.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
.\"
-.TH TCPDUMP 1 "17 September 2015"
+.TH TCPDUMP 1 "8 April 2018"
.SH NAME
tcpdump \- dump traffic on a network
.SH SYNOPSIS
.B \-c
.I count
]
-.br
-.ti +8
[
.B \-C
.I file_size
-] [
-.B \-G
-.I rotate_seconds
-] [
+]
+.ti +8
+[
+.B \-E
+.I spi@ipaddr algo:secret,...
+]
+.ti +8
+[
.B \-F
.I file
]
-.br
-.ti +8
+[
+.B \-G
+.I rotate_seconds
+]
[
.B \-i
.I interface
]
+.ti +8
+[
+.B \-\-immediate\-mode
+]
[
.B \-j
.I tstamp_type
.B \-m
.I module
]
+.ti +8
[
.B \-M
.I secret
]
-.br
-.ti +8
[
.B \-\-number
]
[
+.B \-\-print
+]
+[
.B \-Q
.I in|out|inout
]
.I file
]
[
-.B \-V
-.I file
-]
-[
.B \-s
.I snaplen
]
.I type
]
[
+.B \-\-version
+]
+.ti +8
+[
+.B \-V
+.I file
+]
+[
.B \-w
.I file
]
-.br
-.ti +8
[
.B \-W
.I filecount
]
-.br
-.ti +8
-[
-.B \-E
-.I spi@ipaddr algo:secret,...
-]
-.br
-.ti +8
[
.B \-y
.I datalinktype
]
+.ti +8
[
.B \-z
.I postrotate-command
]
.ti +8
[
-.B \-\-immediate\-mode
-]
-[
-.B \-\-version
-]
-.ti +8
-[
.I expression
]
.br
it will be reported as 0).
.LP
On platforms that support the SIGINFO signal, such as most BSDs
-(including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
+(including macOS) and Digital/Tru64 UNIX, it will report those counts
when it receives a SIGINFO signal (generated, for example, by typing
your ``status'' character, typically control-T, although on some
-platforms, such as Mac OS X, the ``status'' character is not set by
+platforms, such as macOS, the ``status'' character is not set by
default, so you must set it with
.BR stty (1)
in order to use it) and will continue capturing packets. On platforms that
do not support the SIGINFO signal, the same can be achieved by using the
SIGUSR1 signal.
.LP
+Using the SIGUSR2 signal along with the
+.B \-w
+flag will forcibly flush the packet buffer into the output file.
+.LP
Reading packets from a network interface may require that you have
special privileges; see the
-.B pcap (3PCAP)
+.BR pcap (3PCAP)
man page for details. Reading a saved packet file doesn't require
special privileges.
.SH OPTIONS
was built with an older version of
.I libpcap
that lacks the
-.B pcap_findalldevs()
+.BR pcap_findalldevs(3PCAP)
function.
.TP
.B \-e
which should include a time format as defined by
.BR strftime (3).
If no time format is specified, each new file will overwrite the previous.
+Whenever a generated filename is not unique, tcpdump will overwrite the
+preexisting data; providing a time specification that is coarser than the
+capture period is therefore not advised.
.IP
If used in conjunction with the
.B \-C
.PD
Set the time stamp type for the capture to \fItstamp_type\fP. The names
to use for the time stamp types are given in
-.BR pcap-tstamp (@MAN_MISC_INFO@);
+.BR \%pcap-tstamp (@MAN_MISC_INFO@);
not all the types listed there will necessarily be valid for any given
interface.
.TP
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.
-.LP
+.IP
When reading a savefile, convert time stamps to the precision specified
by \fItimestamp_precision\fP, 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.
-.LP
+.IP
The supported values for \fItimestamp_precision\fP are \fBmicro\fP for
microsecond resolution and \fBnano\fP for nanosecond resolution. The
default is microsecond resolution.
mode for some other reason; hence, `-p' cannot be used as an abbreviation for
`ether host {local-hw-addr} or ether broadcast'.
.TP
+.BI \-\-print
+Print parsed packet output, even if the raw packets are being saved to a
+file with the
+.B \-w
+flag.
+.TP
.BI \-Q " direction"
.PD 0
.TP
.BI \-r " file"
Read packets from \fIfile\fR (which was created with the
.B \-w
-option or by other tools that write pcap or pcap-ng files).
+option or by other tools that write pcap or pcapng files).
Standard input is used if \fIfile\fR is ``-''.
.TP
.B \-S
Packets truncated because of a limited snapshot
are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
is the name of the protocol level at which the truncation has occurred.
+.IP
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 \fIsnaplen\fP to the smallest number that will
-capture the protocol information you're interested in.
-Setting
+Note also that taking smaller snapshots will discard data from protocols
+above the transport layer, which loses information that may be
+important. NFS and AFS requests and replies, for example, are very
+large, and much of the detail won't be available if a too-short snapshot
+length is selected.
+.IP
+If you need to reduce the snapshot size below the default, you should
+limit \fIsnaplen\fP to the smallest number that will capture the
+protocol information you're interested in. Setting
\fIsnaplen\fP to 0 sets it to the default of 262144,
for backwards compatibility with recent older versions of
.IR tcpdump .
fractions of a second since that time, on each dump line.
