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ISBN : 978-2-7460-9712-4
EAN : 9782746097124
(Editions ENI)


CentOS 2.1AS







pcap − Packet Capture library


#include <pcap.h>

pcap_t *pcap_open_live(char *device, int snaplen,
int promisc, int to_ms, char *ebuf)
pcap_t *pcap_open_dead(int linktype, int snaplen)
pcap_t *pcap_open_offline(char *fname, char *ebuf)
pcap_dumper_t *pcap_dump_open(pcap_t *p, char *fname)

char errbuf[PCAP_ERRBUF_SIZE];
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_offline(), 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. ebuf is used to return error text and is only set when pcap_open_live() fails and returns NULL.

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(8). The name "-" in a synonym for stdin. ebuf 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_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 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. 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).

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

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().

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, 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 name of the ’’savefile.’’

int 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 under lying 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() returns 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. 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(8), tcpslice(8)


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



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