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PCAP_OPEN_LIVE(3) Library Functions Manual PCAP_OPEN_LIVE(3)


pcap_open_live, pcap_open_offline, pcap_dump_open, pcap_dump_fopen, pcap_lookupdev, pcap_lookupnet, pcap_dispatch, pcap_loop, pcap_dump, pcap_inject, pcap_sendpacket, pcap_compile, pcap_setfilter, pcap_freecode, pcap_next, pcap_next_ex, pcap_setdirection, pcap_datalink, pcap_snapshot, pcap_is_swapped, pcap_major_version, pcap_minor_version, pcap_stats, pcap_file, pcap_fileno, pcap_get_selectable_fd, pcap_perror, pcap_geterr, pcap_strerror, pcap_close, pcap_dump_file, pcap_dump_ftell, pcap_dump_flush, pcap_dump_close, pcap_breakloop, pcap_findalldevs, pcap_freealldevs, pcap_getnonblock, pcap_setnonblock, pcap_set_datalink, pcap_list_datalinks, pcap_free_datalinks, pcap_open_dead, pcap_fopen_offline, pcap_lib_version, pcap_datalink_val_to_name, pcap_datalink_val_to_description, pcap_datalink_name_to_val, pcap_create, pcap_set_snaplen, pcap_set_promisc, pcap_can_set_rfmon, pcap_set_rfmon, pcap_set_timeout, pcap_set_buffer_size, pcap_activate, pcap_statustostr, pcap_offline_filterPacket Capture library


#include <pcap.h>
pcap_t *
pcap_open_live(const char *source, 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);
pcap_lookupnet(const char *device, bpf_u_int32 *netp, bpf_u_int32 *maskp, char *errbuf);
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, const struct pcap_pkthdr *h, const u_char *sp);
pcap_inject(pcap_t *p, const void *buf, size_t len);
pcap_sendpacket(pcap_t *p, const u_char *buf, int size);
pcap_compile(pcap_t *p, struct bpf_program *program, char *buf, int optimize, bpf_u_int32 netmask);
pcap_setfilter(pcap_t *p, struct bpf_program *fp);
pcap_freecode(struct bpf_program *program);
u_char *
pcap_next(pcap_t *p, struct pcap_pkthdr *h);
pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header, const u_char **pkt_data);
pcap_setdirection(pcap_t *p, pcap_direction_t d);
pcap_datalink(pcap_t *p);
pcap_snapshot(pcap_t *p);
pcap_is_swapped(pcap_t *p);
pcap_major_version(pcap_t *p);
pcap_minor_version(pcap_t *p);
pcap_stats(pcap_t *p, struct pcap_stat *ps);
pcap_file(pcap_t *p);
pcap_fileno(pcap_t *p);
pcap_get_selectable_fd(pcap_t *p);
pcap_perror(pcap_t *p, char *prefix);
char *
pcap_geterr(pcap_t *p);
char *
pcap_strerror(int errnum);
pcap_close(pcap_t *p);
pcap_dump_file(pcap_dumper_t *p);
pcap_dump_ftell(pcap_dumper_t *p);
pcap_dump_flush(pcap_dumper_t *p);
pcap_dump_close(pcap_dumper_t *p);
pcap_breakloop(pcap_t *p);
pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf);
pcap_freealldevs(pcap_if_t *alldevs);
pcap_getnonblock(pcap_t *p, char *errbuf);
pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
pcap_set_datalink(pcap_t *p, int dlt);
pcap_list_datalinks(pcap_t *p, int **dlt_buffer);
pcap_free_datalinks(int *dlt_list);
pcap_open_dead(int linktype, int snaplen);
pcap_fopen_offline(FILE *fp, char *errbuf);
const char *
const char *
pcap_datalink_val_to_name(int dlt);
const char *
pcap_datalink_val_to_description(int dlt);
pcap_datalink_name_to_val(const char *name);
pcap_t *
pcap_create(const char *device, char *errbuf);
pcap_set_snaplen(pcap_t *p, int snaplen);
pcap_set_promisc(pcap_t *p, int promisc);
pcap_can_set_rfmon(pcap_t *p);
pcap_set_rfmon(pcap_t *p, int rfmon);
pcap_set_timeout(pcap_t *p, int timeout_ms);
pcap_set_buffer_size(pcap_t *p, int buffer_size);
pcap_activate(pcap_t *p);
const char *
pcap_statustostr(int errnum);
pcap_offline_filter(const struct bpf_program *fp, const struct pcap_pkthdr *h, const u_char *pkt);


pcap_open_live 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 that errbuf in pcap_open_live(), pcap_open_offline(), pcap_findalldevs(), pcap_lookupdev(), pcap_lookupnet(), pcap_getnonblock(), pcap_setnonblock(), pcap_fopen_offline(), and pcap_create() 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. source is a string that specifies the network device to open. snaplen specifies the maximum number of bytes to capture from one 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.) 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 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_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 precedes 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 buf 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.
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_loop() 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 with pcap_free_datalinks(), which frees the list of link-layer header types pointed to by dlt_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.
pcap_create() is used to create a packet capture handle to look at packets on the network. The returned handle must be activated with pcap_activate() before packets can be captured with it; options for the capture, such as promiscuous mode, can be set on the handle before activating it.
pcap_set_snaplen() sets the snapshot length to be used on a capture handle when the handle is activated to snaplen.
pcap_set_promisc() sets whether promiscuous mode should be set on a capture handle when the handle is activated. If promisc is non-zero, promiscuous mode will be set, otherwise it will not be set.
pcap_can_set_rfmon() checks whether monitor mode could be set on a capture handle when the handle is activated.
pcap_set_rfmon() sets whether monitor mode should be set on a capture handle when the handle is activated. If rfmon is non-zero, monitor mode will be set, otherwise it will not be set.
pcap_set_timeout() sets the read timeout that will be used on a capture handle when the handle is activated to to_ms, which is in units of milliseconds.
pcap_set_buffer_size() sets the buffer size that will be used on a capture handle when the handle is activated to buffer_size, which is in units of bytes.
pcap_activate() is used to activate a packet capture handle to look at packets on the network, with the options that were set on the handle being in effect.
pcap_statustostr() converts a PCAP_ERROR_ or PCAP_WARNING_ value returned by a libpcap routine to an error string.
pcap_offline_filter() checks whether a filter matches a packet.


pcap-filter(3), tcpdump(8)


Van Jacobson, Craig Leres, and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
October 17, 2016 OpenBSD-current