pcap

PCAP(3)                     Library Functions Manual                     PCAP(3)



NAME
       pcap - Packet Capture library

SYNOPSIS
       #include <pcap.h>

       char errbuf[PCAP_ERRBUF_SIZE];

       pcap_t *pcap_open_live(const 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(const char *fname, char *errbuf)
       pcap_t *pcap_fopen_offline(FILE *fp, char *errbuf)
       pcap_dumper_t *pcap_dump_open(pcap_t *p, const char *fname)
       pcap_dumper_t *pcap_dump_fopen(pcap_t *p, FILE *fp)

       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 *alldevs)
       char *pcap_lookupdev(char *errbuf)
       int pcap_lookupnet(const char *device, bpf_u_int32 *netp,
               bpf_u_int32 *maskp, char *errbuf)

       typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h,
                                   const u_char *bytes);
       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,
               const char *str, int optimize, bpf_u_int32 netmask)
       int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
       void pcap_freecode(struct bpf_program *)
       int pcap_setdirection(pcap_t *p, pcap_direction_t d)

       const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
       int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
               const u_char **pkt_data)

       void pcap_breakloop(pcap_t *)

       int pcap_inject(pcap_t *p, const void *buf, size_t size)
       int pcap_sendpacket(pcap_t *p, const u_char *buf, int size)

       int pcap_datalink(pcap_t *p)
       int pcap_list_datalinks(pcap_t *p, int **dlt_buf);
       int pcap_set_datalink(pcap_t *p, int dlt);
       int pcap_datalink_name_to_val(const char *name);
       const char *pcap_datalink_val_to_name(int dlt);
       const char *pcap_datalink_val_to_description(int dlt);
       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)
       int pcap_get_selectable_fd(pcap_t *p);
       void pcap_perror(pcap_t *p, char *prefix)
       char *pcap_geterr(pcap_t *p)
       const char *pcap_strerror(int error)
       const char *pcap_lib_version(void)

       void pcap_close(pcap_t *p)
       int pcap_dump_flush(pcap_dumper_t *p)
       long pcap_dump_ftell(pcap_dumper_t *p)
       FILE *pcap_dump_file(pcap_dumper_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_fopen_offline(), pcap_setnonblock(), pcap_getnonblock(),
       pcap_findalldevs(), 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.
       If this value is less than the size of a packet that is captured, only
       the first snaplen bytes of that packet will be captured and provided as
       packet data.  A value of 65535 should be sufficient, on most if not all
       networks, to capture all the data available from the packet.  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 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.  A zero value for to_ms, on platforms that support a read
       timeout, will cause a read to wait forever to allow enough packets to
       arrive, with no timeout.  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 warning 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(1).  The name "-" in a synonym for
       stdin.  Alternatively, you may call pcap_fopen_offline() to read dumped
       data from an existing open stream fp.  Note that on Windows, that stream
       should be opened in binary mode.  errbuf is used to return error text and
       is only set when pcap_open_offline() or pcap_fopen_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. Alternatively, you may call
       pcap_dump_fopen() to write data to an existing open stream fp.  Note that
       on Windows, that stream should be opened in binary mode.  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-readable
                     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

       Note that not all the addresses in the list of addresses are necessarily
       IPv4 or IPv6 addresses - you must check the sa_family member of the
       struct sockaddr before interpreting the contents of the address.

       -1 is returned on failure, in which case errbuf is filled in with an
       appropriate error message; 0 is returned on success.

       pcap_freealldevs() is used to free a list allocated by
       pcap_findalldevs().

       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
       associated 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 reading a
       live capture, or all the packets in the file when reading a ``savefile''.
       callback specifies a pcap_handler 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
       members:

              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 first caplen (as given in the struct
       pcap_pkthdr a pointer to which is passed to the callback routine) bytes
       of data from the packet (which won't necessarily be the entire packet; to
       capture the entire packet, you will have to provide a value for snaplen
       in your call to pcap_open_live() that is sufficiently large to get all of
       the packet's data - a value of 65535 should be sufficient on most if not
       all networks).

