From ImageStream Router Documentation
TCPDUMP(1) TCPDUMP(1)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E algo:secret ] [ expression ]
DESCRIPTION
Tcpdump prints out the headers of packets on a network interface that
match the boolean expression. It can also be run with the -w flag, which
causes it to save the packet data to a file for later analysis, and/or
with the -r flag, which causes it to read from a saved packet file rather
than to read packets from a network interface. In all cases, only pack-
ets 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 ``received by filter'' (the meaning of this depends on the
OS on which you're running tcpdump, and possibly on the way the OS
was configured - if a filter was specified on the command line, on
some OSes it counts packets regardless of whether they were
matched by the filter expression, and on other OSes it counts only
packets that were matched by the filter expression and were pro-
cessed by tcpdump);
packets ``dropped by kernel'' (this is the number of packets that
were dropped, due to a lack of buffer space, by the packet capture
mechanism in the OS on which tcpdump is running, if the OS reports
that information to applications; if not, it will be reported as
0).
On platforms that support the SIGINFO signal, such as most BSDs, it will
report those counts when it receives a SIGINFO signal (generated, for
example, by typing your ``status'' character, typically control-T) and
will continue capturing packets.
Reading packets from a network interface may require that you have spe-
cial privileges:
Under SunOS 3.x or 4.x with NIT or BPF:
You must have read access to /dev/nit or /dev/bpf*.
Under Solaris with DLPI:
You must have read/write access to the network pseudo device, e.g.
/dev/le. On at least some versions of Solaris, however, this is
not sufficient to allow tcpdump to capture in promiscuous mode; on
those versions of Solaris, you must be root, or tcpdump must be
installed setuid to root, in order to capture in promiscuous mode.
Note that, on many (perhaps all) interfaces, if you don't capture
in promiscuous mode, you will not see any outgoing packets, so a
capture not done in promiscuous mode may not be very useful.
Under HP-UX with DLPI:
You must be root or tcpdump must be installed setuid to root.
Under IRIX with snoop:
You must be root or tcpdump must be installed setuid to root.
Under Linux:
You must be root or tcpdump must be installed setuid to root.
Under Ultrix and Digital UNIX/Tru64 UNIX:
Any user may capture network traffic with tcpdump. However, no
user (not even the super-user) can capture in promiscuous mode on
an interface unless the super-user has enabled promiscuous-mode
operation on that interface using pfconfig(8), and no user (not
even the super-user) can capture unicast traffic received by or
sent by the machine on an interface unless the super-user has
enabled copy-all-mode operation on that interface using pfconfig,
so useful packet capture on an interface probably requires that
either promiscuous-mode or copy-all-mode operation, or both modes
of operation, be enabled on that interface.
Under BSD:
You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privileges.
OPTIONS
-a Attempt to convert network and broadcast addresses to names.
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file
is currently larger than file_size and, if so, close the current
savefile and open a new one. Savefiles after the first savefile
will have the name specified with the -w flag, with a number after
it, starting at 2 and continuing upward. The units of file_size
are 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).
-e Print the link-level header on each dump line.
-E Use algo:secret for decrypting IPsec ESP packets. 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
the ascii text for ESP secret key. We cannot take arbitrary
binary value at this moment. The option assumes RFC2406 ESP, not
RFC1827 ESP. The option is only for debugging purposes, and the
use of this option with truly `secret' key is discouraged. By
presenting IPsec secret key onto command line you make it visible
to others, via ps(1) and other occasions.
-f Print `foreign' internet addresses numerically rather than symbol-
ically (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 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.
-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''.
-m Load SMI MIB module definitions from file module. This option can
be used several times to load several MIB modules into tcpdump.
-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 speci-
fication, 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 68 (with SunOS's NIT, the minimum is actually 96). 68
bytes is adequate for IP, ICMP, TCP and UDP but may truncate pro-
tocol information from name server and NFS packets (see below).
Packets truncated because of a limited snapshot are indicated in
the output with ``[|proto]'', where proto is the name of the pro-
tocol level at which the truncation has occurred. Note that tak-
ing larger snapshots both increases the amount of time it takes to
process packets and, effectively, decreases the amount of packet
buffering. This may cause packets to be lost. You should limit
snaplen to the smallest number that will capture the protocol
information you're interested in. Setting snaplen to 0 means use
the required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted the spec-
ified type. Currently known types are cnfp (Cisco NetFlow proto-
col), rpc (Remote Procedure Call), rtp (Real-Time Applications
protocol), rtcp (Real-Time Applications control protocol), snmp
(Simple Network Management 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 (in micro-seconds) between current and previous line
on each dump line.
