each
packet rather than the default of 116. 116 bytes is adequate
for IPv6, ICMP, TCP, and UDP, but may truncate protocol
information from name server and NFS packets (see below).
Packets truncated because of a limited snaplen are indicated in
the output with ``[|proto]'', where proto is the name of the
protocol level at which the truncation has occurred. Taking
larger snapshots both increases the amount of time it takes to
process packets and, effectively, decreases the amount of
packet buffering. This may cause packets to be lost. You
should limit snaplen to the smallest number that will capture
the protocol information you're interested in.
-T type Force packets selected by expression to be interpreted as the
specified type. Currently known types are vrrp (Virtual Router
Redundancy protocol), cnfp (Cisco NetFlow protocol), rpc
(Remote Procedure Call), rtp (Real-Time Applications protocol),
rtcp (Real-Time Applications control protocol), sack (RFC 2018
TCP Selective Acknowledgements Options), tcp (Transmission
Control Protocol), vat (Visual Audio Tool), and wb (distributed
White Board).
-t Do not print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-ttt Print day and month in timestamp.
-tttt Print timestamp difference between packets.
-ttttt Print timestamp difference since the first packet.
-v (Slightly more) verbose output. For example, the time to live
(TTL) and type of service (ToS) information in an IP packet are
printed.
-vv Even more verbose output. For example, additional fields are
printed from NFS reply packets.
-w file Write the raw packets to file rather than parsing and printing
them out. They can be analyzed later with the -r option.
Standard output is used if file is `-'.
-X Print each packet in hex and ASCII. If the -e option is also
specified, the link-level header will be included. The smaller
of the entire packet or snaplen bytes will be printed.
-x Print each packet in hex. If the -e option is also specified,
the link-level header will be included. The smaller of the
entire packet or snaplen bytes will be printed.
-y datalinktype
Set the data link type to use while capturing to datalinktype.
Commonly used types include EN10MB, IEEE802_11, and
IEEE802_11_RADIO. The choices applicable to a particular
device can be listed using -L.
expression selects which packets will be dumped. If no expression is
given, all packets on the net will be dumped. Otherwise, only packets
satisfying expression will be dumped.
The expression consists of one or more primitives. Primitives usually
consist of an id (name or number) preceded by one or more qualifiers.
There are three different kinds of qualifiers:
type Specify which kind of address component 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 Specify a particular transfer direction to and/or from id.
Possible directions are src, dst, src or dst, src and dst, addr1,
addr2, addr3, and addr4. 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. The addr1, addr2, addr3, and addr4 qualifiers
are only valid for IEEE 802.11 Wireless LAN link layers. For null
link layers (i.e., point-to-point protocols such as SLIP (Serial
Line Internet Protocol) or the pflog(4) header), the inbound and
outbound qualifiers can be used to specify a desired direction.
proto Restrict the match to a particular protocol. Possible protocols
are: ah, arp, atalk, decnet, esp, ether, fddi, icmp, icmp6, igmp,
igrp, ip, ip6, lat, mopdl, moprc, pim, rarp, sca, stp, tcp, udp,
and wlan. E.g., ``ether src foo'', ``arp net 128.3'', ``tcp port
21'', ``wlan addr1 0:2:3:4:5:6''. If there is no protocol
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 (Fiber Distributed Data Interface)
headers contain Ethernet-like source and destination addresses,
and often contain Ethernet-like packet types, so you can filter on
these FDDI fields just as with the analogous Ethernet fields.
FDDI headers also contain other fields, but you cannot name them
explicitly in a filter expression.
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 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 IP destination field of the packet is
host, which may be either an address or a name.
src host host True if the IP source field of the packet is host.
host host True if either the IP source or destination of the
packet is host.
Any of the above host expressions can be prepended
with the keywords, ip, arp, or rarp 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(3) for a 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 destination was host.
host must be a name and must be found in both
/etc/hosts and /etc/ethers. An equivalent expression
is
ether host ehost and not host host
which can be used with either names or numbers for
host/ehost.
dst net net True if the IP 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(5) for
details).
src net net True if the IP source address of the packet has a
network number of net.
net net True if either the IP source or destination address of
the packet has a network number of net.
dst port port True if the packet is IP/TCP or IP/UDP and has a
destination port value of port. The port can be a
number or name from services(5) (see tcp(4) and
udp(4)). If a name is used, both the port number and
protocol are checked. If a number or ambiguous name
is used, only the port number is checked; e.g., ``dst
port 513'' will print both TCP/login traffic and
UDP/who traffic, and ``dst 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 proto True if the packet is an IP packet (see ip(4)) of
protocol type proto. proto can be a number or name
from protocols(5), such as icmp, udp, or tcp. These
identifiers are also keywords and must be escaped
using a backslash character (`\').
