1:tcpdump
From Linux Man Pages
tcpdump - dump traffic on a network
Contents |
SYNOPSIS
tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -M secret ]
[ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -Z user ]
[ 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 packets that match expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal
(generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM signal (typically
generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by
a SIGINT or SIGTERM signal or the specified number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts of:
packets ``captured (this is the number of packets that tcpdump has received and processed);
packets ``received by filter (the meaning of this depends on the OS on which you're running tcpdump, and
possibly on the way the OS was configured - if a filter was specified on the command line, on some OSes it
counts packets regardless of whether they were matched by the filter expression and, even if they were
matched by the filter expression, regardless of whether tcpdump has read and processed them yet, on other
OSes it counts only packets that were matched by the filter expression regardless of whether tcpdump has
read and processed them yet, and on other OSes it counts only packets that were matched by the filter
expression and were processed by tcpdump);
packets ``dropped by kernel (this is the number of packets that were dropped, due to a lack of buffer
space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that
information to applications; if not, it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs (including Mac OS X) and Digital/Tru64 UNIX, it
will report those counts when it receives a SIGINFO signal (generated, for example, by typing your ``status
character, typically control-T, although on some platforms, such as Mac OS X, the ``status character is not set
by default, so you must set it with stty(1) in order to use it) and will continue capturing packets.
Reading packets from a network interface may require that you have special privileges:
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 ver-
sions 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 (unless your distribution has a kernel that
supports capability bits such as CAP_NET_RAW and code to allow those capability bits to be given to par-
ticular accounts and to cause those bits to be set on a user's initial processes when they log in, in
which case you must have CAP_NET_RAW in order to capture and CAP_NET_ADMIN to enumerate network devices
with, for example, the -D flag).
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 promiscu-
ous-mode or copy-all-mode operation, or both modes of operation, be enabled on that interface.
Under BSD (this includes Mac OS X):
You must have read access to /dev/bpf*. On BSDs with a devfs (this includes Mac OS X), this might involve
more than just having somebody with super-user access setting the ownership or permissions on the BPF
devices - it might involve configuring devfs to set the ownership or permissions every time the system is
booted, if the system even supports that; if it doesn't support that, you might have to find some other
way to make that happen at boot time.
Reading a saved packet file doesn't require special privileges.
OPTIONS
-A Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file is currently larger than file_size and,
if so, close the current savefile and open a new one. Savefiles after the first savefile will have the
name specified with the -w flag, with a number after it, starting at 1 and continuing upward. The units
of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
-d Dump the compiled packet-matching code in a human readable form to standard output and stop.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (preceded with a count).
-D Print the list of the network interfaces available on the system and on which tcpdump can capture packets.
For each network interface, a number and an interface name, possibly followed by a text description of the
interface, is printed. The interface name or the number can be supplied to the -i flag to specify an
interface on which to capture.
This can be useful on systems that don't have a command to list them (e.g., Windows systems, or UNIX sys-
tems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the interface
name is a somewhat complex string.
The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks the
pcap_findalldevs() function.
-e Print the link-level header on each dump line.
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Secu-
rity Parameter Index value spi. This combination may be repeated with comma or newline seperation.
Note that setting the secret for IPv4 ESP packets is supported at this time.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc.
The ability to decrypt packets is only present if tcpdump was compiled with cryptography enabled.
secret is the ASCII text for ESP secret key. If preceeded by 0x, then a hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The option is only for debugging purposes, and the use
of this option with a true `secret' key is discouraged. By presenting IPsec secret key onto command line
you make it visible to others, via ps(1) and other occasions.
In addition to the above syntax, the syntax file name may be used to have tcpdump read the provided file
in. The file is opened upon receiving the first ESP packet, so any special permissions that tcpdump may
have been given should already have been given up.
-f Print `foreign' IPv4 addresses numerically rather than symbolically (this option is intended to get around
serious brain damage in Sun's NIS server -- usually it hangs forever translating non-local internet num-
bers).
The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the interface on which
capture is being done. If that address or netmask are not available, available, either because the inter-
face on which capture is being done has no address or netmask or because the capture is being done on the
Linux "any" interface, which can capture on more than one interface, this option will not work correctly.
-F Use file as input for the filter expression. An additional expression given on the command line is
ignored.
-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 pack-
ets from all interfaces. Note that captures on the ``any device will not be done in promiscuous mode.
If the -D flag is supported, an interface number as printed by that flag can be used as the interface
argument.
-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 tcpdump.
