NAME
tcpdump - dump traffic on
a network
SYNOPSIS
tcpdump [ -adeflnNOpqStvx
] [ -c count ] [ -F file ]
[ -i interface ] [ -r file ] [ -s snaplen ]
[ -T type ] [ -w file ] [ expression ]
DESCRIPTION
Tcpdump prints
out the headers of packets on a network
interface that match the boolean
expression.
Under SunOS with nit or bpf:
To run tcpdump you must have
read access to /dev/nit
or /dev/bpf*. Under Solaris with
dlpi: You must have read
access to the network pseudo
device, e.g. /dev/le.
Under HP-UX with dlpi: You must be
root or it must be installed
setuid to root. Under IRIX
with snoop: You
must be root or it must be installed
setuid to root. Under
Linux: You must be root or it must
be installed setuid
to root. Under Ultrix and Digital
UNIX: Once the super-user
has enabled promiscuous-mode
operation using
pfconfig(8), any user may run tcpdump.
Under BSD: You must have read
access to /dev/bpf*.
OPTIONS
-a
Attempt to convert network and broadcast addresses
to names.
-c Exit after receiving count packets.
-d
Dump the compiled packet-matching code in a human
readable form to standard output and stop.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump
packet-matching code as decimal numbers (pre-
ceded with a count).
-e Print the link-level header on each dump line.
-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.
This is useful only if you suspect a bug in the
optimizer.
-p
Don't put the interface into promiscuous
mode.
Note that the interface might be in promiscuous
mode for some other reason; hence, `-p' cannot be
used as an abbreviation for `ether host {local-hw-
addr} or ether broadcast'.
-q
Quick (quiet?) output. Print less protocol infor-
mation so output lines are shorter.
-r
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 SunOS's NIT, the mini-
mum is actually 96). 68 bytes is adequate for IP,
ICMP, TCP and UDP but may truncate protocol infor-
mation from name server and NFS packets
(see
below). Packets truncated because of a limited
snapshot are indicated in the
output with
``[|proto]'', where proto is the name of the proto-
col level at which the truncation has occurred.
Note that taking larger snapshots both increases
the amount of time it takes to process packets and,
effectively, decreases the amount of packet buffer-
ing. This may cause packets to be lost.
You
should limit snaplen to the smallest number that
will capture the protocol information you're inter-
ested in.
-T
Force packets selected by "expression" to be inter-
preted the specified type. Currently known types
are rpc (Remote Procedure Call), rtp (Real-Time
Applications protocol), rtcp (Real-Time Applica-
tions control protocol), vat (Visual Audio Tool),
and wb (distributed White Board).
-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.
-vv Even
more verbose output. For example, additional
fields are printed from NFS reply packets.
-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
expression is given, all packets on the net will be
dumped. Otherwise, only packets for which expres-
sion is `true' will be dumped.
The expression consists of one or more primitives.
Primitives usually consist of an id (name or num-
ber) preceded by one or more qualifiers. There are
three different kinds of qualifier:
type qualifiers say what kind of thing
the id
name or number refers to. Possible types
are host, net and port. E.g., `host foo',
`net 128.3', `port 20'. If there is no type
qualifier, host is assumed.
dir qualifiers specify a particular
transfer
direction to and/or from id.
Possible
directions are src, dst, src or dst and src
and dst. E.g., `src foo', `dst net 128.3',
`src or dst port ftp-data'. If there is no
dir qualifier, src or dst is assumed. For
`null' link layers (i.e. point to point pro-
tocols such as slip) the inbound and out-
bound qualifiers can be used to specify a
desired direction.
proto qualifiers restrict the match to a particu-
lar protocol. Possible protos are: ether,
fddi, ip, arp, rarp, decnet, lat, sca,
moprc, mopdl, tcp and udp. E.g., `ether src
foo', `arp net 128.3', `tcp port 21'. If
there is no proto qualifier, all protocols
consistent with the type are assumed. E.g.,
`src foo' means `(ip or arp or rarp) src
foo' (except the latter is not legal syn-
tax), `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 Ether-
net 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 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.
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. Net may
be either a name from /etc/networks or a
network number (see networks(4)
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.
net net mask mask
True if the IP address matches net with the
specific netmask. May be qualified with src
or dst.
net net/len
True if the IP address matches net a netmask
len bits wide. May be qualified with src or
dst.
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 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,
igrp, udp, nd, or tcp. Note that the iden-
tifiers 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
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 (\). [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
Ultrix systems that are configured to run
DECNET.]
decnet dst host
True if the DECNET destination address is
host.
decnet host host
True if either the DECNET source or destina-
tion address is host.
ip, arp, rarp, decnet
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.
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, fddi, ip, arp, rarp,
tcp, udp, or icmp, and indicates the proto-
col 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).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `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:
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.
