.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
.\"
-.TH TCPDUMP 1 "8 April 2018"
+.TH TCPDUMP 1 "29 November 2021"
.SH NAME
tcpdump \- dump traffic on a network
.SH SYNOPSIS
.I count
]
[
+.B \-\-count
+]
+[
.B \-C
.I file_size
]
]
.ti +8
[
+.BI \-\-micro
+]
+[
+.BI \-\-nano
+]
+.ti +8
+[
.I expression
]
.br
.SH DESCRIPTION
.LP
\fITcpdump\fP prints out a description of the contents of packets on a
-network interface that match the boolean \fIexpression\fP; the
+network interface that match the Boolean \fIexpression\fP; 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
.BI \-c " count"
Exit after receiving \fIcount\fP packets.
.TP
+.BI \-\-count
+Print only on stdout the packet count when reading capture file(s) instead
+of parsing/printing the packets. If a filter is specified on the command
+line, \fItcpdump\fP counts only packets that were matched by the filter
+expression.
+.TP
.BI \-C " file_size"
Before writing a raw packet to a savefile, check whether the file is
currently larger than \fIfile_size\fP and, if so, close the current
have the name specified with the
.B \-w
flag, with a number after it, starting at 1 and continuing upward.
-The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
+The default unit of \fIfile_size\fP is millions of bytes (1,000,000 bytes,
not 1,048,576 bytes).
+.IP
+By adding a suffix of k/K, m/M or g/G to the value, the unit
+can be changed to 1,024 (KiB), 1,048,576 (MiB), or 1,073,741,824 (GiB)
+respectively.
.TP
.B \-d
Dump the compiled packet-matching code in a human readable form to
standard output and stop.
+.IP
+Please mind that although code compilation is always DLT-specific,
+typically it is impossible (and unnecessary) to specify which DLT to use
+for the dump because \fItcpdump\fP uses either the DLT of the input pcap
+file specified with
+.BR -r ,
+or the default DLT of the network interface specified with
+.BR -i ,
+or the particular DLT of the network interface specified with
+.B -y
+and
+.B -i
+respectively. In these cases the dump shows the same exact code that
+would filter the input file or the network interface without
+.BR -d .
+.IP
+However, when neither
+.B -r
+nor
+.B -i
+is specified, specifying
+.B -d
+prevents \fItcpdump\fP from guessing a suitable network interface (see
+.BR -i ).
+In this case the DLT defaults to EN10MB and can be set to another valid
+value manually with
+.BR -y .
.TP
.B \-dd
Dump packet-matching code as a
.I 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
+interface, are printed. The interface name or the number can be supplied
to the
.B \-i
flag to specify an interface on which to capture.
was built with an older version of
.I libpcap
that lacks the
-.BR pcap_findalldevs(3PCAP)
+.BR pcap_findalldevs (3PCAP)
function.
.TP
.B \-e
\fIsecret\fP is the ASCII text for ESP secret key.
If preceded by 0x, then a hex value will be read.
.IP
-The option assumes RFC2406 ESP, not RFC1827 ESP.
+The option assumes RFC 2406 ESP, not RFC 1827 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
.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
+pre-existing data; providing a time specification that is coarser than the
capture period is therefore not advised.
.IP
If used in conjunction with the
.TP
.BI \-\-interface= interface
.PD
-Listen on \fIinterface\fP.
-If unspecified, \fItcpdump\fP searches the system interface list for the
-lowest numbered, configured up interface (excluding loopback), which may turn
-out to be, for example, ``eth0''.
+Listen, report the list of link-layer types, report the list of time
+stamp types, or report the results of compiling a filter expression on
+\fIinterface\fP. If unspecified and if the
+.B -d
+flag is not given, \fItcpdump\fP searches the system
+interface list for the lowest numbered, configured up interface
+(excluding loopback), which may turn out to be, for example, ``eth0''.
.IP
On Linux systems with 2.2 or later kernels, an
.I interface
microsecond resolution and \fBnano\fP for nanosecond resolution. The
default is microsecond resolution.