.TP
.B \-ttt
-Print a delta (micro-second resolution) between current and previous line
-on each dump line.
+Print a delta (microsecond or nanosecond resolution depending on the
+.B \-\-time\-stamp-precision
+option) between current and previous line on each dump line.
+The default is microsecond resolution.
.TP
.B \-tttt
Print a timestamp, as hours, minutes, seconds, and fractions of a second
since midnight, preceded by the date, on each dump line.
.TP
.B \-ttttt
-Print a delta (micro-second resolution) between current and first line
-on each dump line.
+Print a delta (microsecond or nanosecond resolution depending on the
+.B \-\-time\-stamp-precision
+option) between current and first line on each dump line.
+The default is microsecond resolution.
.TP
.B \-u
Print undecoded NFS handles.
.PD
If the
.B \-w
-option is not specified, make the printed packet output
+option is not specified, or if it is specified but the
+.B \-\-print
+flag is also 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
was built with an older version of
.I libpcap
that lacks the
-.B pcap_dump_flush()
+.BR pcap_dump_flush(3PCAP)
function.
.TP
.B \-v
.IP
When writing to a file with the
.B \-w
-option, report, every 10 seconds, the number of packets captured.
+option, report, once per second, the number of packets captured.
.TP
.B \-vv
Even more verbose output.
Used in conjunction with the
.B \-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
+created, exiting with status 0 when reaching the limit.
+.IP
+If used in conjunction with both
.B \-C
-as well, the behavior will result in cyclical files per timeslice.
+and
+.B \-G,
+the
+.B \-W
+option will currently be ignored, and will only affect the file name.
.TP
.B \-x
When parsing and printing,
.B
..
.HD
+Timestamps
+.LP
+By default, all output lines are preceded by a timestamp.
+The timestamp
+is the current clock time in the form
+.RS
+.nf
+\fIhh:mm:ss.frac\fP
+.fi
+.RE
+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 network 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.
+.HD
Link Level Headers
.LP
If the '-e' option is given, the link level header is printed out.
destination is the Ethernet broadcast address, the type field
contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
.HD
+IPv4 Packets
+.LP
+If the link-layer header is not being printed, for IPv4 packets,
+\fBIP\fP is printed after the time stamp.
+.LP
+If the
+.B \-v
+flag is specified, information from the IPv4 header is shown in
+parentheses after the \fBIP\fP or the link-layer header.
+The general format of this information is:
+.RS
+.nf
+.sp .5
+tos \fItos\fP, ttl \fIttl\fP, id \fIid\fP, offset \fIoffset\fP, flags [\fIflags\fP], proto \fIproto\fP, length \fIlength\fP, options (\fIoptions\fP)
+.sp .5
+.fi
+.RE
+\fItos\fP is the type of service field; if the ECN bits are non-zero,
+those are reported as \fBECT(1)\fP, \fBECT(0)\fP, or \fBCE\fP.
+\fIttl\fP is the time-to-live; it is not reported if it is zero.
+\fIid\fP is the IP identification field.
+\fIoffset\fP is the fragment offset field; it is printed whether this is
+part of a fragmented datagram or not.
+\fIflags\fP are the MF and DF flags; \fB+\fP is reported if MF is set,
+and \fBDF\fP is reported if F is set. If neither are set, \fB.\fP is
+reported.
+\fIproto\fP is the protocol ID field.
+\fIlength\fP is the total length field.
+\fIoptions\fP are the IP options, if any.
+.LP
+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
+destination. For other protocols, the addresses will be printed, with
+a > separating the source and destination. Higher level protocol
+information, if any, will be printed after that.
+.LP
+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
+.B \-v
+flag, in the IP header information, as described above.
+.HD
TCP Packets
.LP
\fI(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 \fItcpdump\fP will
+with the protocol, this description will not
be of much use to you.)\fP
.LP
-The general format of a tcp protocol line is:
+The general format of a TCP protocol line is:
.RS
.nf
.sp .5
-\fIsrc > dst: flags data-seqno ack window urgent options\fP
+\fIsrc\fP > \fIdst\fP: Flags [\fItcpflags\fP], seq \fIdata-seqno\fP, ack \fIackno\fP, win \fIwindow\fP, urg \fIurgent\fP, options [\fIopts\fP], length \fIlen\fP
.sp .5
.fi
.RE
\fISrc\fP and \fIdst\fP are the source and destination IP
addresses and ports.
-\fIFlags\fP are some combination of S (SYN),
+\fITcpflags\fP 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.
\fIData-seqno\fP describes the portion of sequence space covered
by the data in this packet (see example below).
-\fIAck\fP is sequence number of the next data expected the other
+\fIAckno\fP is sequence number of the next data expected the other
direction on this connection.
\fIWindow\fP is the number of bytes of receive buffer space available
the other direction on this connection.
\fIUrg\fP indicates there is `urgent' data in the packet.
-\fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
+\fIOpts\fP are TCP options (e.g., mss 1024).
