TCPDUMP(1) TCPDUMP(1)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -deflnNOpqStvx ] [ -c count ] [ -F file ]
[ -i interface ] [ -r file ] [ -s snaplen ]
[ -w file ] expression
DESCRIPTION
Tcpdump prints out the headers of packets on a network
interface that match the boolean expression. Under SunOS:
You must be root to invoke tcpdump or it must be installed
setuid to root. Under Ultrix: Any user can invoke tcpdump
once the super-user has enabled promiscuous-mode operation
using pfconfig(8). Under BSD: Access is controlled by the
permissions on /dev/bpf0, etc.
OPTIONS
-c Exit after receiving count packets.
-d Dump the compiled packet-matching code to standard
output and stop.
-e Print the link-level header on each dump line.
-f Print `foreign' internet addresses numerically
rather than symbolically (this option is intended
to get around serious brain damage in Sun's yp
server -- usually it hangs forever translating non-
local internet numbers).
-F Use file as input for the filter expression. An
additional expression given on the command line is
ignored.
-i Listen on interface. If unspecified, tcpdump
searches the system interface list for the lowest
numbered, configured up interface (excluding loop-
back). Ties are broken by choosing the earliest
match.
-l Make stdout line buffered. Useful if you want to
see the data while capturing it. E.g.,
``tcpdump -l | tee dat'' or ``tcpdump -l >
dat & tail -f dat''.
-n Don't convert addresses (i.e., host addresses, port
numbers, etc.) to names.
-N Don't print domain name qualification of host
names. E.g., if you give this flag then tcpdump
will print ``nic'' instead of ``nic.ddn.mil''.
-O Do not run the packet-matching code optimizer.
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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 for
some other reason; hence, `-p' cannot be used as an
abbreviation for `ether host {localhost} or broad-
cast'.
-q Quick (quiet?) output. Print less protocol infor-
mation so output lines are shorter.
-r Read packets from file (which was created with the
-w option). Standard input is used if file is
``-''.
-s Snarf snaplen bytes of data from each packet rather
than the default of 68 (with NIT, the minimum is
actually 96). 68 bytes is adequate for IP, ICMP,
TCP and UDP but may truncate protocol information
from name server and NFS packets (see below).
Packets truncated because of a limited snapshot are
indicated in the output with ``[|proto]'', where
proto is the name of the protocol level at which
the truncation has occured. 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 proto-
col information you're interested in.
-S Print absolute, rather than relative, TCP sequence
numbers.
-t Don't print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-v (Slightly more) verbose output. For example, the
time to live and type of service information in an
IP packet is printed.
-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.
expression
selects which packets will be dumped. If no
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TCPDUMP(1) TCPDUMP(1)
expression is given, all packets on the net will be
dumped. Otherwise, only packets for which expres-
sion is `true' will be dumped.
The expression consists of one or more primitives.
Primitives usually consist of an id (name or num-
ber) preceded by one or more qualifiers. There are
three different kinds of qualifier:
type qualifiers say what kind of thing the id
name or number refers to. Possible types
are host, net and port. E.g., `host foo',
`net 128.3', `port 20'. If there is no type
qualifier, host is assumed.
dir qualifiers specify a particular tranfer
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.
proto qualifiers restrict the match to a particu-
lar protocol. Possible protos are: ether,
ip, arp, rarp, tcp and udp. E.g., `ether
src foo', `arp net 128.3', `tcp port 21'.
If there is no proto qualifier, all proto-
cols 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'.
In addition to the above, there are some special
`primitive' keywords that don't follow the pattern:
gateway, broadcast, less, greater and arithmetic
expressions. All of these are described below.
More complex filter expressions are built up by
using the words and, or and not to combine primi-
tives. E.g., `host foo and not port ftp and not
port ftp-data'. To save typing, identical quali-
fier lists can be omitted. E.g., `tcp dst port ftp
or ftp-data or domain' is exactly the same as `tcp
dst port ftp or tcp dst port ftp-data or tcp dst
port domain'.
Allowable primitives are:
dst host host
True if the IP destination field of the
packet is host, which may be either an
address or a name.
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TCPDUMP(1) TCPDUMP(1)
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(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 desti-
nation address is ehost.
gateway host
True if the packet used host as a gateway.
