.\" @(#) $Header: /tcpdump/master/libpcap/pcap.3,v 1.64.2.4 2005/06/03 20:36:56 guy Exp $ .\" .\" Copyright (c) 1994, 1996, 1997 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that: (1) source code distributions .\" retain the above copyright notice and this paragraph in its entirety, (2) .\" distributions including binary code include the above copyright notice and .\" this paragraph in its entirety in the documentation or other materials .\" provided with the distribution, and (3) all advertising materials mentioning .\" features or use of this software display the following acknowledgement: .\" ``This product includes software developed by the University of California, .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of .\" the University nor the names of its contributors may be used to endorse .\" or promote products derived from this software without specific prior .\" written permission. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .\" $FreeBSD$ .\" .TH PCAP 3 "27 February 2004" .SH NAME pcap \- Packet Capture library .SH SYNOPSIS .nf .ft B #include .ft .LP .nf .ft B char errbuf[PCAP_ERRBUF_SIZE]; .ft .LP .ft B pcap_t *pcap_open_live(const char *device, int snaplen, .ti +8 int promisc, int to_ms, char *errbuf) pcap_t *pcap_open_dead(int linktype, int snaplen) pcap_t *pcap_open_offline(const char *fname, char *errbuf) pcap_t *pcap_fopen_offline(FILE *fp, char *errbuf) pcap_dumper_t *pcap_dump_open(pcap_t *p, const char *fname) pcap_dumper_t *pcap_dump_fopen(pcap_t *p, FILE *fp) .ft .LP .ft B int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf); int pcap_getnonblock(pcap_t *p, char *errbuf); .ft .LP .ft B int pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf) void pcap_freealldevs(pcap_if_t *alldevs) char *pcap_lookupdev(char *errbuf) int pcap_lookupnet(const char *device, bpf_u_int32 *netp, .ti +8 bpf_u_int32 *maskp, char *errbuf) .ft .LP .ft B typedef void (*pcap_handler)(u_char *user, const struct pcap_pkthdr *h, .ti +8 const u_char *bytes); .ft B int pcap_dispatch(pcap_t *p, int cnt, .ti +8 pcap_handler callback, u_char *user) int pcap_loop(pcap_t *p, int cnt, .ti +8 pcap_handler callback, u_char *user) void pcap_dump(u_char *user, struct pcap_pkthdr *h, .ti +8 u_char *sp) .ft .LP .ft B int pcap_compile(pcap_t *p, struct bpf_program *fp, .ti +8 char *str, int optimize, bpf_u_int32 netmask) int pcap_setfilter(pcap_t *p, struct bpf_program *fp) void pcap_freecode(struct bpf_program *) int pcap_setdirection(pcap_t *p, direction_t d) .ft .LP .ft B const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h) int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header, .ti +8 const u_char **pkt_data) .ft .LP .ft B void pcap_breakloop(pcap_t *) .ft .LP .ft B int pcap_inject(pcap_t *p, const void *buf, size_t size) int pcap_sendpacket(pcap_t *p, const u_char *buf, int size) .ft .LP .ft B int pcap_datalink(pcap_t *p) int pcap_list_datalinks(pcap_t *p, int **dlt_buf); int pcap_set_datalink(pcap_t *p, int dlt); int pcap_datalink_name_to_val(const char *name); const char *pcap_datalink_val_to_name(int dlt); const char *pcap_datalink_val_to_description(int dlt); 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, char *prefix) char *pcap_geterr(pcap_t *p) char *pcap_strerror(int error) const char *pcap_lib_version(void) .ft .LP .ft B void pcap_close(pcap_t *p) int pcap_dump_flush(pcap_dumper_t *p) long pcap_dump_ftell(pcap_dumper_t *p) FILE *pcap_dump_file(pcap_dumper_t *p) void pcap_dump_close(pcap_dumper_t *p) .ft .fi .SH DESCRIPTION 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. .PP .SH ROUTINES NOTE: .I errbuf in .BR pcap_open_live() , .BR pcap_open_dead() , .BR pcap_open_offline() , .BR pcap_fopen_offline() , .BR pcap_setnonblock() , .BR pcap_getnonblock() , .BR pcap_findalldevs() , .BR pcap_lookupdev() , and .B pcap_lookupnet() is assumed to be able to hold at least .B PCAP_ERRBUF_SIZE chars. .PP .B pcap_open_live() is used to obtain a packet capture descriptor to look at packets on the network. .I device is a string that specifies the network device to open; on Linux systems with 2.2 or later kernels, a .I device argument of "any" or .B NULL can be used to capture packets from all interfaces. .I snaplen specifies the maximum number of bytes to capture. If this value is less than the size of a packet that is captured, only the first .I snaplen bytes of that packet will be captured and provided as packet data. A value of 65535 