.TP
+.B \-\-micro
+.PD 0
+.TP
+.B \-\-nano
+.PD
+Shorthands for \fB\-\-time\-stamp\-precision=micro\fP or
+\fB\-\-time\-stamp\-precision=nano\fP, adjusting the time stamp
+precision accordingly. When reading packets from a savefile, using
+\fB\-\-micro\fP truncates time stamps if the savefile was created with
+nanosecond precision. In contrast, a savefile created with microsecond
+precision will have trailing zeroes added to the time stamp when
+\fB\-\-nano\fP is used.
+.TP
.B \-K
.PD 0
.TP
\fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
\fBcarp\fR (Common Address Redundancy Protocol),
\fBcnfp\fR (Cisco NetFlow protocol),
+\fBdomain\fR (Domain Name System),
\fBlmp\fR (Link Management Protocol),
\fBpgm\fR (Pragmatic General Multicast),
\fBpgm_zmtp1\fR (ZMTP/1.0 inside PGM/EPGM),
-\fBresp\fR (REdis Serialization Protocol),
+\fBptp\fR (Precision Time Protocol),
+\fBquic\fR (QUIC),
\fBradius\fR (RADIUS),
+\fBresp\fR (REdis Serialization Protocol),
\fBrpc\fR (Remote Procedure Call),
-\fBrtp\fR (Real-Time Applications protocol),
\fBrtcp\fR (Real-Time Applications control protocol),
+\fBrtp\fR (Real-Time Applications protocol),
\fBsnmp\fR (Simple Network Management Protocol),
+\fBsomeip\fR (SOME/IP),
\fBtftp\fR (Trivial File Transfer Protocol),
\fBvat\fR (Visual Audio Tool),
-\fBwb\fR (distributed White Board),
-\fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0)
+\fBvxlan\fR (Virtual eXtensible Local Area Network),
+\fBwb\fR (distributed White Board)
and
-\fBvxlan\fR (Virtual eXtensible Local Area Network).
+\fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0).
.IP
Note that the \fBpgm\fR type above affects UDP interpretation only, the native
PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM is
was built with an older version of
.I libpcap
that lacks the
-.BR pcap_dump_flush(3PCAP)
+.BR pcap_dump_flush (3PCAP)
function.
.TP
.B \-v
.IP
When writing to a file with the
.B \-w
-option, report, once per second, the number of packets captured.
+option and at the same time not reading from a file with the
+.B \-r
+option, report to stderr, once per second, the number of packets captured. In
+Solaris, FreeBSD and possibly other operating systems this periodic update
+currently can cause loss of captured packets on their way from the kernel to
+tcpdump.
.TP
.B \-vv
Even more verbose output.
present and adding one (e.g. .pcap) is recommended.
.IP
See
-.BR pcap-savefile (@MAN_FILE_FORMATS@)
+.BR \%pcap-savefile (@MAN_FILE_FORMATS@)
for a description of the file format.
.TP
-.B \-W
+.BI \-W " filecount"
Used in conjunction with the
.B \-C
option, this will limit the number
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.
+In the current implementation this flag may have the same effect as
+.B \-xx
+if the packet is truncated.
.TP
.B \-xx
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.
+In the current implementation this flag may have the same effect as
+.B \-XX
+if the packet is truncated.
.TP
.B \-XX
When parsing and printing,
.TP
.BI \-\-linktype= datalinktype
.PD
-Set the data link type to use while capturing packets to \fIdatalinktype\fP.
+Set the data link type to use while capturing packets (see
+.BR -L )
+or just compiling and dumping packet-matching code (see
+.BR -d )
+to \fIdatalinktype\fP.
.TP
.BI \-z " postrotate-command"
Used in conjunction with the
only packets for which \fIexpression\fP is `true' will be dumped.
.LP
For the \fIexpression\fP syntax, see
-.BR pcap-filter (@MAN_MISC_INFO@).
+.BR \%pcap-filter (@MAN_MISC_INFO@).
.LP
The \fIexpression\fP argument can be passed to \fItcpdump\fP as either a single
Shell argument, or as multiple Shell arguments, whichever is more convenient.