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.
ether proto proto True if the packet is of ether type proto. proto can
be a number or one of the names ip, ip6, arp, rarp,
atalk, atalkarp, decnet, decdts, decdns, lanbridge,
lat, mopdl, moprc, pup, sca, sprite, stp, vexp, vprod,
or xns. These identifiers are also keywords and must
be escaped using a backslash character (`\'). In the
case of FDDI (e.g., ``fddi protocol arp''), the
protocol identification comes from the 802.2 Logical
Link Control (LLC) header, which is usually layered on
top of the FDDI header. tcpdump assumes, when
filtering on the protocol identifier, that all FDDI
packets include an LLC header, and that the LLC header
is in so-called SNAP format.
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
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 destination
address is host.
ifname interface True if the packet was logged as coming from the
specified interface (applies only to packets logged by
pf(4)).
on interface Synonymous with the ifname modifier.
rnr num True if the packet was logged as matching the
specified PF rule number in the main ruleset (applies
only to packets logged by pf(4)).
rulenum num Synonymous with the rnr modifier.
reason code True if the packet was logged with the specified PF
reason code. The known codes are: match, bad-offset,
fragment, short, normalize, memory, bad-timestamp,
congestion, ip-option, proto-cksum, state-mismatch,
state-insert, state-limit, src-limit, and synproxy
(applies only to packets logged by pf(4)).
rset name True if the packet was logged as matching the
specified PF ruleset name of an anchored ruleset
(applies only to packets logged by pf(4)).
ruleset name Synonymous with the rset modifier.
srnr num True if the packet was logged as matching the
specified PF rule number of an anchored ruleset
(applies only to packets logged by pf(4)).
subrulenum num Synonymous with the srnr modifier.
action act True if PF took the specified action when the packet
was logged. Valid actions are: pass, block, and match
(applies only to packets logged by pf(4)).
wlan addr1 ehost True if the first IEEE 802.11 address is ehost.
wlan addr2 ehost True if the second IEEE 802.11 address is ehost.
wlan addr3 ehost True if the third IEEE 802.11 address is ehost.
wlan addr4 ehost True if the fourth IEEE 802.11 address is ehost. The
fourth address field is only used for WDS (Wireless
Distribution System) frames.
wlan host ehost True if either the first, second, third, or fourth
IEEE 802.11 address is ehost.
type type True if the IEEE 802.11 frame type matches the
specified type. Valid types are: data, mgt, ctl, or a
numeric value.
subtype subtype True if the IEEE 802.11 frame subtype matches the
specified subtype. Valid subtypes are: assocreq,
assocresp, reassocreq, reassocresp, probereq,
proberesp, beacon, atim, disassoc, auth, deauth, data,
or a numeric value.
dir dir True if the IEEE 802.11 frame direction matches the
specified dir. Valid directions are: nods, tods,
fromds, dstods, or a numeric value.
atalk, ip, ip6, arp, decnet, lat, moprc, mopdl, rarp, sca
Abbreviations for: ether proto p where p is one of the
above protocols. tcpdump does not currently know how
to parse lat, moprc, or mopdl.
ah, esp, icmp, icmp6, igmp, igrp, pim, tcp, udp
Abbreviations for: ip proto p where p is one of the
above 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, ip, arp, rarp, tcp, udp,
or icmp, and indicates the protocol layer for the
index operation. 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 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.
Primitives may be combined using a parenthesized group of primitives and
operators. Parentheses are special to the shell and must be escaped.
Allowable primitives and operators are:
Negation (``!'' or ``not'')
Concatenation (``&&'' or ``and'')
Alternation (``||'' or ``or'')
Negation has highest precedence. Alternation and concatenation have
equal precedence and associate left to right. 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 sundown:
# tcpdump host sundown
To print traffic between helios and either hot or ace (the expression is
quoted to prevent the shell from mis-interpreting the parentheses):
# 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:
# 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
connection that involves a non-local host:
# tcpdump 'tcp[13] & 3 != 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[0] != 8 and icmp[0] != 0'
To print and decrypt all ESP packets with SPI 0x00001234:
# tcpdump -E des3-hmac96:ab...def 'ip[20:4] = 0x00001234'
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, protocol, and packet
length are printed.
On the packet filter logging interface pflog(4), logging reason (rule
match, bad-offset, fragment, bad-timestamp, short, normalize, memory),
action taken (pass/block), direction (in/out) and interface information
are printed out for each packet.
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.
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
In this example, Ethernet addresses are in caps and internet addresses in
lower case. The first line says that rtsg sent an arp packet asking for
the Ethernet address of internet host csam. csam replies with its
Ethernet address CSAM.
