--- name: "pcap" section: "3" platform: "linux" package: "libpcap0.8-dev" version: "0.9.8-5" release: "Debian 5.0 (Lenny)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/pcap" --- # pcap(3) Packet Capture library ## NAME pcap - Packet Capture library ## SYNOPSIS ``` #include ``` ``` char errbuf[PCAP_ERRBUF_SIZE]; ``` **pcap\_t \*pcap\_open\_live(const char \*device, int snaplen,** **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)** **int pcap\_setnonblock(pcap\_t \*p, int nonblock, char \*errbuf);** **int pcap\_getnonblock(pcap\_t \*p, char \*errbuf);** **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,** **bpf\_u\_int32 \*maskp, char \*errbuf)** **typedef void (\*pcap\_handler)(u\_char \*user, const struct pcap\_pkthdr \*h,**\ **const u\_char \*bytes);** **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,** **const 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, pcap\_direction\_t d)** **const 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)** **void pcap\_breakloop(pcap\_t \*)** **int pcap\_inject(pcap\_t \*p, const void \*buf, size\_t size)** **int pcap\_sendpacket(pcap\_t \*p, const u\_char \*buf, int size)** **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)** **const char \*pcap\_strerror(int error)** **const char \*pcap\_lib\_version(void)** **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)** ## 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.** ## ROUTINES **NOTE:** *errbuf* **in** **pcap\_open\_live()**, **pcap\_open\_dead()**, **pcap\_open\_offline()**, **pcap\_fopen\_offline()**, **pcap\_setnonblock()**, **pcap\_getnonblock()**, **pcap\_findalldevs()**, **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. If this value is less** **than the size of a packet that is captured, only the first** *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.** *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. A zero value for** *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.** *errbuf* **is used to return error or warning text. It will be set to error text when** **pcap\_open\_live()** **fails and returns** **NULL**. *errbuf* **may also be set to warning text when** **pcap\_open\_live()** **succeds; to detect this case the caller should store a zero-length string in** *errbuf* **before calling** **pcap\_open\_live()** **and display the warning to the user if** *errbuf* **is no longer a zero-length string.** **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)](/tcpdump/8.md)** **and** **[tcpslice(1)](/tcpslice/1.md)**. **The name "-" in a synonym for** **stdin**. **Alternatively, you may call** **pcap\_fopen\_offline()** **to read dumped data from an existing open stream** *fp*. **Note that on Windows, that stream should be opened in binary mode.** *errbuf* **is used to return error text and is only set when** **pcap\_open\_offline()** **or** **pcap\_fopen\_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. Alternatively, you may call** **pcap\_dump\_fopen()** **to write data to an existing open stream** *fp*. **Note that on Windows, that stream should be opened in binary mode.** **If** **NULL** **is returned,** **pcap\_geterr()** **can be used to get the error text.** **pcap\_setnonblock()** **puts a capture descriptor, opened with** **pcap\_open\_live()**, **into \`\`non-blocking'' mode, or takes it out of \`\`non-blocking'' mode,** **depending on whether the** *nonblock* **argument is non-zero or zero. It has no effect on \`\`savefiles''.** **If there is an error, -1 is returned and** *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** **pcap\_dispatch()** **will, if no packets are currently available to be read, return 0** **immediately rather than blocking waiting for packets to arrive.** **pcap\_loop()** **and** **pcap\_next()** **will not work in \`\`non-blocking'' mode.** **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** *errbuf* **is filled in with an appropriate error message.** **pcap\_findalldevs()** **constructs a list of network devices that can be opened with** **pcap\_open\_live()**. **(Note that there may be network devices that cannot be opened with** **pcap\_open\_live()** **by the** **process calling** **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.)