should be sufficient, on most if not all networks, to capture all the data available from the packet. .I 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 .I promisc flag is ignored. .I 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. A zero value for .IR to_ms , on platforms that support a read timeout, will cause a read to wait forever to allow enough packets to arrive, with no timeout. .I errbuf is used to return error or warning text. It will be set to error text when .B pcap_open_live() fails and returns .BR NULL . .I errbuf may also be set to warning text when .B pcap_open_live() succeds; to detect this case the caller should store a zero-length string in .I errbuf before calling .B pcap_open_live() and display the warning to the user if .I errbuf is no longer a zero-length string. .PP .B pcap_open_dead() is used for creating a .B pcap_t structure to use when calling the other functions in libpcap. It is typically used when just using libpcap for compiling BPF code. .PP .B pcap_open_offline() is called to open a ``savefile'' for reading. .I fname specifies the name of the file to open. The file has the same format as those used by .B tcpdump(1) and .BR tcpslice(1) . The name "-" in a synonym for .BR stdin . Alternatively, you may call .B pcap_fopen_offline() to read dumped data from an existing open stream .IR fp . Note that on Windows, that stream should be opened in binary mode. .I errbuf is used to return error text and is only set when .B pcap_open_offline() or .B pcap_fopen_offline() fails and returns .BR NULL . .PP .B pcap_dump_open() is called to open a ``savefile'' for writing. The name "-" in a synonym for .BR stdout . .B NULL is returned on failure. .I p is a .I pcap struct as returned by .B pcap_open_offline() or .BR pcap_open_live() . .I fname specifies the name of the file to open. Alternatively, you may call .B pcap_dump_fopen() to write data to an existing open stream .IR fp . Note that on Windows, that stream should be opened in binary mode. If .B NULL is returned, .B pcap_geterr() can be used to get the error text. .PP .PP .B pcap_setnonblock() puts a capture descriptor, opened with .BR pcap_open_live() , into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode, depending on whether the .I nonblock argument is non-zero or zero. It has no effect on ``savefiles''. If there is an error, \-1 is returned and .I 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 .B pcap_dispatch() will, if no packets are currently available to be read, return 0 immediately rather than blocking waiting for packets to arrive. .B pcap_loop() and .B pcap_next() will not work in ``non-blocking'' mode. .PP .B 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 .I errbuf is filled in with an appropriate error message. .PP .B pcap_findalldevs() constructs a list of network devices that can be opened with .BR pcap_open_live() . (Note that there may be network devices that cannot be opened with .BR pcap_open_live() by the process calling .BR 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.) .I alldevsp is set to point to the first element of the list; each element of the list is of type .BR pcap_if_t , and has the following members: .RS .TP .B next if not .BR NULL , a pointer to the next element in the list; .B NULL for the last element of the list .TP .B name a pointer to a string giving a name for the device to pass to .B pcap_open_live() .TP .B description if not .BR NULL , a pointer to a string giving a human-readable description of the device .TP .B addresses a pointer to the first element of a list of addresses for the interface .TP .B flags interface flags: .RS .TP .B PCAP_IF_LOOPBACK set if the interface is a loopback interface .RE .RE .PP Each element of the list of addresses is of type .BR pcap_addr_t , and has the following members: .RS .TP .B next if not .BR NULL , a pointer to the next element in the list; .B NULL for the last element of the list .TP .B addr a pointer to a .B "struct sockaddr" containing an address .TP .B netmask if not .BR NULL , a pointer to a .B "struct sockaddr" that contains the netmask corresponding to the address pointed to by .B addr .TP .B broadaddr if not .BR NULL , a pointer to a .B "struct sockaddr" that contains the broadcast address corresponding to the address pointed to by .BR addr ; may be null if the interface doesn't support broadcasts .TP .B dstaddr if not .BR NULL , a pointer to a .B "struct sockaddr" that