.\"	$OpenBSD: tcpdump.8,v 1.112 2021/09/07 06:48:42 denis Exp $
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.Dd $Mdocdate: September 7 2021 $
.Dt TCPDUMP 8
.Os
.Sh NAME
.Nm tcpdump
.Nd dump traffic on a network
.Sh SYNOPSIS
.Nm tcpdump
.Op Fl AadefILlNnOopqStvXx
.Op Fl B Ar fildrop
.Op Fl c Ar count
.Op Fl D Ar direction
.Op Fl E Oo Ar espalg : Oc Ns Ar espkey
.Op Fl F Ar file
.Op Fl i Ar interface
.Op Fl r Ar file
.Op Fl s Ar snaplen
.Op Fl T Ar type
.Op Fl w Ar file
.Op Fl y Ar datalinktype
.Op Ar expression
.Sh DESCRIPTION
.Nm
prints out the headers of packets on a network interface that match the boolean
.Ar expression .
You must have read access to
.Pa /dev/bpf .
.Pp
The options are as follows:
.Bl -tag -width "-c count"
.It Fl A
Print each packet in ASCII.
If the
.Fl e
option is also specified, the link-level header will be included.
The smaller of the entire packet or
.Ar snaplen
bytes will be printed.
.It Fl a
Attempt to convert network and broadcast addresses to names.
.It Fl B Ar fildrop
Configure the drop action specified by
.Ar fildrop
to be used when the filter expression matches a packet.
The actions are:
.Pp
.Bl -tag -width "capture" -offset indent -compact
.It Cm pass
Matching packets are accepted and captured.
.It Cm capture
Matching packets are dropped and captured.
.It Cm drop
Matching packets are dropped but not captured.
.El
.Pp
The default action is
.Cm pass .
.It Fl c Ar count
Exit after receiving
.Ar count
packets.
.It Fl D Ar direction
Select packets flowing in the specified
.Ar direction .
Valid directions are:
.Cm in
and
.Cm out .
The default is to accept packets flowing in any direction.
.It Fl d
Dump the compiled packet-matching code in a human readable form to
standard output and stop.
.It Fl dd
Dump packet-matching code as a C program fragment.
.It Fl ddd
Dump packet-matching code as decimal numbers
preceded with a count.
.It Fl E Oo Ar espalg : Oc Ns Ar espkey
Try to decrypt RFC 4835 ESP
.Pq Encapsulating Security Payload
traffic using the specified hex key
.Ar espkey .
Supported algorithms for
.Ar espalg
are:
.Cm aes128 ,
.Cm aes128-hmac96 ,
.Cm blowfish ,
.Cm blowfish-hmac96 ,
.Cm cast ,
.Cm cast-hmac96 ,
.Cm des3 ,
.Cm des3-hmac96 ,
.Cm des
and
.Cm des-hmac96 .
The algorithm defaults to
.Cm aes128-hmac96 .
This option should be used for debugging only, since the key will show up in
.Xr ps 1
output.
.It Fl e
Print the link-level header on each dump line.
.It Fl F Ar file
Use
.Ar file
as input for the filter expression.
Any additional expressions given on the command line are ignored.
.It Fl f
Print
.Dq foreign
internet addresses numerically rather than symbolically.
This option is intended to get around serious brain damage in
Sun's yp server \(em usually it hangs forever translating non-local
internet numbers.
.It Fl I
Print the interface on each dump line.
.It Fl i Ar interface
Listen on
.Ar interface .
If unspecified,
.Nm
searches the system interface list for the lowest numbered, configured
.Dq up
interface
.Pq excluding loopback .
Ties are broken by choosing the earliest match.
.Ar interface
may be either a network interface or a USB interface, for example
.Ar usb0 .
.It Fl L
List the supported data link types for the interface and exit.
.It Fl l
Make stdout line buffered.
Useful if you want to see the data while capturing it.
For example:
.Pp
.Dl # tcpdump -l | tee dat
or
.Dl # tcpdump -l > dat & tail -f dat
.It Fl N
Do not print domain name qualification of host names.
For example, if you specify this flag then
.Nm
will print
.Dq nic
instead of
.Dq nic.ddn.mil .
.It Fl n
Do not convert addresses
.Pq host addresses, port numbers, etc.
to names.
.It Fl O
Do not run the packet-matching code optimizer.
This is useful only if you suspect a bug in the optimizer.
.It Fl o
Print a guess of the possible operating system(s) of hosts that sent
TCP SYN packets.
See
.Xr pf.os 5
for a description of the passive operating system fingerprints.
.It Fl p
Do not put the interface into promiscuous mode.
The interface might be in promiscuous mode for some other reason; hence,
.Fl p
cannot be used as an abbreviation for
.Dq ether host \&"{local-hw-addr}\&"
or
.Dq ether broadcast .
.It Fl q
Quick
.Pq quiet?
output.
Print less protocol information so output lines are shorter.
.It Fl r Ar file
Read packets from a
.Ar file
which was created with the
.Fl w
option.
Standard input is used if
.Ar file
is
.Ql - .
.It Fl S
Print absolute, rather than relative, TCP sequence numbers.
.It Fl s Ar snaplen
Analyze at most the first
.Ar snaplen
bytes of data from each packet rather than the default of 116.
116 bytes is adequate for IPv6, ICMP, TCP, and UDP,
but may truncate protocol information from name server and NFS packets
.Pq see below .
Packets truncated because of a limited
.Ar snaplen
are indicated in the output with
.Dq Op | Ns Em proto ,
where
.Em proto
is the name of the protocol level at which the truncation has occurred.
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
.Ar snaplen
to the smallest number that will capture the protocol information
you're interested in.
.It Fl T Ar type
Force packets selected by
.Ar expression
to be interpreted as the specified
.Ar type .
Currently known types are:
.Pp
.Bl -tag -width "erspan" -offset indent -compact
.It Cm cnfp
Cisco NetFlow protocol
.It Cm erspan
Cisco Encapsulated Remote Switch Port Analyzer (ERSPAN) over GRE
.It Cm geneve
Generic Network Virtualization Encapsulation
.It Cm gre
Generic Routing Encapsulation over UDP
.It Cm mpls
Multiprocol Label Switching over UDP
.It Cm rpc
Remote Procedure Call
.It Cm rtcp
Real-Time Applications control protocol
.It Cm rtp
Real-Time Applications protocol
.It Cm sack
RFC 2018 TCP Selective Acknowledgements Options
.It Cm tcp
Transmission Control Protocol
.It Cm tftp
Trivial File Transfer Protocol
.It Cm vat
Visual Audio Tool
.It Cm vrrp
Virtual Router Redundancy protocol
.It Cm vxlan
Virtual eXtensible Local Area Network
.It Cm wb
distributed White Board
.It Cm wg
WireGuard tunnel
.El
.It Fl t
Do not print a timestamp on each dump line.
