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.TH PCAP 3 "3 January 2001"
.SH NAME
pcap \- Packet Capture library
.SH SYNOPSIS
.nf
.ft B
#include <pcap.h>
.ft
.LP
.nf
.ft B
char errbuf[PCAP_ERRBUF_SIZE];
.ft
.LP
.ft B
pcap_t *pcap_open_live(char *device, int snaplen,
.ti +8
int promisc, int to_ms, char *errbuf)
pcap_t *pcap_open_dead(int linktype, int snaplen)
pcap_t *pcap_open_offline(char *fname, char *errbuf)
pcap_dumper_t *pcap_dump_open(pcap_t *p, char *fname)
.ft
.LP
.ft B
int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int pcap_getnonblock(pcap_t *p, char *errbuf);
.ft
.LP
.ft B
int pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf)
void pcap_freealldevs(pcap_if_t *)
char *pcap_lookupdev(char *errbuf)
int pcap_lookupnet(char *device, bpf_u_int32 *netp,
.ti +8
bpf_u_int32 *maskp, char *errbuf)
.ft
.LP
.ft B
int pcap_dispatch(pcap_t *p, int cnt,
.ti +8
pcap_handler callback, u_char *user)
int pcap_loop(pcap_t *p, int cnt,
.ti +8
pcap_handler callback, u_char *user)
void pcap_dump(u_char *user, struct pcap_pkthdr *h,
.ti +8
u_char *sp)
.ft
.LP
.ft B
int pcap_compile(pcap_t *p, struct bpf_program *fp,
.ti +8
char *str, int optimize, bpf_u_int32 netmask)
int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
void pcap_freecode(struct bpf_program *);
.ft
.LP
.ft B
u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
.ft
.LP
.ft B
int pcap_datalink(pcap_t *p)
int pcap_snapshot(pcap_t *p)
int pcap_is_swapped(pcap_t *p)
int pcap_major_version(pcap_t *p)
int pcap_minor_version(pcap_t *p)
int pcap_stats(pcap_t *p, struct pcap_stat *ps)
FILE *pcap_file(pcap_t *p)
int pcap_fileno(pcap_t *p)
void pcap_perror(pcap_t *p, char *prefix)
char *pcap_geterr(pcap_t *p)
char *pcap_strerror(int error)
.ft
.LP
.ft B
void pcap_close(pcap_t *p)
void pcap_dump_close(pcap_dumper_t *p)
.ft
.fi
.SH DESCRIPTION
The Packet Capture 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.
.PP
.SH ROUTINES
NOTE:
.I errbuf
in
.BR pcap_open_live() ,
.BR pcap_open_dead() ,
.BR pcap_open_offline() ,
.BR pcap_setnonblock() ,
.BR pcap_getnonblock() ,
.BR pcap_findalldevs() ,
.BR pcap_lookupdev() ,
and
.B pcap_lookupnet()
is assumed to be able to hold at least
.B PCAP_ERRBUF_SIZE
chars.
.PP
.B pcap_open_live()
is used to obtain a packet capture descriptor to look
at packets on the network.
.I device
is a string that specifies the network device to open; on Linux systems
with 2.2 or later kernels, a
.I device
argument of "any" or
.B NULL
can be used to capture packets from all interfaces.
.I snaplen
specifies the maximum number of bytes to capture.
.I 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.)  For now, this
doesn't work on the "any" device; if an argument of "any" or NULL is
supplied, the
.I promisc
flag is ignored.
.I to_ms
specifies the read timeout in milliseconds.  The read timeout is used to
arrange that the read not necessarily return immediately when a packet
is seen, but that it wait for some amount of time to allow more packets
to arrive and to read multiple packets from the OS kernel in one
operation.  Not all platforms support a read timeout; on platforms that
don't, the read timeout is ignored.
.I errbuf
is used to return error or warning text.  It will be set to error text when
.B pcap_open_live()
fails and returns
.BR NULL .
.I errbuf
may also be set to warning text when
.B pcap_open_live()
succeds; to detect this case the caller should store a zero-length string in
.I errbuf
before calling
.B pcap_open_live()
and display the warning to the user if
.I errbuf
is no longer a zero-length string.
.PP
.B pcap_open_dead()
is used for creating a
.B pcap_t
structure to use when calling the other functions in libpcap.  It is
typically used when just using libpcap for compiling BPF code.
.PP
.B pcap_open_offline()
is called to open a ``savefile'' for reading.
.I fname
specifies the name of the file to open. The file has
the same format as those used by
.B tcpdump(1)
and
.BR tcpslice(1) .
The name "-" in a synonym for
.BR stdin .
.I errbuf
is used to return error text and is only set when
.B pcap_open_offline()
fails and returns
.BR NULL .
.PP
.B pcap_dump_open()
is called to open a ``savefile'' for writing. The name "-" in a synonym
for
.BR stdout .
