tcpdump
dump traffic on a network
see also :
stty
Synopsis
tcpdump
[ -AbdDefhHIJKlLnNOpqRStuUvxX ] [
-B buffer_size ] [ -c
count ]
[ -C file_size ] [ -G
rotate_seconds ] [ -F file ]
[ -i interface ] [ -j
tstamp_type ] [ -m module ] [
-M secret ]
[ -r file ] [ -s
snaplen ] [ -T type ] [
-w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -z
postrotate-command ] [ -Z user ]
[ expression ]
add an example, a script, a trick and tips
examples
To print all packets arriving at or departing from
sundown:
tcpdump host sundown
To print traffic between helios and either hot or
ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except
helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup:
(note that the expression is quoted to prevent the shell from
(mis-)interpreting the parentheses):
tcpdump ’gateway snup and (port ftp or ftp-data)’
To print traffic neither sourced from nor destined for local
hosts (if you gateway to one other net, this stuff should never
make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of
each TCP conversation that involves a non-local host.
tcpdump ’tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src
and dst net localnet’
To print all IPv4 HTTP packets to and from port 80, i.e. print
only packets that contain data, not, for example, SYN and FIN
packets and ACK-only packets. (IPv6 is left as an exercise for
the reader.)
tcpdump ’tcp port 80 and (((ip[2:2] -
((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) !=
0)’
To print IP packets longer than 576 bytes sent through gateway
snup:
tcpdump ’gateway snup and ip[2:2] > 576’
To print IP broadcast or multicast packets that were not
sent via Ethernet broadcast or multicast:
tcpdump ’ether[0] & 1 = 0 and ip[16] >= 224’
To print all ICMP packets that are not echo requests/replies
(i.e., not ping packets):
tcpdump ’icmp[icmptype] != icmp-echo and icmp[icmptype] !=
icmp-echoreply’
source
config USER_TCPDUMP
bool "tcpdump"
select USER_PCAP
help
Enable building of the tcpdump tools.
source
Linux interface stops receiving packets as seen by tcpdump
This is probably down to your switch.
Sniffing a network in parallel like this either requires a hub or
a switch that has the capability to have a port placed in
'monitor mode'. In this mode all traffic to other ports gets
duplicated to the monitoring port for sniffing / capturing.
Without this monitor port the traffic just won't be coming from
the switch - it knows of no valid reason to send it through that
port.
source
How to make tcpdump (or other tool) to actually dump TCP stream?
tcpdump
normally displays packet information, as
opposed to actual data.
Use the -A
flag to dump ASCII contents. It will
still dump a lot of other data (like ARP and DNS packets, for
example), but you should be able to get what you want through
filters.
source
How to output a simple network activity plot in console in Linux?
Well, if you have perl and tcpdump installed it shouldn't be too
hard to hack something up...
Following script will output something like:
# ./host_traffic google.com
0k
30k ****************
26k **************
24k *************
26k **************
43k **********************
39k ********************
24k *************
15k ********
0k
I'll leave the sent/received bytes display as an exercise.
host_traffic:
#!/usr/bin/perl
$interface = "eth0";
$interval = 1;
$target_host = $ARGV[0];
$bytes = 0;
sub print_stats
{
$nstars = $bytes / 2000; # diagram scale = 2k
$stars = "";
for ($i = 0; $i < $nstars; $i++)
{ $stars .= "*"; }
printf("%4ik $stars\n", $bytes / 1000);
$bytes = 0;
alarm $interval;
}
$SIG{ALRM} = sub { print_stats(); };
alarm $interval;
$pcap_filter = "ip src or dst $target_host";
open(IN, "tcpdump -s 500 -l -n -i $interface '$pcap_filter' | ");
while (my $s = <IN>)
{ # parse tcpdump output
if ($s =~ m|(.*) IP (.*)\.([^.]*) > (.*)\.([^.]*): Flags \[(.*)\],.*, length (.*)|)
{
my ($timestamp, $src, $sport, $dst, $dport, $flags, $len) = ($1, $2, $3, $4, $5, $6, $7);
$bytes += $len;
next;
}
print $s;
}
source
Count exchanged bytes per TCP connection
If you want something fancy and graphical you might take a look
at ntop
- it's in most distro's apt / yum /
whatever.
It monitors the network traffic creating nice graphs and charts
on a per-connection, protocol, host, etc basis and displays it
all through your web browser (has a built in web server).
It's nice, but fairly heavyweight.
source
TCPDUMP -G option
Any chance you have upper-case %S
instead of
%s
? If so, it's doing exactly what you tell it to:
-
-w %S
create a file named
strftime("%S")
seconds into each minute, i.e. 15
for arguments sake
-
-G 60
recreate it every 60 seconds, i.e. at 15
seconds into each minute
If you want to capture files in minute-sized chunks you
can use %s
lower-case, or better still
something like %H%M%S.cap
, these should work:
tcpdump -w %s -G 60
tcpdump -w %H%M%S.cap -G 60
See the strftime
man page for details on the
specifiers. Otherwise what you describe is normal behaviour for
just -G
with no %-specifiers in the -w
option. I'm not aware of any bugs related to the -G
option.