contains the destination address corresponding to the address pointed to by .BR addr ; may be null if the interface isn't a point-to-point interface .RE .PP Note that not all the addresses in the list of addresses are necessarily IPv4 or IPv6 addresses - you must check the .B sa_family member of the .B "struct sockaddr" before interpreting the contents of the address. .PP .B \-1 is returned on failure, in which case .B errbuf is filled in with an appropriate error message; .B 0 is returned on success. .PP .B pcap_freealldevs() is used to free a list allocated by .BR pcap_findalldevs() . .PP .B pcap_lookupdev() returns a pointer to a network device suitable for use with .B pcap_open_live() and .BR pcap_lookupnet() . If there is an error, .B NULL is returned and .I errbuf is filled in with an appropriate error message. .PP .B pcap_lookupnet() is used to determine the network number and mask associated with the network device .BR device . Both .I netp and .I maskp are .I bpf_u_int32 pointers. A return of \-1 indicates an error in which case .I errbuf is filled in with an appropriate error message. .PP .B pcap_dispatch() is used to collect and process packets. .I 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 .I cnt packets may be processed. A .I 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''. .I callback specifies a routine to be called with three arguments: a .I u_char pointer which is passed in from .BR pcap_dispatch() , a .I const struct pcap_pkthdr pointer to a structure with the following members: .RS .TP .B ts a .I struct timeval containing the time when the packet was captured .TP .B caplen a .I bpf_u_int32 giving the number of bytes of the packet that are available from the capture .TP .B len a .I 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) .RE .PP and a .I const u_char pointer to the first .B caplen (as given in the .I struct pcap_pkthdr a pointer to which is passed to the callback routine) bytes of data from the packet (which won't necessarily be the entire packet; to capture the entire packet, you will have to provide a value for .I snaplen in your call to .B pcap_open_live() that is sufficiently large to get all of the packet's data - a value of 65535 should be sufficient on most if not all networks). .PP 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 .B pcap_perror() or .B pcap_geterr() may be used to display the error text. A return of \-2 indicates that the loop terminated due to a call to .B pcap_breakloop() before any packets were processed. .ft B If your application uses pcap_breakloop(), make sure that you explicitly check for \-1 and \-2, rather than just checking for a return value < 0. .ft R .PP .BR NOTE : when reading a live capture, .B 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 .B 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 .B pcap_dispatch() will return after the timeout expires. .PP .B pcap_loop() is similar to .B pcap_dispatch() except it keeps reading packets until .I cnt packets are processed or an error occurs. It does .B not return when live read timeouts occur. Rather, specifying a non-zero read timeout to .B pcap_open_live() and then calling .B pcap_dispatch() allows the reception and processing of any packets that arrive when the timeout occurs. A negative .I cnt causes .B pcap_loop() to loop forever (or at least until an error occurs). \-1 is returned on an error; 0 is returned if .I cnt is exhausted; \-2 is returned if the loop terminated due to a call to .B pcap_breakloop() before any packets were processed. .ft B If your application uses pcap_breakloop(), make sure that you explicitly check for \-1 and \-2, rather than just checking for a return value < 0. .ft R .PP .B pcap_next() reads the next packet (by calling .B pcap_dispatch() with a .I cnt of 1) and returns a .I u_char pointer to the data in that packet. (The .I pcap_pkthdr struct for that packet is not supplied.) .B NULL is returned if an error occured, or 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.'' Unfortunately, there is no way to determine whether an error occured or not. .PP .B pcap_next_ex() reads the next packet and returns a success/failure indication: .RS .TP 1 the packet was read without problems .TP 0 packets are being read from a live capture, and the timeout expired .TP \-1 an error occurred while reading the packet .TP \-2 packets are being read from a ``savefile'', and there are no more packets to read