.fi
.RE
.LP
+To print the TCP packets with flags RST and ACK both set.
+(i.e. select only the RST and ACK flags in the flags field, and if the result
+is "RST and ACK both set", match)
+.RS
+.nf
+.B
+tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'
+.fi
+.RE
+.LP
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.)
packets are assumed to contain an LLC packet.
.LP
\fI(N.B.: The following description assumes familiarity with
-the SLIP compression algorithm described in RFC-1144.)\fP
+the SLIP compression algorithm described in RFC 1144.)\fP
.LP
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
packet type, and compression information are printed out.
.HD
ARP/RARP Packets
.LP
-Arp/rarp output shows the type of request and its arguments.
+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
.sp .5
.fi
.RE
-The first line says that rtsg sent an arp packet asking
+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
TCP Packets
.LP
\fI(N.B.:The following description assumes familiarity with
-the TCP protocol described in RFC-793.
+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.)\fP
(The notation is `first:last' which means `sequence
numbers \fIfirst\fP
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
+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.
.LP
Csam replies with a similar packet except it includes a piggy-backed
-ack for rtsg's SYN.
-Rtsg then acks csam's SYN.
+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
+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.
.sp .5
.fi
.RE
-This says that port \fIwho\fP on host \fIactinide\fP sent a udp
+This says that port \fIwho\fP on host \fIactinide\fP sent a UDP
datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
broadcast address.
The packet contained 84 bytes of user data.
.LP
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.
+In particular, Domain Name service requests (RFC 1034/1035) and Sun
+RPC calls (RFC 1050) to NFS.
.HD
-UDP Name Server Requests
+TCP or UDP Name Server Requests
.LP
\fI(N.B.:The following description assumes familiarity with
-the Domain Service protocol described in RFC-1035.
+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.)\fP
+in Greek.)\fP
.LP
Name server requests are formatted as
.RS
The query id was `3'.
The `+' indicates the \fIrecursion desired\fP flag
was set.
-The query length was 37 bytes, not including the UDP and
+The query length was 37 bytes, excluding the TCP or UDP and
IP protocol headers.
The query operation was the normal one, \fIQuery\fP,
so the op field was omitted.
`must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
is printed, where \fIx\fP is the hex value of header bytes two and three.
.HD
-UDP Name Server Responses
+TCP or UDP Name Server Responses
.LP
Name server responses are formatted as
.RS
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.
+excluding TCP or UDP and IP headers.
The op (Query) and response code
(NoError) were omitted, as was the class (C_IN) of the A record.
.LP
gory details.
.LP
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.
+\%https://download.samba.org/pub/samba/specs/ and other online resources.
The SMB patches were written by Andrew Tridgell
.HD
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.
+the file mode (in octal), the UID and GID, and the file size.
.LP
If the \-v flag is given more than once, even more details are printed.
.LP
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.
+excluding the ATP header.
The `*' on packet 7 indicates that the
EOM bit was set.
.LP
indicates that XO (`exactly once') was \fInot\fP set.
.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@)
+.BR stty (1),
+.BR pcap (3PCAP),
+.BR bpf (4),
+.BR nit (4P),
+.BR \%pcap-savefile (@MAN_FILE_FORMATS@),
+.BR \%pcap-filter (@MAN_MISC_INFO@),
+.BR \%pcap-tstamp (@MAN_MISC_INFO@)
.LP
.RS
-.I http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
+.na
+.I https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
+.ad
.RE
.LP
.SH AUTHORS
.LP
It is currently being maintained by tcpdump.org.
.LP
-The current version is available via http:
+The current version is available via HTTPS:
.LP
.RS
.I https://www.tcpdump.org/
.LP
To report bugs and other problems, contribute patches, request a
-feature, provide generic feedback etc please see the file
+feature, provide generic feedback etc. please see the file
.I CONTRIBUTING
in the tcpdump source tree root.
.LP
.IP
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
+asked to copy only part of a packet to userspace, will not report the
true length of the packet; this would cause most IP packets to get an
error from
.BR tcpdump );
projects / tcpdump / blobdiff