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tcpdump Flags的含义
在看tcpdump输出的时候,看到Flags[S],Flags[.],Flags[S.],Flags[P],
搞不懂这是什么意思,百度搜索[tcpdump Flags ]竟然称心的答复.
闲话少说,看man文档怎么说:
Flags are some combination of S (SYN), F(FIN), P (PUSH), R (RST), U (URG),
W (ECN CWR), E (ECN-Echo) or `.‘ (ACK), or `none‘ if no flags are set.
打开man文档:man tcpdump
搜索:Flags are some combination:
Flags are some combination of S (SYN), F(FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or `.‘ (ACK), or `none‘ if no flags are set. Data-seqno describes the portion of sequence space covered by the data in this packet (see example below). Ack is sequence number of the next data expected the other direction on this connection. Window is the number of bytes of receive buffer space available the other direction on this connection. Urg indicates there is `urgent‘ datain the packet. Options are tcp options enclosed in angle brackets (e.g., <mss 1024>).
【本文谢绝转载原文来自http://990487026.blog.51cto.com】
附:tcpdump man文档:
TCPDUMP(8) System Manager‘s Manual TCPDUMP(8)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
[ -P 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 ]
[ expression ]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on a network inter‐
face that match the boolean expression. It can also be run with the -w flag,
which causes it to save the packet data to a file for later analysis, and/or
with the -r flag, which causes it to read from a saved packet file rather than
to read packets from a network interface (please note tcpdump is protected via
an enforcing apparmor(7) profile in Ubuntu which limits the files tcpdump may
access). 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 inter‐
rupt 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 config‐
ured - 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 expres‐
sion 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 ``sta‐
tus‘‘ 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 priv‐
ileges; 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 cap‐
turing web pages.
-b Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
notation.
-B Set the operating system capture buffer size to buffer_size, in units of
KiB (1024 bytes).
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file is cur‐
rently 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 stan‐
dard output and stop.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (preceded with a count).
-D Print the list of the network interfaces available on the system and on
which tcpdump can capture packets. For each network interface, a number
and an interface name, possibly followed by a text description of the
interface, is printed. The interface name or the number can be supplied
to the -i flag to specify an interface on which to capture.
This can be useful on systems that don‘t have a command to list them
(e.g., Windows systems, or UNIX systems lacking ifconfig -a); the number
can be useful on Windows 2000 and later systems, where the interface name
is a somewhat complex string.
The -D flag will not be supported if tcpdump was built with an older ver‐
sion of libpcap that lacks the pcap_findalldevs() function.
-e Print the link-level header on each dump line. This can be used, for
example, to print MAC layer addresses for protocols such as Ethernet and
IEEE 802.11.
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are
addressed to addr and contain Security Parameter Index value spi. This
combination may be repeated with comma or newline separation.
Note that setting the secret for IPv4 ESP packets is supported at this
time.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc,
or none. The default is des-cbc. The ability to decrypt packets is only
present if tcpdump was compiled with cryptography enabled.
secret is the ASCII text for ESP secret key. If preceded by 0x, then a
hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The option is only for
debugging purposes, and the use of this option with a true `secret‘ key
is discouraged. By presenting IPsec secret key onto command line you
make it visible to others, via ps(1) and other occasions.
In addition to the above syntax, the syntax file name may be used to have
tcpdump read the provided file in. The file is opened upon receiving the
first ESP packet, so any special permissions that tcpdump may have been
given should already have been given up.
-f Print `foreign‘ IPv4 addresses numerically rather than symbolically (this
option is intended to get around serious brain damage in Sun‘s NIS server
— usually it hangs forever translating non-local internet numbers).
The test for `foreign‘ IPv4 addresses is done using the IPv4 address and
netmask of the interface on which capture is being done. If that address
or netmask are not available, available, either because the interface on
which capture is being done has no address or netmask or because the cap‐
ture is being done on the Linux "any" interface, which can capture on
more than one interface, this option will not work correctly.
-F Use file as input for the filter expression. An additional expression
given on the command line is ignored.
-G If specified, rotates the dump file specified with the -w option every
rotate_seconds seconds. Savefiles will have the name specified by -w
which should include a time format as defined by strftime(3). If no time
format is specified, each new file will overwrite the previous.