** *alldevsp* **is set to point to the first element of the list; each element of the** **list is of type** **pcap\_if\_t**, **and has the following members:** - **next** **if not** **NULL**, **a pointer to the next element in the list;** **NULL** **for the last element of the list** - **name** **a pointer to a string giving a name for the device to pass to** **pcap\_open\_live()** - **description** **if not** **NULL**, **a pointer to a string giving a human-readable description of the device** - **addresses** **a pointer to the first element of a list of addresses for the interface** - **flags** **interface flags:** - **PCAP\_IF\_LOOPBACK** **set if the interface is a loopback interface** **Each element of the list of addresses is of type** **pcap\_addr\_t**, **and has the following members:** - **next** **if not** **NULL**, **a pointer to the next element in the list;** **NULL** **for the last element of the list** - **addr** **a pointer to a** **struct sockaddr** **containing an address** - **netmask** **if not** **NULL**, **a pointer to a** **struct sockaddr** **that contains the netmask corresponding to the address pointed to by** **addr** - **broadaddr** **if not** **NULL**, **a pointer to a** **struct sockaddr** **that contains the broadcast address corresponding to the address pointed** **to by** **addr**; **may be null if the interface doesn't support broadcasts** - **dstaddr** **if not** **NULL**, **a pointer to a** **struct sockaddr** **that contains the destination address corresponding to the address pointed** **to by** **addr**; **may be null if the interface isn't a point-to-point interface** **Note that not all the addresses in the list of addresses are** **necessarily IPv4 or IPv6 addresses - you must check the** **sa\_family** **member of the** **struct sockaddr** **before interpreting the contents of the address.** **-1** **is returned on failure, in which case** **errbuf** **is filled in with an appropriate error message;** **0** **is returned on success.** **pcap\_freealldevs()** **is used to free a list allocated by** **pcap\_findalldevs()**. **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** *pcap\_handler* **routine to be called with three arguments:** **a** *u\_char* **pointer which is passed in from** **pcap\_dispatch()**, **a** *const struct pcap\_pkthdr* **pointer to a structure with the following members:** - **ts** **a** *struct timeval* **containing the time when the packet was captured** - **caplen** **a** *bpf\_u\_int32* **giving the number of bytes of the packet that are available from the** **capture** - **len** **a** *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)** **and a** *const u\_char* **pointer to the first** **caplen** **(as given in the** *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** *snaplen* **in your call to** **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).** **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.** **A return of -2 indicates that the loop terminated due to a call to** **pcap\_breakloop()** **before any packets were processed.** **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.** **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). -1 is returned on an error; 0 is returned if *cnt* is exhausted; -2 is returned if the loop terminated due to a call to **pcap\_breakloop()** before any packets were processed. **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.** **pcap\_next()** reads the next packet (by calling **pcap\_dispatch()** with a *cnt* of 1) and returns a *u\_char* pointer to the data in that packet. (The *pcap\_pkthdr* struct for that packet is not supplied.) **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. **pcap\_next\_ex()** reads the next packet and returns a success/failure indication: - 1 the packet was read without problems - 0 packets are being read from a live capture, and the timeout expired - \-1 an error occurred while reading the packet - \-2 packets are being read from a \`\`savefile'', and there are no more packets to read from the savefile. If the packet was read without problems, the pointer pointed to by the *pkt\_header* argument is set to point to the *pcap\_pkthdr* struct for the packet, and the pointer pointed to by the *pkt\_data* argument is set to point to the data in the packet. **pcap\_breakloop()** sets a flag that will force **pcap\_dispatch()** or **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. 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. 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. **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.** **Note also that, in a multi-threaded application, if one thread is** **blocked in** **pcap\_dispatch()**, **pcap\_loop()**, **pcap\_next()**, **or** **pcap\_next\_ex()**, **a call to** **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.** Note that **pcap\_next()** will, on some platforms, loop reading packets from the OS; that loop will not necessarily be terminated by a signal, so **pcap\_breakloop()** should be used to terminate packet processing even if **pcap\_next()** is being used. **pcap\_breakloop()** does not guarantee that no further packets will be processed by **pcap\_dispatch()** or **pcap\_loop()** after it is called; at most one more packet might be processed. If -2 is returned from **pcap\_dispatch()** or **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. **pcap\_inject()** sends a raw packet through the network interface; *buf* points to the data of the packet, including the link-layer header, and *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 **pcap\_perror()** or **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 *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. 