PCAP(3) PCAP(3)
NAME
pcap - Packet Capture library
SYNOPSIS
#include <pcap.h>
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,
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)
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 ker-nels, kernels,
nels, 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 time-out, timeout,
out, 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 dis-play display
play 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(1) and tcpslice(1). 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 nonzero
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 corre-sponding corresponding
sponding to the address pointed to by addr; may be null if the interface doesn't sup-port support
port broadcasts
dstaddr
if not NULL, a pointer to a struct sockaddr that contains the destination address cor-responding corresponding
responding 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 pro-cesses processes
cesses all the packets received in one buffer when reading a live capture, or all the packets in the
file when reading a ``savefile''. callback specifies a routine to be called with three arguments: a
u_char pointer which is passed in from pcap_dispatch(), a 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 cap-ture capture
ture
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 pack-ets packets
ets 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 pro-
cessed 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_break-
loop(), 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 pro-cessing processing
cessing 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 num-ber number
ber 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 send-ing sending
ing packets; as pcap_open_live() doesn't have a flag to indicate whether to open for capturing, send-ing, sending,
ing, 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 what-ever whatever
ever 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 com-patibility.) compatibility.)
patibility.)
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 net-work, network,
work, 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, usu-ally usually
ally 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 fil-ter filter
ter 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 func-tion function
tion 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 cap-tured; captured;
tured; 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 cap-turing capturing
turing 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 num-ber number
ber 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 con-taining containing
taining 1 for Novell 802.3 frames without an 802.2 LLC header or 4 for frames begin-ning beginning
ning 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.shaft-
net.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 associ-
ated 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 cur-
rent system.
pcap_major_version() returns the major number of the file format of the savefile; pcap_minor_ver-
sion() 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 a pcap_stat struct. The values represent packet statistics from
the start of the run to the time of the call. If there is an error or the underlying packet capture
doesn't support packet statistics, -1 is returned and the error text can be obtained with pcap_per-
ror() 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 ``save-
file''.
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 cor-
rectly on BPF devices; pcap_get_selectable_fd() will return a file descriptor on most of those ver-
sions (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(1), tcpslice(1)
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, Uni-
versity 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
27 February 2004 PCAP(3)
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