-M Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5 option
(RFC 2385), if present.
-n Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
-N Don't print domain name qualification of host names. E.g., if you give this flag then tcpdump will print
``nic instead of ``nic.ddn.mil.
-O Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.
-p Don't put the interface into promiscuous mode. Note that the interface might be in promiscuous mode for
some other reason; hence, `-p' cannot be used as an abbreviation for `ether host {local-hw-addr} or ether
broadcast'.
-q Quick (quiet?) output. Print less protocol information so output lines are shorter.
-R Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified, tcpdump will
not print replay prevention field. Since there is no protocol version field in ESP/AH specification, tcp-
dump 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 indi-
cated in the output with ``[|proto], where proto is the name of the protocol level at which the trunca-
tion has occurred. Note that taking larger snapshots both increases the amount of time it takes to pro-
cess 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
aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc (Remote Procedure
Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications control protocol), snmp (Simple
Network Management Protocol), tftp (Trivial File Transfer Protocol), vat (Visual Audio Tool), and wb (dis-
tributed 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.
-U Make output saved via the -w option ``packet-buffered; i.e., as each packet is saved, it will be written
to the output file, rather than being written only when the output buffer fills.
The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks the
pcap_dump_flush() function.
-v When parsing and printing, produce (slightly more) verbose output. For example, the time to live, identi-
fication, total length and options in an IP packet are printed. Also enables additional packet integrity
checks such as verifying the IP and ICMP header checksum.
When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.
-vv Even more verbose output. For example, additional fields are printed from NFS reply packets, and SMB
packets are fully decoded.
-vvv Even more verbose output. For example, telnet SB ... SE options are printed in full. With -X Telnet
options are printed in hex as well.
-w Write the raw packets to file rather than parsing and printing them out. They can later be printed with
the -r option. Standard output is used if file is ``-.
-W Used in conjunction with the -C option, this will limit the number of files created to the specified num-
ber, and begin overwriting files from the beginning, thus creating a 'rotating' buffer. In addition, it
will name the files with enough leading 0s to support the maximum number of files, allowing them to sort
correctly.
-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. Ether-
net), the padding bytes will also be printed when the higher layer packet is shorter than the required
padding.
-xx Print each packet, including its link level header, in hex.
-X Print each packet (minus its link level header) in hex and ASCII. This is very handy for analysing new
protocols.
-XX Print each packet, including its link level header, in hex and ASCII.
-y Set the data link type to use while capturing packets to datalinktype.
-Z Drops privileges (if root) and changes user ID to user and the group ID to the primary group of user.
This behavior can also be enabled by default at compile time.
expression
selects which packets will be dumped. If no expression is given, all packets on the net will be dumped.
Otherwise, only packets for which expression is `true' will be dumped.
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 ,
port and portrange. E.g., `host foo', `net 128.3', `port 20', `portrange 6000-6008'. 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 some link layers, such as SLIP and the
``cooked Linux capture mode used for the ``any device and for some other device types, 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,
wlan, ip, ip6, arp, rarp, decnet, tcp and udp. E.g., `ether src foo', `arp net 128.3', `tcp port
21', `udp portrange 7000-7009'. 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 destina-
tion 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 expression.
Similarly, `tr' and `wlan' are aliases for `ether'; the previous paragraph's statements about FDDI headers
also apply to Token Ring and 802.11 wireless LAN headers. For 802.11 headers, the destination address is
the DA field and the source address is the SA field; the BSSID, RA, and TA fields aren't tested.]
In addition to the above, there are some special `primitive' keywords that don't follow the pattern: gate-
way, 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 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 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-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 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 IPv4 address matches net with the specific netmask. 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 traf-
fic, 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.
dst portrange port1-port2
True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a destination port value between
port1 and port2. port1 and port2 are interpreted in the same fashion as the port parameter for
port.
src portrange port1-port2
True if the packet has a source port value between port1 and port2.
portrange port1-port2
True if either the source or destination port of the packet is between port1 and port2.
Any of the above port or port range 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 IPv4 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 con-
tain, 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 IPv4 broadcast packet. It checks for both the all-zeroes and all-ones
broadcast conventions, and looks up the subnet mask on the interface on which the capture is being
done.