I.e., the ethernet source or destination
address was host but neither the IP source
nor the IP destination was host. Host must
be a name and must be found 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 num-
bers for host / ehost.)
dst net net
True if the IP destination address of the
packet has a network number of net, which
may be either an address or a name.
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
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TCPDUMP(1) TCPDUMP(1)
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 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 expressions can be prepended with
the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets.
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,
udp, nd, or tcp. Note that the identifiers
tcp, udp, and icmp are also keywords and
must be escaped via backslash (\), which is
\\ in the C-shell.
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
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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 protocol
True if the packet is of ether type proto-
col. Protocol can be a number or a name
like ip, arp, or rarp. Note these identi-
fiers are also keywords and must be escaped
via backslash (\).
ip, arp, rarp
Abbreviations for:
ether proto p
where p is one of the above protocols.
tcp, udp, icmp
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, 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[2:2] & 0x1fff =
0' catches only unfragmented datagrams and
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TCPDUMP(1) TCPDUMP(1)
frag zero of fragmented datagrams. This
check is implicitly applied to the tcp and
udp index opertations. 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).
Negation (`!' or `not').
Concatenation (`and').
Alternation (`or').
Negation has highest precedence. Alternation and
concatenation have equal precedence and associate
left to right. Note that explicit and tokens, not
juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the
most recent keyword is assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to tcpdump as
either a single argument or as multiple arguments,
whichever is more convenient. Generally, if the
expression contains Shell metacharacters, it is
easier to pass it as a single, quoted argument.
Multiple arguments are concatenated with spaces
before being parsed.
EXAMPLES
To print all packets arriving at or departing from sun-
down:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except
helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at
Berkeley:
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TCPDUMP(1) TCPDUMP(1)
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup:
(note that the expression is quoted to prevent the shell
from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for
local hosts (if you gateway to one other net, this stuff
should never make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN pack-
ets) of each TCP conversation that involves a non-local
host.
tcpdump 'tcp[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"
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The follow-
ing gives a brief description and examples of most of the
formats.
Link Level Headers
If the '-e' option is given, the link level header is
printed out. On ethernets, the source and destination
addresses, protocol, and packet length are printed.
(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I'' for inbound,
``O'' for outbound), packet type, and compression informa-
tion are printed out. The packet type is printed first.
The three types are ip, utcp, and ctcp. No further link
information is printed for ip packets. For TCP packets,
the connection identifier is printed following the type.
If the packet is compressed, its encoded header is printed
out. The special cases are printed out as *S+n and *SA+n,
where n is the amount by which the sequence number (or
sequence number and ack) has changed. If it is not a spe-
cial case, zero or more changes are printed. A change is
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TCPDUMP(1) TCPDUMP(1)
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 com-
pressed TCP packet, with an implicit connection identi-
fier; the ack has changed by 6, the sequence number by 49,
and the packet ID by 6; there are 3 bytes of data and 6
bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
Arp/rarp output shows the type of request and its argu-
ments. The format is intended to be self explanatory.
Here is a short sample taken from the start of an `rlogin'
from host rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking
for the ethernet address of internet host csam. Csam
replies with its ethernet address (in this example, ether-
net addresses are in caps and internet addresses in lower
case).
This would look less redundant if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet
is broadcast and the second is point-to-point would be
visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the ethernet source address
is RTSG, the destination is the 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
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TCPDUMP(1) TCPDUMP(1)
by the data in this packet (see example below). Ack is
sequence number of the next data expected the other direc-
tion 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 brack-
ets (e.g., <mss 1024>).
Src, dst and flags are always present. The other fields
depend on the contents of the packet's tcp protocol header
and are output only if appropriate.
Here is the opening portion of an rlogin from host rtsg to
host csam.
rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
rtsg.1023 > csam.login: . ack 1 win 4096
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
csam.login > rtsg.1023: . ack 2 win 4096
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
The first line says that tcp port 1023 on rtsg sent a
packet to port login on csam. The S indicates that the
SYN flag was set. The packet sequence number was 768512
and it contained no data. (The notation is
`first:last(nbytes)' which means `sequence numbers first
up to but not including last which is nbytes bytes of user
data'.) There was no piggy-backed ack, the available
receive window was 4096 bytes and there was a max-segment-
size option requesting an mss of 1024 bytes.