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
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 src-os data-seqno ack window urgent options
src and dst are the source and destination IP addresses and ports. flags
is some combination of `S' (SYN), `F' (FIN), `P' (PUSH), or `R' (RST),
`W' (congestion Window reduced), `E' (ecn ECHO) or a single `.' (no
flags). src-os will list a guess of the source host's operating system
if the -o command line flag was passed to tcpdump. data-seqno describes
the portion of sequence space covered by the data in this packet (see
example below). ack is the sequence number of the next data expected by
the other end of this connection. window is the number of bytes of
receive buffer space available at the other end of 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 host 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 contained no data so there is no data sequence number.
The ack sequence number is a 32-bit integer. The first time tcpdump sees
a TCP connection, it prints the sequence number from the packet. On
subsequent packets of the connection, the difference between the current
packet's sequence number and this initial sequence number is printed.
This means that sequence numbers after the first can be interpreted as
relative byte positions in the connection's data stream (with the first
data byte each direction being 1). -S will override this feature,
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 connection). 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.
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
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)
For example:
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 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 was
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, name server or authority
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)
For example:
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 authority records. The first
answer record is type A (address and its data is internet) address
128.32.137.3. The total size of the response was 273 bytes, excluding
UDP and IP headers. The op (Query) and rcode (NoError) were omitted, as
was the class (C_IN) of the A record.
In the second example, helios responds to query op 2 with an rcode 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.
Name server requests and responses tend to be large and the default
snaplen of 96 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.
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.
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
op was a readlink (read symbolic link) on fh (``file handle'')
21,24/10.731657119. If one is lucky, as in this case, the file handle
can be interpreted as a major,minor device number pair, followed by the
inode number and generation number. Wrl replies with a stat of ok and
the contents of the link.
In the third line, sushi asks wrl to look up the name ``xcolors'' in
directory file 9,74/4096.6878. The data printed depends on the operation
type. The format is intended to be self-explanatory if read in
conjunction with an 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, and fragmentation fields, which
have been omitted from this example. In the first line, sushi asks wrl
to read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl
replies with a stat of ok; the packet shown on the second line is the
first fragment of the reply, and hence is only 1472 bytes long. The
other bytes will follow in subsequent fragments, but these fragments do
not have 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.
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 xid (transaction 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
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
For example:
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
If /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-net 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. The broadcast address (255)
is indicated by a net name with no host number; for this reason it is a
good idea to keep node names and net names distinct in /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
icsdi-net host 112 and broadcast on net jssmag. The nbp ID for the
lookup is 190. The second line shows a reply for this request (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 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 ``:n'' following the
transaction ID gives the packet sequence number in the transaction and
the number in parentheses is the amount of data in the packet, excluding
the ATP header. The `*' on packet 7 indicates 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 [+])
A `+' indicates there are more fragments. The last fragment will have no
`+'.
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 fragment info is
printed after the protocol info. Fragments after the first contain no
higher level protocol header and the fragment info is printed after the
source and destination addresses. For example, here is part of an FTP
from arizona.edu to lbl-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 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 and when the
kernel serviced the ``new packet'' interrupt.
IP Checksum Offload
Some network cards support IP checksum offload. Packet headers for such
interfaces erroneously indicate a bad checksum, since the checksum is not
calculated until after tcpdump sees the packet.
SEE ALSOethers(3), pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4),
networks(5), pf.os(5), protocols(5), services(5)
Transmission Control Protocol, RFC 793, September 1981.
Domain Names - Concepts and Facilities, RFC 1034, November 1987.
Domain Names - Implementation and Specification, RFC 1035, November 1987.
RPC: Remote Procedure Call, RFC 1050, April 1988.
Compressing TCP/IP Headers for Low-Speed Serial Links, RFC 1144, February
1990.
TCP Selective Acknowledgement Options, RFC 2018, October 1996.
IP Encapsulating Security Payload (ESP), RFC 2406, November 1998.
AUTHORS
Van Jacobson <van@ee.lbl.gov>, Craig Leres <leres@ee.lbl.gov>, and Steven
McCanne <mccanne@ee.lbl.gov>, all of the Lawrence Berkeley Laboratory,
University of California, Berkeley, CA.
BUGS
Please send bug reports to <tcpdump@ee.lbl.gov> or <libpcap@ee.lbl.gov>.
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 the real query in the answer
section. Some believe that inverse queries are themselves a bug and
prefer to fix the program generating them rather than tcpdump.
Apple Ethertalk DDP packets could be dumped as easily as KIP DDP packets
but aren't. Even if we were inclined to do anything to promote the use
of Ethertalk (we aren't, LBL doesn't allow Ethertalk on any of its
networks so we'd have no way of testing this code).
A packet trace that crosses a daylight saving time change will give
skewed time stamps (the time change is ignored).
Filter expressions that manipulate FDDI headers assume that all FDDI
packets are encapsulated Ethernet packets. This is true for IP, ARP, and
DECNET Phase IV, but is not true for protocols such as ISO CLNS.
Therefore, the filter may inadvertently accept certain packets that do
not properly match the filter expression.
OpenBSD 4.9 February 7, 2011 OpenBSD 4.9