PCAP(3)

NAME
pcap - Packet Capture library

SYNOPSIS
#include <pcap.h>
pcap_t *
pcap_open_live(char *device, int snaplen, int promisc, int to_ms, char *errbuf);
pcap_t *
pcap_open_offline(char *fname, char *errbuf);
pcap_dumper_t *
pcap_dump_open(pcap_t *p, char *fname);
pcap_dumper_t *
pcap_dump_fopen(pcap_t *p, FILE *f);
char *
pcap_lookupdev(char *errbuf);
uint
pcap_lookupnet(const 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_inject(pcap_t *p, void *, size_t);
int
pcap_sendpacket(pcap_t *p, void *, int);
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 *fp);
u_char *
pcap_next(pcap_t *p, struct pcap_pkthdr *h);
int
pcap_next_ex(pcap_t *p, struct pcap_pkthdr **hp, const u_char **pktp);
int
pcap_setdirection(pcap_t *p, pcap_direction_t dir);
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);
int
pcap_get_selectable_fd(pcap_t *);
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);
FILE *
pcap_dump_file(pcap_dumper_t *p);
long
pcap_dump_ftell(pcap_dumper_t *p);
int
pcap_dump_flush(pcap_dumper_t *p);
void
pcap_dump_close(pcap_dumper_t *p);
int
pcap_breakloop(pcap_t *p);
int
pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf);
void
pcap_freealldevs(pcap_if_t *alldevs);
int
pcap_getnonblock(pcap_t *p, char *errbuf);
int
pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int
pcap_set_datalink(pcap_t *, int dlt);
int
pcap_list_datalinks(pcap_t *p, int **dlts);
pcap_t
pcap_open_dead(int linktype, int snaplen);
pcap_t
pcap_fopen_offline(FILE *fp, char *errbuf);
const char *
pcap_lib_version(void);
const char *
pcap_datalink_val_to_name(int);
const char *
pcap_datalink_val_to_description(int);
int
pcap_datalink_name_to_val(const char *);

DESCRIPTION
pcap 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_offline(), 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. 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.) 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_dump_fopen() allows the use of savefile functions on the already-opened stream “f”.

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. 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 with pcap_dispatch().

pcap_inject() uses write(2) to inject a raw packet through the network interface. It returns the number of bytes written or -1 on failure.

pcap_sendpacket() is an alternate interface for packet injection (provided for compatibility). It returns 0 on success or -1 on failure.

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_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_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_loop() may be terminated early through an explicit call to pcap_breakloop(). In this case, the return value of pcap_loop() will be -2.

pcap_next() returns a u_char pointer to the next packet.

pcap_next_ex() reads the next packet and returns a success/failure indication: a return value of 1 indicates success, 0 means that the timeout was exceeded on a live capture, -1 indicates that an error occurred whilst reading the packet and -2 is returned when there are no more packets to read in a savefile.

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 stream associated with the savefile.

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_fileno() and pcap_get_selectable_fd() return 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 error.

pcap_strerror() is provided in case strerror(3) isn't available.

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

pcap_dump_file() returns the stream associated with a savefile.

pcap_dump_ftell() returns the current file offset within a savefile.

pcap_dump_flush() ensures that any buffered data has been written to a savefile.

pcap_dump_close() closes the savefile.

pcap_findalldevs() constructs a linked list of network devices that are suitable for opening with pcap_open_live().

pcap_freealldevs() frees a list of interfaces built by pcap_findalldevs().

pcap_getnonblock() returns 1 if the capture file descriptor is in non-blocking mode, 0 if it is in blocking mode, or -1 on error.

pcap_setnonblock() sets or resets non-blocking mode on a capture file descriptor.

pcap_set_datalink() sets the datalink type on a live capture device that supports multiple datalink types.

pcap_setdirection() is used to limit the direction that packets must be flowing in order to be captured.

pcap_list_datalinks() returns an array of the supported datalink types for an opened live capture device as a -1 terminated array. It is the caller's responsibility to free this list.

pcap_breakloop() safely breaks out of a pcap_loop(). This function sets an internal flag and is safe to be called from inside a signal handler.

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_fopen_offline() may be used to read dumped data from an existing open stream “fp”.

pcap_lib_version() returns a string describing the version of libpcap. pcap_datalink_val_to_name() and pcap_datalink_val_to_description() look up the name or description of a datalink type by number. These functions return NULL if the specified datalink type is not known. pcap_datalink_name_to_val() finds the datalink number for a given datalink name. Returns -1 if the name is not known.

SEE ALSO
tcpdump(8)

AUTHORS
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.

BUGS
Please send bug reports to libpcap@ee.lbl.gov.