.It Fl tt
Print an unformatted timestamp on each dump line.
.It Fl ttt
Print day and month in timestamp.
.It Fl tttt
Print timestamp difference between packets.
.It Fl ttttt
Print timestamp difference since the first packet.
.It Fl v
.Pq Slightly more
verbose output.
For example, the time to live
.Pq TTL
and type of service
.Pq ToS
information in an IP packet are printed.
.It Fl vv
Even more verbose output.
For example, additional fields are printed from NFS reply packets.
.It Fl w Ar file
Write the raw packets to
.Ar file
rather than parsing and printing them out.
They can be analyzed later with the
.Fl r
option.
Standard output is used if
.Ar file
is
.Ql - .
.It Fl X
Print each packet in hex and ASCII.
If the
.Fl e
option is also specified, the link-level header will be included.
The smaller of the entire packet or
.Ar snaplen
bytes will be printed.
.It Fl x
Print each packet in hex.
If the
.Fl e
option is also specified, the link-level header will be included.
The smaller of the entire packet or
.Ar snaplen
bytes will be printed.
.It Fl y Ar datalinktype
Set the data link type to use while capturing to
.Ar datalinktype .
Commonly used types include
.Cm EN10MB ,
.Cm IEEE802_11 ,
and
.Cm IEEE802_11_RADIO .
The choices applicable to a particular device can be listed using
.Fl L .
.El
.Pp
.Ar expression
selects which packets will be dumped.
If no
.Ar expression
is given, all packets on the net will be dumped.
Otherwise, only packets satisfying
.Ar expression
will be dumped.
.Pp
The filter expression consists of one or more
.Em primitives .
Primitives usually consist of an
.Ar id
.Pq name or number
preceded by one or more qualifiers.
There are three different kinds of qualifier:
.Bl -tag -width "proto"
.It Ar type
Specify which kind of address component the
.Ar id
name or number refers to.
Possible types are
.Cm host ,
.Cm net
and
.Cm port .
E.g.,
.Dq host foo ,
.Dq net 128.3 ,
.Dq port 20 .
If there is no type qualifier,
.Cm host
is assumed.
.It Ar dir
Specify a particular transfer direction to and/or from
.Ar id .
Possible directions are
.Cm src ,
.Cm dst ,
.Cm src or dst ,
.Cm src and dst ,
.Cm ra ,
.Cm ta ,
.Cm addr1 ,
.Cm addr2 ,
.Cm addr3 ,
and
.Cm addr4 .
E.g.,
.Dq src foo ,
.Dq dst net 128.3 ,
.Dq src or dst port ftp-data .
If there is no
.Ar dir
qualifier,
.Cm src or dst
is assumed.
The
.Cm ra ,
.Cm ta ,
.Cm addr1 ,
.Cm addr2 ,
.Cm addr3 ,
and
.Cm addr4
qualifiers are only valid for IEEE 802.11 Wireless LAN link layers.
For null link layers (i.e., point-to-point protocols such as SLIP
.Pq Serial Line Internet Protocol
or the
.Xr pflog 4
header), the
.Cm inbound
and
.Cm outbound
qualifiers can be used to specify a desired direction.
.It Ar proto
Restrict the match to a particular protocol.
Possible protocols are:
.Cm ah ,
.Cm arp ,
.Cm atalk ,
.Cm decnet ,
.Cm esp ,
.Cm ether ,
.Cm fddi ,
.Cm icmp ,
.Cm icmp6 ,
.Cm igmp ,
.Cm igrp ,
.Cm ip ,
.Cm ip6 ,
.Cm lat ,
.Cm mopdl ,
.Cm moprc ,
.Cm pim ,
.Cm rarp ,
.Cm sca ,
.Cm stp ,
.Cm tcp ,
.Cm udp ,
and
.Cm wlan .
E.g.,
.Dq ether src foo ,
.Dq arp net 128.3 ,
.Dq tcp port 21 ,
and
.Dq wlan addr2 0:2:3:4:5:6 .
If there is no protocol qualifier,
all protocols consistent with the type are assumed.
E.g.,
.Dq src foo
means
.Do
.Pq ip or arp or rarp
src foo
.Dc
.Pq except the latter is not legal syntax ;
.Dq net bar
means
.Do
.Pq ip or arp or rarp
net bar
.Dc ;
and
.Dq port 53
means
.Do
.Pq TCP or UDP
port 53
.Dc .
.Pp
.Cm fddi
is actually an alias for
.Cm ether ;
the parser treats them identically as meaning
.Qo
the data link level used on the specified network interface
.Qc .
FDDI
.Pq Fiber Distributed Data Interface
headers contain Ethernet-like source and destination addresses,
and often contain Ethernet-like packet types,
so it's possible to filter these FDDI fields just as with the analogous
Ethernet fields.
FDDI headers also contain other fields,
but they cannot be named explicitly in a filter expression.
.Pp
Similarly,
.Cm tr
and
.Cm wlan
are aliases for
.Cm ether ;
the previous paragraph's statements about FDDI headers also apply to Token Ring
and 802.11 wireless LAN headers.
For 802.11 headers, the destination address is the DA field
and the source address is the SA field;
the BSSID, RA, and TA fields aren't tested.
.El
.Pp
In addition to the above, there are some special primitive
keywords that don't follow the pattern:
.Cm gateway ,
.Cm broadcast ,
.Cm less ,
.Cm greater ,
and arithmetic expressions.
All of these are described below.
.Pp
More complex filter expressions are built up by using the words
.Cm and ,
.Cm or ,
and
.Cm not
to combine primitives
e.g.,
.Do
host foo and not port ftp and not port ftp-data
.Dc .
To save typing, identical qualifier lists can be omitted
e.g.,
.Dq tcp dst port ftp or ftp-data or domain
is exactly the same as
.Do
tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain
.Dc .