       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.  A return of -2 indicates that the loop
       terminated due to a call to pcap_breakloop() before any packets were
       processed.  If your application uses pcap_breakloop(), make sure that you
       explicitly check for -1 and -2, rather than just checking for a return
       value < 0.

       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 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).  -1 is returned on an error; 0 is returned if cnt
       is exhausted; -2 is returned if the loop terminated due to a call to
       pcap_breakloop() before any packets were processed.  If your application
       uses pcap_breakloop(), make sure that you explicitly check for -1 and -2,
       rather than just checking for a return value < 0.

       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.)  NULL is returned if
       an error occured, or 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.''  Unfortunately, there is no way to determine whether
       an error occured or not.

       pcap_next_ex() reads the next packet and returns a success/failure
       indication:

              1      the packet was read without problems

              0      packets are being read from a live capture, and the timeout
                     expired

              -1     an error occurred while reading the packet

              -2     packets are being read from a ``savefile'', and there are
                     no more packets to read from the savefile.

       If the packet was read without problems, the pointer pointed to by the
       pkt_header argument is set to point to the pcap_pkthdr struct for the
       packet, and the pointer pointed to by the pkt_data argument is set to
       point to the data in the packet.

       pcap_breakloop() sets a flag that will force pcap_dispatch() or
       pcap_loop() to return rather than looping; they will return the number of
       packets that have been processed so far, or -2 if no packets have been
       processed so far.

       This routine is safe to use inside a signal handler on UNIX or a console
       control handler on Windows, as it merely sets a flag that is checked
       within the loop.

       The flag is checked in loops reading packets from the OS - a signal by
       itself will not necessarily terminate those loops - as well as in loops
       processing a set of packets returned by the OS.  Note that if you are
       catching signals on UNIX systems that support restarting system calls
       after a signal, and calling pcap_breakloop() in the signal handler, you
       must specify, when catching those signals, that system calls should NOT
       be restarted by that signal.  Otherwise, if the signal interrupted a call
       reading packets in a live capture, when your signal handler returns after
       calling pcap_breakloop(), the call will be restarted, and the loop will
       not terminate until more packets arrive and the call completes.

       Note also that, in a multi-threaded application, if one thread is blocked
       in pcap_dispatch(), pcap_loop(), pcap_next(), or pcap_next_ex(), a call
       to pcap_breakloop() in a different thread will not unblock that thread;
       you will need to use whatever mechanism the OS provides for breaking a
       thread out of blocking calls in order to unblock the thread, such as
       thread cancellation in systems that support POSIX threads.

       Note that pcap_next() will, on some platforms, loop reading packets from
       the OS; that loop will not necessarily be terminated by a signal, so
       pcap_breakloop() should be used to terminate packet processing even if
       pcap_next() is being used.

       pcap_breakloop() does not guarantee that no further packets will be
       processed by pcap_dispatch() or pcap_loop() after it is called; at most
       one more packet might be processed.

       If -2 is returned from pcap_dispatch() or pcap_loop(), the flag is
       cleared, so a subsequent call will resume reading packets.  If a positive
       number is returned, the flag is not cleared, so a subsequent call will
       return -2 and clear the flag.

       pcap_inject() sends a raw packet through the network interface; buf
       points to the data of the packet, including the link-layer header, and
       size is the number of bytes in the packet.  It returns the number of
       bytes written on success.  A return of -1 indicates an error in which
       case pcap_perror() or pcap_geterr() may be used to display the error
       text.  Note that, even if you successfully open the network interface,
       you might not have permission to send packets on it, or it might not
       support sending packets; as pcap_open_live() doesn't have a flag to
       indicate whether to open for capturing, sending, or capturing and
       sending, you cannot request an open that supports sending and be notified
       at open time whether sending will be possible.  Note also that some
       devices might not support sending packets.

