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.\"	$OpenBSD: pcap.3,v 1.25 2003/03/06 20:13:15 jmc Exp $
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.Dd July 5, 1999
.Dt PCAP 3
.Os
.Sh NAME
.Nm pcap
.Nd Packet Capture library
.Sh SYNOPSIS
.Fd #include <pcap.h>
.Ft "pcap_t *"
.Fn pcap_open_live "char *device" "int snaplen" "int promisc" "int to_ms" "char *errbuf"
.Ft "pcap_t *"
.Fn pcap_open_offline "char *fname" "char *errbuf"
.Ft "pcap_dumper_t *"
.Fn pcap_dump_open "pcap_t *p" "char *fname"
.Ft "char *"
.Fn pcap_lookupdev "char *errbuf"
.Ft uint
.Fn pcap_lookupnet "char *device" "bpf_u_int32 *netp" "bpf_u_int32 *maskp" "char *errbuf"
.Ft int
.Fn pcap_dispatch "pcap_t *p" "int cnt" "pcap_handler callback" "u_char *user"
.Ft int
.Fn pcap_loop "pcap_t *p" "int cnt" "pcap_handler callback" "u_char *user"
.Ft void
.Fn pcap_dump "u_char *user" "struct pcap_pkthdr *h" "u_char *sp"
.Ft int
.Fn pcap_inject "pcap_t *p" "void *, size_t"
.Ft int
.Fn pcap_compile "pcap_t *p" "struct bpf_program *fp" "char *str" "int optimize" "bpf_u_int32 netmask"
.Ft int
.Fn pcap_setfilter "pcap_t *p" "struct bpf_program *fp"
.Ft void
.Fn pcap_freecode "struct bpf_program *fp"
.Ft "u_char *"
.Fn pcap_next "pcap_t *p" "struct pcap_pkthdr *h"
.Ft int
.Fn pcap_datalink "pcap_t *p"
.Ft int
.Fn pcap_snapshot "pcap_t *p"
.Ft int
.Fn pcap_is_swapped "pcap_t *p"
.Ft int
.Fn pcap_major_version "pcap_t *p"
.Ft int
.Fn pcap_minor_version "pcap_t *p"
.Ft int
.Fn pcap_stats "pcap_t *p" "struct pcap_stat *ps"
.Ft "FILE *"
.Fn pcap_file "pcap_t *p"
.Ft int
.Fn pcap_fileno "pcap_t *p"
.Ft void
.Fn pcap_perror "pcap_t *p" "char *prefix"
.Ft "char *"
.Fn pcap_geterr "pcap_t *p"
.Ft "char *"
.Fn pcap_strerror "int error"
.Ft void
.Fn pcap_close "pcap_t *p"
.Ft void
.Fn pcap_dump_close "pcap_dumper_t *p"
.Sh DESCRIPTION
.Nm
provides a high level interface to packet capture systems.
All packets
on the network, even those destined for other hosts, are accessible
through this mechanism.
.Sh ROUTINES
.Sy Note:
.Fa errbuf
in
.Fn pcap_open_live ,
.Fn pcap_open_offline ,
.Fn pcap_lookupdev ,
and
.Fn pcap_lookupnet
is assumed to be able to hold at least
.Dv PCAP_ERRBUF_SIZE
chars.
.Pp
.Fn pcap_open_live
is used to obtain a packet capture descriptor to look
at packets on the network.
.Fa device
is a string that specifies the network device to open.
.Fa snaplen
specifies the maximum number of bytes to capture.
.Fa promisc
specifies if the interface is to be put into promiscuous mode.
(Note that even if this parameter is false, the interface
could well be in promiscuous mode for some other reason.)
.Fa to_ms
specifies the read timeout in milliseconds.
.Fa errbuf
is used to return error text and is only set when
.Fn pcap_open_live
fails and returns
.Dv NULL .
.Pp
.Fn pcap_open_offline
is called to open a
.Dq savefile
for reading.
.Fa fname
specifies the name of the file to open.
The file has the same format as those used by
.Xr tcpdump 8 .
.\" and
.\" .BR tcpslice(1) .
The name
.Ql -
is a synonym for
.Dv stdin .
.Fa errbuf
is used to return error text and is only set when
.Fn pcap_open_offline
fails and returns
.Dv NULL .
.Pp
.Fn pcap_dump_open
is called to open a
.Dq savefile
for writing. The name
.Ql -
is a synonym for
.Dv stdin .
.Dv NULL
is returned on failure.
.Fa p
is a
.Fa pcap
struct as returned by
.Fn pcap_open_offline
or
.Fn pcap_open_live .
.Fa fname
specifies the name of the file to open.
If
.Dv NULL
is returned,
.Fn pcap_geterr
can be used to get the error text.
.Pp
.Fn pcap_lookupdev
returns a pointer to a network device suitable for use with
.Fn pcap_open_live
and
.Fn pcap_lookupnet .
