tcpdump - dump traffic on a network
tcpdump [ -adeflLnNOpqRStuvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E algo:secret ] [ -y datalinktype ]
[ expression ]
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 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 ``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 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, 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 special
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.
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:
Once the super-user has enabled promiscuous-mode operation using
pfconfig(8), any user may capture network traffic with tcpdump.
Under BSD:
You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privileges.
-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' 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).
-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''.
-L List the known data link types for the interface and exit.
-m Load SMI MIB module definitions from file module. This option
can be used several times to load several MIB modules into tcp-
dump.
-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 68 (with SunOS's NIT, the minimum is actually 96).
68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate
protocol information from name server and NFS packets (see
below). Packets truncated because of a limited snapshot are
indicated in the output with ``[|proto]'', where proto is the
name of the protocol level at which the truncation has 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 means use the required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted the
specified type. Currently known types are cnfp (Cisco NetFlow
protocol), 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.
-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.
-y Set the data link type to use while capturing packets to
datalinktype.
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, lat, sca, moprc, mopdl, iso, esis, isis, icmp,
icmp6, 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 Ethernetlike
packet types, so you can filter on these FDDI fields just
as with the analogous Ethernet fields. FDDI headers also contain
other fields, but you cannot name them explicitly in a filter
expression.
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'
keywords that don't follow the pattern: gateway, broadcast,
less, greater and arithmetic expressions. All of these are
described below.
More complex filter expressions are built up by using the words
and, or and not to combine primitives. E.g., `host foo and not
port ftp and not port ftp-data'. To save typing, identical
qualifier lists can be omitted. E.g., `tcp dst port ftp or ftpdata
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
ethernet source or destination address was host but neither
the IP source nor the IP destination was host. Host
must be a name and must be found 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-toEthernet-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 configuration
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
network 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 protocol are checked. If a number or ambiguous name
is used, only the port number is checked (e.g., dst port
513 will print both tcp/login traffic and udp/who traffic,
and port domain will print both tcp/domain and
udp/domain traffic).
src port port
True if the packet has a source port value of port.
port port
True if either the source or destination port of the
packet is port. Any of the above port expressions can be
prepended with the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
less length
True if the packet has a length less than or equal to
length. This is equivalent to:
len <= length.
greater length
True if the packet has a length greater than or equal to
length. This is equivalent to:
len >= length.
ip proto protocol
True if the packet is an IP packet (see ip(4P)) of protocol
type protocol. Protocol can be a number or one of
the names icmp, 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
protocol. Note that this primitive does not chase the
protocol 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 example,
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
protocols, the protocol identification comes from the
802.2 Logical 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
Ethernet; 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
(Destination Service Access Point) and SSAP (Source Service
Access Point) fields of the LLC header, stp and net-
beui, 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 encapsulation 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
systems that are configured to run DECNET.]
decnet dst host
True if the DECNET destination address is host.
decnet host host
True if either the DECNET source or destination address
is host.
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 pro-
tocol. 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 operator, and special packet data accessors. To
access data inside the packet, use the following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, tr, ip, arp, rarp, tcp, udp,
icmp or ip6, and indicates the protocol layer for the
index operation. Note that tcp, udp and other upperlayer
protocol types only apply to IPv4, not IPv6 (this
will be fixed in the future). The byte offset, relative
to the indicated protocol layer, is given by expr. Size
is optional and indicates the number of bytes in the
field of interest; it can be either one, two, or four,
and defaults to one. The length operator, indicated by
the keyword len, gives the length of the packet.
For example, `ether[0] & 1 != 0' catches all multicast
traffic. The expression `ip[0] & 0xf != 5' catches all
IP packets with options. The expression `ip[6:2] &
0x1fff = 0' catches only unfragmented datagrams and frag
zero of fragmented datagrams. This check is implicitly
applied to the tcp and udp index operations. For
instance, tcp[0] always means the first byte of the TCP
header, and never means the first byte of an intervening
fragment.
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-redi-
rect, icmp-echo, icmp-routeradvert, icmp-routersolicit,
icmp-timxceed, icmp-paramprob, icmp-tstamp, icmp-tstam-
preply, 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 (parentheses
are special to the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation
have equal precedence and associate left to right. Note that
explicit and tokens, not juxtaposition, are now required for
concatenation.
If an identifier is given without a keyword, the most recent
keyword is assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to tcpdump as either a single
argument or as multiple arguments, whichever is more convenient.
Generally, if the expression contains Shell metacharacters, it
is easier to pass it as a single, quoted argument. Multiple
arguments are concatenated with spaces before being parsed.
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 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'
The output of tcpdump is protocol dependent. The following gives a
brief description and examples of most of the formats.
Link Level Headers [Toc] [Back]
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.
(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 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 [Toc] [Back]
(N.B.:The following description assumes familiarity with the TCP proto-
col described in RFC-793. If you are not familiar with the protocol,
neither this description nor tcpdump will be of much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and ports.
Flags are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or
a single `.' (no flags). Data-seqno describes the portion of sequence
space covered by the data in this packet (see example below). Ack is
sequence number of the next data expected the other 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 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'
Note that you should use single quotes or a backslash in the expression
to hide the AND ('&') special character from the shell.
UDP Packets [Toc] [Back]
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 [Toc] [Back]
(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 [Toc] [Back]
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
([email protected]).
NFS Requests and Replies [Toc] [Back]
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 [Toc] [Back]
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 pac
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