The %s
specifier is a GNU extension to
strftime()
, it's been present in glibc
since glibc-2.0 (1997), however it may cause a problem
with non-glibc libraries, though both dietlibc and
uClibc have supported it since 2006 and 2002
respectively. I cannot think of anything else that would cause
strftime()
to not support %s
(unless
you are not running linux: on Solaris I would expect to get a
capture file called %s
which was reset every 60
seconds).
source
tcpdump error: no suitable device found in Linux
You probably need to execute as superuser in order to access the
network device directly.
Sudo the command.
Edit: To clarify, that tcpdump
command won't give
you much useful information either. Try this:
sudo tcpdump host stackoverflow.com
Or, if sudo
is not available:
su - ; tcpdump host stackoverflow.com
source
Detect HTTP sessions using tcpdump
tcpdump -r file.trace - tcp dst port 80
is, as you've discovered, not a valid tcpdump command. If you
remove the extra -
, then you get
tcpdump -r file.trace tcp dst port 80
which is a valid tcpdump command, that will show you only traffic
to TCP port 80. It will not show you traffic
from TCP port 80, so, for example, if
file.trace
has HTTP traffic to and from port 80, it
will show you HTTP request traffic but
NOT HTTP response traffic.
If you want to see traffic to and from port 80, use
tcpdump -r file.trace tcp port 80
However, if there's TCP traffic other than HTTP traffic to or
from port 80, that will also be shown.
In addition, HTTP traffic not going to or from port 80,
such as HTTP-over-SSL/TLS ("https") traffic, or traffic to ports
such as port 8080, will not be shown. To, for example, see
traffic to or from port 80 (for HTTP) and port 443 (for HTTPS),
do
tcpdump -r file.trace tcp port 80 or 443
So what's "not working"?
Is it not showing, for example, HTTPS traffic? If so, you need to
add port 443.
Is it not showing traffic to other ports? If so, you need to add
those ports as well. Tcpdump can't recognize HTTP traffic (and
Wireshark only recognizes it by port number).
Is it showing only requests, not responses? If you want to see
responses, use tcp port
rather than tcp dst
port
. If you only want to see requests, use
tcp dst port
.
source
how to make tcpdump to display ip and port number but not hostname and protocol
Add -n
to your tcpdump
command line.
From the tcpdump manpage:
-n Don't convert addresses (i.e., host addresses, port numbers,
etc.) to names.
source
Trouble with mergecap [ concatenation of pcap files ] - undesired info in output file
I think you must have done something strange, it seems to work
perfectly well for me:
$ head oo.pcap.merged | hexdump -C
00000000 0a 0d 0d 0a 84 00 00 00 4d 3c 2b 1a 01 00 00 00 |........M<+.....|
00000010 ff ff ff ff ff ff ff ff 01 00 52 00 46 69 6c 65 |..........R.File|
00000020 20 63 72 65 61 74 65 64 20 62 79 20 6d 65 72 67 | created by merg|
00000030 69 6e 67 3a 20 0a 46 69 6c 65 31 3a 20 61 61 61 |ing: .File1: aaa|
00000040 61 2e 70 63 61 70 20 0a 46 69 6c 65 32 3a 20 62 |a.pcap .File2: b|
00000050 62 62 62 62 2e 70 63 61 70 20 0a 46 69 6c 65 33 |bbbb.pcap .File3|
00000060 3a 20 63 63 63 63 63 2e 70 63 61 70 20 0a 00 00 |: ccccc.pcap ...|
00000070 04 00 08 00 6d 65 72 67 65 63 61 70 00 00 00 00 |....mergecap....|
So, it is displaying the same behavior you describe, but then
when I try to use tcpdump on the file, it seems to work:
$ tcpdump -r oo.pcap.merged | head
reading from file oo.pcap.merged, link-type EN10MB (Ethernet)
And then continues normally for 369 lines.
tcpdump version 4.3.0
Mergecap 1.8.0
source
Is there a tool to decrypt all SSL/SSH traffic originating from this computer?
I believe encryption keys are not static and they change during
the course of a session. So would be almost impossible to achieve
this.
description
Tcpdump
prints out a description of the contents of packets on a
network interface that match the boolean expression.
It can also be run with the -w flag, which
causes it to save the packet data to a file for later
analysis, and/or with the -r flag, which causes
it to read from a saved packet file rather than to read
packets from a network interface (please note tcpdump
is protected via an enforcing apparmor(7) profile in
Ubuntu which limits the files tcpdump may access). In
all cases, only packets that match expression will be
processed by tcpdump.
Tcpdump
will, if not run with the -c flag, continue
capturing packets until it is interrupted by a SIGINT signal
(generated, for example, by typing your interrupt character,
typically control-C) or a SIGTERM signal (typically
generated with the kill(1) command); if run with the
-c flag, it will capture packets until it is
interrupted by a SIGINT or SIGTERM signal or the specified
number of packets have been processed.
