FreeBSD Man Pagespcap(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_dumper_t *pcap_dump_open(pcap_t *p, const char *fname)
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)
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 *);
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_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_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_setnonblock(), pcap_getnonblock(), pcap_find-
alldevs(), 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 net-
work 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 cap-
ture. If this value is less than the size of a packet that is cap-
tured, 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 promiscu-
ous 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 pack-
ets 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 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. errbuf is used to return error text and is only set
when pcap_open_offline() fails and returns NULL.
pcap_dump_open() is called to open a ``savefile'' for writing. The name
"-" in a synonym for stdout. NULL is returned on failure. p is a pcap
struct as returned by pcap_open_offline() or pcap_open_live(). fname
specifies the name of the file to open. If NULL is returned,
pcap_geterr() can be used to get the error text.
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-read-
able 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
alldevs().
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 asso-
ciated 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 read-
ing 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 mem-
bers:
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 time-
out expires before any packets arrive, or if the file descriptor for
the capture device is in non-blocking mode and no packets were avail-
able 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
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 pack-
ets 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 time-
out 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 con-
sole 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
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 posi-
tive number is returned, the flag is not cleared, so a subsequent call
will return -2 and clear the flag.
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 result-
ing code is performed. netmask specifies the IPv4 netmask of the net-
work 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 pro-
gram, or if packets are being captured on the Linux "any" pseudo-inter-
face 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_com-
pile_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_com-
pile(). -1 is returned on failure, in which case pcap_geterr() may be
used to display the error text; 0 is returned on success.
pcap_freecode() is used to free up allocated memory pointed to by a
bpf_program struct generated by pcap_compile() when that BPF program is
no longer needed, for example after it has been made the filter program
for a pcap structure by a call to pcap_setfilter().
pcap_datalink() returns the link layer type; link layer types it can
return include:
are for the OS of the machine on which the packets are cap-
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 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 (UNCOM-
PRESSED_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
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_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
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
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.
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 fail-
ure.
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 fail-
ure.
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 fail-
ure.
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_is_swapped() returns true if the current ``savefile'' uses a dif-
ferent 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 a pcap_stat struct. The values rep-
resent 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 sup-
ported 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 descrip-
tor 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 work-
around 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.
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_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, 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
27 February 2004 PCAP(3)
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