.Pp
Allowable primitives are:
.Bl -tag -width "ether proto proto"
.It Cm dst host Ar host
True if the IPv4/v6 destination field of the packet is
.Ar host ,
which may be either an address or a name.
.It Cm src host Ar host
True if the IPv4/v6 source field of the packet is
.Ar host .
.It Cm host Ar host
True if either the IPv4/v6 source or destination of the packet is
.Ar host .
.Pp
Any of the above
.Ar host
expressions can be prepended with the keywords,
.Cm ip , arp , rarp ,
or
.Cm ip6 ,
as in:
.Pp
.D1 Cm ip host Ar host
.Pp
which is equivalent to:
.Bd -ragged -offset indent
.Cm ether proto
.Ar ip
.Cm and host
.Ar host
.Ed
.Pp
If
.Ar host
is a name with multiple IP addresses, each address will be checked for a match.
.It Cm ether dst Ar ehost
True if the Ethernet destination address is
.Ar ehost .
.Ar ehost
may be either a name from
.Pa /etc/ethers
or a number (see
.Xr ether_aton 3
for a numeric format).
.It Cm ether src Ar ehost
True if the Ethernet source address is
.Ar ehost .
.It Cm ether host Ar ehost
True if either the Ethernet source or destination address is
.Ar ehost .
.It Cm gateway Ar host
True if the packet used
.Ar host
as a gateway; i.e., the Ethernet source or destination address was
.Ar host
but neither the IP source nor the IP destination was
.Ar host .
.Ar host
must be a name and must be found both by the machine's
host-name-to-IP-address resolution mechanisms (host name file, DNS, NIS,
etc.) and by the machine's host-name-to-Ethernet-address resolution mechanism
(such as
.Pa /etc/ethers ) .
An equivalent expression is:
.Bd -ragged -offset indent
.Cm ether host
.Ar ehost
.Cm and not host
.Ar host
.Ed
.Pp
which can be used with either names or numbers for host/ehost.
This syntax does not work in an IPv6-enabled configuration at this moment.
.It Cm dst net Ar net
True if the IPv4/v6 destination address of the packet has a network
number of
.Ar net ,
which may be either a name from the networks database
(such as
.Pa /etc/networks )
or a network number.
An IPv4 network number can be written as a dotted quad (e.g. 192.168.1.0),
dotted triple (e.g. 192.168.1), dotted pair (e.g 172.16),
or single number (e.g. 10);
the netmask is 255.255.255.255 for a dotted quad
(which means that it's really a host match),
255.255.255.0 for a dotted triple, 255.255.0.0 for a dotted pair,
or 255.0.0.0 for a single number.
An IPv6 network number must be written out fully;
the netmask is ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff,
so IPv6 "network" matches are really always host matches,
and a network match requires a netmask length.
.It Cm src net Ar net
True if the IPv4/v6 source address of the packet has a network number of
.Ar net .
.It Cm net Ar net
True if either the IPv4/v6 source or destination address of the packet
has a network number of
.Ar net .
.It Cm net Ar net Cm mask Ar netmask
True if the IPv4 address matches
.Ar net
with the specific
.Ar netmask .
May be qualified with
.Cm src
or
.Cm dst .
Note that this syntax is not valid for IPv6 networks.
.It Cm net Ar net Ns / Ns Ar len
True if the IPv4/v6 address matches
.Ar net
with a netmask
.Ar len
bits wide.
May be qualified with
.Cm src
or
.Cm dst .
.It Cm dst port Ar port
True if the packet is IP/TCP, IP/UDP, IP6/TCP or IP6/UDP
and has a destination port value of
.Ar port .
The
.Ar port
can be a number or a name used in
.Pa /etc/services
(see
.Xr tcp 4
and
.Xr udp 4 ) .
If a name is used, both the port number and protocol are checked.
If a number or ambiguous name is used,
only the port number is checked (e.g.\&
.Dq dst port 513
will print both
TCP/login traffic and UDP/who traffic, and
.Dq port domain
will print both TCP/domain and UDP/domain traffic).
.It Cm src port Ar port
True if the packet has a source port value of
.Ar port .
.It Cm port Ar port
True if either the source or destination port of the packet is
.Ar port .
.Pp
Any of the above port expressions can be prepended with the keywords
.Cm tcp
or
.Cm udp ,
as in:
.Pp
.D1 Cm tcp src port Ar port
.Pp
which matches only TCP packets whose source port is
.Ar port .
.It Cm less Ar length
True if the packet has a length less than or equal to
.Ar length .
This is equivalent to:
.Pp
.D1 Cm len <= Ar length
.It Cm greater Ar length
True if the packet has a length greater than or equal to
.Ar length .
This is equivalent to:
.Pp
.D1 Cm len >= Ar length
.It Cm sample Ar samplerate
True if the packet has been randomly selected or sampled at a rate of 1 per
.Ar samplerate .
.It Cm ip proto Ar protocol
True if the packet is an IPv4 packet (see
.Xr ip 4 )
of protocol type
.Ar protocol .
.Ar protocol
can be a number, or one of the names from
.Xr protocols 5 ,
such as
.Cm icmp ,
.Cm icmp6 ,
.Cm igmp ,
.Cm igrp ,
.Cm pim ,
.Cm ah ,
.Cm esp ,
.Cm vrrp ,
.Cm udp ,
or
.Cm tcp .
Note that the identifiers
.Cm tcp ,
.Cm udp ,
and
.Cm icmp
are also keywords and must be escaped using a backslash character
.Pq \e .
Note that this primitive does not chase the protocol header chain.
.It Cm ip6 proto Ar protocol
True if the packet is an IPv6 packet of protocol type
.Ar protocol .
Note that this primitive does not chase the protocol header chain.
.It Cm ether broadcast
True if the packet is an Ethernet broadcast packet.
The
.Cm ether
keyword is optional.
.It Cm ip broadcast
True if the packet is an IPv4 broadcast packet.
It checks for both the all-zeroes and all-ones broadcast conventions,
and looks up the subnet mask on the interface on which the capture is
being done.
.Pp
If the subnet mask of the interface on which the capture is being done
is not known, a value of PCAP_NETMASK_UNKNOWN can be supplied;
tests for IPv4 broadcast addresses will fail to compile,
but all other tests in the filter program will be OK.
.It Cm ether multicast
True if the packet is an Ethernet multicast packet.
The
.Cm ether
keyword is optional.
This is shorthand for
.Dq ether[0] & 1 != 0 .