When
tcpdump finishes capturing packets, it will report
counts of:
packets
’’captured’’ (this is the number of
packets that tcpdump has received and processed);
packets
’’received by filter’’ (the meaning
of this depends on the OS on which you’re running
tcpdump, and possibly on the way the OS was
configured - if a filter was specified on the command line,
on some OSes it counts packets regardless of whether they
were matched by the filter expression and, even if they were
matched by the filter expression, regardless of whether
tcpdump has read and processed them yet, on other
OSes it counts only packets that were matched by the filter
expression regardless of whether tcpdump has read and
processed them yet, and on other OSes it counts only packets
that were matched by the filter expression and were
processed by tcpdump);
packets
’’dropped by kernel’’ (this is the
number of packets that were dropped, due to a lack of buffer
space, by the packet capture mechanism in the OS on which
tcpdump is running, if the OS reports that
information to applications; if not, it will be reported as
0).
On platforms
that support the SIGINFO signal, such as most BSDs
(including Mac OS X) and Digital/Tru64 UNIX, it will report
those counts when it receives a SIGINFO signal (generated,
for example, by typing your
’’status’’ character, typically
control-T, although on some platforms, such as Mac OS X, the
’’status’’ character is not set by
default, so you must set it with stty(1) in order to
use it) and will continue capturing packets.
Reading packets
from a network interface may require that you have special
privileges; see the pcap (3PCAP) man page for
details. Reading a saved packet file doesn’t require
special privileges.
options
-A
Print each packet (minus its
link level header) in ASCII. Handy for capturing web
pages.
-b
Print the AS number in BGP packets in ASDOT notation
rather than ASPLAIN notation.
-B
Set the operating system capture buffer size to
buffer_size, in units of KiB (1024 bytes).
-c
Exit after receiving count packets.
-C
Before writing a raw packet to a savefile, check whether
the file is currently larger than file_size and, if
so, close the current savefile and open a new one. Savefiles
after the first savefile will have the name specified with
the -w flag, with a number after it, starting
at 1 and continuing upward. The units of file_size
are millions of bytes (1,000,000 bytes, not 1,048,576
bytes).
-d
Dump the compiled packet-matching code in a human
readable form to standard output and stop.
-dd
Dump packet-matching code as a C program
fragment.
-ddd
Dump packet-matching code as decimal numbers (preceded
with a count).
-D
Print the list of the network interfaces available on
the system and on which tcpdump can capture packets.
For each network interface, a number and an interface name,
possibly followed by a text description of the interface, is
printed. The interface name or the number can be supplied to
the -i flag to specify an interface on which to
capture.
This can be
useful on systems that don’t have a command to list
them (e.g., Windows systems, or UNIX systems lacking
ifconfig -a); the number can be useful on
Windows 2000 and later systems, where the interface name is
a somewhat complex string.
The
-D flag will not be supported if tcpdump
was built with an older version of libpcap that lacks
the pcap_findalldevs() function.
-e
Print the link-level header on
each dump line.
-E
Use spi@ipaddr algo:secret for decrypting IPsec
ESP packets that are addressed to addr and contain
Security Parameter Index value spi. This combination
may be repeated with comma or newline separation.
Note that
setting the secret for IPv4 ESP packets is supported at this
time.
Algorithms may
be des-cbc, 3des-cbc, blowfish-cbc,
rc3-cbc, cast128-cbc, or none. The
default is des-cbc. The ability to decrypt packets is
only present if tcpdump was compiled with
cryptography enabled.
secret
is the ASCII text for ESP secret key. If preceded by 0x,
then a hex value will be read.
The option
assumes RFC2406 ESP, not RFC1827 ESP. The option is only for
debugging purposes, and the use of this option with a true
’secret’ key is discouraged. By presenting IPsec
secret key onto command line you make it visible to others,
via ps(1) and other occasions.
In addition to
the above syntax, the syntax file name may be used to
have tcpdump read the provided file in. The file is opened
upon receiving the first ESP packet, so any special
permissions that tcpdump may have been given should already
have been given up.
-f
Print ’foreign’ IPv4
addresses numerically rather than symbolically (this option
is intended to get around serious brain damage in
Sun’s NIS server — usually it hangs forever
translating non-local internet numbers).
The test for
’foreign’ IPv4 addresses is done using the IPv4
address and netmask of the interface on which capture is
being done. If that address or netmask are not available,
available, either because the interface on which capture is
being done has no address or netmask or because the capture
is being done on the Linux "any" interface, which
can capture on more than one interface, this option will not
work correctly.
-F
Use file as input for the
filter expression. An additional expression given on the
command line is ignored.
-G
If specified, rotates the dump file specified with the
-w option every rotate_seconds seconds.
Savefiles will have the name specified by -w
which should include a time format as defined by
strftime(3). If no time format is specified, each new
file will overwrite the previous.
If used in
conjunction with the -C option, filenames will
take the form of ’file<count>’.
-h
Print the tcpdump and libpcap
version strings, print a usage message, and exit.
-H
Attempt to detect 802.11s draft mesh headers.
-i
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.
On Linux
systems with 2.2 or later kernels, an interface
argument of ’’any’’ can be used to
capture packets from all interfaces. Note that captures on
the ’’any’’ device will not be done
in promiscuous mode.
If the
-D flag is supported, an interface number as
printed by that flag can be used as the interface
argument.
-I
Put the interface in
"monitor mode"; this is supported only on IEEE
802.11 Wi-Fi interfaces, and supported only on some
operating systems.