If the subnet mask of the interface on which the capture is being done is not available, either
because the interface on which capture is being done has no netmask or because the capture is being
done on the Linux "any" interface, which can capture on more than one interface, this check will
not work correctly.
ether multicast
True if the packet is an Ethernet multicast packet. The ether keyword is optional. This is short-
hand for `ether[0] & 1 != 0'.
ip multicast
True if the packet is an IPv4 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'), Token Ring (e.g., `tr protocol arp'), and IEEE
802.11 wireless LANS (e.g., `wlan protocol arp'), for most of those protocols, the protocol identi-
fication comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of
the FDDI, Token Ring, or 802.11 header.
When filtering for most protocol identifiers on FDDI, Token Ring, or 802.11, tcpdump checks only
the protocol ID field of an LLC header in so-called SNAP format with an Organizational Unit Identi-
fier (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 tcpdump checks the DSAP (Destination Service Access Point) and SSAP (Source Service Access
Point) fields of the LLC header;
stp and netbeui
tcpdump checks the DSAP of the LLC header;
atalk tcpdump 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, stp, and netbeui
tcpdump checks for an 802.3 frame and then checks the LLC header as it does for FDDI, Token
Ring, and 802.11;
atalk tcpdump checks both for the AppleTalk etype in an Ethernet frame and for a SNAP-format
packet as it does for FDDI, Token Ring, and 802.11;
aarp tcpdump checks for the AppleTalk ARP etype in either an Ethernet frame or an 802.2 SNAP
frame with an OUI of 0x000000;
ipx tcpdump 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 config-
ured 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.
ifname interface
True if the packet was logged as coming from the specified interface (applies only to packets
logged by OpenBSD's pf(4)).
on interface
Synonymous with the ifname modifier.
rnr num
True if the packet was logged as matching the specified PF rule number (applies only to packets
logged by OpenBSD's pf(4)).
rulenum num
Synonomous with the rnr modifier.
reason code
True if the packet was logged with the specified PF reason code. The known codes are: match, bad-
offset, fragment, short, normalize, and memory (applies only to packets logged by OpenBSD's pf(4)).
rset name
True if the packet was logged as matching the specified PF ruleset name of an anchored ruleset
(applies only to packets logged by pf(4)).
ruleset name
Synonomous with the rset modifier.
srnr num
True if the packet was logged as matching the specified PF rule number of an anchored ruleset
(applies only to packets logged by pf(4)).
subrulenum num
Synonomous with the srnr modifier.
action act
True if PF took the specified action when the packet was logged. Known actions are: pass and block
(applies only to packets logged by OpenBSD's pf(4)).
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 if 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. The vlan [vlan_id] expression may be used more than once, to filter on VLAN hierar-
chies. Each use of that expression increments the filter offsets by 4.
For example:
vlan 100 && vlan 200
filters on VLAN 200 encapsulated within VLAN 100, and
vlan && vlan 300 && ip
filters IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN.
mpls [label_num]
True if the packet is an MPLS packet. If [label_num] is specified, only true is the packet has the
specified label_num. Note that the first mpls keyword encountered in expression changes the decod-
ing offsets for the remainder of expression on the assumption that the packet is a MPLS-encapsu-
lated IP packet. The mpls [label_num] expression may be used more than once, to filter on MPLS
hierarchies. Each use of that expression increments the filter offsets by 4.
For example:
mpls 100000 && mpls 1024
filters packets with an outer label of 100000 and an inner label of 1024, and
mpls && mpls 1024 && host 192.9.200.1
filters packets to or from 192.9.200.1 with an inner label of 1024 and any outer label.
pppoed True if the packet is a PPP-over-Ethernet Discovery packet (Ethernet type 0x8863).
pppoes True if the packet is a PPP-over-Ethernet Session packet (Ethernet type 0x8864). Note that the
first pppoes keyword encountered in expression changes the decoding offsets for the remainder of
expression on the assumption that the packet is a PPPoE session packet.
For example:
pppoes && ip
filters IPv4 protocols encapsulated in PPPoE.
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 protocol. 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.
l1, l2, iih, lsp, snp, csnp, psnp
Abbreviations for IS-IS PDU types.
vpi n True if the packet is an ATM packet, for SunATM on Solaris, with a virtual path identifier of n.
vci n True if the packet is an ATM packet, for SunATM on Solaris, with a virtual channel identifier of n.
lane True if the packet is an ATM packet, for SunATM on Solaris, and is an ATM LANE packet. Note that
the first lane keyword encountered in expression changes the tests done in the remainder of expres-
sion on the assumption that the packet is either a LANE emulated Ethernet packet or a LANE LE Con-
trol packet. If lane isn't specified, the tests are done under the assumption that the packet is
an LLC-encapsulated packet.
