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PCAP(3) PCAP(3) NNAAMMEE pcap - Packet Capture library SSYYNNOOPPSSIISS ##iinncclluuddee <<ppccaapp..hh>> cchhaarr eerrrrbbuuff[[PPCCAAPP__EERRRRBBUUFF__SSIIZZEE]];; ppccaapp__tt **ppccaapp__ooppeenn__lliivvee((cchhaarr **ddeevviiccee,, iinntt ssnnaapplleenn,, iinntt pprroommiisscc,, iinntt ttoo__mmss,, cchhaarr **eerrrrbbuuff)) ppccaapp__tt **ppccaapp__ooppeenn__ddeeaadd((iinntt lliinnkkttyyppee,, iinntt ssnnaapplleenn)) ppccaapp__tt **ppccaapp__ooppeenn__oofffflliinnee((cchhaarr **ffnnaammee,, cchhaarr **eerrrrbbuuff)) ppccaapp__dduummppeerr__tt **ppccaapp__dduummpp__ooppeenn((ppccaapp__tt **pp,, cchhaarr **ffnnaammee)) iinntt ppccaapp__sseettnnoonnbblloocckk((ppccaapp__tt **pp,, iinntt nnoonnbblloocckk,, cchhaarr **eerrrrbbuuff));; iinntt ppccaapp__ggeettnnoonnbblloocckk((ppccaapp__tt **pp,, cchhaarr **eerrrrbbuuff));; iinntt ppccaapp__ffiinnddaallllddeevvss((ppccaapp__iiff__tt ****aallllddeevvsspp,, cchhaarr **eerrrrbbuuff)) vvooiidd ppccaapp__ffrreeeeaallllddeevvss((ppccaapp__iiff__tt **)) cchhaarr **ppccaapp__llooookkuuppddeevv((cchhaarr **eerrrrbbuuff)) iinntt ppccaapp__llooookkuuppnneett((cchhaarr **ddeevviiccee,, bbppff__uu__iinntt3322 **nneettpp,, bbppff__uu__iinntt3322 **mmaasskkpp,, cchhaarr **eerrrrbbuuff)) iinntt ppccaapp__ddiissppaattcchh((ppccaapp__tt **pp,, iinntt ccnntt,, ppccaapp__hhaannddlleerr ccaallllbbaacckk,, uu__cchhaarr **uusseerr)) iinntt ppccaapp__lloooopp((ppccaapp__tt **pp,, iinntt ccnntt,, ppccaapp__hhaannddlleerr ccaallllbbaacckk,, uu__cchhaarr **uusseerr)) vvooiidd ppccaapp__dduummpp((uu__cchhaarr **uusseerr,, ssttrruucctt ppccaapp__ppkktthhddrr **hh,, uu__cchhaarr **sspp)) iinntt ppccaapp__ccoommppiillee((ppccaapp__tt **pp,, ssttrruucctt bbppff__pprrooggrraamm **ffpp,, cchhaarr **ssttrr,, iinntt ooppttiimmiizzee,, bbppff__uu__iinntt3322 nneettmmaasskk)) iinntt ppccaapp__sseettffiilltteerr((ppccaapp__tt **pp,, ssttrruucctt bbppff__pprrooggrraamm **ffpp)) vvooiidd ppccaapp__ffrreeeeccooddee((ssttrruucctt bbppff__pprrooggrraamm **));; uu__cchhaarr **ppccaapp__nneexxtt((ppccaapp__tt **pp,, ssttrruucctt ppccaapp__ppkktthhddrr **hh)) iinntt ppccaapp__ddaattaalliinnkk((ppccaapp__tt **pp)) iinntt ppccaapp__ssnnaappsshhoott((ppccaapp__tt **pp)) iinntt ppccaapp__iiss__sswwaappppeedd((ppccaapp__tt **pp)) iinntt ppccaapp__mmaajjoorr__vveerrssiioonn((ppccaapp__tt **pp)) iinntt ppccaapp__mmiinnoorr__vveerrssiioonn((ppccaapp__tt **pp)) iinntt ppccaapp__ssttaattss((ppccaapp__tt **pp,, ssttrruucctt ppccaapp__ssttaatt **ppss)) FFIILLEE **ppccaapp__ffiillee((ppccaapp__tt **pp)) iinntt ppccaapp__ffiilleennoo((ppccaapp__tt **pp)) vvooiidd ppccaapp__ppeerrrroorr((ppccaapp__tt **pp,, cchhaarr **pprreeffiixx)) cchhaarr **ppccaapp__ggeetteerrrr((ppccaapp__tt **pp)) cchhaarr **ppccaapp__ssttrreerrrroorr((iinntt eerrrroorr)) vvooiidd ppccaapp__cclloossee((ppccaapp__tt **pp)) vvooiidd ppccaapp__dduummpp__cclloossee((ppccaapp__dduummppeerr__tt **pp)) DDEESSCCRRIIPPTTIIOONN 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. RROOUUTTIINNEESS NOTE: _e_r_r_b_u_f in ppccaapp__ooppeenn__lliivvee(()), ppccaapp__ooppeenn__ddeeaadd(()), ppccaapp__ooppeenn__oofffflliinnee(()), ppccaapp__sseettnnoonnbblloocckk(()), ppccaapp__ggeettnnoonnbblloocckk(()), ppccaapp__ffiinnddaallllddeevvss(()), ppccaapp__llooookkuuppddeevv(()), and ppccaapp__llooookkuuppnneett(()) is assumed to be able to hold at least PPCCAAPP__EERRRRBBUUFF__SSIIZZEE chars. ppccaapp__ooppeenn__lliivvee(()) is used to obtain a packet capture descriptor to look at packets on the network. _d_e_v_i_c_e is a string that specifies the network device to open; on Linux systems with 2.2 or later kernels, a _d_e_v_i_c_e argument of "any" or NNUULLLL can be used to capture packets from all interfaces. _s_n_a_p_l_e_n specifies the maximum number of bytes to capture. _p_r_o_m_i_s_c specifies if the interface is to be put into promiscuous mode. (Note that even if this param eter 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 _p_r_o_m_i_s_c flag is ignored. _t_o___m_s specifies the read timeout in milliseconds. The read timeout is used to arrange that the read not necessarily return imme diately 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. _e_r_r_b_u_f is used to return error or warning text. It will be set to error text when ppccaapp__ooppeenn__lliivvee(()) fails and returns NNUULLLL. _e_r_r_b_u_f may also be set to warning text when ppccaapp__ooppeenn__lliivvee(()) suc ceds; to detect this case the caller should store a zero- length string in _e_r_r_b_u_f before calling ppccaapp__ooppeenn__lliivvee(()) and display the warning to the user if _e_r_r_b_u_f is no longer a zero-length string. ppccaapp__ooppeenn__ddeeaadd(()) is used for creating a ppccaapp__tt structure to use when calling the other functions in libpcap. It is typically used when just using libpcap for compiling BPF code. ppccaapp__ooppeenn__oofffflliinnee(()) is called to open a ``savefile'' for reading. _f_n_a_m_e specifies the name of the file to open. The file has the same format as those used by ttccppdduummpp((11)) and ttccppsslliiccee((11)). The name "-" in a synonym for ssttddiinn. _e_r_r_b_u_f is used to return error text and is only set when ppccaapp__ooppeenn__oofffflliinnee(()) fails and returns NNUULLLL. ppccaapp__dduummpp__ooppeenn(()) is called to open a ``savefile'' for writing. The name "-" in a synonym for ssttddoouutt. NNUULLLL is returned on failure. _p is a _p_c_a_p struct as returned by ppccaapp__ooppeenn__oofffflliinnee(()) or ppccaapp__ooppeenn__lliivvee(()). _f_n_a_m_e specifies the name of the file to open. If NNUULLLL is returned, ppccaapp__ggeetteerrrr(()) can be used to get the error text. ppccaapp__sseettnnoonnbblloocckk(()) puts a capture descriptor, opened with ppccaapp__ooppeenn__lliivvee(()), into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode, depending on whether the _n_o_n_b_l_o_c_k argument is non-zero or zero. It has no effect on ``savefiles''. If there is an error, -1 is returned and _e_r_r_b_u_f 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 ppccaapp__ddiiss ppaattcchh(()) will, if no packets are currently available to be read, return 0 immediately rather than blocking waiting for packets to arrive. ppccaapp__lloooopp(()) and ppccaapp__nneexxtt(()) will not work in ``non-blocking'' mode. ppccaapp__ggeettnnoonnbblloocckk(()) 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 _e_r_r_b_u_f is filled in with an appropriate error message. ppccaapp__ffiinnddaallllddeevvss(()) constructs a list of network devices that can be opened with ppccaapp__ooppeenn__lliivvee(()). (Note that there may be network devices that cannot be opened with ppccaapp__ooppeenn__lliivvee(()) by the process calling ppccaapp__ffiinndd aallllddeevvss(()), 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.) _a_l_l_d_e_v_s_p is set to point to the first element of the list; each element of the list is of type ppccaapp__iiff__tt, and has the fol lowing members: nneexxtt if not NNUULLLL, a pointer to the next element in the list; NNUULLLL for the last element of the list nnaammee a pointer to a string giving a name for the device to pass to ppccaapp__ooppeenn__lliivvee(()) ddeessccrriippttiioonn if not NNUULLLL, a pointer to a string giving a human-readable description of the device aaddddrreesssseess a pointer to the