-tttt Print a timestamp in default format proceeded by date on each dump
line.
-u Print undecoded NFS handles.
-v (Slightly more) verbose output. For example, the time to live,
identification, total length and options in an IP packet are
printed. Also enables additional packet integrity checks such as
verifying the IP and ICMP header checksum.
-vv Even more verbose output. For example, additional fields are
printed from NFS reply packets, and SMB packets are fully decoded.
-vvv Even more verbose output. For example, telnet SB ... SE options
are printed in full. With -X telnet options are printed in hex as
well.
-w Write the raw packets to file rather than parsing and printing
them out. They can later be printed with the -r option. Standard
output is used if file is ``-''.
-x Print each packet (minus its link level header) in hex. The
smaller of the entire packet or snaplen bytes will be printed.
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.
-X When printing hex, print ascii too. Thus if -x is also set, the
packet is printed in hex/ascii. This is very handy for analysing
new protocols. Even if -x is not also set, some parts of some
packets may be printed in hex/ascii.
expression
selects which packets will be dumped. If no expression is given,
all packets on the net will be dumped. Otherwise, only packets
for which expression is `true' will be dumped.
The expression consists of one or more primitives. Primitives
usually consist of an id (name or number) preceded by one or more
qualifiers. There are three different kinds of 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 protocols such as slip) the inbound and outbound
qualifiers can be used to specify a desired direction.
proto qualifiers restrict the match to a particular protocol.
Possible protos are: ether, fddi, tr, ip, ip6, arp, rarp,
decnet, tcp and udp. E.g., `ether src foo', `arp net
128.3', `tcp port 21'. If there is no proto qualifier, all
protocols consistent with the type are assumed. E.g., `src
foo' means `(ip or arp or rarp) src foo' (except the latter
is not legal syntax), `net bar' means `(ip or arp or rarp)
net bar' and `port 53' means `(tcp or udp) port 53'.
[`fddi' is actually an alias for `ether'; the parser treats them
identically as meaning ``the data link level used on the specified
network interface.'' FDDI headers contain Ethernet-like source
and destination addresses, and often contain Ethernet-like packet
types, so you can filter on these FDDI fields just as with the
analogous Ethernet fields. FDDI headers also contain other
fields, but you cannot name them explicitly in a filter expres-
sion.
Similarly, `tr' is an alias for `ether'; the previous paragraph's
statements about FDDI headers also apply to Token Ring headers.]
In addition to the above, there are some special `primitive' key-
words that don't follow the pattern: gateway, broadcast, less,
greater and arithmetic expressions. All of these are described
below.
More complex filter expressions are built up by using the words
and, or and not to combine primitives. E.g., `host foo and not
port ftp and not port ftp-data'. To save typing, identical 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 IPv4/v6 destination field of the packet is
host, which may be either an address or a name.
src host host
True if the IPv4/v6 source field of the packet is host.
host host
True if either the IPv4/v6 source or destination of the
packet is host. Any of the above host expressions can be
prepended with the keywords, ip, arp, rarp, or ip6 as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each address
will be checked for a match.
ether dst ehost
True if the ethernet destination address is ehost. Ehost
may be either a name from /etc/ethers or a number (see
ethers(3N) for numeric format).
ether src ehost
True if the ethernet source address is ehost.
ether host ehost
True if either the ethernet source or destination address
is ehost.
gateway host
True if the packet used host as a gateway. I.e., the eth-
ernet 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 both by the machine's host-
name-to-IP-address resolution mechanisms (host name file,
DNS, NIS, etc.) and by the machine's host-name-to-Ethernet-
address resolution mechanism (/etc/ethers, etc.). (An
equivalent expression is
ether host ehost and not host host
which can be used with either names or numbers for host /
ehost.) This syntax does not work in IPv6-enabled configu-
ration at this moment.
dst net net
True if the IPv4/v6 destination address of the packet has a
network number of net. Net may be either a name from
/etc/networks or a network number (see networks(4) for
details).
src net net
True if the IPv4/v6 source address of the packet has a net-
work number of net.