Csam replies with a similar packet except it includes a
piggy-backed ack for rtsg's SYN. Rtsg then acks csam's
SYN. The `.' means no flags were set. The packet con-
tained no data so there is no data sequence number. Note
that the ack sequence number is a small integer (1). The
first time tcpdump sees a tcp `conversation', it prints
the sequence number from the packet. On subsequent pack-
ets of the conversation, the difference between the cur-
rent packet's sequence number and this initial sequence
number is printed. This means that sequence numbers after
the first can be interpreted as relative byte positions in
the conversation's data stream (with the first data byte
each direction being `1'). `-S' will override this fea-
ture, causing the original sequence numbers to be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2
through 20 in the rtsg -> csam side of the conversation).
The PUSH flag is set in the packet. On the 7th line, csam
says it's received data sent by rtsg up to but not includ-
ing byte 21. Most of this data is apparently sitting in
the socket buffer since csam's receive window has gotten
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TCPDUMP(1) TCPDUMP(1)
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 data-
gram to port who on host broadcast, the Internet broadcast
address. The packet contained 84 bytes of user data.
Some UDP services are recognized (from the source or des-
tination port number) and the higher level protocol infor-
mation printed. In particular, Domain Name service
requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to
NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with
the Domain Service protocol described in RFC-1035. If you
are not familiar with the protocol, the following descrip-
tion will appear to be written in greek.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an
address record (qtype=A) associated with the name ucb-
vax.berkeley.edu. The query id was `3'. The `+' indi-
cates the recursion desired flag was set. The query
length was 37 bytes, not including the UDP and IP protocol
headers. The query operation was the normal one, Query,
so the op field was omitted. If the op had been anything
else, it would have been printed between the `3' and the
`+'. Similarly, the qclass was the normal one, CIN, and
omitted. Any other qclass would have been printed immedi-
ately after the `A'.
A few anomalies are checked and may result in extra fields
enclosed in square brackets: If a query contains an
answer, 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)
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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 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 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
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
vs.e2766 > helios.nfs: 136 readdir fh 6.5197 8192 bytes @ 0
helios.nfs > vs.e2766: reply ok 384
vs.e2767 > helios.nfs: 136 lookup fh 6.5197 `RCS'
In the first line, host vs sends a transaction with id
e2766 to helios (note that the number following the src
host is a transaction id, not the source port). The
request was 136 bytes, excluding the UDP and IP headers.
The operation was a readdir (read directory) on file han-
dle (fh) 6.5197. 8192 bytes are read, starting at offset
0. Helios replies `ok' with 384 bytes of data. (The
design of Sun's RPC protocol makes it difficult to inter-
pret replies. I don't bother.)
In the third line, vs asks helios to lookup the name `RCS'
in directory file 6.5197. Note that the data printed
depends on the operation type. The format is intended to
be self explanatory (at least, to me) if read in conjunc-
tion with an NFS protocol spec.
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Note that NFS requests are very large and the above won't
be printed unless snaplen is increased. I use `-s 192' to
watch NFS traffic.
KIP Appletalk (DDP in UDP)
Appletalk DDP packets encapsulated in UDP datagrams are
de-encapsulated and dumped as DDP packets (i.e., all the
UDP header information is discarded). The file
/etc/atalk.names is used to translate appletalk net and
node numbers to names. Lines in this file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of appletalk networks.
The third line gives the name of a particular host (a host
is distinguished from a net by the 3rd octet in the number
- a net number must have two octets and a host number must
have three octets.) The number and name should be sepa-
rated by whitespace (blanks or tabs). The
/etc/atalk.names file may contain blank lines or comment
lines (lines starting with a `#').
Appletalk addresses are printed in the form
net.host.port
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn't exist or doesn't contain
an entry for some appletalk host/net number, addresses are
printed in numeric form.) In the first example, NBP (DDP
port 2) on net 144.1 node 209 is sending to whatever is
listening on port 220 of net icsd node 112. The second
line is the same except the full name of the source node
is known (`office'). The third line is a send from port
235 on net jssmag node 149 to broadcast on the icsd-net
NBP port (note that the broadcast address (255) is indi-
cated by a net name with no host number - for this reason
it's a good idea to keep node names and net names distinct
in /etc/atalk.names).
