906 lines
27 KiB
Plaintext
906 lines
27 KiB
Plaintext
.\" Copyright (c) 1994, 1996, 1997
|
|
.\" The Regents of the University of California. All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that: (1) source code distributions
|
|
.\" retain the above copyright notice and this paragraph in its entirety, (2)
|
|
.\" distributions including binary code include the above copyright notice and
|
|
.\" this paragraph in its entirety in the documentation or other materials
|
|
.\" provided with the distribution, and (3) all advertising materials mentioning
|
|
.\" features or use of this software display the following acknowledgement:
|
|
.\" ``This product includes software developed by the University of California,
|
|
.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
|
|
.\" the University nor the names of its contributors may be used to endorse
|
|
.\" or promote products derived from this software without specific prior
|
|
.\" written permission.
|
|
.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
|
|
.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
|
|
.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
|
|
.\"
|
|
.TH PCAP 3PCAP "16 April 2014"
|
|
.SH NAME
|
|
pcap \- Packet Capture library
|
|
.SH SYNOPSIS
|
|
.nf
|
|
.ft B
|
|
#include <pcap/pcap.h>
|
|
.LP
|
|
.ft B
|
|
.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.
|
|
It also supports saving captured packets to a ``savefile'', and reading
|
|
packets from a ``savefile''.
|
|
.SS Opening a capture handle for reading
|
|
To open a handle for a live capture, given the name of the network or
|
|
other interface on which the capture should be done, call
|
|
.BR pcap_create (),
|
|
set the appropriate options on the handle, and then activate it with
|
|
.BR pcap_activate ().
|
|
.PP
|
|
To obtain a list of devices that can be opened for a live capture, call
|
|
.BR pcap_findalldevs ();
|
|
to free the list returned by
|
|
.BR pcap_findalldevs (),
|
|
call
|
|
.BR pcap_freealldevs ().
|
|
.BR pcap_lookupdev ()
|
|
will return the first device on that list that is not a ``loopback``
|
|
network interface.
|
|
.PP
|
|
To open a handle for a ``savefile'' from which to read packets, given the
|
|
pathname of the ``savefile'', call
|
|
.BR pcap_open_offline ();
|
|
to set up a handle for a ``savefile'', given a
|
|
.B "FILE\ *"
|
|
referring to a file already opened for reading, call
|
|
.BR pcap_fopen_offline ().
|
|
.PP
|
|
In order to get a ``fake''
|
|
.B pcap_t
|
|
for use in routines that require a
|
|
.B pcap_t
|
|
as an argument, such as routines to open a ``savefile'' for writing and
|
|
to compile a filter expression, call
|
|
.BR pcap_open_dead ().
|
|
.PP
|
|
.BR pcap_create (),
|
|
.BR pcap_open_offline (),
|
|
.BR pcap_fopen_offline (),
|
|
and
|
|
.BR pcap_open_dead ()
|
|
return a pointer to a
|
|
.BR pcap_t ,
|
|
which is the handle used for reading packets from the capture stream or
|
|
the ``savefile'', and for finding out information about the capture
|
|
stream or ``savefile''.
|
|
To close a handle, use
|
|
.BR pcap_close ().
|
|
.PP
|
|
The options that can be set on a capture handle include
|
|
.IP "snapshot length"
|
|
If, when capturing, you capture the entire contents of the packet, that
|
|
requires more CPU time to copy the packet to your application, more disk
|
|
and possibly network bandwidth to write the packet data to a file, and
|
|
more disk space to save the packet. If you don't need the entire
|
|
contents of the packet - for example, if you are only interested in the
|
|
TCP headers of packets - you can set the "snapshot length" for the
|
|
capture to an appropriate value. If the snapshot length is set to
|
|
.IR snaplen ,
|
|
and
|
|
.I snaplen
|
|
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.
|
|
.IP
|
|
A snapshot length of 65535 should be sufficient, on most if not all
|
|
networks, to capture all the data available from the packet.
|
|
.IP
|
|
The snapshot length is set with
|
|
.BR pcap_set_snaplen ().
|
|
.IP "promiscuous mode"
|
|
On broadcast LANs such as Ethernet, if the network isn't switched, or if
|
|
the adapter is connected to a "mirror port" on a switch to which all
|
|
packets passing through the switch are sent, a network adapter receives
|
|
all packets on the LAN, including unicast or multicast packets not sent
|
|
to a network address that the network adapter isn't configured to
|
|
recognize.
|
|
.IP
|
|
Normally, the adapter will discard those packets; however, many network
|
|
adapters support "promiscuous mode", which is a mode in which all
|
|
packets, even if they are not sent to an address that the adapter
|
|
recognizes, are provided to the host. This is useful for passively
|
|
capturing traffic between two or more other hosts for analysis.