llc True if the packet is an ATM packet, for SunATM on Solaris, and is an LLC-encapsulated packet.
oamf4s True if the packet is an ATM packet, for SunATM on Solaris, and is a segment OAM F4 flow cell
(VPI=0 & VCI=3).
oamf4e True if the packet is an ATM packet, for SunATM on Solaris, and is an end-to-end OAM F4 flow cell
(VPI=0 & VCI=4).
oamf4 True if the packet is an ATM packet, for SunATM on Solaris, and is a segment or end-to-end OAM F4
flow cell (VPI=0 & (VCI=3 | VCI=4)).
oam True if the packet is an ATM packet, for SunATM on Solaris, and is a segment or end-to-end OAM F4
flow cell (VPI=0 & (VCI=3 | VCI=4)).
metac True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit
(VPI=0 & VCI=1).
bcc True if the packet is an ATM packet, for SunATM on Solaris, and is on a broadcast signaling circuit
(VPI=0 & VCI=2).
sc True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit (VPI=0 &
VCI=5).
ilmic True if the packet is an ATM packet, for SunATM on Solaris, and is on an ILMI circuit (VPI=0 &
VCI=16).
connectmsg
True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit and is a
Q.2931 Setup, Call Proceeding, Connect, Connect Ack, Release, or Release Done message.
metaconnect
True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit and
is a Q.2931 Setup, Call Proceeding, Connect, Release, or Release Done message.
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 opera-
tors [+, -, *, /, &, |, <<, >>], a length operator, and special packet data accessors. Note that
all comparisons are unsigned, so that, for example, 0x80000000 and 0xffffffff are > 0. To access
data inside the packet, use the following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, ip6 or
radio, and indicates the protocol layer for the index operation. (ether, fddi, wlan, tr, ppp, slip
and link all refer to the link layer. radio refers to the "radio header" added to some 802.11 cap-
tures.) 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 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 IPv4 packets with options. The expression `ip[6:2] & 0x1fff = 0' catches only unfrag-
mented IPv4 datagrams and frag zero of fragmented IPv4 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 follow-
ing 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-source-
quench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-timxceed, icmp-param-
prob, 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-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.
EXAMPLES
To print all packets arriving at or departing from sundown:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell
from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff
should never make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local
host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example,
SYN and FIN packets and ACK-only packets. (IPv6 is left as an exercise for the reader.)
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following gives a brief description and examples of most of the
formats.
Link Level Headers
If the '-e' option is given, the link level header is printed out. On Ethernets, the source and destination
addresses, protocol, and packet length are printed.
On FDDI networks, the '-e' option causes tcpdump to print the `frame control' field, the source and destination
addresses, and the packet length. (The `frame control' field governs the interpretation of the rest of the
packet. Normal packets (such as those containing IP datagrams) are `async' packets, with a priority value
between 0 and 7; for example, `async4'. Such packets are assumed to contain an 802.2 Logical Link Control (LLC)
packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.
On Token Ring networks, the '-e' option causes tcpdump to print the `access control' and `frame control' fields,
the source and destination addresses, and the packet length. As on FDDI networks, packets are assumed to contain
an LLC packet. Regardless of whether the '-e' option is specified or not, the source routing information is
printed for source-routed packets.
On 802.11 networks, the '-e' option causes tcpdump to print the `frame control' fields, all of the addresses in
the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet.
(N.B.: The following description assumes familiarity with the SLIP compression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I for inbound, ``O for outbound), packet type, and compression infor-
mation are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp. No further
link information is printed for ip packets. For TCP packets, the connection identifier is printed following the
type. If the packet is compressed, its encoded header is printed out. The special cases are printed out as *S+n
and *SA+n, where n is the amount by which the sequence number (or sequence number and ack) has changed. If it is
not a special case, zero or more changes are printed. A change is indicated by U (urgent pointer), W (window), A
(ack), S (sequence number), and I (packet ID), followed by a delta (+n or -n), or a new value (=n). Finally, the
amount of data in the packet and compressed header length are printed.
For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier;
the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6
bytes of compressed header:
O ctcp * A+6 S+49 I+6 3(6)
ARP/RARP Packets
Arp/rarp output shows the type of request and its arguments. The format is intended to be self explanatory.
Here is a short sample taken from the start of an `rlogin' from host rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking for the Ethernet address of internet host csam. Csam
replies with its Ethernet address (in this example, Ethernet addresses are in caps and internet addresses in
lower case).