NBP (name binding protocol) and ATP (Appletalk transaction
protocol) packets have their contents interpreted. Other
protocols just dump the protocol name (or number if no
name is registered for the protocol) and packet size.
NBP packets are formatted like the following examples:
icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
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The first line is a name lookup request for laserwriters
sent by net icsd host 112 and broadcast on net jssmag.
The nbp id for the lookup is 190. The second line shows a
reply for this request (note that it has the same id) from
host jssmag.209 saying that it has a laserwriter resource
named "RM1140" registered on port 250. The third line is
another reply to the same request saying host techpit has
laserwriter "techpit" registered on port 186.
ATP packet formatting is demonstrated by the following
example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios
by requesting up to 8 packets (the `<0-7>'). The hex num-
ber at the end of the line is the value of the `userdata'
field in the request.
Helios responds with 8 512-byte packets. The `:digit'
following the transaction id gives the packet sequence
number in the transaction and the number in parens is the
amount of data in the packet, excluding the atp header.
The `*' on packet 7 indicates that the EOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmit-
ted. Helios resends them then jssmag.209 releases the
transaction. Finally, jssmag.209 initiates the next
request. The `*' on the request indicates that XO
(`exactly once') was not set.
IP Fragmentation
Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments. The
second indicates this is the last fragment.)
Id is the fragment id. Size is the fragment size (in
bytes) excluding the IP header. Offset is this fragment's
offset (in bytes) in the original datagram.
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TCPDUMP(1) TCPDUMP(1)
The fragment information is output for each fragment. The
first fragment contains the higher level protocol header
and the frag info is printed after the protocol info.
Fragments after the first contain no higher level protocol
header and the frag info is printed after the source and
destination addresses. For example, here is part of an
ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connec-
tion that doesn't appear to handle 576 byte datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First,
addresses in the 2nd line don't include port numbers.
This is because the TCP protocol information is all in the
first fragment and we have no idea what the port or
sequence numbers are when we print the later fragments.
Second, the tcp sequence information in the first line is
printed as if there were 308 bytes of user data when, in
fact, there are 512 bytes (308 in the first frag and 204
in the second). If you are looking for holes in the
sequence space or trying to match up acks with packets,
this can fool you.
A packet with the IP don't fragment flag is marked with a
trailing (DF).
Timestamps
By default, all output lines are preceded by a timestamp.
The timestamp is the current clock time in the form
hh:mm:ss.frac
and is as accurate as the kernel's clock (e.g., +-10ms on
a Sun-3). 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 (of course, with Sun's lousy clock
resolution this time lag is negligible.)
SEE ALSO
traffic(1C), nit(4P), bpf(4)
AUTHORS
Van Jacobson (van@helios.ee.lbl.gov), Craig Leres
(leres@helios.ee.lbl.gov) and Steven McCanne
(mccanne@helios.ee.lbl.gov), all of Lawrence Berkeley Lab-
oratory, University of California, Berkeley, CA.
BUGS
The clock resolution on most Suns is pathetic (20ms). If
you want to use the timestamp to generate some of the
important performance distributions (like packet interar-
rival time) it's best to watch something that generates
packets slowly (like an Arpanet gateway or a MicroVax
4 Jan 1992 15
TCPDUMP(1) TCPDUMP(1)
running VMS).
NIT doesn't let you watch your own outbound traffic, BPF
will. We recommend that you use the latter.
tcpdump for Ultrix requires Ultrix version 4.0 or later;
the kernel has to have been built with the packetfilter
pseudo-device driver (see packetfilter(4)). As of this
writing, Ultrix does not let you watch either your own
outbound or inbound traffic.
Under SunOS 4.1, the packet capture code (or Streams NIT)
is not what you'd call efficient. Don't plan on doing
much with your Sun while you're monitoring a busy network.
On Sun systems prior to release 3.2, NIT is very buggy.
If run on an old system, tcpdump may crash the machine.
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 themselves a bug and prefer to fix the program gener-
ating 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
would have no way of testing this code.
A packet trace that crosses a daylight savings time change
will give skewed time stamps (the time change is ignored).
4 Jan 1992 16