first element of a list of addresses for the interface ffllaaggss interface flags: PPCCAAPP__IIFF__LLOOOOPPBBAACCKK set if the interface is a loopback interface Each element of the list of addresses is of type ppccaapp__aaddddrr__tt, and has the following members: nneexxtt if not NNUULLLL, a pointer to the next element in the list; NNUULLLL for the last element of the list aaddddrr a pointer to a ssttrruucctt ssoocckkaaddddrr containing an address nneettmmaasskk if not NNUULLLL, a pointer to a ssttrruucctt ssoocckkaaddddrr that contains the netmask corresponding to the address pointed to by aaddddrr bbrrooaaddaaddddrr if not NNUULLLL, a pointer to a ssttrruucctt ssoocckkaaddddrr that contains the broadcast address corre sponding to the address pointed to by aaddddrr; may be null if the interface doesn't support broadcasts ddssttaaddddrr if not NNUULLLL, a pointer to a ssttrruucctt ssoocckkaaddddrr that contains the destination address corre sponding to the address pointed to by aaddddrr; may be null if the interface isn't a point- to-point interface ppccaapp__ffrreeeeaallllddeevvss(()) is used to free a list allocated by ppccaapp__ffiinnddaallllddeevvss(()). ppccaapp__llooookkuuppddeevv(()) returns a pointer to a network device suitable for use with ppccaapp__ooppeenn__lliivvee(()) and ppccaapp__llooookkuupp nneett(()). If there is an error, NNUULLLL is returned and _e_r_r_b_u_f is filled in with an appropriate error message. ppccaapp__llooookkuuppnneett(()) is used to determine the network number and mask associated with the network device ddeevviiccee. Both _n_e_t_p and _m_a_s_k_p are _b_p_f___u___i_n_t_3_2 pointers. A return of -1 indicates an error in which case _e_r_r_b_u_f is filled in with an appropriate error message. ppccaapp__ddiissppaattcchh(()) is used to collect and process packets. _c_n_t 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 _c_n_t packets may be pro cessed. A _c_n_t 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''. _c_a_l_l_b_a_c_k speci fies a routine to be called with three arguments: a _u___c_h_a_r pointer which is passed in from ppccaapp__ddiissppaattcchh(()), a pointer to the _p_c_a_p___p_k_t_h_d_r struct (which precede the actual net work headers and data), and a _u___c_h_a_r 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 ppccaapp__ppeerrrroorr(()) or ppccaapp__ggeetteerrrr(()) may be used to display the error text. NNOOTTEE: when reading a live capture, ppccaapp__ddiissppaattcchh(()) 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 NNOOTT 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 ppccaapp__ddiiss ppaattcchh(()) will return after the timeout expires. ppccaapp__lloooopp(()) is similar to ppccaapp__ddiissppaattcchh(()) except it keeps reading packets until _c_n_t packets are processed or an error occurs. It does nnoott return when live read timeouts occur. Rather, specifying a non-zero read timeout to ppccaapp__ooppeenn__lliivvee(()) and then calling ppccaapp__ddiissppaattcchh(()) allows the reception and processing of any packets that arrive when the timeout occurs. A negative _c_n_t causes ppccaapp__lloooopp(()) to loop forever (or at least until an error occurs). ppccaapp__nneexxtt(()) reads the next packet (by calling ppccaapp__ddiiss ppaattcchh(()) with a _c_n_t of 1) and returns a _u___c_h_a_r pointer to the data in that packet. (The _p_c_a_p___p_k_t_h_d_r struct for that packet is not supplied.) ppccaapp__dduummpp(()) outputs a packet to the ``savefile'' opened with ppccaapp__dduummpp__ooppeenn(()). Note that its calling arguments are suitable for use with ppccaapp__ddiissppaattcchh(()) or ppccaapp__lloooopp(()). If called directly, the _u_s_e_r