.It Cm ip multicast
True if the packet is an IPv4 multicast packet.
.It Cm ip6 multicast
True if the packet is an IPv6 multicast packet.
.It Cm ether proto Ar protocol
True if the packet is of ether type
.Ar protocol .
.Ar protocol
can be a number, or one of the names
.Cm ip ,
.Cm ip6 ,
.Cm arp ,
.Cm rarp ,
.Cm atalk ,
.Cm atalkarp ,
.Cm decnet ,
.Cm decdts ,
.Cm decdns ,
.Cm lanbridge ,
.Cm lat ,
.Cm mopdl ,
.Cm moprc ,
.Cm pup ,
.Cm sca ,
.Cm sprite ,
.Cm stp ,
.Cm vexp ,
.Cm vprod ,
or
.Cm xns .
These identifiers are also keywords and must be escaped
using a backslash character
.Pq Sq \e .
.Pp
In the case of FDDI (e.g.,
.Dq fddi protocol arp ) ,
and IEEE 802.11 wireless LANS (such as
.Dq wlan protocol arp ) ,
for most of those protocols
the protocol identification comes from the 802.2 Logical Link Control
.Pq LLC
header, which is usually layered on top of the FDDI or 802.11 header.
.Pp
When filtering for most protocol identifiers on FDDI or 802.11,
the filter checks only the protocol ID field of an LLC header
in so-called SNAP format with an Organizational Unit Identifier (OUI) of
0x000000, for encapsulated Ethernet; it doesn't check whether the packet
is in SNAP format with an OUI of 0x000000.
The exceptions are:
.Bl -tag -width "atalk"
.It iso
The filter checks the DSAP (Destination Service Access Point) and
SSAP (Source Service Access Point) fields of the LLC header.
.It stp
The filter checks the DSAP of the LLC header.
.It atalk
The filter checks for a SNAP-format packet with an OUI of 0x080007
and the AppleTalk etype.
.El
.Pp
In the case of Ethernet, the filter checks the Ethernet type field
for most of those protocols.
The exceptions are:
.Bl -tag -width "iso and stp"
.It iso and stp
The filter checks for an 802.3 frame and then checks the LLC header as
it does for FDDI and 802.11.
.It atalk
The filter checks both for the AppleTalk etype in an Ethernet frame and
for a SNAP-format packet as it does for FDDI, Token Ring, and 802.11.
.El
.It Cm decnet src Ar host
True if the DECNET source address is
.Ar host ,
which may be an address of the form
.Dq 10.123 ,
or a DECNET host name.
DECNET host name support is only available on systems that are
configured to run DECNET.
.It Cm decnet dst Ar host
True if the DECNET destination address is
.Ar host .
.It Cm decnet host Ar host
True if either the DECNET source or destination address is
.Ar host .
.It Cm ifname Ar interface
True if the packet was logged as coming from the specified interface
(applies only to packets logged by
.Xr pf 4 ) .
.It Cm on Ar interface
Synonymous with the
.Cm ifname
modifier.
.It Cm rnr Ar num
True if the packet was logged as matching the specified PF rule number
in the main ruleset (applies only to packets logged by
.Xr pf 4 ) .
.It Cm rulenum Ar num
Synonymous with the
.Cm rnr
modifier.
.It Cm reason Ar code
True if the packet was logged with the specified PF reason code.
Known codes are:
.Cm match ,
.Cm bad-offset ,
.Cm fragment ,
.Cm short ,
.Cm normalize ,
.Cm memory ,
.Cm bad-timestamp ,
.Cm congestion ,
.Cm ip-option ,
.Cm proto-cksum ,
.Cm state-mismatch ,
.Cm state-insert ,
.Cm state-limit ,
.Cm src-limit ,
and
.Cm synproxy
(applies only to packets logged by
.Xr pf 4 ) .
.It Cm rset Ar name
True if the packet was logged as matching the specified PF ruleset
name of an anchored ruleset (applies only to packets logged by
.Xr pf 4 ) .
.It Cm ruleset Ar name
Synonymous with the
.Cm rset
modifier.
.It Cm srnr Ar num
True if the packet was logged as matching the specified PF rule number
of an anchored ruleset (applies only to packets logged by
.Xr pf 4 ) .
.It Cm subrulenum Ar num
Synonymous with the
.Cm srnr
modifier.
.It Cm action Ar act
True if PF took the specified action when the packet was logged.
Known actions are:
.Cm pass
and
.Cm block ,
.Cm nat ,
.Cm rdr ,
.Cm binat ,
.Cm match
and
.Cm scrub
(applies only to packets logged by
.Xr pf 4 ) .
.It Cm ip , ip6 , arp , rarp , atalk , decnet , iso , stp
Abbreviations for
.Cm ether proto Ar p ,
where
.Ar p
is one of the above protocols.
.It Cm lat , moprc , mopdl
Abbreviations for
.Cm ether proto Ar p ,
where
.Ar p
is one of the above protocols.
.Cm tcpdump
does not currently know how to parse these.
.It Xo
.Cm ah ,
.Cm esp ,
.Cm icmp ,
.Cm icmp6 ,
.Cm igmp ,
.Cm igrp ,
.Cm pim ,
.Cm tcp ,
.Cm udp
.Xc
Abbreviations for
.Cm ip proto Ar p
or
.Cm ip6 proto Ar p ,
where
.Ar p
is one of the above protocols.
.It Cm wlan addr1 Ar ehost
True if the first IEEE 802.11 address is
.Ar ehost .
.It Cm wlan addr2 Ar ehost
True if the second IEEE 802.11 address is
.Ar ehost .
.It Cm wlan addr3 Ar ehost
True if the third IEEE 802.11 address is
.Ar ehost .
.It Cm wlan addr4 Ar ehost
True if the fourth IEEE 802.11 address is
.Ar ehost .
The fourth address field is only used for
WDS (Wireless Distribution System) frames.
.It Cm wlan host Ar ehost
True if either the first, second, third, or fourth
IEEE 802.11 address is
.Ar ehost .
.It Cm type Ar wlan_type
True if the IEEE 802.11 frame type matches the specified
.Ar wlan_type .
Valid types are:
.Cm mgt ,
.Cm ctl ,
.Cm data ,
or a numeric value.
.It Cm type Ar wlan_type Cm subtype Ar wlan_subtype
True if the IEEE 802.11 frame type matches the specified
.Ar wlan_type
and frame subtype matches the specified
.Ar wlan_subtype .
