pcap
PCAP(3) PCAP(3)
NAME
pcap - Packet Capture library
SYNOPSIS
#include <pcap.h>
char errbuf[PCAP_ERRBUF_SIZE];
pcap_t *pcap_open_live(char *device, int snaplen,
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)
int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int pcap_getnonblock(pcap_t *p, char *errbuf);
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,
bpf_u_int32 *maskp, char *errbuf)
int pcap_dispatch(pcap_t *p, int cnt,
pcap_handler callback, u_char *user)
int pcap_loop(pcap_t *p, int cnt,
pcap_handler callback, u_char *user)
void pcap_dump(u_char *user, struct pcap_pkthdr *h,
u_char *sp)
int pcap_compile(pcap_t *p, struct bpf_program *fp,
char *str, int optimize, bpf_u_int32 netmask)
int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
void pcap_freecode(struct bpf_program *);
u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
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)
void pcap_close(pcap_t *p)
void pcap_dump_close(pcap_dumper_t *p)
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.
ROUTINES
NOTE: errbuf in pcap_open_live(), pcap_open_dead(),
pcap_open_offline(), pcap_setnonblock(), pcap_getnonblock(), pcap_find-
alldevs(), pcap_lookupdev(), and pcap_lookupnet() is assumed to be able
to hold at least PCAP_ERRBUF_SIZE chars.
pcap_open_live() is used to obtain a packet capture descriptor to look
at packets on the network. device is a string that specifies the net-
work device to open; on Linux systems with 2.2 or later kernels, a
device argument of "any" or NULL can be used to capture packets from
all interfaces. snaplen specifies the maximum number of bytes to cap-
ture. 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 promisc flag is ignored. to_ms specifies the read time-
out 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. errbuf is used to return error or warning text.
It will be set to error text when pcap_open_live() fails and returns
NULL. errbuf may also be set to warning text when pcap_open_live()
succeds; to detect this case the caller should store a zero-length
string in errbuf before calling pcap_open_live() and display the warn-
ing to the user if errbuf is no longer a zero-length string.
pcap_open_dead() is used for creating a pcap_t structure to use when
calling the other functions in libpcap. It is typically used when just
using libpcap for compiling BPF code.
pcap_open_offline() is called to open a ``savefile'' for reading.
fname specifies the name of the file to open. The file has the same
format as those used by tcpdump(8) and tcpslice(8). The name "-" in a
synonym for stdin. errbuf is used to return error text and is only set
when pcap_open_offline() fails and returns NULL.
pcap_dump_open() is called to open a ``savefile'' for writing. The name
"-" in a synonym for stdout. NULL is returned on failure. p is a pcap
struct as returned by pcap_open_offline() or pcap_open_live(). fname
specifies the name of the file to open. If NULL is returned,
pcap_geterr() can be used to get the error text.
pcap_setnonblock() puts a capture descriptor, opened with
pcap_open_live(), into ``non-blocking'' mode, or takes it out of ``non-
blocking'' mode, depending on whether the nonblock argument is non-zero
or zero. It has no effect on ``savefiles''. If there is an error, -1
is returned and 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 pcap_dispatch() will, if no packets
are currently available to be read, return 0 immediately rather than
blocking waiting for packets to arrive. pcap_loop() and pcap_next()
will not work in ``non-blocking'' mode.
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 errbuf is filled in with an appropriate
error message.
pcap_findalldevs() constructs a list of network devices that can be
opened with pcap_open_live(). (Note that there may be network devices
that cannot be opened with pcap_open_live() by the process calling
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.) alldevsp is set to point to the first
element of the list; each element of the list is of type pcap_if_t, and
has the following members:
next if not NULL, a pointer to the next element in the list;
NULL for the last element of the list
name a pointer to a string giving a name for the device to
pass to pcap_open_live()
description
if not NULL, a pointer to a string giving a human-read-
able description of the device
addresses
a pointer to the first element of a list of addresses for
the interface
flags interface flags:
PCAP_IF_LOOPBACK
set if the interface is a loopback interface
Each element of the list of addresses is of type pcap_addr_t, and has
the following members:
next if not NULL, a pointer to the next element in the list;
NULL for the last element of the list
addr a pointer to a struct sockaddr containing an address
netmask
if not NULL, a pointer to a struct sockaddr that contains
the netmask corresponding to the address pointed to by
addr
broadaddr
if not NULL, a pointer to a struct sockaddr that contains
the broadcast address corresponding to the address
pointed to by addr; may be null if the interface doesn't
support broadcasts
dstaddr
if not NULL, a pointer to a struct sockaddr that contains
the destination address corresponding to the address
pointed to by addr; may be null if the interface isn't a
point-to-point interface
pcap_freealldevs() is used to free a list allocated by pcap_find-
alldevs().