.B NULL
is returned on failure.
.I p
is a
.I pcap
struct as returned by
.B pcap_open_offline()
or
.BR pcap_open_live() .
.I fname
specifies the name of the file to open.
If
.B NULL
is returned,
.B pcap_geterr()
can be used to get the error text.
.PP
.B pcap_setnonblock()
puts a capture descriptor, opened with
.BR pcap_open_live() ,
into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode,
depending on whether the
.I nonblock
argument is non-zero or zero.  It has no effect on ``savefiles''.
If there is an error, \-1 is returned and
.I errbuf
is filled in with an appropriate error message; otherwise, 0 is
returned.
In
``non-blocking'' mode, an attempt to read from the capture descriptor
with
.B pcap_dispatch()
will, if no packets are currently available to be read, return 0
immediately rather than blocking waiting for packets to arrive.
.B pcap_loop()
and
.B pcap_next()
will not work in ``non-blocking'' mode.
.PP
.B pcap_getnonblock()
returns the current ``non-blocking'' state of the capture descriptor; it
always returns 0 on ``savefiles''.
If there is an error, \-1 is returned and
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_findalldevs()
constructs a list of network devices that can be opened with
.BR pcap_open_live() .
(Note that there may be network devices that cannot be opened with
.BR pcap_open_live()
by the
process calling
.BR pcap_findalldevs() ,
because, for example, that process might not have sufficient privileges
to open them for capturing; if so, those devices will not appear on the
list.)
.I alldevsp
is set to point to the first element of the list; each element of the
list is of type
.BR pcap_if_t ,
and has the following members:
.RS
.TP
.B next
if not
.BR NULL ,
a pointer to the next element in the list;
.B NULL
for the last element of the list
.TP
.B name
a pointer to a string giving a name for the device to pass to
.B pcap_open_live()
.TP
.B description
if not
.BR NULL ,
a pointer to a string giving a human-readable description of the device
.TP
.B addresses
a pointer to the first element of a list of addresses for the interface
.TP
.B flags
interface flags:
.RS
.TP
.B PCAP_IF_LOOPBACK
set if the interface is a loopback interface
.RE
.RE
.PP
Each element of the list of addresses is of type
.BR pcap_addr_t ,
and has the following members:
.RS
.TP
.B next
if not
.BR NULL ,
a pointer to the next element in the list;
.B NULL
for the last element of the list
.TP
.B addr
a pointer to a
.B "struct sockaddr"
containing an address
.TP
.B netmask
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the netmask corresponding to the address pointed to by
.B addr
.TP
.B broadaddr
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the broadcast address corresponding to the address pointed
to by
.BR addr ;
may be null if the interface doesn't support broadcasts
.TP
.B dstaddr
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the destination address corresponding to the address pointed
to by
.BR addr ;
may be null if the interface isn't a point-to-point interface
.RE
.PP
.B pcap_freealldevs()
is used to free a list allocated by
.BR pcap_findalldevs() .
.PP
.B pcap_lookupdev()
returns a pointer to a network device suitable for use with
.B pcap_open_live()
and
.BR pcap_lookupnet() .
If there is an error,
.B NULL
is returned and
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_lookupnet()
is used to determine the network number and mask
associated with the network device
.BR device .
Both
.I netp
and
.I maskp
are
.I bpf_u_int32
pointers.
A return of \-1 indicates an error in which case
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_dispatch()
is used to collect and process packets.
.I cnt
specifies the maximum number of packets to process before returning.
This is not a minimum number; when reading a live capture, only one
bufferful of packets is read at a time, so fewer than
.I cnt
packets may be processed. A
.I cnt
of \-1 processes all the packets received in one buffer when reading a
live capture, or all the packets in the file when reading a
``savefile''.
.I callback
specifies a routine to be called with three arguments:
a
.I u_char
pointer which is passed in from
.BR pcap_dispatch() ,
a
.I const struct pcap_pkthdr
pointer to a structure with the following members:
.RS
.TP
.B ts
a
.I struct timeval
containing the time when the packet was captured
.TP
.B caplen
a
.I bpf_u_int32
giving the number of bytes of the packet that are available from the
capture
.TP
.B len
a
.I bpf_u_int32
giving the length of the packet, in bytes (which might be more than the
number of bytes available from the capture, if the length of the packet
is larger than the maximum number of bytes to capture)
.RE
.PP
and a
.I const u_char
pointer to the packet data.
.PP
The number of packets read is returned.
0 is returned if no packets were read from a live capture (if, for
example, they were discarded because they didn't pass the packet filter,
or if, on platforms that support a read timeout that starts before any
packets arrive, the timeout expires before any packets arrive, or if the
file descriptor for the capture device is in non-blocking mode and no
packets were available to be read) or if no more packets are available
in a ``savefile.'' A return of \-1 indicates
an error in which case
.B pcap_perror()
or
.B pcap_geterr()
may be used to display the error text.
