pcap - Packet Capture library
SYNOPSIS
#include <pcap.h>
pcap_t *pcap_open_live(char *device, int snaplen,
int promisc, int to_ms, char *ebuf)
pcap_t *pcap_open_dead(int linktype, int snaplen)
pcap_t *pcap_open_offline(char *fname, char *ebuf)
pcap_dumper_t *pcap_dump_open(pcap_t *p, char *fname)
char errbuf[PCAP_ERRBUF_SIZE];
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.
NOTE: errbuf in pcap_open_live(), pcap_open_offline(),
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 network 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 capture. promisc specifies if the interface is to be
put into promiscuous mode. (Note that even if this param
eter 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 timeout in milliseconds. The read timeout is
used to arrange that the read not necessarily return imme
diately 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. ebuf is used to
return error text and is only set when pcap_open_live()
fails and returns NULL.
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.
ebuf 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_lookupdev() returns a pointer to a network device
suitable for use with pcap_open_live() and pcap_lookup
net(). 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 associated with the network device device. Both
netp and maskp are bpf_u_int32 pointers. A return of -1
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 pro
cessed. A 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''. callback speci
fies a routine to be called with three arguments: a u_char
pointer which is passed in from pcap_dispatch(), a pointer
to the pcap_pkthdr struct (which precede the actual net
work headers and data), and a 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 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 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_dis
patch() will return after the timeout expires.
pcap_loop() is similar to pcap_dispatch() except it keeps
reading packets 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).
pcap_next() returns a u_char pointer to the next packet.
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().
filter program. program is a pointer to a bpf_program
struct and is filled in by pcap_compile(). optimize con
trols whether optimization on the resulting code is per
formed. 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_compile_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_compile(). -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_com
pile() 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, e.g.
DLT_EN10MB.
pcap_snapshot() returns the snapshot length specified when
pcap_open_live was called.
pcap_is_swapped() returns true if the current ``savefile''
uses a different byte order than the current system.
pcap_major_version() returns the major number of the ver
sion of the pcap used to write the savefile.
pcap_minor_version() returns the minor number of the ver
sion of the pcap used to write the savefile.
pcap_file() returns the name of the ``savefile.''
int pcap_stats() returns 0 and fills in a 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 pcap_perror() or pcap_geterr().
``savefile.''
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 deal
locates 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 Cali
fornia, 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 enhance
ments, etc. to:
tcpdump-workers@tcpdump.org
Please send source code contributions, etc. to:
patches@tcpdump.org
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