net net
True if either the IPv4/v6 source or destination address of
the packet has a network number of net.
net net mask netmask
True if the IP address matches net with the specific net-
mask. May be qualified with src or dst. Note that this
syntax is not valid for IPv6 net.
net net/len
True if the IPv4/v6 address matches net with a netmask len
bits wide. May be qualified with src or dst.
dst port port
True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp
and has a destination port value of port. The port can be
a number or a name used in /etc/services (see tcp(4P) and
udp(4P)). If a name is used, both the port number and pro-
tocol 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, icmp6, igmp, igrp, pim, ah, esp, vrrp, udp, or
tcp. Note that the identifiers tcp, udp, and icmp are also
keywords and must be escaped via backslash (\), which is \\
in the C-shell. Note that this primitive does not chase
the protocol header chain.
ip6 proto protocol
True if the packet is an IPv6 packet of protocol type pro-
tocol. Note that this primitive does not chase the proto-
col header chain.
ip6 protochain protocol
True if the packet is IPv6 packet, and contains protocol
header with type protocol in its protocol header chain.
For example,
ip6 protochain 6
matches any IPv6 packet with TCP protocol header in the
protocol header chain. The packet may contain, for exam-
ple, authentication header, routing header, or hop-by-hop
option header, between IPv6 header and TCP header. The BPF
code emitted by this primitive is complex and cannot be
optimized by BPF optimizer code in tcpdump, so this can be
somewhat slow.
ip protochain protocol
Equivalent to ip6 protochain protocol, but this is for
IPv4.
ether broadcast
True if the packet is an ethernet broadcast packet. The
ether keyword is optional.
ip broadcast
True if the packet is an IP broadcast packet. It checks
for both the all-zeroes and all-ones 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.
ip6 multicast
True if the packet is an IPv6 multicast packet.
ether proto protocol
True if the packet is of ether type protocol. Protocol can
be a number or one of the names ip, ip6, arp, rarp, atalk,
aarp, decnet, sca, lat, mopdl, moprc, iso, stp, ipx, or
netbeui. Note these identifiers are also keywords and must
be escaped via backslash (\).
[In the case of FDDI (e.g., `fddi protocol arp') and Token
Ring (e.g., `tr protocol arp'), for most of those proto-
cols, the protocol identification comes from the 802.2 Log-
ical Link Control (LLC) header, which is usually layered on
top of the FDDI or Token Ring header.
When filtering for most protocol identifiers on FDDI or
Token Ring, tcpdump checks only the protocol ID field of an
LLC header in so-called SNAP format with an Organizational
Unit Identifier (OUI) of 0x000000, for encapsulated Ether-
net; it doesn't check whether the packet is in SNAP format
with an OUI of 0x000000.
The exceptions are iso, for which it checks the DSAP (Des-
tination Service Access Point) and SSAP (Source Service
Access Point) fields of the LLC header, stp and netbeui,
where it checks the DSAP of the LLC header, and atalk,
where it checks for a SNAP-format packet with an OUI of
0x080007 and the Appletalk etype.
In the case of Ethernet, tcpdump checks the Ethernet type
field for most of those protocols; the exceptions are iso,
sap, and netbeui, for which it checks for an 802.3 frame
and then checks the LLC header as it does for FDDI and
Token Ring, atalk, where it checks both for the Appletalk
etype in an Ethernet frame and for a SNAP-format packet as
it does for FDDI and Token Ring, aarp, where it checks for
the Appletalk ARP etype in either an Ethernet frame or an
802.2 SNAP frame with an OUI of 0x000000, and ipx, where it
checks for the IPX etype in an Ethernet frame, the IPX DSAP
in the LLC header, the 802.3 with no LLC header encapsula-
tion of IPX, and the IPX etype in a SNAP frame.]
decnet src host
True if the DECNET source address is host, which may be an
address of the form ``10.123'', or a DECNET host name.
[DECNET host name support is only available on Ultrix sys-
tems that are configured to run DECNET.]
decnet dst host
True if the DECNET destination address is host.
decnet host host
True if either the DECNET source or destination address is
host.
ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
Abbreviations for:
ether proto p
where p is one of the above protocols.
lat, moprc, mopdl
Abbreviations for:
ether proto p
where p is one of the above protocols. Note that tcpdump
does not currently know how to parse these protocols.
vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN packet. If
[vlan_id] is specified, only true is the packet has the
specified vlan_id. Note that the first vlan keyword
encountered in expression changes the decoding offsets for
the remainder of expression on the assumption that the
packet is a VLAN packet.
tcp, udp, icmp
Abbreviations for:
ip proto p or ip6 proto p
where p is one of the above protocols.
iso proto protocol
True if the packet is an OSI packet of protocol type proto-
col. Protocol can be a number or one of the names clnp,
esis, or isis.
clnp, esis, isis
Abbreviations for:
iso proto p
where p is one of the above protocols. Note that tcpdump
does an incomplete job of parsing these protocols.
expr relop expr
True if the relation holds, where relop is one of >, <, >=,
<=, =, !=, and expr is an arithmetic expression composed of
integer constants (expressed in standard C syntax), the
normal binary operators [+, -, *, /, &, |], a length opera-
tor, and special packet data accessors. To access data
inside the packet, use the following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, tr, ppp, slip, link, ip, arp,
rarp, tcp, udp, icmp or ip6, and indicates the protocol
layer for the index operation. (ether, fddi, tr, ppp, slip
and link all refer to the link layer.) Note that tcp, udp
and other upper-layer protocol types only apply to IPv4,
not IPv6 (this will be fixed in the future). The byte off-
set, 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.
Some offsets and field values may be expressed as names
rather than as numeric values. The following protocol
header field offsets are available: icmptype (ICMP type
field), icmpcode (ICMP code field), and tcpflags (TCP flags
field).
The following ICMP type field values are available: icmp-
echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect,
icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-
timxceed, icmp-paramprob, icmp-tstamp, icmp-tstampreply,
icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-maskreply.
The following TCP flags field values are available: tcp-
fin, tcp-syn, tcp-rst, tcp-push, tcp-push, tcp-ack, tcp-
urg.
Primitives may be combined using:
A parenthesized group of primitives and operators (paren-
theses 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 con-
catenation.
If an identifier is given without a keyword, the most recent key-
word 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 argu-
ments are concatenated with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
tcpdump host sundown
To print traffic between helios and either hot or ace:
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 loc
alnet'
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 ether-
net broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not
ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following gives a brief
description and examples of most of the formats.
Link Level Headers
If the '-e' option is given, the link level header is printed out. On
ethernets, the source and destination addresses, 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.
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.
(N.B.: The following description assumes familiarity with the SLIP com-
pression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out-
bound), packet type, and compression information are printed out. The
packet type is printed first. The three types are ip, utcp, and ctcp.
No further link information is printed for ip packets. For TCP packets,
the connection identifier is printed following the type. If the packet
is compressed, its encoded header is printed out. The special cases are
printed out as *S+n and *SA+n, where n is the amount by which the
sequence number (or sequence number and ack) has changed. If it is not a
special case, zero or more changes are printed. A change is indicated by
U (urgent pointer), W (window), A (ack), S (sequence number), and I
(packet ID), followed by a delta (+n or -n), or a new value (=n).
Finally, the amount of data in the packet and compressed header length
are printed.
For example, the following line shows an outbound compressed TCP packet,
with an implicit connection identifier; the ack has changed by 6, the
sequence number by 49, and the packet ID by 6; there are 3 bytes of data
and 6 bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
Arp/rarp output shows the type of request and its arguments. The format
is intended to be self explanatory. Here is a short sample taken from
the start of an `rlogin' from host rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking for the ethernet
address of internet host csam. Csam replies with its ethernet address
(in this example, ethernet addresses are in caps and internet addresses
in lower case).
This would look less redundant if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet is broadcast
and the second is point-to-point would be visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the ethernet source address is RTSG, the
destination is the ethernet broadcast address, the type field contained
hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
TCP Packets
(N.B.:The following description assumes familiarity with the TCP protocol
described in RFC-793. If you are not familiar with the protocol, neither
this description nor tcpdump will be of much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and ports. Flags
are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single
`.' (no flags). Data-seqno describes the portion of sequence space cov-
ered 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 no flags were
set. The packet contained no data so there is no data sequence number.
Note that the ack sequence number is a small integer (1). The first time
tcpdump sees a tcp `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 out-
put.
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 ini-
tializes 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 tcp-
dump.