If used in conjunction with the -C option, filenames will take the form
of `file<count>‘.
-h Print the tcpdump and libpcap version strings, print a usage message, and
exit.
-H Attempt to detect 802.11s draft mesh headers.
-i Listen on interface. If unspecified, tcpdump searches the system inter‐
face list for the lowest numbered, configured up interface (excluding
loopback), which may turn out to be, for example, ``eth0‘‘.
On Linux systems with 2.2 or later kernels, an interface argument of
``any‘‘ can be used to capture packets from all interfaces. Note that
captures on the ``any‘‘ device will not be done in promiscuous mode.
If the -D flag is supported, an interface number as printed by that flag
can be used as the interface argument.
-I Put the interface in "monitor mode"; this is supported only on IEEE
802.11 Wi-Fi interfaces, and supported only on some operating systems.
Note that in monitor mode the adapter might disassociate from the network
with which it‘s associated, so that you will not be able to use any wire‐
less networks with that adapter. This could prevent accessing files on a
network server, or resolving host names or network addresses, if you are
capturing in monitor mode and are not connected to another network with
another adapter.
This flag will affect the output of the -L flag. If -I isn‘t specified,
only those link-layer types available when not in monitor mode will be
shown; if -I is specified, only those link-layer types available when in
monitor mode will be shown.
-j Set the time stamp type for the capture to tstamp_type. The names to use
for the time stamp types are given in pcap-tstamp-type(7); not all the
types listed there will necessarily be valid for any given interface.
-J List the supported time stamp types for the interface and exit. If the
time stamp type cannot be set for the interface, no time stamp types are
listed.
-K Don‘t attempt to verify IP, TCP, or UDP checksums. This is useful for
interfaces that perform some or all of those checksum calculation in
hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
-l Make stdout line buffered. Useful if you want to see the data while cap‐
turing it. E.g.,
tcpdump -l | tee dat
or
tcpdump -l > dat & tail -f dat
Note that on Windows,``line buffered‘‘ means ``unbuffered‘‘, so that Win‐
Dump will write each character individually if -l is specified.
-U is similar to -l in its behavior, but it will cause output to be
``packet-buffered‘‘, so that the output is written to stdout at the end
of each packet rather than at the end of each line; this is buffered on
all platforms, including Windows.
-L List the known data link types for the interface, in the specified mode,
and exit. The list of known data link types may be dependent on the
specified mode; for example, on some platforms, a Wi-Fi interface might
support one set of data link types when not in monitor mode (for example,
it might support only fake Ethernet headers, or might support 802.11
headers but not support 802.11 headers with radio information) and
another set of data link types when in monitor mode (for example, it
might support 802.11 headers, or 802.11 headers with radio information,
only in monitor mode).
-m Load SMI MIB module definitions from file module. This option can be
used several times to load several MIB modules into tcpdump.
-M Use secret as a shared secret for validating the digests found in TCP
segments with the TCP-MD5 option (RFC 2385), if present.
-n Don‘t convert addresses (i.e., host addresses, port numbers, etc.) to
names.
-N Don‘t print domain name qualification of host names. E.g., if you give
this flag then tcpdump will print ``nic‘‘ instead of ``nic.ddn.mil‘‘.
-O Do not run the packet-matching code optimizer. This is useful only if
you suspect a bug in the optimizer.
-p Don‘t put the interface into promiscuous mode. Note that the interface
might be in promiscuous mode for some other reason; hence, `-p‘ cannot be
used as an abbreviation for `ether host {local-hw-addr} or ether broad‐
cast‘.
-P Choose send/receive direction direction for which packets should be cap‐
tured. Possible values are `in‘, `out‘ and `inout‘. Not available on all
platforms.
-q Quick (quiet?) output. Print less protocol information so output lines
are shorter.
-R Assume ESP/AH packets to be based on old specification (RFC1825 to
RFC1829). If specified, tcpdump will not print replay prevention field.
Since there is no protocol version field in ESP/AH specification, tcpdump
cannot deduce the version of ESP/AH protocol.
-r Read packets from file (which was created with the -w option). Standard
input is used if file is ``-‘‘.
-S Print absolute, rather than relative, TCP sequence numbers.