from the savefile. .RE .PP If the packet was read without problems, the pointer pointed to by the .I pkt_header argument is set to point to the .I pcap_pkthdr struct for the packet, and the pointer pointed to by the .I pkt_data argument is set to point to the data in the packet. .PP .B pcap_breakloop() sets a flag that will force .B pcap_dispatch() or .B pcap_loop() to return rather than looping; they will return the number of packets that have been processed so far, or \-2 if no packets have been processed so far. .PP This routine is safe to use inside a signal handler on UNIX or a console control handler on Windows, as it merely sets a flag that is checked within the loop. .PP The flag is checked in loops reading packets from the OS - a signal by itself will not necessarily terminate those loops - as well as in loops processing a set of packets returned by the OS. .ft B Note that if you are catching signals on UNIX systems that support restarting system calls after a signal, and calling pcap_breakloop() in the signal handler, you must specify, when catching those signals, that system calls should NOT be restarted by that signal. Otherwise, if the signal interrupted a call reading packets in a live capture, when your signal handler returns after calling pcap_breakloop(), the call will be restarted, and the loop will not terminate until more packets arrive and the call completes. .PP Note also that, in a multi-threaded application, if one thread is blocked in .BR pcap_dispatch() , .BR pcap_loop() , .BR pcap_next() , or .BR pcap_next_ex() , a call to .B pcap_breakloop() in a different thread will not unblock that thread; you will need to use whatever mechanism the OS provides for breaking a thread out of blocking calls in order to unblock the thread, such as thread cancellation in systems that support POSIX threads. .ft R .PP Note that .B pcap_next() will, on some platforms, loop reading packets from the OS; that loop will not necessarily be terminated by a signal, so .B pcap_breakloop() should be used to terminate packet processing even if .B pcap_next() is being used. .PP .B pcap_breakloop() does not guarantee that no further packets will be processed by .B pcap_dispatch() or .B pcap_loop() after it is called; at most one more packet might be processed. .PP If \-2 is returned from .B pcap_dispatch() or .BR pcap_loop() , the flag is cleared, so a subsequent call will resume reading packets. If a positive number is returned, the flag is not cleared, so a subsequent call will return \-2 and clear the flag. .PP .B pcap_inject() sends a raw packet through the network interface; .I buf points to the data of the packet, including the link-layer header, and .I size is the number of bytes in the packet. It returns the number of bytes written on success. A return of \-1 indicates an error in which case .B pcap_perror() or .B pcap_geterr() may be used to display the error text. Note that, even if you successfully open the network interface, you might not have permission to send packets on it, or it might not support sending packets; as .I pcap_open_live() doesn't have a flag to indicate whether to open for capturing, sending, or capturing and sending, you cannot request an open that supports sending and be notified at open time whether sending will be possible. Note also that some devices might not support sending packets. .PP Note that, on some platforms, the link-layer header of the packet that's sent might not be the same as the link-layer header of the packet supplied to .BR pcap_inject() , as the source link-layer address, if the header contains such an address, might be changed to be the address assigned to the interface on which the packet it sent, if the platform doesn't support sending completely raw and unchanged packets. .PP .B pcap_sendpacket() is like .BR pcap_inject() , but it returns 0 on success and \-1 on failure. .RB ( pcap_inject() comes from OpenBSD; .B pcap_sendpacket() comes from WinPcap. Both are provided for compatibility.) .PP .B pcap_dump() outputs a packet to the ``savefile'' opened with .BR pcap_dump_open() . Note that its calling arguments are suitable for use with .B pcap_dispatch() or .BR pcap_loop() . If called directly, the .I user parameter is of type .I pcap_dumper_t as returned by .BR pcap_dump_open() . .PP .B pcap_compile() is used to compile the string .I str into a filter program. .I program is a pointer to a .I bpf_program struct and is filled