Note that in
monitor mode the adapter might disassociate from the network
with which it’s associated, so that you will not be
able to use any wireless networks with that adapter. This
could prevent accessing files on a network server, or
resolving host names or network addresses, if you are
capturing in monitor mode and are not connected to another
network with another adapter.
This flag will
affect the output of the -L flag. If
-I isn’t specified, only those link-layer
types available when not in monitor mode will be shown; if
-I is specified, only those link-layer types
available when in monitor mode will be shown.
-j
Set the time stamp type for the
capture to tstamp_type. The names to use for the time
stamp types are given in pcap-tstamp-type(7); not all
the types listed there will necessarily be valid for any
given interface.
-J
List the supported time stamp types for the interface
and exit. If the time stamp type cannot be set for the
interface, no time stamp types are listed.
-K
Don’t attempt to verify IP, TCP, or UDP checksums.
This is useful for interfaces that perform some or all of
those checksum calculation in hardware; otherwise, all
outgoing TCP checksums will be flagged as bad.
-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
Note that on
Windows,’’line buffered’’ means
’’unbuffered’’, so that WinDump will
write each character individually if -l is
specified.
-U
is similar to -l in its behavior, but it will
cause output to be
’’packet-buffered’’, so that the
output is written to stdout at the end of each packet rather
than at the end of each line; this is buffered on all
platforms, including Windows.
-L
List the known data link types
for the interface, in the specified mode, and exit. The list
of known data link types may be dependent on the specified
mode; for example, on some platforms, a Wi-Fi interface
might support one set of data link types when not in monitor
mode (for example, it might support only fake Ethernet
headers, or might support 802.11 headers but not support
802.11 headers with radio information) and another set of
data link types when in monitor mode (for example, it might
support 802.11 headers, or 802.11 headers with radio
information, only in monitor mode).
-m
Load SMI MIB module definitions from file module.
This option can be used several times to load several MIB
modules into tcpdump.
-M
Use secret as a shared secret for validating the
digests found in TCP segments with the TCP-MD5 option (RFC
2385), if present.
-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 information
so output lines are shorter.
-R
Assume ESP/AH packets to be based on old specification
(RFC1825 to RFC1829). If specified, tcpdump will not
print replay prevention field. Since there is no protocol
version field in ESP/AH specification, tcpdump cannot
deduce the version of ESP/AH protocol.
-r
Read packets from file (which was created with
the -w option). Standard input is used if
file is ’’-’’.
-S
Print absolute, rather than relative, TCP sequence
numbers.
-s
Snarf snaplen bytes of data from each packet
rather than the default of 65535 bytes. Packets truncated
because of a limited snapshot are indicated in the output
with ’’[|proto]’’, where
proto is the name of the protocol level at which the
truncation has occurred. Note that taking larger snapshots
both increases the amount of time it takes to process
packets and, effectively, decreases the amount of packet
buffering. This may cause packets to be lost. You should
limit snaplen to the smallest number that will
capture the protocol information you’re interested in.
Setting snaplen to 0 sets it to the default of 65535,
for backwards compatibility with recent older versions of
tcpdump.
-T
Force packets selected by "expression"
to be interpreted the specified type. Currently known
types are aodv (Ad-hoc On-demand Distance Vector
protocol), cnfp (Cisco NetFlow protocol), rpc
(Remote Procedure Call), rtp (Real-Time Applications
protocol), rtcp (Real-Time Applications control
protocol), snmp (Simple Network Management Protocol),
tftp (Trivial File Transfer Protocol), vat
(Visual Audio Tool), and wb (distributed White
Board).
-t
Don’t print a timestamp on each dump
line.
-tt
Print an unformatted timestamp on each dump line.
-ttt
Print a delta (micro-second resolution) between current
and previous line on each dump line.
-tttt
Print a timestamp in default format proceeded by date on
each dump line.
-ttttt
Print a delta (micro-second resolution) between current
and first line on each dump line.
-u
Print undecoded NFS handles.
-U
If the -w option is not specified, make the
printed packet output
’’packet-buffered’’; i.e., as the
description of the contents of each packet is printed, it
will be written to the standard output, rather than, when
not writing to a terminal, being written only when the
output buffer fills.
If the
-w option is specified, make the saved raw
packet output ’’packet-buffered’’;
i.e., as each packet is saved, it will be written to the
output file, rather than being written only when the output
buffer fills.
The
-U flag will not be supported if tcpdump
was built with an older version of libpcap that lacks
the pcap_dump_flush() function.
-v
When parsing and printing,
produce (slightly more) verbose output. For example, the
time to live, identification, total length and options in an
IP packet are printed. Also enables additional packet
integrity checks such as verifying the IP and ICMP header
checksum.
When writing to
a file with the -w option, report, every 10
seconds, the number of packets captured.
-vv
Even more verbose output. For
example, additional fields are printed from NFS reply
packets, and SMB packets are fully decoded.
-vvv
Even more verbose output. For example, telnet SB
... SE options are printed in full. With
-X Telnet options are printed in hex as
well.
-w
Write the raw packets to file rather than parsing
and printing them out. They can later be printed with the
-r option. Standard output is used if file is
’’-’’.