.PP
.BR NOTE :
when reading a live capture,
.B pcap_dispatch()
will not necessarily return when the read times out; on some platforms,
the read timeout isn't supported, and, on other platforms, the timer
doesn't start until at least one packet arrives.  This means that the
read timeout should
.B NOT
be used in, for example, an interactive application, to allow the packet
capture loop to ``poll'' for user input periodically, as there's no
guarantee that
.B pcap_dispatch()
will return after the timeout expires.
.PP
.B pcap_loop()
is similar to
.B pcap_dispatch()
except it keeps reading packets until
.I cnt
packets are processed or an error occurs.
It does
.B not
return when live read timeouts occur.
Rather, specifying a non-zero read timeout to
.B pcap_open_live()
and then calling
.B pcap_dispatch()
allows the reception and processing of any packets that arrive when the
timeout occurs.
A negative
.I cnt
causes
.B pcap_loop()
to loop forever (or at least until an error occurs).  A negative number
is returned on an error; 0 is returned if
.I cnt
is exhausted.
.PP
.B pcap_next()
reads the next packet (by calling
.B pcap_dispatch()
with a
.I cnt
of 1) and returns a
.I u_char
pointer to the data in that packet.  (The
.I pcap_pkthdr
struct for that packet is not supplied.)
.PP
.B pcap_dump()
outputs a packet to the ``savefile'' opened with
.BR pcap_dump_open() .
Note that its calling arguments are suitable for use with
.B pcap_dispatch()
or
.BR pcap_loop() .
If called directly, the 
.I user
parameter is of type 
.I pcap_dumper_t
as returned by
.BR pcap_dump_open() .
.PP
.B pcap_compile()
is used to compile the string
.I str
into a filter program.
.I program
is a pointer to a
.I bpf_program
struct and is filled in by
.BR pcap_compile() .
.I optimize
controls whether optimization on the resulting code is performed.
.I netmask
specifies the netmask of the local net.
A return of \-1 indicates an error in which case
.BR pcap_geterr()
may be used to display the error text.
.PP
.B pcap_compile_nopcap()
is similar to
.B pcap_compile()
except that instead of passing a pcap structure, one passes the
snaplen and linktype explicitly.  It is intended to be used for
compiling filters for direct BPF usage, without necessarily having
called
.BR pcap_open() .
A return of \-1 indicates an error; the error text is unavailable.
.RB ( pcap_compile_nopcap()
is a wrapper around
.BR pcap_open_dead() ,
.BR pcap_compile() ,
and
.BR pcap_close() ;
the latter three routines can be used directly in order to get the error
text for a compilation error.)
.B
.PP
.B pcap_setfilter()
is used to specify a filter program.
.I fp
is a pointer to a
.I bpf_program
struct, usually the result of a call to
.BR pcap_compile() .
.B \-1
is returned on failure, in which case
.BR pcap_geterr()
may be used to display the error text;
.B 0
is returned on success.
.PP
.B pcap_freecode()
is used to free up allocated memory pointed to by a
.I bpf_program
struct generated by
.B 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
.BR pcap_setfilter() .
.PP
.B pcap_datalink()
returns the link layer type; link layer types it can return include:
.PP
.RS 5
.TP 5
.B DLT_NULL
BSD loopback encapsulation; the link layer header is a 4-byte field, in
.I host
byte order, containing a PF_ value from
.B socket.h
for the network-layer protocol of the packet
.IP
Note that ``host byte order'' is the byte order of the machine on which
the packets are captured, and the PF_ values are for the OS of the
machine on which the packets are captured; if a live capture is being
done, ``host byte order'' is the byte order of the machine capturing the
packets, and the PF_ values are those of the OS of the machine capturing
the packets, but if a ``savefile'' is being read, the byte order and PF_
values are
.I not
necessarily those of the machine reading the capture file.