pcap_lookupdev() returns a pointer to a network device suitable for use
with pcap_open_live() and pcap_lookupnet(). If there is an error, NULL
is returned and errbuf is filled in with an appropriate error message.
pcap_lookupnet() is used to determine the network number and mask asso-
ciated with the network device device. Both netp and maskp are
bpf_u_int32 pointers. A return of -1 indicates an error in which case
errbuf is filled in with an appropriate error message.
pcap_dispatch() is used to collect and process packets. 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 cnt packets may be processed.
A cnt of -1 processes all the packets received in one buffer when read-
ing a live capture, or all the packets in the file when reading a
``savefile''. callback specifies a routine to be called with three
arguments: a u_char pointer which is passed in from pcap_dispatch(), a
const struct pcap_pkthdr pointer to a structure with the following mem-
bers:
ts a struct timeval containing the time when the packet was
captured
caplen a bpf_u_int32 giving the number of bytes of the packet
that are available from the capture
len a 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)
and a const u_char pointer to the packet data.
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 time-
out expires before any packets arrive, or if the file descriptor for
the capture device is in non-blocking mode and no packets were avail-
able to be read) or if no more packets are available in a ``savefile.''
A return of -1 indicates an error in which case pcap_perror() or
pcap_geterr() may be used to display the error text.
NOTE: when reading a live capture, 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
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 pcap_dispatch() will return after the timeout
expires.
pcap_loop() is similar to pcap_dispatch() except it keeps reading pack-
ets until cnt packets are processed or an error occurs. It does not
return when live read timeouts occur. Rather, specifying a non-zero
read timeout to pcap_open_live() and then calling pcap_dispatch()
allows the reception and processing of any packets that arrive when the
timeout occurs. A negative cnt causes pcap_loop() to loop forever (or
at least until an error occurs). A negative number is returned on an
error; 0 is returned if cnt is exhausted.
pcap_next() reads the next packet (by calling pcap_dispatch() with a
cnt of 1) and returns a u_char pointer to the data in that packet.
(The pcap_pkthdr struct for that packet is not supplied.)
pcap_dump() outputs a packet to the ``savefile'' opened with
pcap_dump_open(). Note that its calling arguments are suitable for use
with pcap_dispatch() or pcap_loop(). If called directly, the user
parameter is of type pcap_dumper_t as returned by pcap_dump_open().
pcap_compile() is used to compile the string str into a filter program.
program is a pointer to a bpf_program struct and is filled in by
pcap_compile(). optimize controls whether optimization on the result-
ing code is performed. netmask specifies the netmask of the local net.
A return of -1 indicates an error in which case pcap_geterr() may be
used to display the error text.
pcap_compile_nopcap() is similar to 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 pcap_open(). A return of
-1 indicates an error; the error text is unavailable. (pcap_com-
pile_nopcap() is a wrapper around pcap_open_dead(), pcap_compile(), and
pcap_close(); the latter three routines can be used directly in order
to get the error text for a compilation error.)
pcap_setfilter() is used to specify a filter program. fp is a pointer
to a bpf_program struct, usually the result of a call to pcap_com-
pile(). -1 is returned on failure, in which case pcap_geterr() may be
used to display the error text; 0 is returned on success.
pcap_freecode() is used to free up allocated memory pointed to by a
bpf_program struct generated by 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 pcap_setfilter().
pcap_datalink() returns the link layer type; link layer types it can
return include:
DLT_NULL
BSD loopback encapsulation; the link layer header is a 4-byte
field, in host byte order, containing a PF_ value from
socket.h for the network-layer protocol of the packet
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 cap-
tured; 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 not necessarily those of the machine
reading the capture file.