in by .BR pcap_compile() . .I optimize controls whether optimization on the resulting code is performed. .I netmask specifies the IPv4 netmask of the network on which packets are being captured; it is used only when checking for IPv4 broadcast addresses in the filter program. If the netmask of the network on which packets are being captured isn't known to the program, or if packets are being captured on the Linux "any" pseudo-interface that can capture on more than one network, a value of 0 can be supplied; tests for IPv4 broadcast addreses won't be done correctly, but all other tests in the filter program will be OK. A return of \-1 indicates an error in which case .BR pcap_geterr() may be used to display the error text. .PP .B pcap_compile_nopcap() is similar to .B 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 .BR pcap_open() . A return of \-1 indicates an error; the error text is unavailable. .RB ( pcap_compile_nopcap() is a wrapper around .BR pcap_open_dead() , .BR pcap_compile() , and .BR pcap_close() ; the latter three routines can be used directly in order to get the error text for a compilation error.) .B .PP .B pcap_setfilter() is used to specify a filter program. .I fp is a pointer to a .I bpf_program struct, usually the result of a call to .BR pcap_compile() . .B \-1 is returned on failure, in which case .BR pcap_geterr() may be used to display the error text; .B 0 is returned on success. .PP .B pcap_freecode() is used to free up allocated memory pointed to by a .I bpf_program struct generated by .B 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 .BR pcap_setfilter() . .PP .B pcap_setdirection() is used to specify a direction that packets will be captured. .I direction_t is one of the constants .BR D_IN , .B D_OUT or .BR D_INOUT . .B D_IN will only capture packets received by the device, .B D_OUT will only capture packets sent by the device and .B D_INOUT will capture packets received by or sent by the device. .B D_INOUT is the default setting if this function is not called. This isn't necessarily supported on all platforms; some platforms might return an error, and some other platforms might not support .BR D_OUT . This operation is not supported if a ``savefile'' is being read. .B \-1 is returned on failure, .B 0 is returned on success. .PP .B pcap_datalink() returns the link layer type; link layer types it can return include: .PP .RS 5 .TP 5 .B DLT_NULL BSD loopback encapsulation; the link layer header is a 4-byte field, in .I host byte order, containing a PF_ value from .B socket.h for the network-layer protocol of the packet. .IP 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 .I not necessarily those of the machine reading the capture file. .TP 5 .B DLT_EN10MB Ethernet (10Mb, 100Mb, 1000Mb, and up) .TP 5 .B DLT_IEEE802 IEEE 802.5 Token Ring .TP 5 .B DLT_ARCNET ARCNET .TP 5 .B DLT_SLIP SLIP; the link layer header contains, in order: .RS 10 .LP a 1-byte flag, which is 0 for packets received by the machine and 1 for packets sent by the machine; .LP a 1-byte field, the upper 4 bits of which indicate the type of packet, as per RFC 1144: .RS 5 .TP 5 0x40 an unmodified IP datagram (TYPE_IP); .TP 5 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; .TP 5 0x80 a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the first byte of the compressed TCP/IP datagram header; .RE .LP for UNCOMPRESSED_TCP, the rest of the modified IP header, and for COMPRESSED_TCP, the compressed TCP/IP datagram header; .RE .RS 5 .LP for a total of 16 bytes; the uncompressed IP datagram follows the header. .RE .TP 5 .B 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. .TP 5 .B DLT_FDDI FDDI .TP 5 .B DLT_ATM_RFC1483 RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2 LLC header. .TP 5 .B DLT_RAW raw IP; the packet begins with an IP header. .TP 5 .B 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. .TP 5 .B DLT_PPP_ETHER PPPoE; the packet begins with a PPPoE header, as per RFC 2516. .TP 5 .B DLT_C_HDLC Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547. .TP 5 .B DLT_IEEE802_11 IEEE 802.11 wireless LAN .TP 5 .B DLT_FRELAY Frame Relay .TP 5 .B DLT_LOOP OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in .I network byte order, containing a PF_ value from OpenBSD's .B socket.h for the network-layer protocol of the packet. .IP Note that, if a ``savefile'' is being read, those PF_ values are .I not necessarily those of the machine reading the capture