This would look less redundant if we had done tcpdump -n:
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be
visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the Ethernet source address is RTSG, the destination is the Ethernet broadcast
address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
TCP Packets
(N.B.:The following description assumes familiarity with the TCP protocol described in RFC-793. If you are not
familiar with the protocol, neither this description nor tcpdump will be of much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F
(FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single `.' (no flags). Data-seqno describes the por-
tion 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-seg-
ment-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 cur-
rent packet's sequence number and this initial sequence number is printed. This means that sequence numbers
after the first can be interpreted as relative byte positions in the conversation's data stream (with the first
data byte each direction being `1'). `-S' will override this 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 conversa-
tion). 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 pack-
ets 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 pre-
serve 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
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 broad-
cast address. The packet contained 84 bytes of user data.
Some UDP services are recognized (from the source or destination port number) and the higher level protocol
information printed. In particular, Domain Name service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to
NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC-1035. If
you are not familiar with the protocol, the following description will appear to be written in greek.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu.(37)
Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucb-
vax.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 prim-
itive decoding of IPX and NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that
with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory details.
For information on SMB packet formats and what all te fields mean see www.cifs.org or the pub/samba/specs/ direc-
tory on your favorite 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 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 num-
ber 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 exam-
ple.) 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 cor-
responding 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) ser-
vice, 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 file-
name 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 argu-
ments 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 fol-
low the corresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the
UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and node num-
bers to names. Lines in this file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host (a
host is distinguished from a net by the 3rd octet in the number - a net number must have two octets and a host
number must have three octets.) The number and name should be separated by whitespace (blanks or tabs). The
/etc/atalk.names file may contain blank lines or comment lines (lines starting with a `#').
AppleTalk addresses are printed in the form
net.host.port
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses
are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to what-
ever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the
source node is known (`office'). The third line is a send from port 235 on net jssmag node 149 to broadcast on
the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number - for
this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted.
Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet
size.
NBP packets are formatted like the following examples:
icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
The first line is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag.
The nbp id for the lookup is 190. The second line shows a reply for this request (note that it has the same id)
from host jssmag.209 saying that it has a laserwriter resource named "RM1140" registered on port 250. The third
line is another reply to the same request saying host techpit has laserwriter "techpit" registered on port 186.
ATP packet formatting is demonstrated by the following example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the `<0-7>'). The hex
number at the end of the line is the value of the `userdata' field in the request.
Helios responds with 8 512-byte packets. The `:digit' following the transaction id gives the packet sequence
number in the transaction and the number in parens is the amount of data in the packet, excluding the atp header.
The `*' on packet 7 indicates that the EOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios resends them then jssmag.209 releases the
transaction. Finally, jssmag.209 initiates the next request. The `*' on the request indicates that XO (`exactly
once') was not set.
IP Fragmentation
Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments. The second indicates this is the last fragment.)
Id is the fragment id. Size is the fragment size (in bytes) excluding the IP header. Offset is this fragment's
offset (in bytes) in the original datagram.
The fragment information is output for each fragment. The first fragment contains the higher level protocol
header and the frag info is printed after the protocol info. Fragments after the first contain no higher level
protocol header and the frag info is printed after the source and destination addresses. For example, here is
part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle 576 byte
datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in the 2nd line don't include port numbers. This is
because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence
numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed
as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the
second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can
fool you.
A packet with the IP don't fragment flag is marked with a trailing (DF).
Timestamps
By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form
hh:mm:ss.frac
and is as accurate as the kernel's clock. The timestamp reflects the time the kernel first saw the packet. No
attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire
and when the kernel serviced the `new packet' interrupt.
RELATED
stty(1), pcap(3), bpf(4), nit(4P), pfconfig(8)
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 recommend that you use the latter.
On Linux systems with 2.0[.x] kernels:
packets on the loopback device will be seen twice;
packet filtering cannot be done in the kernel, so that all packets must be copied from the kernel in order
to be filtered in user mode;
all of a packet, not just the part that's within the snapshot length, will be copied from the kernel (the
2.0[.x] packet capture mechanism, if asked to copy only part of a packet to userland, will not report the
true length of the packet; this would cause most IP packets to get an error from tcpdump);
capturing on some PPP devices won't work correctly.
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher
level protocol.
Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real
query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the pro-
gram generating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is
ignored).
Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token
Ring packets.
Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets
with both To DS and From DS set.
ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behav-
ior.
Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only
looks at IPv4 packets.
CATEGORY
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