DLT_EN10MB
Ethernet (10Mb, 100Mb, 1000Mb, and up)
DLT_IEEE802
IEEE 802.5 Token Ring
DLT_ARCNET
ARCNET
DLT_SLIP
SLIP; the link layer header contains, in order:
a 1-byte flag, which is 0 for packets received by the
machine and 1 for packets sent by the machine;
a 1-byte field, the upper 4 bits of which indicate the
type of packet, as per RFC 1144:
0x40 an unmodified IP datagram (TYPE_IP);
0x70 an uncompressed-TCP IP datagram (UNCOM-
PRESSED_TCP), with that byte being the first
byte of the raw IP header on the wire, con-
taining the connection number in the protocol
field;
0x80 a compressed-TCP IP datagram (COMPRESSED_TCP),
with that byte being the first byte of the
compressed TCP/IP datagram header;
for UNCOMPRESSED_TCP, the rest of the modified IP
header, and for COMPRESSED_TCP, the compressed TCP/IP
datagram header;
for a total of 16 bytes; the uncompressed IP datagram follows
the header
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
DLT_FDDI
FDDI
DLT_ATM_RFC1483
RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an
IEEE 802.2 LLC header
DLT_RAW
raw IP; the packet begins with an IP header
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
DLT_PPP_ETHER
PPPoE; the packet begins with a PPPoE header, as per RFC 2516
DLT_C_HDLC
Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
DLT_IEEE802_11
IEEE 802.11 wireless LAN
DLT_LOOP
OpenBSD loopback encapsulation; the link layer header is a
4-byte field, in network byte order, containing a PF_ value
from OpenBSD's socket.h for the network-layer protocol of the
packet
Note that, if a ``savefile'' is being read, those PF_ values
are not necessarily those of the machine reading the capture
file.
DLT_LINUX_SLL
Linux "cooked" capture encapsulation; the link layer header
contains, in order:
a 2-byte "packet type", in network byte order, which is
one of:
0 packet was sent to us by somebody else
1 packet was broadcast by somebody else
2 packet was multicast, but not broadcast, by
somebody else
3 packet was sent by somebody else to somebody
else
4 packet was sent by us
a 2-byte field, in network byte order, containing a
Linux ARPHRD_ value for the link layer device type;
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);
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);
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.
DLT_LTALK
Apple LocalTalk; the packet begins with an AppleTalk LLAP
header
pcap_snapshot() returns the snapshot length specified when
pcap_open_live was called.
pcap_is_swapped() returns true if the current ``savefile'' uses a dif-
ferent byte order than the current system.
pcap_major_version() returns the major number of the version of the
pcap used to write the savefile.
pcap_minor_version() returns the minor number of the version of the
pcap used to write the savefile.
pcap_file() returns the standard I/O stream of the ``savefile,'' if a
``savefile'' was opened with pcap_open_offline(), or NULL, if a network
device was opened with pcap_open_live().
pcap_stats() returns 0 and fills in a pcap_stat struct. The values rep-
resent 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 pcap_perror() or pcap_geterr(). pcap_stats() is sup-
ported only on live captures, not on ``savefiles''; no statistics are
stored in ``savefiles'', so no statistics are available when reading
from a ``savefile''.
pcap_fileno() returns the file descriptor number from which captured
packets are read, if a network device was opened with pcap_open_live(),
or -1, if a ``savefile'' was opened with pcap_open_offline().
pcap_perror() prints the text of the last pcap library error on stderr,
prefixed by prefix.
pcap_geterr() returns the error text pertaining to the last pcap
library error. NOTE: the pointer it returns will no longer point to a
valid error message string after the pcap_t passed to it is closed; you
must use or copy the string before closing the pcap_t.
pcap_strerror() is provided in case strerror(1) isn't available.
pcap_close() closes the files associated with p and deallocates
resources.
pcap_dump_close() closes the ``savefile.''
SEE ALSO
tcpdump(8), tcpslice(8)
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
Berkeley National Laboratory, University of California, Berkeley, CA.
The current version is available from "The Tcpdump Group"'s Web site at
http://www.tcpdump.org/
BUGS
Please send problems, bugs, questions, desirable enhancements, etc. to:
tcpdump-workers@tcpdump.org
Please send source code contributions, etc. to:
patches@tcpdump.org
3 January 2001 PCAP(3)