-s Snarf snaplen bytes of data from each packet rather than the default of
65535 bytes. Packets truncated because of a limited snapshot are indi‐
cated in the output with ``[|proto]‘‘, where proto is the name of the
protocol level at which the truncation has occurred. Note that taking
larger snapshots both increases the amount of time it takes to process
packets and, effectively, decreases the amount of packet buffering. This
may cause packets to be lost. You should limit snaplen to the smallest
number that will capture the protocol information you‘re interested in.
Setting snaplen to 0 sets it to the default of 65535, for backwards com‐
patibility with recent older versions of tcpdump.
-T Force packets selected by "expression" to be interpreted the specified
type. Currently known types are aodv (Ad-hoc On-demand Distance Vector
protocol), carp (Common Address Redundancy Protocol), cnfp (Cisco NetFlow
protocol), lmp (Link Management Protocol), pgm (Pragmatic General Multi‐
cast), pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), radius (RADIUS), rpc (Remote
Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time
Applications control protocol), snmp (Simple Network Management Proto‐
col), 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).
Note that the pgm type above affects UDP interpretation only, the native
PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated
PGM is often called "EPGM" or "PGM/UDP".
Note that the pgm_zmtp1 type above affects interpretation of both native
PGM and UDP at once. During 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 an unformatted timestamp on each dump line.
-ttt Print a delta (micro-second resolution) between current and previous line
on each dump line.
-tttt Print a timestamp in default format proceeded by date on each dump line.
-ttttt Print a delta (micro-second resolution) between current and first line on
each dump line.
-u Print undecoded NFS handles.
-U If the -w option is not specified, make the printed packet output
``packet-buffered‘‘; i.e., as the description of the contents of each
packet is printed, it will be written to the standard output, rather
than, when not writing to a terminal, being written only when the output
buffer fills.
If the -w option is specified, make the saved raw packet output ``packet-
buffered‘‘; i.e., as each packet is saved, it will be written to the out‐
put 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 ver‐
sion of libpcap that lacks the pcap_dump_flush() function.
-v When parsing and printing, produce (slightly more) verbose output. For
example, the time to live, identification, total length and options in an
IP packet are printed. Also enables additional packet integrity checks
such as verifying the IP and ICMP header checksum.
When writing to a file with the -w option, report, every 10 seconds, the
number of packets captured.
-vv Even more verbose output. For example, additional fields are printed
from NFS reply packets, and SMB packets are fully decoded.
-vvv Even more verbose output. For example, telnet SB ... SE options are
printed in full. With -X Telnet options are printed in hex as well.
-V Read a list of filenames from file. Standard input is used if file is
``-‘‘.
-w Write the raw packets to file rather than parsing and printing them out.
They can later be printed with the -r option. Standard output is used if
file is ``-‘‘.
This output will be buffered if written to a file or pipe, so a program
reading from the file or pipe may not see packets for an arbitrary amount
of time after they are received. Use the -U flag to cause packets to be
written as soon as they are received.
The MIME type application/vnd.tcpdump.pcap has been registered with IANA
for pcap files. The filename extension .pcap appears to be the most com‐
monly 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.
See pcap-savefile(5) for a description of the file format.
-W Used in conjunction with the -C option, this will limit the number of
files created to the specified number, and begin overwriting files from
the beginning, thus creating a ‘rotating‘ buffer. In addition, it will
name the files with enough leading 0s to support the maximum number of
files, allowing them to sort correctly.
Used in conjunction with the -G option, this will limit the number of
rotated dump files that get created, exiting with status 0 when reaching
the limit. If used with -C as well, the behavior will result in cyclical
files per timeslice.
-x When parsing and printing, in addition to printing the headers of each
packet, print the data of each packet (minus its link level header) in
hex. The smaller of the entire packet or snaplen bytes will be printed.
Note that this is the entire link-layer packet, so for link layers that
pad (e.g. Ethernet), the padding bytes will also be printed when the
higher layer packet is shorter than the required padding.
-xx When parsing and printing, in addition to printing the headers of each
packet, print the data of each packet, including its link level header,
in hex.
-X When parsing and printing, in addition to printing the headers of each
packet, print the data of each packet (minus its link level header) in
hex and ASCII. This is very handy for analysing new protocols.