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 **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. Even worse, some drivers on some platforms might change the link-layer type field to whatever value libpcap used when attaching to the device, even on platforms that *do* nominally support sending completely raw and unchanged packets. **pcap\_sendpacket()** is like **pcap\_inject()**, but it returns 0 on success and -1 on failure. (**pcap\_inject()** comes from OpenBSD; **pcap\_sendpacket()** comes from WinPcap. Both are provided for compatibility.) **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()**. If called directly, the *user* parameter is of type *pcap\_dumper\_t* as returned by **pcap\_dump\_open()**. **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 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 **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\_setdirection()** is used to specify a direction that packets will be captured. *pcap\_direction\_t* is one of the constants **PCAP\_D\_IN**, **PCAP\_D\_OUT** or **PCAP\_D\_INOUT**. **PCAP\_D\_IN** will only capture packets received by the device, **PCAP\_D\_OUT** will only capture packets sent by the device and **PCAP\_D\_INOUT** will capture packets received by or sent by the device. **PCAP\_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 **PCAP\_D\_OUT**. This operation is not supported if a \`\`savefile'' is being read. **-1** is returned on failure, **0** is returned on success. **pcap\_datalink()** returns the link layer type; link layer types it can return include: - **DLT\_NULL** BSD loopback encapsulation; the link layer header is a 4-byte field, in *host* byte order, containing a PF\_ value from **socket.h** 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 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 *not* necessarily those of the machine reading the capture file. - **DLT\_EN10MB** Ethernet (10Mb, 100Mb, 1000Mb, and up) - **DLT\_IEEE802** IEEE 802.5 Token Ring - **DLT\_ARCNET** ARCNET - **DLT\_SLIP** 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, containing the connection number in the protocol field; - 0x80 a compressed-TCP IP datagram (COMPRESSED\_TCP), with that byte being the first byte of the compressed TCP/IP datagram header; for UNCOMPRESSED\_TCP, the rest of the modified 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. - **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. - **DLT\_FDDI** FDDI - **DLT\_ATM\_RFC1483** RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2 LLC header. - **DLT\_RAW** raw IP; the packet begins with an IP header. - **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. - **DLT\_PPP\_ETHER** PPPoE; the packet begins with a PPPoE header, as per RFC 2516. - **DLT\_C\_HDLC** Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547. - **DLT\_IEEE802\_11** IEEE 802.11 wireless LAN - **DLT\_FRELAY** Frame Relay - **DLT\_LOOP** OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in *network* byte order, containing a PF\_ value from OpenBSD's **socket.h** for the network-layer protocol of the packet. - Note that, if a \`\`savefile'' is being read, those PF\_ values are *not* necessarily those of the machine reading the capture file. - **DLT\_LINUX\_SLL** 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, containing a Linux ARPHRD\_ value for the link layer device type; 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); 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 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. - **DLT\_LTALK** Apple LocalTalk; the packet begins with an AppleTalk LLAP header. - **DLT\_PFLOG** OpenBSD pflog; the link layer header contains, in order: a 1-byte header length, in host byte order; a 4-byte PF\_ value, in host byte order; a 2-byte action code, in network byte order, which is one of: - 0 passed - 1 dropped - 2 scrubbed a 2-byte reason code, in network byte order, which is one of: - 0 match - 1 bad offset - 2 fragment - 3 short - 4 normalize - 5 memory a 16-character interface name; a 16-character ruleset name (only meaningful if subrule is set); a 4-byte rule number, in network byte order; a 4-byte subrule number, in network byte order; a 1-byte direction, in network byte order, which is one of: - 0 incoming or outgoing - 1 incoming - 2 outgoing - **DLT\_PRISM\_HEADER** Prism monitor mode information followed by an 802.11 header. - **DLT\_IP\_OVER\_FC** RFC 2625 IP-over-Fibre Channel, with the link-layer header being the Network\_Header