This output
will be buffered if written to a file or pipe, so a program
reading from the file or pipe may not see packets for an
arbitrary amount of time after they are received. Use the
-U flag to cause packets to be written as soon
as they are received.
See
pcap-savefile(5) for a description of the file
format.
-W
Used in conjunction with the
-C option, this will limit the number of files
created to the specified number, and begin overwriting files
from the beginning, thus creating a ’rotating’
buffer. In addition, it will name the files with enough
leading 0s to support the maximum number of files, allowing
them to sort correctly.
Used in
conjunction with the -G option, this will limit
the number of rotated dump files that get created, exiting
with status 0 when reaching the limit. If used with
-C as well, the behavior will result in
cyclical files per timeslice.
-x
When parsing and printing, in
addition to printing the headers of each packet, print the
data of each packet (minus its link level header) in hex.
The smaller of the entire packet or snaplen bytes
will be printed. Note that this is the entire link-layer
packet, so for link layers that pad (e.g. Ethernet), the
padding bytes will also be printed when the higher layer
packet is shorter than the required padding.
-xx
When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet,
including its link level header, in hex.
-X
When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet (minus
its link level header) in hex and ASCII. This is very handy
for analysing new protocols.
-XX
When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet,
including its link level header, in hex and
ASCII.
-y
Set the data link type to use while capturing packets to
datalinktype.
-z
Used in conjunction with the -C or -G
options, this will make tcpdump run " command
file " where file is the savefile being
closed after each rotation. For example, specifying
-z gzip or -z bzip2 will compress
each savefile using gzip or bzip2.
Note that
tcpdump will run the command in parallel to the capture,
using the lowest priority so that this doesn’t disturb
the capture process.
And in case you
would like to use a command that itself takes flags or
different arguments, you can always write a shell script
that will take the savefile name as the only argument, make
the flags & arguments arrangements and execute the
command that you want.
-Z
If tcpdump is running as
root, after opening the capture device or input savefile,
but before opening any savefiles for output, change the user
ID to user and the group ID to the primary group of
user.
This behavior
can also be enabled by default at compile time.
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
expression is ’true’ will be dumped.
For the
expression syntax, see pcap-filter(7).
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.
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 FDDI networks, the ’-e’ option causes tcpdump to print
the ’frame control’ field, the source and destination addresses,
and the packet length. (The ’frame control’ field governs the
interpretation of the rest of the packet. Normal packets (such as
those containing IP datagrams) are ’async’ packets, with a
priority value between 0 and 7; for example, ’async4’.
Such packets are assumed to contain an 802.2 Logical Link Control
(LLC) packet; the LLC header is printed if it is not an
ISO datagram or a so-called SNAP packet.
On Token Ring networks, the ’-e’ option causes tcpdump to
print the ’access control’ and ’frame control’ fields, the source
and destination addresses, and the packet length. As on FDDI
networks, packets are assumed to contain an LLC packet.
Regardless of whether the ’-e’ option is specified or not, the
source routing information is printed for source-routed packets.
On 802.11 networks, the ’-e’ option causes tcpdump to
print the ’frame control’ fields, all of the addresses in the
802.11 header, and the packet length. As on FDDI networks,
packets are assumed to contain an LLC 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 information are
printed out. The packet type is printed first. The three types
are ip, utcp, and ctcp. No further link
information is printed for ip packets. For TCP packets,
the connection identifier is printed following the type. If the
packet is compressed, its encoded header is printed out. The
special cases are printed out as *S+n and
*SA+n, where n is the amount by which the
sequence number (or sequence number and ack) has changed. If it
is not a special case, zero or more changes are printed. A change
is indicated by U (urgent pointer), W (window), A (ack), S
(sequence number), and I (packet ID), followed by a delta (+n or
-n), or a new value (=n). Finally, the amount of data in the
packet and compressed header length are printed.
For example, the following line shows an outbound compressed TCP
packet, with an implicit connection identifier; the ack has
changed by 6, the sequence number by 49, and the packet ID by 6;
there are 3 bytes of data and 6 bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
Arp/rarp output shows the type of request and its arguments. The
format is intended to be self explanatory. Here is a short sample
taken from the start of an ’rlogin’ from host rtsg to host
csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking for the
Ethernet address of internet host csam. Csam replies with its
Ethernet address (in this example, Ethernet addresses are in caps
and internet addresses in lower case).
This would look less redundant if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet
is broadcast and the second is point-to-point would be visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the Ethernet source address is
RTSG, the destination is the Ethernet broadcast address, the type
field contained hex 0806 (type ETHER_ARP) and the total length
was 64 bytes.
TCP Packets
(N.B.:The following description assumes familiarity with the
TCP protocol described in RFC-793. If you are not familiar with
the protocol, neither this description nor tcpdump will be of
much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP
addresses and ports. Flags are some combination of S
(SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E
(ECN-Echo) or ’.’ (ACK), or ’none’ if no flags are set.
Data-seqno describes the portion of sequence space covered
by the data in this packet (see example below). Ack is
sequence number of the next data expected the other direction on
this connection. Window is the number of bytes of receive
buffer space available the other direction on this connection.