-XX When parsing and printing, in addition to printing the headers of each
packet, print the data of each packet, including its link level header,
in hex and ASCII.
-y Set the data link type to use while capturing packets to datalinktype.
-z Used in conjunction with the -C or -G options, this will make tcpdump run
" command file " where file is the savefile being closed after each rota‐
tion. For example, specifying -z gzip or -z bzip2 will compress each
savefile using gzip or bzip2.
Note that tcpdump will run the command in parallel to the capture, using
the lowest priority so that this doesn‘t disturb the capture process.
And in case you would like to use a command that itself takes flags or
different arguments, you can always write a shell script that will take
the savefile name as the only argument, make the flags & arguments
arrangements and execute the command that you want.
-Z If tcpdump is running as root, after opening the capture device or input
savefile, but before opening any savefiles for output, change the user ID
to user and the group ID to the primary group of user.
This behavior can also be enabled by default at compile time.
expression
selects which packets will be dumped. If no expression is given, all
packets on the net will be dumped. Otherwise, only packets for which
expression is `true‘ will be dumped.
For the expression syntax, see pcap-filter(7).
The expression argument can be passed to tcpdump as either a single Shell
argument, or as multiple Shell arguments, whichever is more convenient.
Generally, if the expression contains Shell metacharacters, such as back‐
slashes used to escape protocol names, it is easier to pass it as a sin‐
gle, 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:
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 expres‐
sion 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 gate‐
way 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 conver‐
sation 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 Ether‐
nets, 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 con‐
trol‘ 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 infor‐
mation 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 num‐
ber), 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 destina‐
tion is the Ethernet broadcast address, the type field contained hex 0806 (type
ETHER_ARP) and the total length was 64 bytes.
TCP Packets
(N.B.:The following description assumes familiarity with the TCP protocol
described in RFC-793. If you are not familiar with the protocol, neither this
description nor tcpdump will be of much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and ports. Flags are
some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E
(ECN-Echo) or `.‘ (ACK), or `none‘ if no flags are set. Data-seqno describes
the portion of sequence space covered by the data in this packet (see example
below). Ack is sequence number of the next data expected the other direction on
this connection. Window is the number of bytes of receive buffer space avail‐
able 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 win‐
dow was 4096 bytes and there was a max-segment-size option requesting an mss of
1024 bytes.
Csam replies with a similar packet except it includes a piggy-backed ack for
rtsg‘s SYN. Rtsg then acks csam‘s SYN. The `.‘ means the ACK flag was set.
The packet contained no data so there is no data sequence number. Note that the
ack sequence number is a small integer (1). The first time tcpdump sees a tcp
`conversation‘, it prints the sequence number from the packet. On subsequent
packets of the conversation, the difference between the current packet‘s
sequence number and this initial sequence number is printed. This means that
sequence numbers after the first can be interpreted as relative byte positions
in the conversation‘s data stream (with the first data byte each direction being
`1‘). `-S‘ will override this feature, causing the original sequence numbers to
be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the
rtsg → csam side of the conversation). The PUSH flag is set in the packet. On
the 7th line, csam says it‘s received data sent by rtsg up to but not including
byte 21. Most of this data is apparently sitting in the socket buffer since
csam‘s receive window has gotten 19 bytes smaller. Csam also sends one byte of
data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of
urgent, pushed data to rtsg.
If the snapshot was small enough that tcpdump didn‘t capture the full TCP
header, it interprets as much of the header as it can and then reports
``[|tcp]‘‘ to indicate the remainder could not be interpreted. If the header
contains a bogus option (one with a length that‘s either too small or beyond the
end of the header), tcpdump reports it as ``[bad opt]‘‘ and does not interpret
any further options (since it‘s impossible to tell where they start). If the
header length indicates options are present but the IP datagram length is not
long enough for the options to actually be there, tcpdump reports it as ``[bad
hdr length]‘‘.
Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
There are 8 bits in the control bits section of the TCP header:
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
Let‘s assume that we want to watch packets used in establishing a TCP connec‐
tion. Recall that TCP uses a 3-way handshake protocol when it initializes a new
connection; the connection sequence with regard to the TCP control bits is
1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK
Now we‘re interested in capturing packets that have only the SYN bit set (Step
1). Note that we don‘t want packets from step 2 (SYN-ACK), just a plain initial
SYN. What we need is a correct filter expression for tcpdump.