.Pp
If the specified
.Ar wlan_type
is
.Cm mgtv ,
then valid values for
.Ar wlan_subtype
are
.Cm assoc-req ,
.Cm assoc-resp ,
.Cm reassoc-req ,
.Cm reassoc-resp ,
.Cm probe-req ,
.Cm probe-resp ,
.Cm beacon ,
.Cm atim ,
.Cm disassoc ,
.Cm auth ,
and
.Cm deauth .
.Pp
If the specified
.Ar wlan_type
is
.Cm ctl ,
then valid values for
.Ar wlan_subtype
are
.Cm ps-poll ,
.Cm rts ,
.Cm cts ,
.Cm ack ,
.Cm cf-end ,
and
.Cm cf-end-ack .
.Pp
If the specified
.Ar wlan_type
is
.Cm data ,
then valid values for
.Ar wlan_subtype
are
.Cm data ,
.Cm data-cf-ack ,
.Cm data-cf-poll ,
.Cm data-cf-ack-poll ,
.Cm null ,
.Cm cf-ack ,
.Cm cf-poll ,
.Cm cf-ack-poll ,
.Cm qos-data ,
.Cm qos-data-cf-ack ,
.Cm qos-data-cf-poll ,
.Cm qos-data-cf-ack-poll ,
.Cm qos ,
.Cm qos-cf-poll ,
and
.Cm qos-cf-ack-poll .
.It Cm subtype Ar wlan_subtype
True if the IEEE 802.11 frame subtype matches the specified
.Ar wlan_subtype
and frame has the type to which the specified
.Ar wlan_subtype
belongs.
.It Cm dir Ar dir
True if the IEEE 802.11 frame direction matches the specified
.Cm dir .
Valid directions are:
.Cm nods ,
.Cm tods ,
.Cm fromds ,
.Cm dstods ,
or a numeric value.
.It Cm vlan Op Ar vlan_id
True if the packet is an IEEE 802.1Q VLAN packet.
If
.Ar vlan_id
is specified, only true if the packet has the specified ID.
Note that the first
.Cm vlan
keyword encountered in
.Ar expression
changes the decoding offsets for the remainder of
.Ar expression
on the assumption that the packet is a VLAN packet.
This expression may be used more than once, to filter on VLAN hierarchies.
Each use of that expression increments the filter offsets by 4.
.Pp
For example,
to filter on VLAN 200 encapsulated within VLAN 100:
.Pp
.Dl vlan 100 && vlan 200
.Pp
To filter IPv4 protocols encapsulated in VLAN 300 encapsulated within any
higher order VLAN:
.Pp
.Dl vlan && vlan 300 && ip
.It Cm mpls Op Ar label
True if the packet is an MPLS (Multi-Protocol Label Switching) packet.
If
.Ar label
is specified, only true if the packet has the specified label.
Note that the first
.Cm mpls
keyword encountered in
.Ar expression
changes the decoding offsets for the remainder of
.Ar expression
on the assumption that the packet is an MPLS packet.
This expression may be used more than once, to filter on MPLS labels.
Each use of that expression increments the filter offsets by 4.
.Pp
For example,
to filter on MPLS label 42 first and requires the next label to be 12:
.Pp
.Dl mpls 42 && mpls 12
.Pp
To filter on network 192.0.2.0/24 transported inside packets with label 42:
.Pp
.Dl mpls 42 && net 192.0.2.0/24
.It Ar expr relop expr
True if the relation holds, where
.Ar relop
is one of
.Sq > ,
.Sq < ,
.Sq >= ,
.Sq <= ,
.Sq = ,
.Sq != ,
and
.Ar expr
is an arithmetic expression composed of integer constants
(expressed in standard C syntax), the normal binary operators
.Pf ( Sq + ,
.Sq - ,
.Sq * ,
.Sq / ,
.Sq & ,
.Sq | ,
.Sq << ,
.Sq >> ) ,
a length operator, a random operator, and special packet data accessors.
Note that all comparisons are unsigned, so that, for example,
0x80000000 and 0xffffffff are > 0.
To access data inside the packet, use the following syntax:
.Pp
.D1 Ar proto Ns Op Ar expr : Ns Ar size
.Pp
.Ar proto
is one of
.Cm ether ,
.Cm fddi ,
.Cm tr ,
.Cm wlan ,
.Cm ppp ,
.Cm slip ,
.Cm link ,
.Cm ip ,
.Cm arp ,
.Cm rarp ,
.Cm tcp ,
.Cm udp ,
.Cm icmp ,
.Cm ip6 ,
or
.Cm radio ,
and indicates the protocol layer for the index operation
.Pf ( Cm ether ,
.Cm fddi ,
.Cm wlan ,
.Cm tr ,
.Cm ppp ,
.Cm slip ,
and
.Cm link
all refer to the link layer;
.Cm radio
refers to the "radio header" added to some 802.11 captures).
Note that
.Cm tcp ,
.Cm udp ,
and other upper-layer protocol types only apply to IPv4, not IPv6
(this will be fixed in the future).
The byte offset, relative to the indicated protocol layer, is given by
.Ar expr .
.Ar size
is optional and indicates the number of bytes in the field of interest;
it can be either one, two, or four, and defaults to one.
The length operator, indicated by the keyword
.Cm len ,
gives the length of the packet.
The random operator, indicated by the keyword
.Cm random ,
generates a random number.
.Pp
For example,
.Dq ether[0] & 1 != 0
catches all multicast traffic.
The expression
.Dq ip[0] & 0xf != 5
catches all IPv4 packets with options.
The expression
.Dq ip[6:2] & 0x1fff = 0
catches only unfragmented IPv4 datagrams and frag zero of fragmented
IPv4 datagrams.
This check is implicitly applied to the
.Cm tcp
and
.Cm udp
index operations.
For instance,
.Dq tcp[0]
always means the first byte of the TCP header,
and never means the first byte of an intervening fragment.
.Pp
Some offsets and field values may be expressed as names rather than
as numeric values.
The following protocol header field offsets are available:
.Cm icmptype
(ICMP type field),
.Cm icmpcode
(ICMP code field), and
.Cm tcpflags
(TCP flags field).