       Note that, on some platforms, the link-layer header of the packet that's
       sent might not be the same as the link-layer header of the packet
       supplied to pcap_inject(), as the source link-layer address, if the
       header contains such an address, might be changed to be the address
       assigned to the interface on which the packet it sent, if the platform
       doesn't support sending completely raw and unchanged packets.  Even
       worse, some drivers on some platforms might change the link-layer type
       field to whatever value libpcap used when attaching to the device, even
       on platforms that do nominally support sending completely raw and
       unchanged packets.

       pcap_sendpacket() is like pcap_inject(), but it returns 0 on success and
       -1 on failure.  (pcap_inject() comes from OpenBSD; pcap_sendpacket()
       comes from WinPcap.  Both are provided for compatibility.)

       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 resulting
       code is performed.  netmask specifies the IPv4 netmask of the network on
       which packets are being captured; it is used only when checking for IPv4
       broadcast addresses in the filter program.  If the netmask of the network
       on which packets are being captured isn't known to the program, or if
       packets are being captured on the Linux "any" pseudo-interface that can
       capture on more than one network, a value of 0 can be supplied; tests for
       IPv4 broadcast addreses won't be done correctly, but all other tests in
       the filter program will be OK.  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_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_setdirection() is used to specify a direction that packets will be
       captured.  pcap_direction_t is one of the constants PCAP_D_IN, PCAP_D_OUT
       or PCAP_D_INOUT.  PCAP_D_IN will only capture packets received by the
       device, PCAP_D_OUT will only capture packets sent by the device and
       PCAP_D_INOUT will capture packets received by or sent by the device.
       PCAP_D_INOUT is the default setting if this function is not called.  This
       isn't necessarily supported on all platforms; some platforms might return
       an error, and some other platforms might not support PCAP_D_OUT.  This
       operation is not supported if a ``savefile'' is being read.  -1 is
       returned on failure, 0 is returned on success.

       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 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 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
                                (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;

                           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_FRELAY
                 Frame Relay

            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.

            DLT_PFLOG
                 OpenBSD pflog; the link layer header contains, in order:

                      a 1-byte header length, in host byte order;

                      a 4-byte PF_ value, in host byte order;

                      a 2-byte action code, in network byte order, which is one
                      of:

                           0    passed

                           1    dropped

                           2    scrubbed

                      a 2-byte reason code, in network byte order, which is one
                      of:

                           0    match

                           1    bad offset

                           2    fragment

                           3    short

                           4    normalize

                           5    memory

                      a 16-character interface name;

                      a 16-character ruleset name (only meaningful if subrule is
                      set);

                      a 4-byte rule number, in network byte order;

                      a 4-byte subrule number, in network byte order;

                      a 1-byte direction, in network byte order, which is one
                      of:

                           0    incoming or outgoing

                           1    incoming

                           2    outgoing

            DLT_PRISM_HEADER
                 Prism monitor mode information followed by an 802.11 header.

            DLT_IP_OVER_FC
                 RFC 2625 IP-over-Fibre Channel, with the link-layer header
                 being the Network_Header as described in that RFC.

            DLT_SUNATM
                 SunATM devices; the link layer header contains, in order:

                      a 1-byte flag field, containing a direction flag in the
                      uppermost bit, which is set for packets transmitted by the
                      machine and clear for packets received by the machine, and
                      a 4-byte traffic type in the low-order 4 bits, which is
                      one of:

                           0    raw traffic

                           1    LANE traffic

                           2    LLC-encapsulated traffic

                           3    MARS traffic

                           4    IFMP traffic

                           5    ILMI traffic

                           6    Q.2931 traffic

                      a 1-byte VPI value;

                      a 2-byte VCI field, in network byte order.

            DLT_IEEE802_11_RADIO
                 link-layer information followed by an 802.11 header - see
                 http://www.shaftnet.org/~pizza/software/capturefrm.txt for a
                 description of the link-layer information.