parameter is of type _p_c_a_p___d_u_m_p_e_r___t as returned by ppccaapp__dduummpp__ooppeenn(()). ppccaapp__ccoommppiillee(()) is used to compile the string _s_t_r into a filter program. _p_r_o_g_r_a_m is a pointer to a _b_p_f___p_r_o_g_r_a_m struct and is filled in by ppccaapp__ccoommppiillee(()). _o_p_t_i_m_i_z_e con trols whether optimization on the resulting code is per formed. _n_e_t_m_a_s_k specifies the netmask of the local net. A return of -1 indicates an error in which case ppccaapp__ggeetteerrrr(()) may be used to display the error text. ppccaapp__ccoommppiillee__nnooppccaapp(()) is similar to ppccaapp__ccoommppiillee(()) 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 ppccaapp__ooppeenn(()). A return of -1 indicates an error; the error text is unavailable. (ppccaapp__ccoommppiillee__nnooppccaapp(()) is a wrapper around ppccaapp__ooppeenn__ddeeaadd(()), ppccaapp__ccoommppiillee(()), and ppccaapp__cclloossee(()); the latter three routines can be used directly in order to get the error text for a compilation error.) ppccaapp__sseettffiilltteerr(()) is used to specify a filter program. _f_p is a pointer to a _b_p_f___p_r_o_g_r_a_m struct, usually the result of a call to ppccaapp__ccoommppiillee(()). --11 is returned on failure, in which case ppccaapp__ggeetteerrrr(()) may be used to display the error text; 00 is returned on success. ppccaapp__ffrreeeeccooddee(()) is used to free up allocated memory pointed to by a _b_p_f___p_r_o_g_r_a_m struct generated by ppccaapp__ccoomm ppiillee(()) 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 ppccaapp__sseettffiilltteerr(()). ppccaapp__ddaattaalliinnkk(()) returns the link layer type; link layer types it can return include: DDLLTT__NNUULLLL BSD loopback encapsulation; the link layer header is a 4-byte field, in _h_o_s_t byte order, containing a PF_ value from ssoocckkeett..hh for the network-layer protocol of the packet Note that ``host byte order'' is the byte order of the machine on which the packets are cap tured, 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 _n_o_t necessarily those of the machine reading the capture file. DDLLTT__EENN1100MMBB Ethernet (10Mb, 100Mb, 1000Mb, and up) DDLLTT__IIEEEEEE880022 IEEE 802.5 Token Ring DDLLTT__AARRCCNNEETT ARCNET DDLLTT__SSLLIIPP SLIP; the link layer header contains, in order: a 1-byte flag, which is 0 for packets received by the machine and 1 for packets sent by the machine; a 1-byte field, the upper 4 bits of which indicate the type of packet, as per RFC 1144: 0x40 an unmodified IP datagram (TYPE_IP); 0x70 an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being the first byte of the raw IP header on the wire, con taining the connection number in the protocol field; 0x80 a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the first byte of the com pressed TCP/IP datagram header; for UNCOMPRESSED_TCP, the rest of the modi fied IP header, and for COMPRESSED_TCP, the compressed TCP/IP datagram header; for a total of 16 bytes; the uncompressed IP datagram follows the header DDLLTT__PPPPPP 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 DDLLTT__FFDDDDII FDDI DDLLTT__AATTMM__RRFFCC11448833 RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2 LLC header DDLLTT__RRAAWW raw IP; the packet begins with an IP header DDLLTT__PPPPPP__SSEERRIIAALL 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 DDLLTT__PPPPPP__EETTHHEERR PPPoE; the packet begins with a PPPoE header, as per RFC 2516 DDLLTT__CC__HHDDLLCC Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547 DDLLTT__IIEEEEEE880022__1111 IEEE 802.11 wireless LAN DDLLTT__LLOOOOPP OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in _n_e_t_w_o_r_k