Recall the structure of a TCP header without options:
0 15 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------
A TCP header usually holds 20 octets of data, unless options are present. The
first line of the graph contains octets 0 - 3, the second line shows octets 4 -
7 etc.
Starting to count with 0, the relevant TCP control bits are contained in octet
13:
0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |
Let‘s have a closer look at octet no. 13:
| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|
These are the TCP control bits we are interested in. We have numbered the bits
in this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while
the URG bit is number 5.
Recall that we want to capture packets with only SYN set. Let‘s see what hap‐
pens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Looking at the control bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in network byte
order, the binary value of this octet is
00000010
and its decimal representation is
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
We‘re almost done, because now we know that if only SYN is set, the value of the
13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in
network byte order, must be exactly 2.
This relationship can be expressed as
tcp[13] == 2
We can use this expression as the filter for tcpdump in order to watch packets
which have only SYN set:
tcpdump -i xl0 tcp[13] == 2
The expression says "let the 13th octet of a TCP datagram have the decimal value
2", which is exactly what we want.
Now, let‘s assume that we need to capture SYN packets, but we don‘t care if ACK
or any other TCP control bit is set at the same time. Let‘s see what happens to
octet 13 when a TCP datagram with SYN-ACK set arrives:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is
00010010
which translates to decimal
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
Now we can‘t just use ‘tcp[13] == 18‘ in the tcpdump filter expression, because
that would select only those packets that have SYN-ACK set, but not those with
only SYN set. Remember that we don‘t care if ACK or any other control bit is
set as long as SYN is set.
In order to achieve our goal, we need to logically AND the binary value of octet
13 with some other value to preserve the SYN bit. We know that we want SYN to
be set in any case, so we‘ll logically AND the value in the 13th octet with the
binary value of a SYN:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
We see that this AND operation delivers the same result regardless whether ACK
or another TCP control bit is set. The decimal representation of the AND value
as well as the result of this operation is 2 (binary 00000010), so we know that
for packets with SYN set the following relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 ‘tcp[13] & 2 == 2‘
Some offsets and field values may be expressed as names rather than as numeric
values. For example tcp[13] may be replaced with tcp[tcpflags]. The following
TCP flag field values are also available: tcp-fin, tcp-syn, tcp-rst, tcp-push,
tcp-act, tcp-urg.
This can be demonstrated as:
tcpdump -i xl0 ‘tcp[tcpflags] & tcp-push != 0‘
Note that you should use single quotes or a backslash in the expression to hide
the AND (‘&‘) special character from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp datagram to port who on host
broadcast, the Internet broadcast address. The packet contained 84 bytes of
user data.
Some UDP services are recognized (from the source or destination port number)
and the higher level protocol information printed. In particular, Domain Name
service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with the Domain Service pro‐
tocol described in RFC-1035. If you are not familiar with the protocol, the
following description will appear to be written in greek.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an address record (qtype=A)
associated with the name ucbvax.berkeley.edu. The query id was `3‘. The `+‘
indicates the recursion desired flag was set. The query length was 37 bytes,
not including the UDP and IP protocol headers. The query operation was the nor‐
mal one, Query, so the op field was omitted. If the op had been anything else,
it would have been printed between the `3‘ and the `+‘. Similarly, the qclass
was the normal one, C_IN, and omitted. Any other qclass would have been printed
immediately after the `A‘.
A few anomalies are checked and may result in extra fields enclosed in square
brackets: If a query contains an answer, authority records or additional
records section, ancount, nscount, or arcount are printed as `[na]‘, `[nn]‘ or
`[nau]‘ where n is the appropriate count. If any of the response bits are set
(AA, RA or rcode) or any of the `must be zero‘ bits are set in bytes two and
three, `[b2&3=x]‘ is printed, where x is the hex value of header bytes two and
three.
UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo with 3 answer
records, 3 name server records and 7 additional records. The first answer
record is type A (address) and its data is internet address 128.32.137.3. The
total size of the response was 273 bytes, excluding UDP and IP headers. The op
(Query) and response code (NoError) were omitted, as was the class (C_IN) of the
A record.