file. .TP 5 .B DLT_LINUX_SLL Linux "cooked" capture encapsulation; the link layer header contains, in order: .RS 10 .LP a 2-byte "packet type", in network byte order, which is one of: .RS 5 .TP 5 0 packet was sent to us by somebody else .TP 5 1 packet was broadcast by somebody else .TP 5 2 packet was multicast, but not broadcast, by somebody else .TP 5 3 packet was sent by somebody else to somebody else .TP 5 4 packet was sent by us .RE .LP a 2-byte field, in network byte order, containing a Linux ARPHRD_ value for the link layer device type; .LP 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); .LP 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); .LP 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. .RE .TP 5 .B DLT_LTALK Apple LocalTalk; the packet begins with an AppleTalk LLAP header. .TP 5 .B DLT_PFLOG OpenBSD pflog; the link layer header contains, in order: .RS 10 .LP a 1-byte header length, in host byte order; .LP a 4-byte PF_ value, in host byte order; .LP a 2-byte action code, in network byte order, which is one of: .RS 5 .TP 5 0 passed .TP 5 1 dropped .TP 5 2 scrubbed .RE .LP a 2-byte reason code, in network byte order, which is one of: .RS 5 .TP 5 0 match .TP 5 1 bad offset .TP 5 2 fragment .TP 5 3 short .TP 5 4 normalize .TP 5 5 memory .RE .LP a 16-character interface name; .LP a 16-character ruleset name (only meaningful if subrule is set); .LP a 4-byte rule number, in network byte order; .LP a 4-byte subrule number, in network byte order; .LP a 1-byte direction, in network byte order, which is one of: .RS 5 .TP 5 0 incoming or outgoing .TP 5 1 incoming .TP 5 2 outgoing .RE .RE .TP 5 .B DLT_PRISM_HEADER Prism monitor mode information followed by an 802.11 header. .TP 5 .B DLT_IP_OVER_FC RFC 2625 IP-over-Fibre Channel, with the link-layer header being the Network_Header as described in that RFC. .TP 5 .B DLT_SUNATM SunATM devices; the link layer header contains, in order: .RS 10 .LP a 1-byte flag field, containing a direction flag in the uppermost bit, which is set for packets transmitted by the machine and clear for packets received by the machine, and a 4-byte traffic type in the low-order 4 bits, which is one of: .RS 5 .TP 5 0 raw traffic .TP 5 1 LANE traffic .TP 5 2 LLC-encapsulated traffic .TP 5 3 MARS traffic .TP 5 4 IFMP traffic .TP 5 5 ILMI traffic .TP 5 6 Q.2931 traffic .RE .LP a 1-byte VPI value; .LP a 2-byte VCI field, in network byte order. .RE .TP 5 .B DLT_IEEE802_11_RADIO link-layer information followed by an 802.11 header - see http://www.shaftnet.org/~pizza/software/capturefrm.txt for a description of the link-layer information. .TP 5 .B DLT_ARCNET_LINUX ARCNET, with no exception frames, reassembled packets rather than raw frames, and an extra 16-bit offset field between the destination host and type bytes. .TP 5 .B DLT_LINUX_IRDA Linux-IrDA packets, with a .B DLT_LINUX_SLL header followed by the IrLAP header. .RE .PP .B pcap_list_datalinks() is used to get a list of the supported data link types of the interface associated with the pcap descriptor. .B pcap_list_datalinks() allocates an array to hold the list and sets .IR *dlt_buf . The caller is responsible for freeing the array. .B \-1 is returned on failure; otherwise, the number of data link types in the array is returned. .PP .B pcap_set_datalink() is used to set the current data link type of the pcap descriptor to the type specified by .IR dlt . .B \-1 is returned on failure. .PP .B pcap_datalink_name_to_val() translates a data link type name, which is a .B DLT_ name with the .B DLT_ removed, to the corresponding data link type value. The translation is case-insensitive. .B \-1 is returned on failure. .PP .B pcap_datalink_val_to_name() translates a data link type value to the corresponding data link type name. NULL is returned on failure. .PP .B pcap_datalink_val_to_description() translates a data link type value to a short description of that data link type. NULL is returned on failure. .PP .B pcap_list_datalinks() is used to get a list of the supported data link types of the interface associated with the pcap descriptor. .B pcap_list_datalinks() allocates an array to hold the list and sets .IR *dlt_buf . The caller is responsible for freeing the array. .B \-1 is returned on failure; otherwise, the number of data link types in the array is returned. .PP .B pcap_set_datalink() is used to set the current data link type of the pcap descriptor to the type specified by .IR dlt . .B \-1 is returned on failure. .PP .B pcap_snapshot() returns the snapshot length specified when .B pcap_open_live() was called. .PP .B pcap_is_swapped() returns true if the current ``savefile'' uses a different byte order than the current system. .PP .B pcap_major_version() returns the major number of the file format of the savefile; .B pcap_minor_version() returns the minor number of the file format of the savefile. The version number is stored in the header of the savefile. .PP .B pcap_file() returns the standard I/O stream of the ``savefile,'' if a ``savefile'' was opened with .BR pcap_open_offline() , or NULL, if a network device was opened with .BR pcap_open_live() . .PP .B pcap_stats() returns 0 and fills in a .B 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 .B pcap_perror() or .BR pcap_geterr() . .B 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''. .PP .B pcap_fileno() returns the file descriptor number from which captured packets are read, if a network device was opened with .BR pcap_open_live() , or \-1, if a ``savefile'' was opened with .BR pcap_open_offline() . .PP .B pcap_get_selectable_fd() returns, on UNIX, a file descriptor number for a file descriptor on which one can do a .B select() or .B poll() to wait for it to be possible to read packets without blocking, if such a descriptor exists, or \-1, if no such descriptor exists. Some network devices opened with .B pcap_open_live() do not support .B select() or .B poll() (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so \-1 is returned for those devices. .PP Note that on most versions of most BSDs (including Mac OS X) .B select() and .B poll() do not work correctly on BPF devices; .B pcap_get_selectable_fd() will return a file descriptor on most of those versions (the exceptions being FreeBSD 4.3 and 4.4), a simple .B select() or .B poll() will not return even after a timeout specified in .B pcap_open_live() expires. To work around this, an application that uses .B select() or .B poll() to wait for packets to arrive must put the .B pcap_t in non-blocking mode, and must arrange that the .B select() or .B poll() have a timeout less than or equal to the timeout specified in .BR pcap_open_live() , and must try to read packets after that timeout expires, regardless of whether .B select() or .B poll() indicated that the file descriptor for the .B pcap_t is ready to be read or not. (That workaround will not work in FreeBSD 4.3 and later; however, in FreeBSD 4.6 and later, .B select() and .B poll() work correctly on BPF devices, so the workaround isn't necessary, although it does no harm.) .PP .B pcap_get_selectable_fd() is not available on Windows. .PP .B pcap_perror() prints the text of the last pcap library error on .BR stderr , prefixed by .IR prefix . .PP .B pcap_geterr() returns the error text pertaining to the last pcap library error. .BR NOTE : the pointer it returns will no longer point to a valid error message string after the .B pcap_t passed to it is closed; you must use or copy the string before closing the .BR pcap_t . .PP .B pcap_strerror() is provided in case .BR strerror (1) isn't available. .PP .B pcap_lib_version() returns a pointer to a string giving information about the version of the libpcap library being used; note that it contains more information than just a version number. .PP .B pcap_close() closes the files associated with .I p and deallocates resources. .PP .B pcap_dump_file() returns the standard I/O stream of the ``savefile'' opened by .BR pcap_dump_open(). .PP .B pcap_dump_flush() flushes the output buffer to the ``savefile,'' so that any packets written with .B pcap_dump() but not yet written to the ``savefile'' will be written. .B \-1 is returned on error, 0 on success. .PP .B pcap_dump_ftell() returns the current file position for the ``savefile'', representing the number of bytes written by .B pcap_dump_open() and .BR pcap_dump() . .B \-1 is returned on error. .PP .B pcap_dump_close() closes the ``savefile.'' .PP .SH SEE ALSO tcpdump(1), tcpslice(1) .SH AUTHORS The original authors are: .LP Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. .LP The current version is available from "The Tcpdump Group"'s Web site at .LP .RS .I http://www.tcpdump.org/ .RE .SH BUGS Please send problems, bugs, questions, desirable enhancements, etc. to: .LP .RS tcpdump-workers@tcpdump.org .RE .LP Please send source code contributions, etc. to: .LP .RS patches@tcpdump.org .RE