byte order, containing a PF_ value from OpenBSD's ssoocckkeett..hh for the network-layer protocol of the packet Note that, if a ``savefile'' is being read, those PF_ values are _n_o_t necessarily those of the machine reading the capture file. DDLLTT__LLIINNUUXX__SSLLLL Linux "cooked" capture encapsulation; the link layer header contains, in order: a 2-byte "packet type", in network byte order, which is one of: 0 packet was sent to us by somebody else 1 packet was broadcast by somebody else 2 packet was multicast, but not broadcast, by somebody else 3 packet was sent by somebody else to somebody else 4 packet was sent by us a 2-byte field, in network byte order, con taining a Linux ARPHRD_ value for the link layer device type; a 2-byte field, in network byte order, con taining the length of the link layer address of the sender of the packet (which could be 0); 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); a 2-byte field containing an Ethernet pro tocol type, in network byte order, or con taining 1 for Novell 802.3 frames without an 802.2 LLC header or 4 for frames begin ning with an 802.2 LLC header. DDLLTT__LLTTAALLKK Apple LocalTalk; the packet begins with an AppleTalk LLAP header ppccaapp__ssnnaappsshhoott(()) returns the snapshot length specified when ppccaapp__ooppeenn__lliivvee was called. ppccaapp__iiss__sswwaappppeedd(()) returns true if the current ``savefile'' uses a different byte order than the current system. ppccaapp__mmaajjoorr__vveerrssiioonn(()) returns the major number of the ver sion of the pcap used to write the savefile. ppccaapp__mmiinnoorr__vveerrssiioonn(()) returns the minor number of the ver sion of the pcap used to write the savefile. ppccaapp__ffiillee(()) returns the standard I/O stream of the ``save file,'' if a ``savefile'' was opened with ppccaapp__ooppeenn__oofffflliinnee(()), or NULL, if a network device was opened with ppccaapp__ooppeenn__lliivvee(()). ppccaapp__ssttaattss(()) returns 0 and fills in a ppccaapp__ssttaatt 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 ppccaapp__ppeerrrroorr(()) or ppccaapp__ggeetteerrrr(()). ppccaapp__ssttaattss(()) is supported only on live captures, not on ``savefiles''; no statistics are stored in ``savefiles'', so no statistics are available when reading from a ``save file''. ppccaapp__ffiilleennoo(()) returns the file descriptor number from which captured packets are read, if a network device was opened with ppccaapp__ooppeenn__lliivvee(()), or -1, if a ``savefile'' was opened with ppccaapp__ooppeenn__oofffflliinnee(()). ppccaapp__ppeerrrroorr(()) prints the text of the last pcap library error on ssttddeerrrr, prefixed by _p_r_e_f_i_x. ppccaapp__ggeetteerrrr(()) returns the error text pertaining to the last pcap library error. NNOOTTEE: the pointer it returns will no longer point to a valid error message string after the ppccaapp__tt passed to it is closed; you must use or copy the string before closing the ppccaapp__tt. ppccaapp__ssttrreerrrroorr(()) is provided in case ssttrreerrrroorr(1) isn't available. ppccaapp__cclloossee(()) closes the files associated with _p and deal locates resources. ppccaapp__dduummpp__cclloossee(()) closes the ``savefile.'' SSEEEE AALLSSOO tcpdump(1), tcpslice(1) AAUUTTHHOORRSS The original authors are: Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of Cali fornia, Berkeley, CA. The current version is available from "The Tcpdump Group"'s Web site at _h_t_t_p_:_/_/_w_w_w_._t_c_p_d_u_m_p_._o_r_g_/ BBUUGGSS Please send problems, bugs, questions, desirable enhance ments, etc. to: tcpdump-workers@tcpdump.org Please send source code contributions, etc. to: patches@tcpdump.org 3 January 2001 PCAP(3)