            DLT_ARCNET_LINUX
                 ARCNET, with no exception frames, reassembled packets rather
                 than raw frames, and an extra 16-bit offset field between the
                 destination host and type bytes.

            DLT_LINUX_IRDA
                 Linux-IrDA packets, with a DLT_LINUX_SLL header followed by the
                 IrLAP header.

            DLT_LINUX_LAPD
                 LAPD (Q.921) frames, with a DLT_LINUX_SLL header captured via
                 vISDN.

       pcap_list_datalinks() is used to get a list of the supported data link
       types of the interface associated with the pcap descriptor.
       pcap_list_datalinks() allocates an array to hold the list and sets
       *dlt_buf.  The caller is responsible for freeing the array.  -1 is
       returned on failure; otherwise, the number of data link types in the
       array is returned.

       pcap_set_datalink() is used to set the current data link type of the pcap
       descriptor to the type specified by dlt.  -1 is returned on failure.

       pcap_datalink_name_to_val() translates a data link type name, which is a
       DLT_ name with the DLT_ removed, to the corresponding data link type
       value.  The translation is case-insensitive.  -1 is returned on failure.

       pcap_datalink_val_to_name() translates a data link type value to the
       corresponding data link type name.  NULL is returned on failure.

       pcap_datalink_val_to_description() translates a data link type value to a
       short description of that data link type.  NULL is returned on failure.

       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 file format of the
       savefile; pcap_minor_version() returns the minor number of the file
       format of the savefile.  The version number is stored in the header of
       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 the pcap_stat structure pointed to by
       its second argument.  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 pcap_perror() or
       pcap_geterr().  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''.

       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_get_selectable_fd() returns, on UNIX, a file descriptor number for a
       file descriptor on which one can do a select() or poll() to wait for it
       to be possible to read packets without blocking, if such a descriptor
       exists, or -1, if no such descriptor exists.  Some network devices opened
       with pcap_open_live() do not support select() or poll() (for example,
       regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices),
       so -1 is returned for those devices.

       Note that on most versions of most BSDs (including Mac OS X) select() and
       poll() do not work correctly on BPF devices; pcap_get_selectable_fd()
       will return a file descriptor on most of those versions (the exceptions
       being FreeBSD 4.3 and 4.4), a simple select() or poll() will not return
       even after a timeout specified in pcap_open_live() expires.  To work
       around this, an application that uses select() or poll() to wait for
       packets to arrive must put the pcap_t in non-blocking mode, and must
       arrange that the select() or poll() have a timeout less than or equal to
       the timeout specified in pcap_open_live(), and must try to read packets
       after that timeout expires, regardless of whether select() or poll()
       indicated that the file descriptor for the pcap_t is ready to be read or
       not.  (That workaround will not work in FreeBSD 4.3 and later; however,
       in FreeBSD 4.6 and later, select() and poll() work correctly on BPF
       devices, so the workaround isn't necessary, although it does no harm.)

       pcap_get_selectable_fd() is not available on Windows.

       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_lib_version() returns a pointer to a string giving information about
       the version of the libpcap library being used; note that it contains more
       information than just a version number.

       pcap_close() closes the files associated with p and deallocates
       resources.

       pcap_dump_file() returns the standard I/O stream of the ``savefile''
       opened by pcap_dump_open().

       pcap_dump_flush() flushes the output buffer to the ``savefile,'' so that
       any packets written with pcap_dump() but not yet written to the
       ``savefile'' will be written.  -1 is returned on error, 0 on success.

       pcap_dump_ftell() returns the current file position for the ``savefile'',
       representing the number of bytes written by pcap_dump_open() and
       pcap_dump().  -1 is returned on error.

       pcap_dump_close() closes the ``savefile.''

SEE ALSO
       tcpdump(8), tcpslice(1)

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



                                27 February 2004                         PCAP(3)