pcap.3

用来监视网络通信数据的源代码和应用程序,方便网络程序底层开发.

.PP
.B pcap_loop()
is similar to
.B pcap_dispatch()
except it keeps reading packets until
.I cnt
packets are processed or an error occurs.
It does
.B not
return when live read timeouts occur.
Rather, specifying a non-zero read timeout to
.B pcap_open_live()
and then calling
.B pcap_dispatch()
allows the reception and processing of any packets that arrive when the
timeout occurs.
A negative
.I cnt
causes
.B pcap_loop()
to loop forever (or at least until an error occurs).  A negative number
is returned on an error; 0 is returned if
.I cnt
is exhausted.
.PP
.B pcap_next()
reads the next packet (by calling
.B pcap_dispatch()
with a
.I cnt
of 1) and returns a
.I u_char
pointer to the data in that packet.  (The
.I pcap_pkthdr
struct for that packet is not supplied.)
.B NULL
is returned if an error occured, or if no packets were read from a live
capture (if, for example, they were discarded because they didn't pass
the packet filter, or if, on platforms that support a read timeout that
starts before any packets arrive, the timeout expires before any packets
arrive, or if the file descriptor for the capture device is in
non-blocking mode and no packets were available to be read), or if no
more packets are available in a ``savefile.''  Unfortunately, there is
no way to determine whether an error occured or not.
.PP
.B pcap_dump()
outputs a packet to the ``savefile'' opened with
.BR pcap_dump_open() .
Note that its calling arguments are suitable for use with
.B pcap_dispatch()
or
.BR pcap_loop() .
If called directly, the 
.I user
parameter is of type 
.I pcap_dumper_t
as returned by
.BR pcap_dump_open() .
.PP
.B pcap_compile()
is used to compile the string
.I str
into a filter program.
.I program
is a pointer to a
.I bpf_program
struct and is filled in by
.BR pcap_compile() .
.I optimize
controls whether optimization on the resulting code is performed.
.I netmask
specifies the IPv4 netmask of the network on which packets are being
captured; it is used only when checking for IPv4 broadcast addresses in
the filter program.  If the netmask of the network on which packets are
being captured isn't known to the program, or if packets are being
captured on the Linux "any" pseudo-interface that can capture on more
than one network, a value of 0 can be supplied; tests for IPv4 broadcast
addreses won't be done correctly, but all other tests in the filter
program will be OK.  A return of \-1 indicates an error in which case
.BR pcap_geterr()
may be used to display the error text.
.PP
.B pcap_compile_nopcap()
is similar to
.B pcap_compile()
except that instead of passing a pcap structure, one passes the
snaplen and linktype explicitly.  It is intended to be used for
compiling filters for direct BPF usage, without necessarily having
called
.BR pcap_open() .
A return of \-1 indicates an error; the error text is unavailable.
.RB ( pcap_compile_nopcap()
is a wrapper around
.BR pcap_open_dead() ,
.BR pcap_compile() ,
and
.BR pcap_close() ;
the latter three routines can be used directly in order to get the error
text for a compilation error.)
.B
.PP
.B pcap_setfilter()
is used to specify a filter program.
.I fp
is a pointer to a
.I bpf_program
struct, usually the result of a call to
.BR pcap_compile() .
.B \-1
is returned on failure, in which case
.BR pcap_geterr()
may be used to display the error text;
.B 0
is returned on success.
.PP
.B pcap_freecode()
is used to free up allocated memory pointed to by a
.I bpf_program
struct generated by
.B pcap_compile()
when that BPF program is no longer needed, for example after it
has been made the filter program for a pcap structure by a call to
.BR pcap_setfilter() .
.PP
.B pcap_datalink()
returns the link layer type; link layer types it can return include:
.PP
.RS 5
.TP 5
.B DLT_NULL
BSD loopback encapsulation; the link layer header is a 4-byte field, in
.I host
byte order, containing a PF_ value from
.B socket.h
for the network-layer protocol of the packet
.IP
Note that ``host byte order'' is the byte order of the machine on which
the packets are captured, and the PF_ values are for the OS of the
machine on which the packets are captured; if a live capture is being
done, ``host byte order'' is the byte order of the machine capturing the
packets, and the PF_ values are those of the OS of the machine capturing
the packets, but if a ``savefile'' is being read, the byte order and PF_
values are
.I not
necessarily those of the machine reading the capture file.