If there is an error,
.Dv NULL
is returned and
.Fa errbuf
is filled in with an appropriate error message.
.Pp
.Fn pcap_lookupnet
is used to determine the network number and mask
associated with the network device
.Fa device .
Both
.Fa netp
and
.Fa maskp
are
.Fa bpf_u_int32
pointers.
A return of \-1 indicates an error in which case
.Fa errbuf
is filled in with an appropriate error message.
.Pp
.Fn pcap_dispatch
is used to collect and process packets.
.Fa cnt
specifies the maximum number of packets to process before returning.
A
.Fa cnt
of \-1 processes all the packets received in one buffer.
A
.Fa cnt
of 0 processes all packets until an error occurs,
.Tn EOF
is reached, or the read times out (when doing live reads and a non-zero
read timeout is specified).
.Fa callback
specifies a routine to be called with three arguments: a
.Fa u_char
pointer which is passed in from
.Fn pcap_dispatch ,
a pointer to the
.Fa pcap_pkthdr
struct (which precede the actual network headers and data),
and a
.Fa u_char
pointer to the packet data.
The number of packets read is returned.
Zero is returned when
.Tn EOF
is reached in a savefile.
A return of \-1 indicates an error in which case
.Fn pcap_perror
or
.Fn pcap_geterr
may be used to display the error text.
.Pp
.Fn pcap_dump
outputs a packet to the savefile opened with
.Fn pcap_dump_open .
Note that its calling arguments are suitable for use with
.Fn pcap_dispatch .
.Pp
.Fn pcap_inject
uses
.Xr write 2
to inject a raw packet through the network interface.
.Pp
.Fn pcap_compile
is used to compile the string
.Fa str
into a filter program.
.Fa fp
is a pointer to a
.Fa bpf_program
struct and is filled in by
.Fn pcap_compile .
.Fa optimize
controls whether optimization on the resulting code is performed.
.Fa netmask
specifies the netmask of the local net.
.Pp
.Fn pcap_setfilter
is used to specify a filter program.
.Fa fp
is a pointer to an array of
.Fa bpf_program
struct, usually the result of a call to
.Fn pcap_compile .
\-1
is returned on failure;
0
is returned on success.
.Pp
.Fn pcap_freecode
is used to free up allocated memory pointed to by a
.Fa bpf_program
struct generated by
.Fn pcap_compile
when that BPF program is no longer needed, for example after it has
been made the filter program for a pcap structure by a call to
.Fn pcap_setfilter .
.Pp
.Fn pcap_loop
is similar to
.Fn pcap_dispatch
except it keeps reading packets until
.Fa cnt
packets are processed or an error occurs.
It does
.Em not
return when live read timeouts occur.
Rather, specifying a non-zero read timeout to
.Fn pcap_open_live
and then calling
.Fn pcap_dispatch
allows the reception and processing of any packets that arrive when the
timeout occurs.
A negative
.Fa cnt
causes
.Fn pcap_loop
to loop forever (or at least until an error occurs).
.Pp
.Fn pcap_next
returns a
.Fa u_char
pointer to the next packet.
.Pp
.Fn pcap_datalink
returns the link layer type, e.g.,
.Tn DLT_EN10MB .
.Pp
.Fn pcap_snapshot
returns the snapshot length specified when
.Fn pcap_open_live
was called.
.Pp
.Fn pcap_is_swapped
returns true if the current savefile
uses a different byte order than the current system.
.Pp
.Fn pcap_major_version
returns the major number of the version of the pcap used to write the savefile.
.Pp
.Fn pcap_minor_version
returns the minor number of the version of the pcap used to write the savefile.
.Pp
.Fn pcap_file
returns the stream associated with the savefile.
.Pp
.Fn pcap_stats
returns 0 and fills in a
.Fa pcap_stat
struct.
The values represent packet statistics from the start of the
run to the time of the call.
If there is an error or the underlying
packet capture doesn't support packet statistics, \-1 is returned and
the error text can be obtained with
.Fn pcap_perror
or
.Fn pcap_geterr .
.Pp
.Fn pcap_fileno
returns the file descriptor number of the savefile.
.Pp
.Fn pcap_perror
prints the text of the last pcap library error on
.Dv stderr ,
prefixed by
.Fa prefix .
.Pp
.Fn pcap_geterr
returns the error text pertaining to the last pcap library error.
.Pp
.Fn pcap_strerror
is provided in case
.Xr strerror 3
isn't available.
.Pp
.Fn pcap_close
closes the files associated with
.Fa p
and deallocates resources.
.Pp
.Fn pcap_dump_close
closes the savefile.
.Sh SEE ALSO
.Xr tcpdump 8
.\" , tcpslice(1)
.Sh AUTHORS
Van Jacobson,
Craig Leres and
Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.Sh BUGS
Please send bug reports to
.Pa libpcap@ee.lbl.gov .