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 deci-
mal 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 representa-
tion of the AND value as well as the result of this operation is 2
(binary 00000010), so we know that for packets with SYN set the following
relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'
Note that you should use single quotes or a backslash in the expression
to hide the AND ('&') special character from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp datagram to port who
on host broadcast, the Internet broadcast address. The packet contained
84 bytes of user data.
Some UDP services are recognized (from the source or destination port
number) and the higher level protocol information printed. In particu-
lar, 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 Ser-
vice protocol described in RFC-1035. If you are not familiar with the
protocol, the following description will appear to be written in greek.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an address record
(qtype=A) associated with the name ucbvax.berkeley.edu. The query id was
`3'. The `+' indicates the recursion desired flag was set. The query
length was 37 bytes, not including the UDP and IP protocol headers. The
query operation was the normal one, Query, so the op field was omitted.
If the op had been anything else, it would have been printed between the
`3' and the `+'. Similarly, the qclass was the normal one, C_IN, and
omitted. Any other qclass would have been printed immediately after the
`A'.
A few anomalies are checked and may result in extra fields enclosed in
square brackets: If a query contains an answer, authority records or
additional records section, ancount, nscount, or arcount are printed as
`[na]', `[nn]' or `[nau]' where n is the appropriate count. If any of
the response bits are set (AA, RA or rcode) or any of the `must be zero'
bits are set in bytes two and three, `[b2&3=x]' is printed, where x is
the hex value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo with 3
answer records, 3 name server records and 7 additional records. The
first answer record is type A (address) and its data is internet address
128.32.137.3. The total size of the response was 273 bytes, excluding
UDP and IP headers. The op (Query) and response code (NoError) were
omitted, as was the class (C_IN) of the A record.
In the second example, helios responds to query 2 with a response code of
non-existent domain (NXDomain) with no answers, one name server and no
authority records. The `*' indicates that the authoritative answer bit
was set. Since there were no answers, no type, class or data were
printed.
Other flag characters that might appear are `-' (recursion available, RA,
not set) and `|' (truncated message, TC, set). If the `question' section
doesn't contain exactly one entry, `[nq]' is printed.
Note that name server requests and responses tend to be large and the
default snaplen of 68 bytes may not capture enough of the packet to
print. Use the -s flag to increase the snaplen if you need to seriously
investigate name server traffic. `-s 128' has worked well for me.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI
SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed
decode done if -v is used. Be warned that with -v a single SMB packet
may take up a page or more, so only use -v if you really want all the
gory details.
If you are decoding SMB sessions containing unicode strings then you may
wish to set the environment variable USE_UNICODE to 1. A patch to auto-
detect unicode srings would be welcome.
For information on SMB packet formats and what all te fields mean see
www.cifs.org or the pub/samba/specs/ directory on your favourite
samba.org mirror site. The SMB patches were written by Andrew Tridgell
(tridge@samba.org).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with id 6709 to wrl
(note that the number following the src host is a transaction id, not the
source port). The request was 112 bytes, excluding the UDP and IP head-
ers. 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 direc-
tory 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 frag-
ment 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 dis-
carded). 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 con-
tain 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, jss-
mag.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 origi-
nal 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 ari-
zona.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:32
8@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 infor-
mation 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
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berke-
ley National Laboratory, University of California, Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available via http:
http://www.tcpdump.org/
The original distribution is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric
Young's SSLeay library, under specific configuration.
BUGS
Please send problems, bugs, questions, desirable enhancements, etc. to:
tcpdump-workers@tcpdump.org
Please send source code contributions, etc. to:
patches@tcpdump.org
NIT doesn't let you watch your own outbound traffic, BPF will. We recom-
mend 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) ques-
tion 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 that manipulate FDDI or Token Ring headers assume that
all FDDI and Token Ring packets are SNAP-encapsulated Ethernet packets.
This is true for IP, ARP, and DECNET Phase IV, but is not true for proto-
cols such as ISO CLNS. Therefore, the filter may inadvertently accept
certain packets that do not properly match the filter expression.
Filter expressions on fields other than those that manipulate Token Ring
headers will not correctly handle source-routed Token Ring packets.
ip6 proto should chase header chain, but at this moment it does not. ip6
protochain is supplied for this behavior.
Arithmetic expression against transport layer headers, like tcp[0], does
not work against IPv6 packets. It only looks at IPv4 packets.
3 January 2001 TCPDUMP(1)
