tcpdump – dump traffic on a network
SYNOPSIS
tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
[ –number ] [ -Q in|out|inout ]
[ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,… ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
[ –time-stamp-precision=tstamp_precision ]
[ –immediate-mode ] [ –version ]
[ expression ]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on a network interface that match the boolean
expression; the description is preceded by a time stamp, printed, by default, as hours, minutes, seconds,
and fractions of a second since midnight. 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. It can also be run with the -V flag,
which causes it to read a list of saved packet files. 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:
dump, 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 pro‐
cessed 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 pack‐
ets that were matched by the filter expression and were processed by tcpdump);
fer 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 “sta‐
tus 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 pack‐
ets. On platforms that do not support the SIGINFO signal, the same can be achieved by using the SIGUSR1 sig‐
nal.
Reading packets from a network interface may require that you have special privileges; see the pcap (3PCAP)
man page for details. Reading a saved packet file doesn’t require special privileges.
OPTIONS
-A Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
-b Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.
-B buffer_size
–buffer-size=buffer_size
-c count
-C 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
–list-interfaces
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.
systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the
interface name is a somewhat complex string.
the pcap_findalldevs() function.
-e Print the link-level header on each dump line. This can be used, for example, to print MAC layer
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain
tion.
des-cbc. The ability to decrypt packets is only present if tcpdump was compiled with cryptography
enabled.
use of this option with a true `secret’ key is discouraged. By presenting IPsec secret key onto com‐
mand line you make it visible to others, via ps(1) and other occasions.
file in. The file is opened upon receiving the first ESP packet, so any special permissions that tcp‐
dump 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
internet numbers).
which capture is being done. If that address or netmask are not available, available, either because
the interface on which capture is being done has no address or netmask or because the capture is
being done on the Linux “any” interface, which can capture on more than one interface, this option
will not work correctly.
-F file
ignored.
-G rotate_seconds
files will have the name specified by -w which should include a time format as defined by strf‐
time(3). If no time format is specified, each new file will overwrite the previous.
-h
–help Print the tcpdump and libpcap version strings, print a usage message, and exit.
–version
-H Attempt to detect 802.11s draft mesh headers.
-i interface
–interface=interface
指定监听端口。
如果未指定，tcpdump会在系统接口列表中搜索最小编号的、已启动接口（但不包括lo），这可能是“eth0”。
在Linux 2.2+上，可以使用“any”来捕获所有接口的数据包。注意，“any”捕获不会在混杂模式下进行。
如果支持使用-D选项，那么由-D选项打印的接口编号可以用作interface参数，如果系统上的接口没有使用该数字作为接口名称。
-I
–monitor-mode
ported only on some operating systems.
so that you will not be able to use any wireless networks with that adapter. This could prevent
accessing files on a network server, or resolving host names or network addresses, if you are captur‐
ing in monitor mode and are not connected to another network with another adapter.
available when not in monitor mode will be shown; if -I is specified, only those link-layer types
available when in monitor mode will be shown.
–immediate-mode
rather than being buffered for efficiency. This is the default when printing packets rather than
saving packets to a “savefile” if the packets are being printed to a terminal rather than to a file
or pipe.
-j tstamp_type
–time-stamp-type=tstamp_type
are given in pcap-tstamp(7); not all the types listed there will necessarily be valid for any given
interface.
-J
–list-time-stamp-types
for the interface, no time stamp types are listed.
–time-stamp-precision=tstamp_precision
ability of high precision time stamps (nanoseconds) and their actual accuracy is platform and hard‐
ware dependent. Also note that when writing captures made with nanosecond accuracy to a savefile,
the time stamps are written with nanosecond resolution, and the file is written with a different
magic number, to indicate that the time stamps are in seconds and nanoseconds; not all programs that
read pcap savefiles will be able to read those captures.
When reading a savefile, convert time stamps to the precision specified by timestamp_precision, and display
them with that resolution. If the precision specified is less than the precision of time stamps in the
file, the conversion will lose precision.
The supported values for timestamp_precision are micro for microsecond resolution and nano for nanosecond
resolution. The default is microsecond resolution.
-K
–dont-verify-checksums
or all of those checksum calculation in hardware; otherwise, all outgoing TCP checksums will be
flagged as bad.
-l Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,
ter individually if -l is specified.
output is written to stdout at the end of each packet rather than at the end of each line; this is
buffered on all platforms, including Windows.
-L
–list-data-link-types
data link types may be dependent on the specified mode; for example, on some platforms, a Wi-Fi
interface might support one set of data link types when not in monitor mode (for example, it might
support only fake Ethernet headers, or might support 802.11 headers but not support 802.11 headers
with radio information) and another set of data link types when in monitor mode (for example, it
might support 802.11 headers, or 802.11 headers with radio information, only in monitor mode).
