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.Dd September 25, 2002
.Os
.Dt PCAP 3
.Sh NAME
.Nm pcap
.Nd packet capture library
.Sh LIBRARY
.Lb libpcap
.Sh SYNOPSIS
.In pcap.h
.Ft pcap_t *
.Fn pcap_open_live "char *device" "int snaplen" "int promisc" \
"int to_ms" "char *ebuf"
.Ft pcap_t *
.Fn pcap_open_offline "char *fname" "char *ebuf"
.Ft pcap_dumper_t *
.Fn pcap_dump_open "pcap_t *p" "char *fname"
.Ft char
.Dv errbuf[PCAP_ERRBUF_SIZE];
.Ft char *
.Fn pcap_lookupdev "char *errbuf"
.Ft int
.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_compile "pcap_t *p" "struct bpf_program *fp" \
"char *str" "int optimize" "bpf_u_int32 netmask"
.Ft int
.Fn pcap_compile_nopcap "int snaplen" "int linktype" \
"struct bpf_program *fp" "char *str" "int optimize" \
"bpf_uint32 netmask" "char *errbuf"
.Ft int
.Fn pcap_setfilter "pcap_t *p" "struct bpf_program *fp"
.Ft int
.Fn pcap_set_datalink "pcap_t *p" "int dlt"
.Ft int
.Fn pcap_list_datalinks "pcap_t *p" "int **dlt_buf"
.Ft const 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
The
.Nm
library 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
.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 ebuf
is used to return error text and is only set when
.Fn pcap_open_live
fails and returns
.Em 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 .
The name
.Dq \&-
is a synonym for
.Em stdin .
.Fa ebuf
is used to return error text and is only set when
.Fn pcap_open_offline
fails and returns
.Em NULL .
.Pp
.Fn pcap_dump_open
is called to open a
.Dq savefile
for writing.
The name
.Dq \&-
is a synonym
for
.Em stdout .
.Em 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
.Em 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,
.Em 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
.Em 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 (or
.Em EOF
is reached).
.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
.Em EOF
is reached in a
.Dq 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
.Dq savefile
opened with
.Fn pcap_dump_open .
Note that its calling arguments are suitable for use with
.Fn pcap_dispatch .
.Pp
.Fn pcap_compile
is used to compile the string
.Fa str
into a filter program.
.Fa program
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_compile_nopcap
is similar to
.Fn pcap_compile
except that instead of passing a pcap descriptor, one passes the
snaplen, linktype, and error buffer (which must be
.Em PCAP_ERRBUF_SIZE
in length) explicitly.
It is intended to be used for compiling filters for direct
bpf usage, without necessarily having called
.Fn pcap_open .
.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 .
.Em \-1
is returned on failure;
.Em 0
is returned on success.
.Pp
.Fn pcap_set_datalink
is used to set the current data link type of the pcap descriptor
to the type specified by
.Fa dlt .
This operation is supported only of the interface associated with
the pcap descriptor supports multiple data link types.
.Em \-1
is return on failure;
.Em 0
is returned on success.
.Pp
.Fn pcap_list_datalinks
is used to get a list of the support data link types of the interface
associated with the pcap descriptor.
.Fn pcap_list_datalinks
allocates an array to hold the list and sets
.Fa *dlt_buf .
The caller is responsible for freeing the array.
.Em \-1
is returned on failure;
otherwise, the number of data link types in the array is returned.
.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.
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.
.Em 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
.Dq 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 standard I/O stream of the
.Dq savefile ,
if a
.Dq savefile
was opened with
.Fn pcap_open_offline ,
or NULL, if a network device was opened with
.Fn pcap_open_live .
.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 under lying
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 from which captured packets are read
if a network device was opened with
.Fn pcap_open_live ,
or \-1, if a
.Dq savefile
was opened with
.Fn pcap_open_offline .
.Pp
.Fn pcap_perror
prints the text of the last pcap library error on
.Em stderr ,
prefixed by
.Em 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
.Dq savefile .
.Sh SEE ALSO
.Xr tcpdump 8
.Sh AUTHORS
The original authors are:
.Lp
Van Jacobson,
Craig Leres and
Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.\" .Lp
.\" The current version is available from "The Tcpdump Group"'s Web site at
.\" .Lp
.\" .RS
.\" .Em http://www.tcpdump.org/
.\" .RE
.\" .Sh BUGS
.\" Please send problems, bugs, questions, desirable enhancements, etc. to:
.\" .Lp
.\" .RS
.\" tcpdump-workers@tcpdump.org
.\" .RE
.\" .Lp
.\" Please send source code contributions, etc. to:
.\" .Lp
.\" .RS
.\" patches@tcpdump.org
.\" .RE