pcap - Packet Capture library

#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)

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_dead(), pcap_open_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. 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. 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(1) and tcpslice(1). 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-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

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 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 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_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). 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 resulting 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_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_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_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 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 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 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_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.''

tcpdump(1), tcpslice(1)

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/

Please send problems, bugs, questions, desirable enhancements, etc. to:

tcpdump-workers@tcpdump.org

Please send source code contributions, etc. to:

patches@tcpdump.org