.TP 5
.B DLT_EN10MB
Ethernet (10Mb, 100Mb, 1000Mb, and up)
.TP 5
.B DLT_IEEE802
IEEE 802.5 Token Ring
.TP 5
.B DLT_ARCNET
ARCNET
.TP 5
.B DLT_SLIP
SLIP; the link layer header contains, in order:
.RS 10
.LP
a 1-byte flag, which is 0 for packets received by the machine and 1 for
packets sent by the machine;
.LP
a 1-byte field, the upper 4 bits of which indicate the type of packet,
as per RFC 1144:
.RS 5
.TP 5
0x40
an unmodified IP datagram (TYPE_IP);
.TP 5
0x70
an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being
the first byte of the raw IP header on the wire, containing the
connection number in the protocol field;
.TP 5
0x80
a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the
first byte of the compressed TCP/IP datagram header;
.RE
.LP
for UNCOMPRESSED_TCP, the rest of the modified IP header, and for
COMPRESSED_TCP, the compressed TCP/IP datagram header;
.RE
.RS 5
.LP
for a total of 16 bytes; the uncompressed IP datagram follows the header
.RE
.TP 5
.B DLT_PPP
PPP; if the first 2 bytes are 0xff and 0x03, it's PPP in HDLC-like
framing, with the PPP header following those two bytes, otherwise it's
PPP without framing, and the packet begins with the PPP header
.TP 5
.B DLT_FDDI
FDDI
.TP 5
.B DLT_ATM_RFC1483
RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2
LLC header
.TP 5
.B DLT_RAW
raw IP; the packet begins with an IP header
.TP 5
.B DLT_PPP_SERIAL
PPP in HDLC-like framing, as per RFC 1662, or Cisco PPP with HDLC
framing, as per section 4.3.1 of RFC 1547; the first byte will be 0xFF
for PPP in HDLC-like framing, and will be 0x0F or 0x8F for Cisco PPP
with HDLC framing
.TP 5
.B DLT_PPP_ETHER
PPPoE; the packet begins with a PPPoE header, as per RFC 2516
.TP 5
.B DLT_C_HDLC
Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
.TP 5
.B DLT_IEEE802_11
IEEE 802.11 wireless LAN
.TP 5
.B DLT_FRELAY
Frame Relay
.TP 5
.B DLT_LOOP
OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in
.I network
byte order, containing a PF_ value from OpenBSD's
.B socket.h
for the network-layer protocol of the packet
.IP
Note that, if a ``savefile'' is being read, those PF_ values are
.I not
necessarily those of the machine reading the capture file.
.TP 5
.B DLT_LINUX_SLL
Linux "cooked" capture encapsulation; the link layer header contains, in
order:
.RS 10
.LP
a 2-byte "packet type", in network byte order, which is one of:
.RS 5
.TP 5
0
packet was sent to us by somebody else
.TP 5
1
packet was broadcast by somebody else
.TP 5
2
packet was multicast, but not broadcast, by somebody else
.TP 5
3
packet was sent by somebody else to somebody else
.TP 5
4
packet was sent by us
.RE
.LP
a 2-byte field, in network byte order, containing a Linux ARPHRD_ value
for the link layer device type;
.LP
a 2-byte field, in network byte order, containing the length of the
link layer address of the sender of the packet (which could be 0);
.LP
an 8-byte field containing that number of bytes of the link layer header
(if there are more than 8 bytes, only the first 8 are present);
.LP
a 2-byte field containing an Ethernet protocol type, in network byte
order, or containing 1 for Novell 802.3 frames without an 802.2 LLC
header or 4 for frames beginning with an 802.2 LLC header.
.RE
.TP 5
.B DLT_LTALK
Apple LocalTalk; the packet begins with an AppleTalk LLAP header
.TP 5
.B DLT_SUNATM
SunATM devices; the link layer header contains, in order:
.RS 10
.LP
a 1-byte flag field, containing a direction flag in the uppermost bit,
which is set for packets transmitted by the machine and clear for
packets received by the machine, and a 4-byte traffic type in the
low-order 4 bits, which is one of:
.RS 5
.TP 5
0
raw traffic
.TP 5
1
LANE traffic
.TP 5
2
LLC-encapsulated traffic
.TP 5
3
MARS traffic
.TP 5
4
IFMP traffic
.TP 5
5
ILMI traffic
.TP 5
6
Q.2931 traffic
.RE
.LP
a 1-byte VPI value;
.LP
a 2-byte VCI field, in network byte order.