-m module
eral MIB modules into tcpdump.
-M secret
option (RFC 2385), if present.
-n
不转化地址，显示数据类型的IP地址、端口号。
-N Don’t print domain name qualification of host names. E.g., if you give this flag then tcpdump will
-#
–number
-O
–no-optimize
mizer.
-p
–no-promiscuous-mode
for some other reason; hence, `-p’ cannot be used as an abbreviation for `ether host {local-hw-addr}
or ether broadcast’.
-Q direction
–direction=direction
`in’, `out’ and `inout’. Not available on all platforms.
-q Quick (quiet?) output. Print less protocol information so output lines are shorter.
-r file
pcap-ng files). Standard input is used if file is “-”.
-S
–absolute-tcp-sequence-numbers
-s snaplen
–snapshot-length=snaplen
cated because of a limited snapshot are indicated in the output with “[|proto]”, where proto is the
name of the protocol level at which the truncation has occurred. Note that taking larger snapshots
both increases the amount of time it takes to process packets and, effectively, decreases the amount
of packet buffering. This may cause packets to be lost. You should limit snaplen to the smallest
number that will capture the protocol information you’re interested in. Setting snaplen to 0 sets it
to the default of 262144, for backwards compatibility with recent older versions of tcpdump.
-T type
are aodv (Ad-hoc On-demand Distance Vector protocol), carp (Common Address Redundancy Protocol), cnfp
(Cisco NetFlow protocol), lmp (Link Management Protocol), pgm (Pragmatic General Multicast),
pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), resp (REdis Serialization Protocol), radius (RADIUS), 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), wb (distributed White Board), zmtp1 (ZeroMQ Message Transport Protocol 1.0) and
vxlan (Virtual eXtensible Local Area Network).
IP protocol 113 regardless. UDP-encapsulated PGM is often called “EPGM” or “PGM/UDP”.
the native PGM decoding the application data of an ODATA/RDATA packet would be decoded as a ZeroMQ
datagram with ZMTP/1.0 frames. During the UDP decoding in addition to that any UDP packet would be
treated as an encapsulated PGM packet.
-t Don’t print a timestamp on each dump line.
-tt Print the timestamp, as seconds since January 1, 1970, 00:00:00, UTC, and fractions of a second since
-ttt Print a delta (micro-second resolution) between current and previous line on each dump line.
-tttt Print a timestamp, as hours, minutes, seconds, and fractions of a second since midnight, preceded by
-ttttt Print a delta (micro-second resolution) between current and first line on each dump line.
-u Print undecoded NFS handles.
-U
–packet-buffered
description of the contents of each packet is printed, it will be written to the standard output,
rather than, when not writing to a terminal, being written only when the output buffer fills.
packet is saved, it will be written to the output file, rather than being written only when the out‐
put buffer fills.
the pcap_dump_flush() function.
-v When parsing and printing, produce (slightly more) verbose output. For example, the time to live,
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
-vvv Even more verbose output. For example, telnet SB … SE options are printed in full. With -X Telnet
-V file
-w file
with the -r option. Standard output is used if file is “-”.
may not see packets for an arbitrary amount of time after they are received. Use the -U flag to
cause packets to be written as soon as they are received.
extension .pcap appears to be the most commonly used along with .cap and .dmp. Tcpdump itself doesn’t
check the extension when reading capture files and doesn’t add an extension when writing them (it
uses magic numbers in the file header instead). However, many operating systems and applications will
use the extension if it is present and adding one (e.g. .pcap) is recommended.
-W Used in conjunction with the -C option, this will limit the number of files created to the specified
tion, it will name the files with enough leading 0s to support the maximum number of files, allowing
them to sort correctly.
created, exiting with status 0 when reaching the limit. If used with -C as well, the behavior will
result in cyclical files per timeslice.
-x When parsing and printing, in addition to printing the headers of each packet, print the data of each
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 When parsing and printing, in addition to printing the headers of each packet, print the data of each
-X When parsing and printing, in addition to printing the headers of each packet, print the data of each
cols.
-XX When parsing and printing, in addition to printing the headers of each packet, print the data of each
-y datalinktype
–linktype=datalinktype
-z postrotate-command
where file is the savefile being closed after each rotation. For example, specifying -z gzip or -z
bzip2 will compress each savefile using gzip or bzip2.
this doesn’t disturb the capture process.
always write a shell script that will take the savefile name as the only argument, make the flags &
arguments arrangements and execute the command that you want.
-Z user
–relinquish-privileges=user
any savefiles for output, change the user ID to user and the group ID to the primary group of user.
expression
dumped. Otherwise, only packets for which expression is `true’ will be dumped.