Urg indicates there is ’urgent’ data in the packet.
Options are tcp options enclosed in angle brackets (e.g.,
<mss 1024>).
Src, dst and flags are always present. The other
fields depend on the contents of the packet’s tcp protocol header
and are 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 the ACK flag was set. The packet contained no data so
there is no data sequence number. Note that the ack sequence
number is a small integer (1). The first time tcpdump sees
a tcp ’conversation’, it prints the sequence number from the
packet. On subsequent packets of the conversation, the difference
between the current packet’s sequence number and this initial
sequence number is printed. This means that sequence numbers
after the first can be interpreted as relative byte positions in
the conversation’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 conversation). The PUSH
flag is set in the packet. On the 7th line, csam says it’s
received data sent by rtsg up to but not including byte 21. Most
of this data is apparently sitting in the socket buffer since
csam’s receive window has gotten 19 bytes smaller. Csam also
sends one byte of data to rtsg in this packet. On the 8th and 9th
lines, csam sends two bytes of urgent, pushed data to rtsg.
If the snapshot was small enough that tcpdump didn’t
capture the full TCP header, it interprets as much of the header
as it can and then reports ’’[|tcp]’’ to indicate the
remainder could not be interpreted. If the header contains a
bogus option (one with a length that’s either too small or beyond
the end of the header), tcpdump reports it as ’’[bad
opt]’’ and does not interpret any further options (since it’s
impossible to tell where they start). If the header length
indicates options are present but the IP datagram length is not
long enough for the options to actually be there, tcpdump
reports it as ’’[bad hdr length]’’.
Capturing TCP packets with particular flag combinations
(SYN-ACK, URG-ACK, etc.)
There are 8 bits in the control bits section of the TCP header:
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
Let’s assume that we want to watch packets used in establishing a
TCP connection. Recall that TCP uses a 3-way handshake protocol
when it initializes a new connection; the connection sequence
with regard to the TCP control bits is
1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK
Now we’re interested in capturing packets that have only the SYN
bit set (Step 1). Note that we don’t want packets from step 2
(SYN-ACK), just a plain initial SYN. What we need is a correct
filter expression for tcpdump.
Recall the structure of a TCP header without options:
0 15 31
| source port | destination port |
| sequence number |
| acknowledgment number |
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
| TCP checksum | urgent pointer |
-----------------------------------------------------------------
A TCP header usually holds 20 octets of data, unless options are
present. The first line of the graph contains octets 0 - 3, the
second line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are
contained in octet 13:
0 7| 15| 23| 31
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
| | 13th octet | | |
Let’s have a closer look at octet no. 13:
| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|
These are the TCP control bits we are interested in. We have
numbered the bits in this octet from 0 to 7, right to left, so
the PSH bit is bit number 3, while the URG bit is number 5.
Recall that we want to capture packets with only SYN set. Let’s
see what happens to octet 13 if a TCP datagram arrives with the
SYN bit set in its header:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Looking at the control bits section we see that only bit number 1
(SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in
network byte order, the binary value of this octet is
00000010
and its decimal representation is
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
We’re almost done, because now we know that if only SYN is set,
the value of the 13th octet in the TCP header, when interpreted
as a 8-bit unsigned integer in network byte order, must be
exactly 2.
This relationship can be expressed as
tcp[13] == 2
We can use this expression as the filter for tcpdump in
order to watch packets which have only SYN set:
tcpdump -i xl0 tcp[13] == 2
The expression says "let the 13th octet of a TCP datagram have
the decimal value 2", which is exactly what we want.
Now, let’s assume that we need to capture SYN packets, but we
don’t care if ACK or any other TCP control bit is set at the same
time. Let’s see what happens to octet 13 when a TCP datagram with
SYN-ACK set arrives:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the 13th octet. The binary value of
octet 13 is
00010010
which translates to decimal
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
Now we can’t just use ’tcp[13] == 18’ in the tcpdump
filter expression, because that would select only those packets
that have SYN-ACK set, but not those with only SYN set. Remember
that we don’t care if ACK or any other control bit is set as long
as SYN is set.
In order to achieve our goal, we need to logically AND the binary
value of octet 13 with some other value to preserve the SYN bit.
We know that we want SYN to be set in any case, so we’ll
logically AND the value in the 13th octet with the binary value
of a SYN:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set. The
decimal representation of the AND value as well as the result of
this operation is 2 (binary 00000010), so we know that for
packets with SYN set the following relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 ’tcp[13] & 2 == 2’
Some offsets and field values may be expressed as names rather
than as numeric values. For example tcp[13] may be replaced with
tcp[tcpflags]. The following TCP flag field values are also
available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-act, tcp-urg.
This can be demonstrated as:
tcpdump -i xl0 ’tcp[tcpflags] & tcp-push != 0’
Note that you should use single quotes or a backslash in the
expression to hide the AND (’&’) special character from the
shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp
datagram to port who on host broadcast, the
Internet broadcast address. The packet contained 84 bytes of user
data.
Some UDP services are recognized (from the source or destination
port number) and the higher level protocol information printed.