On FDDI networks, the
'-e' option causes tcpdump to print
the `frame control' field,
the source and destination
addresses, and
the packet length. (The `frame control'
field governs the
interpretation of the rest of the
packet. Normal packets
(such as those containing IP data-
grams) are `async' packets,
with a priority value between
0 and 7; for example,
`async4'. Such packets are assumed
to contain an 802.2 Logical
Link Control (LLC) packet; the
LLC header is
printed if it is not an ISO datagram or a
so-called SNAP packet.
(N.B.: The following description
assumes familiarity with
the SLIP compression algorithm
described in RFC-1144.)
On SLIP links,
a direction indicator (``I'' for inbound,
``O'' for outbound), packet
type, and compression informa-
tion are printed
out. The packet type is printed first.
The three types are ip, utcp,
and ctcp. No further link
information is
printed for ip packets. For TCP packets,
the connection identifier
is printed following the type.
If the packet is compressed,
its encoded header is printed
out. The special cases
are printed out as *S+n and *SA+n,
where n is
the amount by which the sequence number (or
sequence number and ack) has
changed. If it is not a spe-
cial case, zero or more
changes are printed. A change is
indicated by U (urgent pointer),
W (window), A (ack), S
(sequence number), and
I (packet ID), followed by a delta
(+n or -n), or a new value
(=n). Finally, the amount of
data in the
packet and compressed header length are
printed.
For example, the following
line shows an outbound com-
pressed TCP packet,
with an implicit connection identi-
fier; the ack has changed
by 6, the sequence number by 49,
and the packet
ID by 6; there are 3 bytes of data and 6
bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
Arp/rarp output shows the type
of request and its argu-
ments. The
format is intended to be self explanatory.
Here is a short sample taken
from the start of an `rlogin'
from host rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line
says that rtsg sent an arp packet asking
for the ethernet address
of internet host csam. Csam
replies with its ethernet
address (in this example, ether-
net addresses are in caps
and internet addresses in lower
case).
This would look less redundant
if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump
-e, the fact that the first packet
is broadcast and the second
is point-to-point would be
visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this
says the ethernet source address
is RTSG, the
destination is the ethernet broadcast
address, the
type field contained hex 0806 (type
ETHER_ARP) and the total length
was 64 bytes.
TCP Packets
(N.B.:The following description
assumes familiarity with
the TCP protocol
described in RFC-793. If you are not
familiar with the protocol,
neither this description nor
tcpdump will be of much use
to you.)
The general format of a tcp
protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst
are the source and destination IP addresses
and ports. Flags are
some combination of S (SYN), F
(FIN), P (PUSH)
or R (RST) or a single `.' (no flags).
Data-seqno describes the portion
of sequence space covered
by the data
in this packet (see example below). Ack is
sequence number of the next
data expected the other 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 <ms
s 1024>
rtsg.1023 > csam.login: . ack 1 win 4096
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
csam.login > rtsg.1023: . ack 2 win 4096
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
The first line says that tcp
port 1023 on rtsg sent a
packet to port
login on csam. The S indicates that the
SYN flag was set. The
packet sequence number was 768512
and it
contained no data. (The
notation is
`first:last(nbytes)' which
means `sequence numbers first
up to but not including last
which is nbytes bytes of user
data'.) There was
no piggy-backed ack, the available
receive window was 4096 bytes
and there was a max-segment-
size option requesting an
mss of 1024 bytes.
Csam replies with a similar
packet except it includes a
piggy-backed ack
for rtsg's SYN. Rtsg then acks csam's
SYN. The `.' means no
flags were set. The packet con-
tained no data so there
is no data sequence number. Note
that the ack sequence number
is a small integer (1). The
first time tcpdump
sees a tcp `conversation', it prints
the sequence number from the
packet. On subsequent pack-
ets of the conversation,
the difference between the cur-
rent packet's sequence number
and this initial sequence
number is printed. This
means that sequence numbers after
the first can be interpreted
as relative byte positions in
the conversation's
data stream (with the first data byte
each direction being `1').
`-S' will override this fea-
ture, causing the original
sequence numbers to be output.
On the 6th line, rtsg sends
csam 19 bytes of data (bytes 2
through 20 in the rtsg
-> csam side of the conversation).
The PUSH flag is set in the
packet. On the 7th line, csam
says it's received data sent
by rtsg up to but not includ-
ing byte 21. Most of
this data is apparently sitting in
the socket buffer
since csam's receive window has gotten
19 bytes smaller. Csam
also sends one byte of data to
rtsg in this packet.
On the 8th and 9th lines, csam sends
two bytes of urgent, pushed
data to rtsg.
If the snapshot was small enough
that tcpdump didn't cap-
ture the full
TCP header, it interprets as much of the
header as it can and then
reports ``[|tcp]'' to indicate
the remainder
could not be interpreted. If the header
contains a bogus option (one
with a length that's either
too small or
beyond the end of the header), tcpdump
reports it as ``[bad opt]''
and does not interpret any
further options (since
it's impossible to tell where they
start). If the header
length indicates options are pre-
sent but the IP datagram length
is not long enough for the
options to actually be there,
tcpdump reports it as ``[bad
hdr length]''.