.Pp
The following ICMP type field values are available:
.Cm icmp-echoreply ,
.Cm icmp-unreach ,
.Cm icmp-sourcequench ,
.Cm icmp-redirect ,
.Cm icmp-echo ,
.Cm icmp-routeradvert ,
.Cm icmp-routersolicit ,
.Cm icmp-timxceed ,
.Cm icmp-paramprob ,
.Cm icmp-tstamp ,
.Cm icmp-tstampreply ,
.Cm icmp-ireq ,
.Cm icmp-ireqreply ,
.Cm icmp-maskreq ,
.Cm and
.Cm icmp-maskreply .
.Pp
The following TCP flags field values are available:
.Cm tcp-fin ,
.Cm tcp-syn ,
.Cm tcp-rst ,
.Cm tcp-push ,
.Cm tcp-ack ,
.Cm tcp-urg .
.El
.Pp
Primitives may be combined using
a parenthesized group of primitives and operators.
Parentheses are special to the shell and must be escaped.
Allowable primitives and operators are:
.Bd -ragged -offset indent
Negation
.Po
.Dq Cm \&!
or
.Dq Cm not
.Pc
.Pp
Concatenation
.Po
.Dq Cm &&
or
.Dq Cm and
.Pc
.Pp
Alternation
.Po
.Dq Cm ||
or
.Dq Cm or
.Pc
.Ed
.Pp
Negation has highest precedence.
Alternation and concatenation have equal precedence and associate
left to right.
Explicit
.Cm and
tokens, not juxtaposition,
are now required for concatenation.
.Pp
If an identifier is given without a keyword, the most recent keyword
is assumed.
For example,
For example,
.Bd -ragged -offset indent
.Cm not host
vs
.Cm and
ace
.Ed
.Pp
is short for
.Bd -ragged -offset indent
.Cm not host
vs
.Cm and host
ace
.Ed
.Pp
which should not be confused with
.Bd -ragged -offset indent
.Cm not
.Pq Cm host No vs Cm or No ace
.Ed
.Sh EXAMPLES
To print all packets arriving at or departing from sundown:
.Pp
.Dl # tcpdump host sundown
.Pp
To print traffic between helios and either hot or ace
(the expression is quoted to prevent the shell from misinterpreting
the parentheses):
.Pp
.Dl # tcpdump 'host helios and (hot or ace)'
.Pp
To print all IP packets between ace and any host except helios:
.Pp
.Dl # tcpdump ip host ace and not helios
.Pp
To print all traffic between local hosts and hosts at Berkeley:
.Pp
.Dl # tcpdump net ucb-ether
.Pp
To print all FTP traffic through internet gateway snup:
.Pp
.Dl # tcpdump 'gateway snup and (port ftp or ftp-data)'
.Pp
To print traffic neither sourced from nor destined for local network
192.168.7.0/24 (if you gateway to one other net, this stuff should
never make it onto your local network):
.Pp
.Dl # tcpdump ip and not net 192.168.7.0/24
.Pp
To print the start and end packets
.Pq the SYN and FIN packets
of each TCP connection that involves a host that is not in local
network 192.168.7.0/24:
.Bd -literal -offset indent
# tcpdump 'tcp[13] & 3 != 0 and not src and dst net 192.168.7.0/24'
.Ed
.Pp
To print only the SYN packets of HTTP connections:
.Pp
.Dl # tcpdump 'tcp[tcpflags] = tcp-syn and port http'
.Pp
To print IP packets longer than 576 bytes sent through gateway snup:
.Pp
.Dl # tcpdump 'gateway snup and ip[2:2] > 576'
.Pp
To print IP broadcast or multicast packets that were
.Em not
sent via Ethernet broadcast or multicast:
.Bd -literal -offset indent
# tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
.Ed
.Pp
To print all ICMP packets that are not echo requests/replies
.Pq i.e., not ping packets :
.Pp
.Dl # tcpdump 'icmp[0] != 8 and icmp[0] != 0'
.Pp
To print only echo request ICMP packets:
.Pp
.Dl # tcpdump 'icmp[icmptype] = icmp-echo'
.Pp
To print and decrypt all ESP packets with SPI 0x00001234:
.Pp
.Dl # tcpdump -E des3-hmac96:ab...def 'ip[20:4] = 0x00001234'
.Pp
To print raw wireless frames passing the iwn0 interface:
.Dl # tcpdump -i iwn0 -y IEEE802_11_RADIO -v
.Sh OUTPUT FORMAT
The output of
.Nm
is protocol dependent.
The following gives a brief description and examples of most of the formats.
.Ss Link Level Headers
If the
.Fl e
option is given, the link level header is printed out.
On Ethernets, the source and destination addresses, protocol,
and packet length are printed.
.Pp
On the packet filter logging interface
.Xr pflog 4 ,
logging reason
.Pq rule match, bad-offset, fragment, bad-timestamp, short, normalize, memory ,
action taken
.Pq pass/block ,
direction
.Pq in/out
and interface information are printed out for each packet.
.Pp
On FDDI networks, the
.Fl e
option causes
.Nm
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
.Pq such as those containing IP datagrams
are
.Dq async
packets, with a priority value between 0 and 7; for example,
.Sy async4 .
Such packets are assumed to contain an 802.2 Logical Link Control
.Pq LLC
packet; the LLC header is printed if it is
.Em not
an ISO datagram or a so-called SNAP packet.
.Pp
The following description assumes familiarity with the
SLIP compression algorithm described in RFC 1144.
.Pp
On SLIP links, a direction indicator
.Po
.Ql I
for inbound,
.Ql O
for outbound
.Pc ,
packet type, and compression information are printed out.
The packet type is printed first.
The three types are
.Cm ip ,
.Cm utcp ,
and
.Cm 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
.Cm *S+ Ns Ar n
and
.Cm *SA+ Ns Ar n ,
where
.Ar n
is the amount by which the sequence number
.Pq 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
.Sq U
.Pq urgent pointer ,
.Sq W
.Pq window ,
.Sq A
.Pq ack ,
.Sq S
.Pq sequence number ,
and
.Sq I
.Pq packet ID ,
followed by a delta
.Pq +n or -n ,
or a new value
.Pq =n .
Finally, the amount of data in the packet and compressed header length
are printed.
.Pp
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:
.Bd -ragged -offset indent
O
.Cm ctcp No *
.Cm A No +6
.Cm S No +49
.Cm I No +6 3
.Pq 6
.Ed
.Ss 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:
.Bd -literal -offset indent
arp who-has csam tell rtsg
arp reply csam is-at CSAM
.Ed
.Pp
In this example, Ethernet addresses are in caps and internet addresses
in lower case.