|
|
.IP
|
|
Note that even if an application does not set promiscuous mode, the
|
|
adapter could well be in promiscuous mode for some other reason.
|
|
.IP
|
|
For now, this doesn't work on the "any" device; if an argument of "any"
|
|
or NULL is supplied, the setting of promiscuous mode is ignored.
|
|
.IP
|
|
Promiscuous mode is set with
|
|
.BR pcap_set_promisc ().
|
|
.IP "monitor mode"
|
|
On IEEE 802.11 wireless LANs, even if an adapter is in promiscuous mode,
|
|
it will supply to the host only frames for the network with which it's
|
|
associated. It might also supply only data frames, not management or
|
|
control frames, and might not provide the 802.11 header or radio
|
|
information pseudo-header for those frames.
|
|
.IP
|
|
In "monitor mode", sometimes also called "rfmon mode" (for "Radio
|
|
Frequency MONitor"), the adapter will supply all frames that it
|
|
receives, with 802.11 headers, and might supply a pseudo-header with
|
|
radio information about the frame as well.
|
|
.IP
|
|
Note that in monitor mode the adapter might disassociate from the
|
|
network with which it's associated, so that you will not be able to use
|
|
any wireless networks with that adapter. This could prevent accessing
|
|
files on a network server, or resolving host names or network addresses,
|
|
if you are capturing in monitor mode and are not connected to another
|
|
network with another adapter.
|
|
.IP
|
|
Monitor mode is set with
|
|
.BR pcap_set_rfmon (),
|
|
and
|
|
.BR pcap_can_set_rfmon ()
|
|
can be used to determine whether an adapter can be put into monitor
|
|
mode.
|
|
.IP "read timeout"
|
|
If, when capturing, packets are delivered as soon as they arrive, the
|
|
application capturing the packets will be woken up for each packet as it
|
|
arrives, and might have to make one or more calls to the operating
|
|
system to fetch each packet.
|
|
.IP
|
|
If, instead, packets are not delivered as soon as they arrive, but are
|
|
delivered after a short delay (called a "read timeout"), more than one
|
|
packet can be accumulated before the packets are delivered, so that a
|
|
single wakeup would be done for multiple packets, and each set of calls
|
|
made to the operating system would supply multiple packets, rather than
|
|
a single packet. This reduces the per-packet CPU overhead if packets
|
|
are arriving at a high rate, increasing the number of packets per second
|
|
that can be captured.
|
|
.IP
|
|
The read timeout is required so that an application won't wait for the
|
|
operating system's capture buffer to fill up before packets are
|
|
delivered; if packets are arriving slowly, that wait could take an
|
|
arbitrarily long period of time.
|
|
.IP
|
|
Not all platforms support a read timeout; on platforms that
|
|
don't, the read timeout is ignored. A zero value for the timeout,
|
|
on platforms that support a read timeout,
|
|
will cause a read to wait forever to allow enough packets to
|
|
arrive, with no timeout.
|
|
.IP
|
|
.BR NOTE :
|
|
the read timeout cannot be used to cause calls that read
|
|
packets to return within a limited period of time, because, 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, for example, in an interactive application to allow the packet
|
|
capture loop to ``poll'' for user input periodically, as there's no
|
|
guarantee that a call reading packets will return after the timeout
|
|
expires even if no packets have arrived.
|
|
.IP
|
|
The read timeout is set with
|
|
.BR pcap_set_timeout ().
|
|
.IP "buffer size"
|
|
Packets that arrive for a capture are stored in a buffer, so that they
|
|
do not have to be read by the application as soon as they arrive. On
|
|
some platforms, the buffer's size can be set; a size that's too small
|
|
could mean that, if too many packets are being captured and the snapshot
|
|
length doesn't limit the amount of data that's buffered, packets could
|
|
be dropped if the buffer fills up before the application can read
|
|
packets from it, while a size that's too large could use more
|
|
non-pageable operating system memory than is necessary to prevent
|
|
packets from being dropped.
|
|
.IP
|
|
The buffer size is set with
|
|
.BR pcap_set_buffer_size ().
|
|
.IP "timestamp type"
|
|
On some platforms, the time stamp given to packets on live captures can
|
|
come from different sources that can have different resolutions or that
|
|
can have different relationships to the time values for the current time
|
|
supplied by routines on the native operating system. See
|
|
.BR pcap-tstamp (7)
|
|
for a list of time stamp types.
|
|
.IP
|
|
The time stamp type is set with
|
|
.BR pcap_set_tstamp_type ().
