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PCAP(3) OpenBSD Programmer's Manual PCAP(3)
pcap - Packet Capture library
pcap_open_live(char *device, int snaplen, int promisc, int to_ms,
pcap_open_offline(char *fname, char *errbuf);
pcap_dump_open(pcap_t *p, char *fname);
pcap_lookupnet(char *device, bpf_u_int32 *netp, bpf_u_int32 *maskp,
pcap_dispatch(pcap_t *p, int cnt, pcap_handler callback, u_char *user);
pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user);
pcap_dump(u_char *user, struct pcap_pkthdr *h, u_char *sp);
pcap_inject(pcap_t *p, void *, size_t);
pcap_compile(pcap_t *p, struct bpf_program *fp, char *str, int optimize,
pcap_setfilter(pcap_t *p, struct bpf_program *fp);
pcap_next(pcap_t *p, struct pcap_pkthdr *h);
pcap_stats(pcap_t *p, struct pcap_stat *ps);
pcap_perror(pcap_t *p, char *prefix);
pcap provides a high level interface to packet capture systems. All pack-
ets 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_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. 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.) to_ms specifies the
read timeout in milliseconds. errbuf is used to return error text and is
only set when pcap_open_live() fails and returns NULL.
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). The name `-' is 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
`-' is a synonym for stdin. 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_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 associ-
ated 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 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. A cnt of -1
processes all the packets received in one buffer. A cnt of 0 processes
all packets until an error occurs, EOF is reached, or the read times out
(when doing live reads and a non-zero read timeout is specified).
callback specifies 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 network headers and data),
and a u_char pointer to the packet data. The number of packets read is
returned. Zero is returned when EOF is reached 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.
pcap_dump() outputs a packet to the savefile opened with
pcap_dump_open(). Note that its calling arguments are suitable for use
pcap_inject() uses write(2) to inject a raw packet through the network
pcap_compile() is used to compile the string str into a filter program.
fp is a pointer to a bpf_program struct and is filled in by
pcap_compile(). optimize controls whether optimization on the resulting
code is performed. netmask specifies the netmask of the local net.
pcap_setfilter() is used to specify a filter program. fp is a pointer to
an array of bpf_program struct, usually the result of a call to
pcap_compile(). -1 is returned on failure; 0 is returned on success.
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 time-
out to pcap_open_live() and then calling pcap_dispatch() allows the re-
ception and processing of any packets that arrive when the timeout oc-
curs. A negative cnt causes pcap_loop() to loop forever (or at least un-
til an error occurs).
pcap_next() returns a u_char pointer to the next packet.
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 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 name of the savefile.
pcap_stats() returns 0 and fills in a pcap_stat struct. The values repre-
sent 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().
pcap_fileno() returns the file descriptor number of the 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
pcap_strerror() is provided in case strerror(1) isn't available.
pcap_close() closes the files associated with p and deallocates re-
pcap_dump_close() closes the savefile.
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berke-
ley National Laboratory, University of California, Berkeley, CA.
Please send bug reports to firstname.lastname@example.org.
OpenBSD 2.6 July 5, 1999 4
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for pcap(3) functions
Up to: Communication Debugging - Status, tracing, and debugging communications and protocols.
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