.TP 5
.B DLT_EN10MB
Ethernet (10Mb, 100Mb, 1000Mb, and up)
.TP 5
.B DLT_IEEE802
IEEE 802.5 Token Ring
.TP 5
.B DLT_ARCNET
ARCNET
.TP 5
.B DLT_SLIP
SLIP; the link layer header contains, in order:
.RS 10
.LP
a 1-byte flag, which is 0 for packets received by the machine and 1 for
packets sent by the machine;
.LP
a 1-byte field, the upper 4 bits of which indicate the type of packet,
as per RFC 1144:
.RS 5
.TP 5
0x40
an unmodified IP datagram (TYPE_IP);
.TP 5
0x70
an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being
the first byte of the raw IP header on the wire, containing the
connection number in the protocol field;
.TP 5
0x80
a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the
first byte of the compressed TCP/IP datagram header;
.RE
.LP
for UNCOMPRESSED_TCP, the rest of the modified IP header, and for
COMPRESSED_TCP, the compressed TCP/IP datagram header;
.RE
.RS 5
.LP
for a total of 16 bytes; the uncompressed IP datagram follows the header
.RE
.TP 5
.B DLT_PPP
PPP; if the first 2 bytes are 0xff and 0x03, it's PPP in HDLC-like
framing, with the PPP header following those two bytes, otherwise it's
PPP without framing, and the packet begins with the PPP header
.TP 5
.B DLT_FDDI
FDDI
.TP 5
.B DLT_ATM_RFC1483
RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2
LLC header
.TP 5
.B DLT_RAW
raw IP; the packet begins with an IP header
.TP 5
.B DLT_PPP_SERIAL
PPP in HDLC-like framing, as per RFC 1662, or Cisco PPP with HDLC
framing, as per section 4.3.1 of RFC 1547; the first byte will be 0xFF
for PPP in HDLC-like framing, and will be 0x0F or 0x8F for Cisco PPP
with HDLC framing
.TP 5
.B DLT_PPP_ETHER
PPPoE; the packet begins with a PPPoE header, as per RFC 2516
.TP 5
.B DLT_C_HDLC
Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
.TP 5
.B DLT_IEEE802_11
IEEE 802.11 wireless LAN
.TP 5
.B DLT_LOOP
OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in
.I network
byte order, containing a PF_ value from OpenBSD's
.B socket.h
for the network-layer protocol of the packet
.IP
Note that, if a ``savefile'' is being read, those PF_ values are
.I not
necessarily those of the machine reading the capture file.
.TP 5
.B DLT_LINUX_SLL
Linux "cooked" capture encapsulation; the link layer header contains, in
order:
.RS 10
.LP
a 2-byte "packet type", in network byte order, which is one of:
.RS 5
.TP 5
0
packet was sent to us by somebody else
.TP 5
1
packet was broadcast by somebody else
.TP 5
2
packet was multicast, but not broadcast, by somebody else
.TP 5
3
packet was sent by somebody else to somebody else
.TP 5
4
packet was sent by us
.RE
.LP
a 2-byte field, in network byte order, containing a Linux ARPHRD_ value
for the link layer device type;
.LP
a 2-byte field, in network byte order, containing the length of the
link layer address of the sender of the packet (which could be 0);
.LP
an 8-byte field containing that number of bytes of the link layer header
(if there are more than 8 bytes, only the first 8 are present);
.LP
a 2-byte field containing an Ethernet protocol type, in network byte
order, or containing 1 for Novell 802.3 frames without an 802.2 LLC
header or 4 for frames beginning with an 802.2 LLC header.
.RE
.TP 5
.B DLT_LTALK
Apple LocalTalk; the packet begins with an AppleTalk LLAP header
.RE
.PP
.B pcap_snapshot()
returns the snapshot length specified when
.B pcap_open_live
was called.
.PP
.B pcap_is_swapped()
returns true if the current ``savefile'' uses a different byte order
than the current system.
.PP
.B pcap_major_version()
returns the major number of the version of the pcap used to write the
savefile.
.PP
.B pcap_minor_version()
returns the minor number of the version of the pcap used to write the
savefile.
.PP
.B pcap_file()
returns the standard I/O stream of the ``savefile,'' if a ``savefile''
was opened with
.BR pcap_open_offline() ,
or NULL, if a network device was opened with
.BR pcap_open_live() .
.PP
.B pcap_stats()
returns 0 and fills in a
.B 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
.B pcap_perror()
or
.BR pcap_geterr() .
.B pcap_stats()
is supported only on live captures, not on ``savefiles''; no statistics
are stored in ``savefiles'', so no statistics are available when reading
from a ``savefile''.
.PP
.B pcap_fileno()
returns the file descriptor number from which captured packets are read,
if a network device was opened with
.BR pcap_open_live() ,
or \-1, if a ``savefile'' was opened with
.BR pcap_open_offline() .
.PP
.B pcap_perror()
prints the text of the last pcap library error on
.BR stderr ,
prefixed by
.IR prefix .
.PP
.B pcap_geterr()
returns the error text pertaining to the last pcap library error.
.BR NOTE :
the pointer it returns will no longer point to a valid error message
string after the
.B pcap_t
passed to it is closed; you must use or copy the string before closing
the
.BR pcap_t .
.PP
.B pcap_strerror()
is provided in case
.BR strerror (1)
isn't available.
.PP
.B pcap_close()
closes the files associated with
.I p
and deallocates resources.
.PP
.B pcap_dump_close()
closes the ``savefile.''
.PP
.SH SEE ALSO
tcpdump(1), tcpslice(1)
.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
.I 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