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.

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.TH PCAP 3 "3 January 2001"
.SH NAME
pcap \- Packet Capture library
.SH SYNOPSIS
.nf
.ft B
#include <pcap.h>
.ft
.LP
.nf
.ft B
char errbuf[PCAP_ERRBUF_SIZE];
.ft
.LP
.ft B
pcap_t *pcap_open_live(const char *device, int snaplen,
.ti +8
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)
.ft
.LP
.ft B
int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
int pcap_getnonblock(pcap_t *p, char *errbuf);
.ft
.LP
.ft B
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,
.ti +8
bpf_u_int32 *maskp, char *errbuf)
.ft
.LP
.ft B
int pcap_dispatch(pcap_t *p, int cnt,
.ti +8
pcap_handler callback, u_char *user)
int pcap_loop(pcap_t *p, int cnt,
.ti +8
pcap_handler callback, u_char *user)
void pcap_dump(u_char *user, struct pcap_pkthdr *h,
.ti +8
u_char *sp)
.ft
.LP
.ft B
int pcap_compile(pcap_t *p, struct bpf_program *fp,
.ti +8
char *str, int optimize, bpf_u_int32 netmask)
int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
void pcap_freecode(struct bpf_program *);
.ft
.LP
.ft B
const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
.ft
.LP
.ft B
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);
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)
void pcap_perror(pcap_t *p, char *prefix)
char *pcap_geterr(pcap_t *p)
char *pcap_strerror(int error)
.ft
.LP
.ft B
void pcap_close(pcap_t *p)
int pcap_dump_flush(pcap_dumper_t *p)
void pcap_dump_close(pcap_dumper_t *p)
.ft
.fi
.SH 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.
.PP
.SH ROUTINES
NOTE:
.I errbuf
in
.BR pcap_open_live() ,
.BR pcap_open_dead() ,
.BR pcap_open_offline() ,
.BR pcap_setnonblock() ,
.BR pcap_getnonblock() ,
.BR pcap_findalldevs() ,
.BR pcap_lookupdev() ,
and
.B pcap_lookupnet()
is assumed to be able to hold at least
.B PCAP_ERRBUF_SIZE
chars.
.PP
.B pcap_open_live()
is used to obtain a packet capture descriptor to look
at packets on the network.
.I device
is a string that specifies the network device to open; on Linux systems
with 2.2 or later kernels, a
.I device
argument of "any" or
.B NULL
can be used to capture packets from all interfaces.
.I 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
.I 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.
.I 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
.I promisc
flag is ignored.
.I 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
.IR 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.
.I errbuf
is used to return error or warning text.  It will be set to error text when
.B pcap_open_live()
fails and returns
.BR NULL .
.I errbuf
may also be set to warning text when
.B pcap_open_live()
succeds; to detect this case the caller should store a zero-length string in
.I errbuf
before calling
.B pcap_open_live()
and display the warning to the user if
.I errbuf
is no longer a zero-length string.
.PP
.B pcap_open_dead()
is used for creating a
.B pcap_t
structure to use when calling the other functions in libpcap.  It is
typically used when just using libpcap for compiling BPF code.
.PP
.B pcap_open_offline()
is called to open a ``savefile'' for reading.
.I fname
specifies the name of the file to open. The file has
the same format as those used by
.B tcpdump(1)
and
.BR tcpslice(1) .
The name "-" in a synonym for
.BR stdin .
.I errbuf
is used to return error text and is only set when
.B pcap_open_offline()
fails and returns
.BR NULL .
.PP
.B pcap_dump_open()
is called to open a ``savefile'' for writing. The name "-" in a synonym
for
.BR stdout .
.B NULL
is returned on failure.
.I p
is a
.I pcap
struct as returned by
.B pcap_open_offline()
or
.BR pcap_open_live() .
.I fname
specifies the name of the file to open.
If
.B NULL
is returned,
.B pcap_geterr()
can be used to get the error text.
.PP
.B pcap_setnonblock()
puts a capture descriptor, opened with
.BR pcap_open_live() ,
into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode,
depending on whether the
.I nonblock
argument is non-zero or zero.  It has no effect on ``savefiles''.
If there is an error, \-1 is returned and
.I 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
.B pcap_dispatch()
will, if no packets are currently available to be read, return 0
immediately rather than blocking waiting for packets to arrive.
.B pcap_loop()
and
.B pcap_next()
will not work in ``non-blocking'' mode.
.PP
.B 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
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_findalldevs()
constructs a list of network devices that can be opened with
.BR pcap_open_live() .
(Note that there may be network devices that cannot be opened with
.BR pcap_open_live()
by the
process calling
.BR 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.)
.I alldevsp
is set to point to the first element of the list; each element of the
list is of type
.BR pcap_if_t ,
and has the following members:
.RS
.TP
.B next
if not
.BR NULL ,
a pointer to the next element in the list;
.B NULL
for the last element of the list
.TP
.B name
a pointer to a string giving a name for the device to pass to
.B pcap_open_live()
.TP
.B description
if not
.BR NULL ,
a pointer to a string giving a human-readable description of the device
.TP
.B addresses
a pointer to the first element of a list of addresses for the interface
.TP
.B flags
interface flags:
.RS
.TP
.B PCAP_IF_LOOPBACK
set if the interface is a loopback interface
.RE
.RE
.PP
Each element of the list of addresses is of type
.BR pcap_addr_t ,
and has the following members:
.RS
.TP
.B next
if not
.BR NULL ,
a pointer to the next element in the list;
.B NULL
for the last element of the list
.TP
.B addr
a pointer to a
.B "struct sockaddr"
containing an address
.TP
.B netmask
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the netmask corresponding to the address pointed to by
.B addr
.TP
.B broadaddr
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the broadcast address corresponding to the address pointed
to by
.BR addr ;
may be null if the interface doesn't support broadcasts
.TP
.B dstaddr
if not
.BR NULL ,
a pointer to a
.B "struct sockaddr"
that contains the destination address corresponding to the address pointed
to by
.BR addr ;
may be null if the interface isn't a point-to-point interface
.RE
.PP
.B pcap_freealldevs()
is used to free a list allocated by
.BR pcap_findalldevs() .
.PP
.B pcap_lookupdev()
returns a pointer to a network device suitable for use with
.B pcap_open_live()
and
.BR pcap_lookupnet() .
If there is an error,
.B NULL
is returned and
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_lookupnet()
is used to determine the network number and mask
associated with the network device
.BR device .
Both
.I netp
and
.I maskp
are
.I bpf_u_int32
pointers.
A return of \-1 indicates an error in which case
.I errbuf
is filled in with an appropriate error message.
.PP
.B pcap_dispatch()
is used to collect and process packets.
.I 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
.I cnt
packets may be processed. A
.I 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''.
.I callback
specifies a routine to be called with three arguments:
a
.I u_char
pointer which is passed in from
.BR pcap_dispatch() ,
a
.I const struct pcap_pkthdr
pointer to a structure with the following members:
.RS
.TP
.B ts
a
.I struct timeval
containing the time when the packet was captured
.TP
.B caplen
a
.I bpf_u_int32
giving the number of bytes of the packet that are available from the
capture
.TP
.B len
a
.I 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)
.RE
.PP
and a
.I const u_char
pointer to the first
.B caplen
(as given in the
.I 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
.I snaplen
in your call to
.B 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).
.PP
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
.B pcap_perror()
or
.B pcap_geterr()
may be used to display the error text.
.PP
.BR NOTE :
when reading a live capture,
.B 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
.B 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
.B pcap_dispatch()
will return after the timeout expires.