Shell arguments, whichever is more convenient. Generally, if the expression contains Shell metachar‐
acters, such as backslashes used to escape protocol names, it is easier to pass it as a single,
quoted argument rather than to escape the Shell metacharacters. Multiple arguments are concatenated
with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
To print traffic between helios and either hot or ace:
To print all IP packets between ace and any host except helios:
To print all traffic between local hosts and hosts at Berkeley:
To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the
shell from (mis-)interpreting the parentheses):
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).
To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-
local host.
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.)
To print IP packets longer than 576 bytes sent through gateway snup:
To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:
To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following gives a brief description and examples of most
of the formats.
Timestamps
By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the
form
and is as accurate as the kernel’s clock. The timestamp reflects the time the kernel applied a time stamp
to the packet. No attempt is made to account for the time lag between when the network interface finished
receiving the packet from the network and when the kernel applied a time stamp to the packet; that time lag
could include a delay between the time when the network interface finished receiving a packet from the net‐
work and the time when an interrupt was delivered to the kernel to get it to read the packet and a delay
between the time when the kernel serviced the `new packet’ interrupt and the time when it applied a time
stamp to the packet.
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 desti‐
nation addresses, and the packet length. (The `frame control’ field governs the interpretation of the rest
of the packet. Normal packets (such as those containing IP datagrams) are `async’ packets, with a priority
value between 0 and 7; for example, `async4′. Such packets are assumed to contain an 802.2 Logical Link
Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.
On Token Ring networks, the ‘-e’ option causes tcpdump to print the `access control’ and `frame control’
fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are
assumed to contain an LLC packet. Regardless of whether the ‘-e’ option is specified or not, the source
routing information is printed for source-routed packets.
On 802.11 networks, the ‘-e’ option causes tcpdump to print the `frame control’ fields, all of the addresses
in the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC
packet.
(N.B.: The following description assumes familiarity with the SLIP compression algorithm described in
RFC-1144.)
On SLIP links, a direction indicator (“I for inbound, ``O for outbound), packet type, and compression
information are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp. No
further link information is printed for ip packets. For TCP packets, the connection identifier is printed
following the type. If the packet is compressed, its encoded header is printed out. The special cases are
printed out as *S+n and *SA+n, where n is the amount by which the sequence number (or sequence number and
ack) has changed. If it is not a special case, zero or more changes are printed. A change is indicated by
U (urgent pointer), W (window), A (ack), S (sequence number), and I (packet ID), followed by a delta (+n or
-n), or a new value (=n). Finally, the amount of data in the packet and compressed header length are
printed.
For example, the following line shows an outbound compressed TCP packet, with an implicit connection identi‐
fier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data
and 6 bytes of compressed header:
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 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 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:
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 broad‐
cast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
IPv4 Packets
If the link-layer header is not being printed, for IPv4 packets, IP is printed after the time stamp.
If the -v flag is specified, information from the IPv4 header is shown in parentheses after the IP or the
link-layer header. The general format of this information is:
tos is the type of service field; if the ECN bits are non-zero, those are reported as ECT(1), ECT(0), or CE.
ttl is the time-to-live; it is not reported if it is zero. id is the IP identification field. offset is
the fragment offset field; it is printed whether this is part of a fragmented datagram or not. flags are
the MF and DF flags; + is reported if MF is set, and DFP is reported if F is set. If neither are set, . is
reported. proto is the protocol ID field. length is the total length field. options are the IP options,
if any.
Next, for TCP and UDP packets, the source and destination IP addresses and TCP or UDP ports, with a dot
between each IP address and its corresponding port, will be printed, with a > separating the source and des‐
tination. For other protocols, the addresses will be printed, with a > separating the source and destina‐
tion. Higher level protocol information, if any, will be printed after that.
For fragmented IP datagrams, the first fragment contains the higher level protocol header; fragments after
the first contain no higher level protocol header. Fragmentation information will be printed only with the
-v flag, in the IP header information, as described above.
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, this description will not be of much use to you.)
The general format of a TCP protocol line is:
Src and dst are the source and destination IP addresses and ports. Tcpflags are some combination of S
(SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or `.’ (ACK), or `none’ if no flags
are set. Data-seqno describes the portion of sequence space covered by the data in this packet (see example
below). Ackno 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 indi‐
cates there is `urgent’ data in the packet. Opts are TCP options (e.g., mss 1024). Len is the length of
payload data.
Iptype, 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.
IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 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’ which means `sequence numbers first up to but not including last.) There was no piggy-backed
ack, the available receive window was 4096 bytes and there was a max-segment-size option requesting an mss
of 1024 bytes.