UDP Packets
UDP format is illustrated by
this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port
who on host actinide sent a udp data-
gram to port who on host broadcast,
the Internet broadcast
address. The packet
contained 84 bytes of user data.
Some UDP services are
recognized (from the source or des-
tination port number) and
the higher level protocol infor-
mation printed.
In particular, Domain Name service
requests (RFC-1034/1035) and
Sun RPC calls (RFC-1050) to
NFS.
UDP Name Server Requests
(N.B.:The following
description assumes familiarity with
the Domain Service protocol
described in RFC-1035. If you
are not familiar with the
protocol, the following descrip-
tion will appear to be written
in greek.)
Name server requests are formatted
as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain
server on helios for an
address record
(qtype=A) associated with the name ucb-
vax.berkeley.edu. The
query id was `3'. The `+' indi-
cates the recursion
desired flag was set. The query
length was 37 bytes, not including
the UDP and IP protocol
headers. The
query operation was the normal one, Query,
so the op field was omitted.
If the op had been anything
else, it would
have been printed between the `3' and the
`+'. Similarly, the
qclass was the normal one, C_IN, and
omitted. Any other qclass
would have been printed immedi-
ately after the `A'.
A few anomalies are checked
and may result in extra fields
enclosed in square
brackets: If a query contains an
answer, 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)
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 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.
NFS Requests and Replies
Sun NFS (Network
File System) requests and replies are
printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi
sends a transaction with id
6709 to wrl (note
that the number following the src host
is a transaction id, not the
source port). The request
was 112 bytes,
excluding the UDP and IP headers. The
operation was a readlink (read
symbolic link) on file han-
dle (fh) 21,24/10.731657119.
(If one is lucky, as in this
case, the file handle can
be interpreted as a major,minor
device number pair,
followed by the inode number and gen-
eration number.) Wrl
replies `ok' with the contents of
the link.
In the third
line, sushi asks wrl to lookup the name
`xcolors' in directory file
9,74/4096.6878. Note that the
data printed depends on the
operation type. The format is
intended to be self explanatory
if read in conjunction
with an NFS protocol spec.
If the -v (verbose) flag
is given, additional information
is printed. For example:
sushi.1372a > wrl.nfs:
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header
TTL, ID, 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 oper-
ation. Instead,
tcpdump keeps track of ``recent''
requests, and matches them
to the replies using the trans-
action ID. If
a reply does not closely follow the corre-
sponding request, it might
not be parsable.
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:LaserWrite
r@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@
*" 186
The first line
is a name lookup request for laserwriters
sent by net icsd host 112
and broadcast on net jssmag.
The nbp id for the lookup
is 190. The second line shows a
reply for this request (note
that it has the same id) from
host jssmag.209 saying
that it has a laserwriter resource
named "RM1140" registered
on port 250. The third line is
another reply to the
same request saying host techpit has
laserwriter "techpit" registered
on port 186.
ATP packet formatting is
demonstrated by the following
example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction
id 12266 with host helios
by requesting up to 8 packets
(the `<0-7>'). The hex num-
ber at the end of the
line is the value of the `userdata'
field in the request.
Helios responds with 8
512-byte packets. The `:digit'
following the
transaction id gives the packet sequence
number in the transaction
and the number in parens is the
amount of data
in the packet, excluding the atp header.
The `*' on packet 7 indicates
that the EOM bit was set.
Jssmag.209 then requests that
packets 3 & 5 be retransmit-
ted. Helios
resends them then jssmag.209 releases the
transaction. Finally,
jssmag.209 initiates the next
request. The
`*' on the request indicates that XO
(`exactly once') was not set.
IP Fragmentation
Fragmented Internet datagrams
are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates
there are more fragments. The
second indicates this is the
last fragment.)
Id is the
fragment id. Size is the fragment size (in
bytes) excluding the IP header.
Offset is this fragment's
offset (in bytes) in the original
datagram.
The fragment information is
output for each fragment. The
first fragment contains the
higher level protocol header
and the frag
info is printed after the protocol info.
Fragments after the first
contain no higher level protocol
header and the
frag info is printed after the source and
destination addresses.
For example, here is part of an
ftp from arizona.edu to lbl-rtsg.arpa
over a CSNET connec-
tion that doesn't appear to
handle 576 byte datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (fra
g 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.
SEE ALSO
traffic(1C), nit(4P), bpf(4),
pcap(3)
AUTHORS
Van Jacobson, Craig Leres
and Steven McCanne, all of the
Lawrence Berkeley National
Laboratory, University of Cali-
fornia, Berkeley, CA.
The current version is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
BUGS
Please send bug reports to
tcpdump@ee.lbl.gov.
NIT doesn't let you watch your
own outbound traffic, BPF
will. We recommend that
you use the latter.
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).
Filters 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 fil-
ter may inadvertently
accept certain packets that do not
properly match the filter
expression.
30 June 1997
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