PCAP_OPEN_LIVE(3) | Library Functions Manual | PCAP_OPEN_LIVE(3)

# NAME

`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_set_immediate_mode`, `pcap_activate`, `pcap_statustostr`, `pcap_offline_filter` — Packet Capture library

# SYNOPSIS

Packet capture Library (libpcap, -lpcap)
`#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);

int
`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, const struct pcap_pkthdr *h, const u_char *sp);

int
`pcap_inject`(pcap_t *p, const void *buf, size_t len);

int
`pcap_sendpacket`(pcap_t *p, const u_char *buf, int size);

int
`pcap_compile`(pcap_t *p, struct bpf_program *program, const char *buf, int optimize, bpf_u_int32 netmask);

int
`pcap_setfilter`(pcap_t *p, struct bpf_program *fp);

void
`pcap_freecode`(struct bpf_program *program);

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);

int
`pcap_setdirection`(pcap_t *p, pcap_direction_t d);

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 *p);

void
`pcap_perror`(pcap_t *p, const char *prefix);

char *
`pcap_geterr`(pcap_t *p);

const char *
`pcap_strerror`(int errnum);

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);

void
`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 *p, int dlt);

int
`pcap_list_datalinks`(pcap_t *p, int **dlt_buffer);

void
`pcap_free_datalinks`(int *dlt_list);

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 dlt);

const char *
`pcap_datalink_val_to_description`(int dlt);

int
`pcap_datalink_name_to_val`(const char *name);

pcap_t *
`pcap_create`(const char *device, char *errbuf);

int
`pcap_set_snaplen`(pcap_t *p, int snaplen);

int
`pcap_set_promisc`(pcap_t *p, int promisc);

int
`pcap_can_set_rfmon`(pcap_t *p);

int
`pcap_set_rfmon`(pcap_t *p, int rfmon);

int
`pcap_set_timeout`(pcap_t *p, int timeout_ms);

int
`pcap_set_buffer_size`(pcap_t *p, int buffer_size);

int
`pcap_set_immediate_mode`(pcap_t *p, int immediate);

int
`pcap_activate`(pcap_t *p);

const char *
`pcap_statustostr`(int errnum);

int
`pcap_offline_filter`(const struct bpf_program *fp, const struct pcap_pkthdr *h, const u_char *pkt);

# DESCRIPTION

`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 a save file previously opened using `pcap_dump_open`() or `pcap_dump_fopen`(). Note that the user argument contains the handle to the save file and should be of type pcap_dumper_t *. This makes `pcap_dump`() a suitable callback function for use as an argument to `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. The direction is either `PCAP_D_INOUT`, `PCAP_D_IN` or `PCAP_D_OUT`. Direction is only relevant to live captures. When reading from a dump file, `pcap_setdirection`() has no effect.

`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_set_immediate_mode`() sets whether immediate mode should be set on a capture handle when the handle is activated. If immediate is non-zero, immediate mode will be set, otherwise it will not be set.

`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.

# SEE ALSO

pcap-filter(5), tcpdump(8)

# AUTHORS

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

June 13, 2025 | Debian