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 CSAM.
.Pp
This would look less redundant if we had done
.Nm
.Fl n :
.Bd -literal -offset indent
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
.Ed
.Pp
If we had done
.Nm
.Fl e ,
the fact that the first packet is
broadcast and the second is point-to-point would be visible:
.Bd -literal -offset indent
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
.Ed
.Pp
For the first packet this says the Ethernet source address is RTSG,
the destination is the Ethernet broadcast address,
the type field contained hex 0806 (type
.Dv ETHER_ARP )
and the total length was 64 bytes.
.Ss TCP Packets
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
.Nm
will be of much use to you.
.Pp
The general format of a TCP protocol line is:
.Bd -ragged -offset indent
.Ar src No > Ar dst :
.Ar flags src-os data-seqno ack window urgent options
.Ed
.Pp
.Ar src
and
.Ar dst
are the source and destination IP addresses and ports.
.Ar flags
is some combination of
.Sq S
.Pq SYN ,
.Sq F
.Pq FIN ,
.Sq P
.Pq PUSH ,
or
.Sq R
.Pq RST ,
.Sq W
.Pq congestion Window reduced ,
.Sq E
.Pq ecn ECHO
or a single
.Ql \&.
.Pq no flags .
.Ar src-os
will list a guess of the source host's operating system if the
.Fl o
command line flag was passed to
.Nm tcpdump .
.Ar data-seqno
describes the portion of sequence space covered
by the data in this packet
.Pq see example below .
.Ar ack
is the sequence number of the next data expected by the other
end of this connection.
.Ar window
is the number of bytes of receive buffer space available
at the other end of this connection.
.Ar urgent
indicates there is urgent data in the packet.
.Ar options
are TCP options enclosed in angle brackets e.g.,
<mss 1024>.
.Pp
.Ar src , dst
and
.Ar flags
are always present.
The other fields depend on the contents of the packet's TCP protocol header and
are output only if appropriate.
.Pp
Here is the opening portion of an rlogin from host rtsg to host csam.
.Bd -unfilled -offset 2n
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
.Ed
.Pp
The first line says that TCP port 1023 on rtsg sent a packet
to port login on host csam.
The
.Ql S
indicates that the SYN flag was set.
The packet sequence number was 768512 and it contained no data.
The notation is
.Sm off
.So
.Ar first : last
.Po Ar nbytes
.Pc
.Sc
.Sm on
which means sequence numbers
.Ar first
up to but not including
.Ar last
which is
.Ar nbytes
bytes of user data.
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.
.Pp
Csam replies with a similar packet except it includes a piggy-backed
ack for rtsg's SYN.
Rtsg then acks csam's SYN.
The
.Ql \&.
means no flags were set.
The packet contained no data so there is no data sequence number.
The ack sequence number is a 32-bit integer.
The first time
.Nm
sees a TCP connection, it prints the sequence number from the packet.
On subsequent packets of the connection, 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 connection's data stream
.Po
with the first data byte each direction being 1
.Pc .
.Fl S
will override this
feature, causing the original sequence numbers to be output.
.Pp
On the 6th line, rtsg sends csam 19 bytes of data
.Po
bytes 2 through 20
in the rtsg -> csam side of the connection
.Pc .
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.
.Ss UDP Packets
UDP format is illustrated by this rwho packet:
.Pp
.D1 actinide.who > broadcast.who: udp 84
.Pp
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.
.Pp
Some UDP services are recognized
.Pq from the source or destination port number
and the higher level protocol information printed.
In particular, Domain Name service requests
.Pq RFC 1034/1035
and Sun RPC calls
.Pq RFC 1050
to NFS.
.Ss UDP Name Server Requests
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.
.Pp
Name server requests are formatted as
.Bd -ragged -offset indent
.Ar src
>
.Ar dst :
.Ar id op Ns ?\&
.Ar flags qtype qclass name
.Pq Ar len
.Ed
.Pp
For example:
.Pp
.D1 h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
.Pp
Host h2opolo asked the domain server on helios for an address record
.Pq Ar qtype Ns =A
associated with the name
ucbvax.berkeley.edu.
The query
.Ar id
was 3.
The
.Ql +
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
.Pq Query
so the
.Ar op
field was omitted.
If
.Ar op
had been anything else, it would have been printed between the 3 and the
.Ql + .
Similarly, the
.Ar qclass
was the normal one
.Pq C_IN
and was omitted.
Any other
.Ar qclass
would have been printed immediately after the A.
.Pp
A few anomalies are checked and may result in extra fields enclosed in
square brackets: if a query contains an answer, name server or
authority section,
.Ar ancount ,
.Ar nscount ,
or
.Ar arcount
are printed as
.Dq Bq Ar n Ns a ,
.Dq Bq Ar n Ns n ,
or
.Dq Bq Ar n Ns au
where
.Ar n
is the appropriate count.
If any of the response bits are set
.Po
AA, RA or rcode
.Pc
or any of the
.Dq must be zero
bits are set in bytes two and three,
.Dq Bq b2&3= Ns Ar x
is printed, where
.Ar x
is the hex value of header bytes two and three.
.Ss UDP Name Server Responses
Name server responses are formatted as
.Bd -ragged -offset indent
.Ar src No > Ar dst :
.Ar id op rcode flags
.Ar a
/
.Ar n
/
.Ar au
.Ar type class data
.Pq Ar len
.Ed
.Pp
For example:
.Bd -unfilled -offset indent
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
.Ed
.Pp
In the first example, helios responds to query
.Ar id
3 from h2opolo
with 3 answer records, 3 name server records and 7 authority records.
The first answer record is type A
.Pq 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
.Ar op
.Pq Query
and
.Ar rcode
.Pq NoError
were omitted, as was the
.Ar class
.Pq C_IN
of the A record.
.Pp
In the second example, helios responds to query
.Ar op
2 with an
.Ar rcode
of non-existent domain
.Pq NXDomain
with no answers,
one name server and no authority records.
The
.Ql *
indicates that the authoritative answer bit was set.
Since there were no answers, no
.Ar type ,
.Ar class
or
.Ar data
were printed.
.Pp
Other flag characters that might appear are
.Sq -
(recursion available, RA,
.Em not
set)
and
.Sq |
.Pq truncated message, TC, set .