In the second example, helios responds to query 2 with a response code of non-
existent domain (NXDomain) with no answers, one name server and no authority
records. The `*‘ indicates that the authoritative answer bit was set. Since
there were no answers, no type, class or data were printed.
Other flag characters that might appear are `-‘ (recursion available, RA, not
set) and `|‘ (truncated message, TC, set). If the `question‘ section doesn‘t
contain exactly one entry, `[nq]‘ is printed.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137,
UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI SMB data is
also done.
By default a fairly minimal decode is done, with a much more detailed decode
done if -v is used. Be warned that with -v a single SMB packet may take up a
page or more, so only use -v if you really want all the gory details.
For information on SMB packet formats and what all the fields mean see
www.cifs.org or the pub/samba/specs/ directory on your favorite samba.org mirror
site. The SMB patches were written by Andrew Tridgell (tridge@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 number and generation number.) Wrl
replies `ok‘ with the contents of the link.
In the third line, sushi asks wrl to lookup the name `xcolors‘ in directory file
9,74/4096.6878. Note that the data printed depends on the operation type. The
format is intended to be self explanatory if read in conjunction with an NFS
protocol spec.
If the -v (verbose) flag is given, additional information is printed. For exam‐
ple:
sushi.1372a > wrl.nfs:
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header TTL, ID, length, and fragmentation fields, which
have been omitted from this example.) In the first line, sushi asks wrl to read
8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl replies `ok‘; the
packet shown on the second line is the first fragment of the reply, and hence is
only 1472 bytes long (the other bytes will follow in subsequent fragments, but
these fragments do not have NFS or even UDP headers and so might not be printed,
depending on the filter expression used). Because the -v flag is given, some of
the file attributes (which are returned in addition to the file data) are
printed: the file type (``REG‘‘, for regular file), the file mode (in octal),
the uid and gid, and the file size.
If the -v flag is given more than once, even more details are printed.
Note that NFS requests are very large and much of the detail won‘t be printed
unless snaplen is increased. Try using `-s 192‘ to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC operation. Instead, tcp‐
dump keeps track of ``recent‘‘ requests, and matches them to the replies using
the transaction ID. If a reply does not closely follow the corresponding
request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printed as:
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was a RX data
packet to the fs (fileserver) service, and is the start of an RPC call. The RPC
call was a rename, with the old directory file id of 536876964/1/1 and an old
filename of `.newsrc.new‘, and a new directory file id of 536876964/1/1 and a
new filename of `.newsrc‘. The host pike responds with a RPC reply to the
rename call (which was successful, because it was a data packet and not an abort
packet).
In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs
have at least some of the arguments decoded (generally only the `interesting‘
arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probably not be useful
to people who are not familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and additional
header information is printed, such as the 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 informa‐
tion 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 pro‐
tocol).
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, tcp‐
dump 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 corre‐
sponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and
dumped as DDP packets (i.e., all the UDP header information is discarded). The
file /etc/atalk.names is used to translate AppleTalk net and node numbers to
names. Lines in this file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of AppleTalk networks. The third line gives
the name of a particular host (a host is distinguished from a net by the 3rd
octet in the number - a net number must have two octets and a host number must
have three octets.) The number and name should be separated by whitespace
(blanks or tabs). The /etc/atalk.names file may contain blank lines or comment
lines (lines starting with a `#‘).
AppleTalk addresses are printed in the form
net.host.port
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn‘t exist or doesn‘t contain an entry for some
AppleTalk host/net number, addresses are printed in numeric form.) In the first
example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is lis‐
tening 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 con‐
tains 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.
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-
tstamp-type(7), apparmor(7)
http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley
National Laboratory, University of California, Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available via http:
http://www.tcpdump.org/
The original distribution is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric
Young‘s SSLeay library, under specific configurations.
BUGS
Please send problems, bugs, questions, desirable enhancements, patches etc. to:
tcpdump-workers@lists.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 sec‐
tion 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 cor‐
rectly 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 pro‐
tochain 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.
12 July 2012 TCPDUMP(8)
本文出自 “魂斗罗” 博客，谢绝转载！
tcpdump Flags的含义
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