+\fILen\fP is the length of payload data.
.LP
-\fISrc, dst\fP and \fIflags\fP are always present.
+\fIIptype\fR, \fISrc\fP, \fIdst\fP, and \fIflags\fP are always present.
The other fields
-depend on the contents of the packet's tcp protocol header and
+depend on the contents of the packet's TCP protocol header and
are output only if appropriate.
.LP
Here is the opening portion of an rlogin from host \fIrtsg\fP to
.RS
.nf
.sp .5
-\s-2\f(CWrtsg.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\fR\s+2
+\f(CWIP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
+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\fR
.sp .5
.fi
.RE
-The first line says that tcp port 1023 on rtsg sent a packet
+The first line says that TCP port 1023 on rtsg sent a packet
to port \fIlogin\fP
on csam.
The \fBS\fP indicates that the \fISYN\fP flag was set.
The packet sequence number was 768512 and it contained no data.
-(The notation is `first:last(nbytes)' which means `sequence
+(The notation is `first:last' which means `sequence
numbers \fIfirst\fP
-up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
+up to but not including \fIlast\fP'.)
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.
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.
+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 \fItcpdump\fP sees a
-tcp `conversation', it prints the sequence number from the packet.
+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.
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.
+tcp-push, tcp-ack, tcp-urg.
.PP
This can be demonstrated as:
.RS
.LP
If the \-v flag is given more than once, even more details are printed.
.LP
-Note that NFS requests are very large and much of the detail won't be printed
-unless \fIsnaplen\fP is increased.
-Try using `\fB\-s 192\fP' to watch
-NFS traffic.
-.LP
NFS reply packets do not explicitly identify the RPC operation.
Instead,
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
beacon packets (because abort packets are used to signify a yes vote
for the Ubik protocol).
.LP
-Note that AFS requests are very large and many of the arguments won't
-be printed unless \fIsnaplen\fP is increased.
-Try using `\fB-s 256\fP'
-to watch AFS traffic.
-.LP
AFS reply packets do not explicitly identify the RPC operation.
Instead,
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
.RS
.nf
.sp .5
-\s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
+\f(CWicsd-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\fR\s+2
+techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR
.sp .5
.fi
.RE
.RS
.nf
.sp .5
-\s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
+\f(CWjssmag.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 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
-jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
+jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR
.sp .5
.fi
.RE
The `*' on the request
indicates that XO (`exactly once') was \fInot\fP set.
-.HD
-IP Fragmentation
-.LP
-Fragmented Internet datagrams are printed as
-.RS
-.nf
-.sp .5
-\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
-\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
-.sp .5
-.fi
-.RE
-(The first form indicates there are more fragments.
-The second
-indicates this is the last fragment.)
-.LP
-\fIId\fP is the fragment id.
-\fISize\fP is the fragment
-size (in bytes) excluding the IP header.
-\fIOffset\fP is this
-fragment's offset (in bytes) in the original datagram.
-.LP
-The fragment information is output for each fragment.
-The first
-fragment contains the higher level protocol header and the frag
-info is printed after the protocol info.
-Fragments
-after the first contain no higher level protocol header and the
-frag info is printed after the source and destination addresses.
-For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
-over a CSNET connection that doesn't appear to handle 576 byte datagrams:
-.RS
-.nf
-.sp .5
-\s-2\f(CWarizona.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\fP\s+2
-.sp .5
-.fi
-.RE
-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.
-.LP
-A packet with the IP \fIdon't fragment\fP flag is marked with a
-trailing \fB(DF)\fP.
-.HD
-Timestamps
-.LP
-By default, all output lines are preceded by a timestamp.
-The timestamp
-is the current clock time in the form
-.RS
-.nf
-\fIhh:mm:ss.frac\fP
-.fi
-.RE
-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 network 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.
.SH "SEE ALSO"
-stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
-pcap-filter(@MAN_MISC_INFO@), pcap-tstamp(@MAN_MISC_INFO@)
+stty(1), pcap(3PCAP), bpf(4), nit(4P), \%pcap-savefile(@MAN_FILE_FORMATS@),
+\%pcap-filter(@MAN_MISC_INFO@), \%pcap-tstamp(@MAN_MISC_INFO@)
.LP
.RS
.I http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
The current version is available via http:
.LP
.RS
-.I http://www.tcpdump.org/
+.I https://www.tcpdump.org/
.RE
.LP
The original distribution is available via anonymous ftp:
.RE
.LP
IPv6/IPsec support is added by WIDE/KAME project.
-This program uses Eric Young's SSLeay library, under specific configurations.
+This program uses OpenSSL/LibreSSL, under specific configurations.
.SH BUGS
-Please send problems, bugs, questions, desirable enhancements, patches
-etc. to:
.LP
-.RS
-.RE
+To report bugs and other problems, contribute patches, request a
+feature, provide generic feedback etc please see the file
+.I CONTRIBUTING
+in the tcpdump source tree root.
.LP
NIT doesn't let you watch your own outbound traffic, BPF will.
We recommend that you use the latter.
projects / tcpdump / blobdiff