In particular, Domain Name service requests (RFC-1034/1035) and
Sun RPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with the
Domain Service protocol described in RFC-1035. If you are not
familiar with the protocol, the following description will appear
to be written in greek.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for
an address record (qtype=A) associated with the name
ucbvax.berkeley.edu. The query id was ’3’. The ’+’
indicates the recursion desired flag was set. The query
length was 37 bytes, not including the UDP and IP protocol
headers. The query operation was the normal one, Query, so
the op field was omitted. If the op had been anything else, it
would have been printed between the ’3’ and the ’+’. Similarly,
the qclass was the normal one, C_IN, and omitted. Any
other qclass would have been printed immediately after the ’A’.
A few anomalies are checked and may result in extra fields
enclosed in square brackets: If a query contains an answer,
authority records or additional records section, ancount,
nscount, or arcount are printed as ’[na]’,
’[nn]’ or ’[nau]’ where n is the appropriate
count. If any of the response bits are set (AA, RA or rcode) or
any of the ’must be zero’ bits are set in bytes two and three,
’[b2&3=x]’ is printed, where x is the hex value
of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data
(len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from
h2opolo with 3 answer records, 3 name server records and 7
additional records. The first answer record is type A (address)
and its data is internet address 128.32.137.3. The total size of
the response was 273 bytes, excluding UDP and IP headers. The op
(Query) and response code (NoError) were omitted, as was the
class (C_IN) of the A record.
In the second example, helios responds to query 2 with a
response code of non-existent domain (NXDomain) with no answers,
one name server and no authority records. The ’*’ indicates that
the authoritative answer bit was set. Since there were no
answers, no type, class or data were printed.
Other flag characters that might appear are ’-’ (recursion
available, RA, not set) and ’|’ (truncated message, TC,
set). If the ’question’ section doesn’t contain exactly one
entry, ’[nq]’ is printed.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT
decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive
decoding of IPX and NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more
detailed decode done if -v is used. Be warned that with -v a
single SMB packet may take up a page or more, so only use -v if
you really want all the gory details.
For information on SMB packet formats and what all the fields
mean see www.cifs.org or the pub/samba/specs/ directory on your
favorite samba.org mirror site. The SMB patches were written by
Andrew Tridgell (tridge[:at:]samba[:dot:]org).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed
as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with id
6709 to wrl (note that the number following the src
host is a transaction id, not the source port). The
request was 112 bytes, excluding the UDP and IP headers. The
operation was a readlink (read symbolic link) on file
handle (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 generation
number.) Wrl replies ’ok’ with the contents of the link.
In the third line, sushi asks wrl to lookup the
name ’xcolors’ in directory file 9,74/4096.6878. Note that
the data printed depends on the operation type. The format is
intended to be self explanatory if read in conjunction with an
NFS protocol spec.
If the -v (verbose) flag is given, additional information is
printed. For example:
sushi.1372a > wrl.nfs:
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header TTL, ID, length, and fragmentation
fields, which have been omitted from this example.) In the first
line, sushi asks wrl to read 8192 bytes from file
21,11/12.195, at byte offset 24576. Wrl replies ’ok’; the
packet shown on the second line is the first fragment of the
reply, and hence is only 1472 bytes long (the other bytes will
follow in subsequent fragments, but these fragments do not have
NFS or even UDP headers and so might not be printed, depending on
the filter expression used). Because the -v flag is given, some
of the file attributes (which are returned in addition to the
file data) are printed: the file type (’’REG’’, for regular
file), the file mode (in octal), the uid and gid, and the file
size.
If the -v flag is given more than once, even more details are
printed.
Note that NFS requests are very large and much of the detail
won’t be printed unless snaplen is increased. Try using
’-s 192’ to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump keeps track of ’’recent’’ requests, and
matches them to the replies using the transaction ID. If a reply
does not closely follow the corresponding request, it might not
be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are
printed as:
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name
args
src.sport > dst.dport: rx packet-type service reply call-name
args
elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was
a RX data packet to the fs (fileserver) service, and is the start
of an RPC call. The RPC call was a rename, with the old directory
file id of 536876964/1/1 and an old filename of ’.newsrc.new’,
and a new directory file id of 536876964/1/1 and a new filename
of ’.newsrc’. The host pike responds with a RPC reply to the
rename call (which was successful, because it was a data packet
and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name.
Most AFS RPCs have at least some of the arguments decoded
(generally only the ’interesting’ arguments, for some definition
of interesting).
The format is intended to be self-describing, but it will
probably not be useful to people who are not familiar with the
workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets
and additional header information is printed, such as the the RX
call ID, call number, sequence number, serial number, and the RX
packet flags.
If the -v flag is given twice, additional information is printed,
such as the the RX call ID, serial number, and the RX packet
flags. The MTU negotiation information is also printed from RX
ack packets.
If the -v flag is given three times, the security index and
service id are printed.
Error codes are printed for abort packets, with the exception of
Ubik beacon packets (because abort packets are used to signify a
yes vote for the Ubik protocol).
Note that AFS requests are very large and many of the arguments
won’t be printed unless snaplen is increased. Try using
’-s 256’ to watch AFS traffic.
AFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump keeps track of ’’recent’’ requests, and
matches them to the replies using the call number and service ID.