If the question section doesn't contain exactly one entry,
.Dq Bq Ar n Ns q
is printed.
.Pp
Name server requests and responses tend to be large and the default
.Ar snaplen
of 96 bytes may not capture enough of the packet to print.
Use the
.Fl s
flag to increase the
.Ar snaplen
if you need to seriously investigate name server traffic.
.Dq Fl s No 128
has worked well for me.
.Ss NFS Requests and Replies
Sun NFS
.Pq Network File System
requests and replies are printed as:
.Bd -ragged -offset indent
.Ar src . Ns Ar xid
>
.Ar dst . Ns nfs :
.Ar len op args
.Pp
.Ar src . Ns nfs
>
.Ar dst . Ns Ar xid :
reply
.Ar stat len op results
.Ed
.Bd -unfilled -offset indent
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
.Ed
.Pp
In the first line, host sushi sends a transaction with ID 6709 to wrl.
The number following the src host is a transaction ID,
.Em not
the source port.
The request was 112 bytes, excluding the UDP and IP headers.
The
.Ar op
was a readlink
.Pq read symbolic link
on fh
.Pq Dq file handle
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 with a
.Ar stat
of ok and the contents of the link.
.Pp
In the third line, sushi asks wrl to look up the name
.Dq xcolors
in directory file 9,74/4096.6878.
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.
.Pp
If the
.Fl v
.Pq verbose
flag is given, additional information is printed.
For example:
.Bd -unfilled -offset indent
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
.Ed
.Pp
.Fl v
also prints the IP header TTL, ID, 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 with a
.Ar stat of
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
.Fl v
flag is given, some of the file attributes
.Po
which are returned in addition to the file data
.Pc
are printed: the file type
.Pq So REG Sc , No for regular file ,
the file mode
.Pq in octal ,
the UID and GID, and the file size.
.Pp
If the
.Fl v
flag is given more than once, even more details are printed.
.Pp
NFS requests are very large and much of the detail won't be printed unless
.Ar snaplen
is increased.
Try using
.Dq Fl s No 192
to watch NFS traffic.
.Pp
NFS reply packets do not explicitly identify the RPC operation.
Instead,
.Nm
keeps track of
.Dq recent
requests, and matches them to the replies using the
.Ar xid
.Pq transaction ID .
If a reply does not closely follow the corresponding request,
it might not be parsable.
.Ss IP Fragmentation
Fragmented Internet datagrams are printed as
.Bd -ragged -offset indent
.Po
.Cm frag Ar id
:
.Ar size
@
.Ar offset
.Op +
.Pc
.Ed
.Pp
A
.Ql +
indicates there are more fragments.
The last fragment will have no
.Ql + .
.Pp
.Ar id
is the fragment ID.
.Ar size
is the fragment size
.Pq in bytes
excluding the IP header.
.Ar offset
is this fragment's offset
.Pq in bytes
in the original datagram.
.Pp
The fragment information is output for each fragment.
The first fragment contains the higher level protocol header and the fragment
info is printed after the protocol info.
Fragments after the first contain no higher level protocol header and the
fragment 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:
.Bd -unfilled -offset indent
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
.Ed
.Pp
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
.Po
308 in the first frag and 204 in the second
.Pc .
If you are looking for holes in the sequence space or trying to match up acks
with packets, this can fool you.
.Pp
A packet with the IP
.Sy don't fragment
flag is marked with a trailing
.Dq Pq DF .
.Ss Timestamps
By default, all output lines are preceded by a timestamp.
The timestamp is the current clock time in the form
.Sm off
.Ar hh : mm : ss . frac
.Sm on
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
.Dq new packet
interrupt.
.Ss IP and Protocol Checksum Offload
Some network cards support IP and/or protocol checksum offload.
Packet headers for such interfaces erroneously indicate a bad checksum,
since the checksum is not calculated until after
.Nm
sees the packet.
.Sh SEE ALSO
.\" traffic(1C), nit(4P),
.Xr ether_aton 3 ,
.Xr pcap_open_live 3 ,
.Xr bpf 4 ,
.Xr ip 4 ,
.Xr pf 4 ,
.Xr pflog 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr hosts 5 ,
.Xr pcap-filter 5 ,
.Xr pf.os 5 ,
.Xr protocols 5 ,
.Xr services 5
.Sh STANDARDS
.Rs
.%D September 1981
.%R RFC 793
.%T Transmission Control Protocol
.Re
.Pp
.Rs
.%A P. Mockapetris
.%D November 1987
.%R RFC 1034
.%T Domain Names \(en Concepts and Facilities
.Re
.Pp
.Rs
.%A P. Mockapetris
.%D November 1987
.%R RFC 1035
.%T Domain Names \(en Implementation and Specification
.Re
.Pp
.Rs
.%D April 1988
.%R RFC 1050
.%T RPC: Remote Procedure Call Protocol Specification
.Re
.Pp
.Rs
.%A V. Jacobson
.%D February 1990
.%R RFC 1144
.%T Compressing TCP/IP Headers for Low-Speed Serial Links
.Re
.Pp
.Rs
.%A M. Mathis
.%A J. Mahdavi
.%A S. Floyd
.%A A. Romanow
.%D October 1996
.%R RFC 2018
.%T TCP Selective Acknowledgement Options
.Re
.Pp
.Rs
.%A V. Manral
.%D April 2007
.%R RFC 4835
.%T Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)
.Re
.Sh AUTHORS
.An -nosplit
.An Van Jacobson Aq Mt van@ee.lbl.gov ,
.An Craig Leres Aq Mt leres@ee.lbl.gov ,
and
.An Steven McCanne Aq Mt mccanne@ee.lbl.gov ,
all of the Lawrence Berkeley Laboratory, University of California, Berkeley, CA.
.Sh BUGS
Some attempt should be made to reassemble IP fragments,
or at least to compute the right length for the higher level protocol.
.Pp
Name server inverse queries are not dumped correctly: The
.Pq empty
question section is printed rather than the 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
.Nm tcpdump .
.Pp
A packet trace that crosses a daylight saving time change will give
skewed time stamps
.Pq the time change is ignored .
.Pp
Filter expressions that manipulate FDDI headers assume that all FDDI packets
are encapsulated Ethernet packets.
This is true for IP, ARP, and DECNET Phase IV,
but is not true for protocols such as ISO CLNS.
Therefore, the filter may inadvertently accept certain packets that
do not properly match the filter expression.