Csam replies with a similar packet except it includes a piggy-backed ack for rtsg’s SYN. Rtsg then acks
csam’s SYN. The `.’ means the ACK flag was set. The packet contained no data so there is no data sequence
number or length. 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 conversa‐
tion, 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 conver‐
sation). 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:
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
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 con‐
tains 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 data‐
gram arrives with the SYN bit set in its header:
|—————|
|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
and its decimal representation is
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
We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set:
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:
|—————|
|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
which translates to decimal
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:
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:
This points us to the tcpdump filter expression
Some offsets and field values may be expressed as names rather than as numeric values. For example tcp[13]
may be replaced with tcp[tcpflags]. The following TCP flag field values are also available: tcp-fin, tcp-
syn, tcp-rst, tcp-push, tcp-act, tcp-urg.
This can be demonstrated as:
Note that you should use single quotes or a backslash in the expression to hide the AND (‘&’) special char‐
acter from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
This says that port who on host actinide sent a udp datagram to port who on host broadcast, the Internet
broadcast address. The packet contained 84 bytes of user data.
Some UDP services are recognized (from the source or destination port number) and the higher level protocol
information printed. In particular, Domain Name service requests (RFC-1034/1035) and Sun RPC calls
(RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC-1035.
If you are not familiar with the protocol, the following description will appear to be written in greek.)
Name server requests are formatted as
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
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 mes‐
sage, TC, set). If the `question’ section doesn’t contain exactly one entry, `[nq]’ is printed.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some
primitive decoding of IPX and NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned
that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory
details.
For information on SMB packet formats and what all the fields mean see www.cifs.org or the pub/samba/specs/
directory on your favorite samba.org mirror site. The SMB patches were written by Andrew Tridgell
(tridge@samba.org).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as:
src.nfs > dst.dport: NFS reply xid xid reply stat len op results
sushi.1023 > wrl.nfs: NFS request xid 26377
In the first line, host sushi sends a transaction with id 26377 to wrl. The request was 112 bytes, exclud‐
ing 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.) In the second line, wrl replies
`ok’ with the same transaction id and the contents of the link.
In the third line, sushi asks (using a new transaction id) wrl to lookup the name `xcolors’ in directory
file 9,74/4096.6878. In the fourth line, wrl sends a reply with the respective transaction id.
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. Also note that older versions of tcpdump printed NFS packets
in a slightly different format: the transaction id (xid) would be printed instead of the non-NFS port number
of the packet.
If the -v (verbose) flag is given, additional information is printed. For example:
(-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
- Wrl replies `ok’; the packet shown on the second line is the first fragment of the reply, and hence
is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not
have NFS or even UDP headers and so might not be printed, depending on the filter expression used). Because
the -v flag is given, some of the file attributes (which are returned in addition to the file data) are
printed: the file type (“REG”, for regular file), the file mode (in octal), the uid and gid, and the file
size.
If the -v flag is given more than once, even more details are printed.
Note that NFS requests are very large and much of the detail won’t be printed unless snaplen is increased.
Try using `-s 192′ to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent”
requests, and matches them to the replies using the transaction ID. If a reply does not closely follow the
corresponding request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printed as:
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs:
new fid 536876964/1/1 “.newsrc”
In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver)
service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of
536876964/1/1 and an old filename of `.newsrc.new’, and a new directory file id of 536876964/1/1 and a new
filename of `.newsrc’. The host pike responds with a RPC reply to the rename call (which was successful,
because it was a data packet and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the
arguments decoded (generally only the `interesting’ arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probably not be useful to people who are not
familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is
printed, such as the 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 RX call ID, serial number, and
the RX packet flags. The MTU negotiation information is also printed from RX ack packets.
If the -v flag is given three times, the security index and service id are printed.
Error codes are printed for abort packets, with the exception of Ubik beacon packets (because abort packets
are used to signify a yes vote for the Ubik protocol).
Note that AFS requests are very large and many of the arguments won’t be printed unless snaplen is
increased. Try using `-s 256′ to watch AFS traffic.
AFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent”
requests, and matches them to the replies using the call number and service ID. If a reply does not closely
follow the corresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all
the UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and
node numbers to names. Lines in this file have the form
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
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn’t exist or doesn’t contain an entry for some AppleTalk host/net number,
addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is
sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the
full name of the source node is known (`office’). The third line is a send from port 235 on net jssmag node
149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name
with no host number – for this reason it’s a good idea to keep node names and net names distinct in
/etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents inter‐
preted. 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:
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 jss‐
mag. 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:
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.
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp(7)
BUGS
To report a security issue please send an e-mail to security@tcpdump.org.
To report bugs and other problems, contribute patches, request a feature, provide generic feedback etc
please see the file CONTRIBUTING in the tcpdump source tree root.
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:
order to be filtered in user mode;
(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);
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the
higher level protocol.
Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than
real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix
the program generating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is
ignored).
Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed
Token Ring packets.
Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data pack‐
ets with both To DS and From DS set.
ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this
behavior.
Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It
only looks at IPv4 packets.
参考文献
- man 8 tcpdump, version 4.9.2