If a reply does not closely follow the corresponding request, it
might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are
de-encapsulated and dumped as DDP packets (i.e., all the UDP
header information is discarded). The file
/etc/atalk.names is used to translate AppleTalk net and
node numbers to names. Lines in this file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of AppleTalk networks. The
third line gives the name of a particular host (a host is
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
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn’t exist or doesn’t contain
an entry for some AppleTalk host/net number, addresses are
printed in numeric form.) In the first example, NBP (DDP port 2)
on net 144.1 node 209 is sending to whatever is listening on port
220 of net icsd node 112. The second line is the same except the
full name of the source node is known (’office’). The third line
is a send from port 235 on net jssmag node 149 to broadcast on
the icsd-net NBP port (note that the broadcast address (255) is
indicated by a net name with no host number - for this reason
it’s a good idea to keep node names and net names distinct in
/etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction
protocol) packets have their contents interpreted. Other
protocols just dump the protocol name (or number if no name is
registered for the protocol) and packet size.
NBP packets are formatted like the following examples:
icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
The first line is a name lookup request for laserwriters sent by
net icsd host 112 and broadcast on net jssmag. The nbp id for the
lookup is 190. The second line shows a reply for this request
(note that it has the same id) from host jssmag.209 saying that
it has a laserwriter resource named "RM1140" registered on port
250. The third line is another reply to the same request saying
host techpit has laserwriter "techpit" registered on port 186.
ATP packet formatting is demonstrated by the following
example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by
requesting up to 8 packets (the ’<0-7>’). The hex number at
the end of the line is the value of the ’userdata’ field in the
request.
Helios responds with 8 512-byte packets. The ’:digit’ following
the transaction id gives the packet sequence number in the
transaction and the number in parens is the amount of data in the
packet, excluding the atp header. The ’*’ on packet 7 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+)
(frag
id:size@offset)
(The first form indicates there are more fragments. The second
indicates this is the last fragment.)
Id is the fragment id. Size is the fragment size
(in bytes) excluding the IP header. Offset is this
fragment’s offset (in bytes) in the original datagram.
The fragment information is output for each fragment. The first
fragment contains the higher level protocol header and the frag
info is printed after the protocol info. Fragments after the
first contain no higher level protocol header and the frag info
is printed after the source and destination addresses. For
example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
over a CSNET connection that doesn’t appear to handle 576 byte
datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in
the 2nd line don’t include port numbers. This is because the TCP
protocol information is all in the first fragment and we have no
idea what the port or sequence numbers are when we print the
later fragments. Second, the tcp sequence information in the
first line is printed as if there were 308 bytes of user data
when, in fact, there are 512 bytes (308 in the first frag and 204
in the second). If you are looking for holes in the sequence
space or trying to match up acks with packets, this can fool you.
A packet with the IP don’t fragment flag is marked with a
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.
bugs
Please send
problems, bugs, questions, desirable enhancements, patches
etc. to:
tcpdump-workers[:at:]lists.tcpdump[:dot:]org
NIT
doesn’t let you watch your own outbound traffic, BPF
will. We recommend that you use the latter.
On Linux
systems with 2.0[.x] kernels:
packets on the
loopback device will be seen twice;
packet
filtering cannot be done in the kernel, so that all packets
must be copied from the kernel in order to be filtered in
user mode;
all of a
packet, not just the part that’s within the snapshot
length, will be copied from the kernel (the 2.0[.x] packet
capture mechanism, if asked to copy only part of a packet to
userland, will not report the true length of the packet;
this would cause most IP packets to get an error from
tcpdump);
capturing on
some PPP devices won’t work correctly.
We recommend
that you upgrade to a 2.2 or later kernel.
Some attempt
should be made to reassemble IP fragments or, at least to
compute the right length for the higher level protocol.
Name server
inverse queries are not dumped correctly: the (empty)
question section is printed rather than real query in the
answer section. Some believe that inverse queries are
themselves a bug and prefer to fix the program generating
them rather than tcpdump.
A packet trace
that crosses a daylight savings time change will give skewed
time stamps (the time change is ignored).
Filter
expressions on fields other than those in Token Ring headers
will not correctly handle source-routed Token Ring
packets.
Filter
expressions on fields other than those in 802.11 headers
will not correctly handle 802.11 data packets with both To
DS and From DS set.
ip6
proto should chase header chain, but at this moment it
does not. ip6 protochain is supplied for this
behavior.
Arithmetic
expression against transport layer headers, like
tcp[0], does not work against IPv6 packets. It only
looks at IPv4 packets.
see also
stty ,
pcap(3PCAP), bpf, nit(4P), pcap-savefile,
pcap-filter, pcap-tstamp-type, apparmor
authors
The original
authors are:
Van Jacobson,
Craig Leres and Steven McCanne, all of the Lawrence Berkeley
National Laboratory, University of California, Berkeley,
CA.
It is currently
being maintained by tcpdump.org.
The current
version is available via http:
http://www.tcpdump.org/
The original
distribution is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
IPv6/IPsec
support is added by WIDE/KAME project. This program uses
Eric Young’s SSLeay library, under specific
configurations.