path: root/lib/libpcap/pcap-filter.5

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.Dd $Mdocdate: August 3 2020 $
.Dt PCAP-FILTER 5
.Os
.Sh NAME
.Nm pcap-filter
.Nd packet filter syntax
.Sh DESCRIPTION
.Xr pcap_compile 3
compiles pcap filters for software such as
.Xr tcpdump 8 .
The resulting filter program can then be applied to
some stream of packets to determine which packets will be supplied to
.Xr pcap_loop 3 ,
.Xr pcap_dispatch 3 ,
.Xr pcap_next 3 ,
or
.Xr pcap_next_ex 3 .
.Pp
The filter expression consists of one or more
.Em primitives .
Primitives usually consist of an ID (name or number)
preceded by one or more qualifiers.
There are three different kinds of qualifier:
.Bl -tag -width "proto"
.It type
Type qualifiers say what kind of thing the ID name or number refers to.
Possible types are
.Cm host ,
.Cm net ,
and
.Cm port .
For example,
.Dq host foo ,
.Dq net 128.3 ,
and
.Dq port 20 .
If there is no type qualifier,
.Cm host
is assumed.
.It dir
Dir qualifiers specify a particular transfer direction to and/or from an 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 .
For example,
.Cm src foo ,
.Cm dst net 128.3 ,
.Cm src or dst port ftp-data .
If there is no 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 some link layers, such as SLIP and the "cooked" Linux capture mode
used for the "any" device and for some other device types, the
.Cm inbound
and
.Cm outbound
qualifiers can be used to specify a desired direction.
.It proto
Proto qualifiers restrict the match to a particular protocol.
Possible
protos are:
.Cm ether ,
.Cm fddi ,
.Cm tr ,
.Cm wlan ,
.Cm ip ,
.Cm ip6 ,
.Cm arp ,
.Cm rarp ,
.Cm decnet ,
.Cm tcp ,
and
.Cm udp .
For example,
.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 proto qualifier,
all protocols consistent with the type are assumed.
For example,
.Dq src foo
means
.Dq (ip or arp or rarp) src foo
(except the latter is not legal syntax);
.Dq net bar
means
.Dq (ip or arp or rarp) net bar ;
and
.Dq port 53
means
.Dq (tcp or udp) port 53 .
.Pp
.Cm fddi
is actually an alias for
.Cm ether ;
the parser treats them identically as meaning
"the data link level used on the specified network interface".
FDDI 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 primitives 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.
For example,
.Dq host foo and not port ftp and not port ftp-data .
To save typing, identical qualifier lists can be omitted,
so that
.Dq tcp dst port ftp or ftp-data or domain
is exactly the same as
.Dq tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain .
.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 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 ,
which may be either a name from
.Pa /etc/ethers
or a number (see
.Xr ether_aton 3
for 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 host
True if the packet used
.Ar host
as a gateway.
That is,
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 ff:ff:ff:ff:ff:ff:ff:ff,
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 .
.It Cm less Ar length
True if the packet has a length less than or equal to
.Ar length .
This is equivalent to
.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
.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
.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 available,
this check will not work correctly.
.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 decnet ,
.Cm sca ,
.Cm lat ,
or
.Cm stp .
Note these identifiers are also keywords
and must be escaped using a backslash character
.Pq \e .
.Pp
In the case of FDDI (such as "fddi protocol arp")
and IEEE 802.11 wireless LANS (such as "wlan protocol arp"),
for most of those protocols
the protocol identification comes from
the 802.2 Logical Link Control (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 "10.123", or a DECNET hostname.
DECNET hostname support is only available on ULTRIX 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
(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.
The known codes are:
.Cm match ,
.Cm bad-offset ,
.Cm fragment ,
.Cm short ,
.Cm normalize ,
and
.Cm memory
(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
and, with later versions of
.Xr pf 4 ,
.Cm nat ,
.Cm rdr ,
.Cm binat
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.
Note that not all applications using
.Xr pcap_open_live 3
currently know how to parse these protocols.
.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 ,
and
.Cm data .
.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 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 Cm tcp , udp , icmp
Abbreviations for
.Cm ip proto Ar p
or
.Cm ip6 proto Ar p ,
where
.Ar p
is one of the above protocols.
.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
.Ar len ,
gives the length of the packet.
The random operator, indicated by the keyword
.Ar 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
.Ar 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.
.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.
Note that 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,
.Dq not host vs and ace
is short for
.Dq not host vs and host ace ,
which shouldn't be confused with
.Dq not (\& host vs or ace )\& .
.Sh EXAMPLES
To select all packets arriving at or departing from
.Dq sundown :
.Pp
.Dl host sundown
.Pp
To select traffic between
.Dq helios
and either
.Dq hot
or
.Dq ace :
.Pp
.Dl host helios and \e( hot or ace \e)
.Pp
To select all IP packets between
.Dq ace
and any host except
.Dq helios :
.Pp
.Dl ip host ace and not helios
.Pp
To select all traffic between local hosts and hosts at Berkeley:
.Pp
.Dl net ucb-ether
.Pp
To select all FTP traffic through internet gateway
.Dq snup :
.Pp
.Dl gateway snup and (port ftp or ftp-data)
.Pp
To select 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 net):
.Pp
.Dl ip and not net 192.168.7.0/24
.Pp
To select the start and end packets (the SYN and FIN packets) of each
TCP connection that involves a host not in local network 192.168.7.0/24:
.Bd -literal -offset indent
tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst \e
	net 192.168.7.0/24
.Ed
.Pp
To select all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data and not, for example, SYN and FIN packets and
ACK-only packets
(IPv6 is left as an exercise for the reader):
.Bd -literal -offset indent
tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) \e
	- ((tcp[12]&0xf0)>>2)) != 0)
.Ed
.Pp
To select IP packets longer than 576 bytes sent through gateway
.Dq snup :
.Pp
.Dl gateway snup and ip[2:2] > 576
.Pp
To select IP broadcast or multicast packets
that were not sent via Ethernet broadcast or multicast:
.Pp
.Dl ether[0] & 1 = 0 and ip[16] >= 224
.Pp
To select all ICMP packets that are not echo requests/replies (i.e. not ping packets):
.Pp
.Dl icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply
.Sh SEE ALSO
.Xr pcap_open_live 3 ,
.Xr tcpdump 8
.Sh AUTHORS
.An -nosplit
The original authors are
.An Van Jacobson ,
.An Craig Leres ,
and
.An Steven McCanne ,
all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.\" Fixes should be submitted to http://sourceforge.net/tracker/?group_id=53067