as described in that RFC. - **DLT\_SUNATM** SunATM devices; the link layer header contains, in order: 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: - 0 raw traffic - 1 LANE traffic - 2 LLC-encapsulated traffic - 3 MARS traffic - 4 IFMP traffic - 5 ILMI traffic - 6 Q.2931 traffic a 1-byte VPI value; a 2-byte VCI field, in network byte order. - **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. - **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. - **DLT\_LINUX\_IRDA** Linux-IrDA packets, with a **DLT\_LINUX\_SLL** header followed by the IrLAP header. - **DLT\_LINUX\_LAPD** LAPD (Q.921) frames, with a **DLT\_LINUX\_SLL** header captured via vISDN. **pcap\_list\_datalinks()** is used to get a list of the supported data link types of the interface associated with the pcap descriptor. **pcap\_list\_datalinks()** allocates an array to hold the list and sets *\*dlt\_buf*. The caller is responsible for freeing the array. **-1** is returned on failure; otherwise, the number of data link types in the array is returned. **pcap\_set\_datalink()** is used to set the current data link type of the pcap descriptor to the type specified by *dlt*. **-1** is returned on failure. **pcap\_datalink\_name\_to\_val()** translates a data link type name, which is a **DLT\_** name with the **DLT\_** removed, to the corresponding data link type value. The translation is case-insensitive. **-1** is returned on failure. **pcap\_datalink\_val\_to\_name()** translates a data link type value to the corresponding data link type name. NULL is returned on failure. **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. **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 file format of the savefile; **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. **pcap\_file()** returns the standard I/O stream of the \`\`savefile,'' if a \`\`savefile'' was opened with **pcap\_open\_offline()**, or NULL, if a network device was opened with **pcap\_open\_live()**. **pcap\_stats()** returns 0 and fills in the **pcap\_stat** structure pointed to by its second argument. 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\_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''. **pcap\_fileno()** returns the file descriptor number from which captured packets are read, if a network device was opened with **pcap\_open\_live()**, or -1, if a \`\`savefile'' was opened with **pcap\_open\_offline()**. **pcap\_get\_selectable\_fd()** returns, on UNIX, a file descriptor number for a file descriptor on which one can do a **select()** or **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 **pcap\_open\_live()** do not support **select()** or **poll()** (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned for those devices. Note that on most versions of most BSDs (including Mac OS X) **select()** and **poll()** do not work correctly on BPF devices; **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 **select()** or **poll()** will not return even after a timeout specified in **pcap\_open\_live()** expires. To work around this, an application that uses **select()** or **poll()** to wait for packets to arrive must put the **pcap\_t** in non-blocking mode, and must arrange that the **select()** or **poll()** have a timeout less than or equal to the timeout specified in **pcap\_open\_live()**, and must try to read packets after that timeout expires, regardless of whether **select()** or **poll()** indicated that the file descriptor for the **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, **select()** and **poll()** work correctly on BPF devices, so the workaround isn't necessary, although it does no harm.) **pcap\_get\_selectable\_fd()** is not available on Windows. **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\_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. **pcap\_close()** closes the files associated with *p* and deallocates resources. **pcap\_dump\_file()** returns the standard I/O stream of the \`\`savefile'' opened by **pcap\_dump\_open().** **pcap\_dump\_flush()** flushes the output buffer to the \`\`savefile,'' so that any packets written with **pcap\_dump()** but not yet written to the \`\`savefile'' will be written. **-1** is returned on error, 0 on success. **pcap\_dump\_ftell()** returns the current file position for the \`\`savefile'', representing the number of bytes written by **pcap\_dump\_open()** and **pcap\_dump()**. **-1** is returned on error. **pcap\_dump\_close()** closes the \`\`savefile.'' ## SEE ALSO [tcpdump(8)](/tcpdump/8.md), [tcpslice(1)](/tcpslice/1.md) ## AUTHORS 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 *http://www.tcpdump.org/* ## BUGS Please send problems, bugs, questions, desirable enhancements, etc. to: tcpdump-workers@tcpdump.org Please send source code contributions, etc. to: patches@tcpdump.org