.RE
.TP 5
.B DLT_IP_OVER_FC
RFC 2625 IP-over-Fibre Channel, with the link-layer header being the
Network_Header as described in that RFC.
.RE
.PP
.B pcap_list_datalinks()
is used to get a list of the supported data link types of the interface
associated with the pcap descriptor.
.B pcap_list_datalinks()
allocates an array to hold the list and sets
.IR *dlt_buf .
The caller is responsible for freeing the array.
.B \-1
is returned on failure;
otherwise, the number of data link types in the array is returned.
.PP
.B pcap_set_datalink()
is used to set the current data link type of the pcap descriptor
to the type specified by
.IR dlt .
.B \-1
is returned on failure.
.PP
.B pcap_datalink_name_to_val()
translates a data link type name, which is a
.B DLT_
name with the
.B DLT_
removed, to the corresponding data link type value.  The translation
is case-insensitive.  
is used to set the current data link type of the pcap descriptor
NULL is returned on failure.
.PP
.B pcap_datalink_val_to_name()
translates a data link type value to the corresponding data link type
name.
.B \-1
is returned on failure.
.PP
.B pcap_snapshot()
returns the snapshot length specified when
.B pcap_open_live()
was called.
.PP
.B pcap_is_swapped()
returns true if the current ``savefile'' uses a different byte order
than the current system.
.PP
.B pcap_major_version()
returns the major number of the version of the pcap used to write the
savefile.
.PP
.B pcap_minor_version()
returns the minor number of the version of the pcap used to write the
savefile.
.PP
.B pcap_file()
returns the standard I/O stream of the ``savefile,'' if a ``savefile''
was opened with
.BR pcap_open_offline() ,
or NULL, if a network device was opened with
.BR pcap_open_live() .
.PP
.B pcap_stats()
returns 0 and fills in a
.B pcap_stat
struct. The values represent packet statistics from the start of the
run to the time of the call. If there is an error or the underlying
packet capture doesn't support packet statistics, \-1 is returned and
the error text can be obtained with
.B pcap_perror()
or
.BR pcap_geterr() .
.B pcap_stats()
is supported only on live captures, not on ``savefiles''; no statistics
are stored in ``savefiles'', so no statistics are available when reading
from a ``savefile''.
.PP
.B pcap_fileno()
returns the file descriptor number from which captured packets are read,
if a network device was opened with
.BR pcap_open_live() ,
or \-1, if a ``savefile'' was opened with
.BR pcap_open_offline() .
.PP
.B pcap_perror()
prints the text of the last pcap library error on
.BR stderr ,
prefixed by
.IR prefix .
.PP
.B pcap_geterr()
returns the error text pertaining to the last pcap library error.
.BR NOTE :
the pointer it returns will no longer point to a valid error message
string after the
.B pcap_t
passed to it is closed; you must use or copy the string before closing
the
.BR pcap_t .
.PP
.B pcap_strerror()
is provided in case
.BR strerror (1)
isn't available.
.PP
.B pcap_close()
closes the files associated with
.I p
and deallocates resources.
.PP
.B pcap_dump_flush()
flushes the output buffer to the ``savefile,'' so that any packets
written with
.B pcap_dump()
but not yet written to the ``savefile'' will be written.
.B \-1
is returned on error, 0 on success.
.PP
.B pcap_dump_close()
closes the ``savefile.''
.PP
.SH SEE ALSO
tcpdump(1), tcpslice(1)
.SH AUTHORS
The original authors are:
.LP
Van Jacobson,
Craig Leres and
Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.LP
The current version is available from "The Tcpdump Group"'s Web site at
.LP
.RS
.I http://www.tcpdump.org/
.RE
.SH BUGS
Please send problems, bugs, questions, desirable enhancements, etc. to:
.LP
.RS
[e-mail]
.RE
.LP
Please send source code contributions, etc. to:
.LP
.RS
[e-mail]
.RE