|
|
.PP
|
|
Reading packets from a network interface may require that you have
|
|
special privileges:
|
|
.TP
|
|
.B Under SunOS 3.x or 4.x with NIT or BPF:
|
|
You must have read access to
|
|
.I /dev/nit
|
|
or
|
|
.IR /dev/bpf* .
|
|
.TP
|
|
.B Under Solaris with DLPI:
|
|
You must have read/write access to the network pseudo device, e.g.
|
|
.IR /dev/le .
|
|
On at least some versions of Solaris, however, this is not sufficient to
|
|
allow
|
|
.I tcpdump
|
|
to capture in promiscuous mode; on those versions of Solaris, you must
|
|
be root, or the application capturing packets
|
|
must be installed setuid to root, in order to capture in promiscuous
|
|
mode. Note that, on many (perhaps all) interfaces, if you don't capture
|
|
in promiscuous mode, you will not see any outgoing packets, so a capture
|
|
not done in promiscuous mode may not be very useful.
|
|
.IP
|
|
In newer versions of Solaris, you must have been given the
|
|
.B net_rawaccess
|
|
privilege; this is both necessary and sufficient to give you access to the
|
|
network pseudo-device - there is no need to change the privileges on
|
|
that device. A user can be given that privilege by, for example, adding
|
|
that privilege to the user's
|
|
.B defaultpriv
|
|
key with the
|
|
.B usermod (1M)
|
|
command.
|
|
.TP
|
|
.B Under HP-UX with DLPI:
|
|
You must be root or the application capturing packets must be installed
|
|
setuid to root.
|
|
.TP
|
|
.B Under IRIX with snoop:
|
|
You must be root or the application capturing packets must be installed
|
|
setuid to root.
|
|
.TP
|
|
.B Under Linux:
|
|
You must be root or the application capturing packets must be installed
|
|
setuid to root (unless your distribution has a kernel
|
|
that supports capability bits such as CAP_NET_RAW and code to allow
|
|
those capability bits to be given to particular accounts and to cause
|
|
those bits to be set on a user's initial processes when they log in, in
|
|
which case you must have CAP_NET_RAW in order to capture and
|
|
CAP_NET_ADMIN to enumerate network devices with, for example, the
|
|
.B \-D
|
|
flag).
|
|
.TP
|
|
.B Under ULTRIX and Digital UNIX/Tru64 UNIX:
|
|
Any user may capture network traffic.
|
|
However, no user (not even the super-user) can capture in promiscuous
|
|
mode on an interface unless the super-user has enabled promiscuous-mode
|
|
operation on that interface using
|
|
.IR pfconfig (8),
|
|
and no user (not even the super-user) can capture unicast traffic
|
|
received by or sent by the machine on an interface unless the super-user
|
|
has enabled copy-all-mode operation on that interface using
|
|
.IR pfconfig ,
|
|
so
|
|
.I useful
|
|
packet capture on an interface probably requires that either
|
|
promiscuous-mode or copy-all-mode operation, or both modes of
|
|
operation, be enabled on that interface.
|
|
.TP
|
|
.B Under BSD (this includes Mac OS X):
|
|
You must have read access to
|
|
.I /dev/bpf*
|
|
on systems that don't have a cloning BPF device, or to
|
|
.I /dev/bpf
|
|
on systems that do.
|
|
On BSDs with a devfs (this includes Mac OS X), this might involve more
|
|
than just having somebody with super-user access setting the ownership
|
|
or permissions on the BPF devices - it might involve configuring devfs
|
|
to set the ownership or permissions every time the system is booted,
|
|
if the system even supports that; if it doesn't support that, you might
|
|
have to find some other way to make that happen at boot time.
|
|
.PP
|
|
Reading a saved packet file doesn't require special privileges.
|
|
.PP
|
|
The packets read from the handle may include a ``pseudo-header''
|
|
containing various forms of packet meta-data, and probably includes a
|
|
link-layer header whose contents can differ for different network
|
|
interfaces. To determine the format of the packets supplied by the
|
|
handle, call
|
|
.BR pcap_datalink ();
|
|
.I http://www.tcpdump.org/linktypes.html
|
|
lists the values it returns and describes the packet formats that
|
|
correspond to those values.
|
|
.PP
|
|
Do
|
|
.B NOT
|
|
assume that the packets for a given capture or ``savefile`` will have
|
|
any given link-layer header type, such as
|
|
.B DLT_EN10MB
|
|
for Ethernet. For example, the "any" device on Linux will have a
|
|
link-layer header type of
|
|
.B DLT_LINUX_SLL
|
|
even if all devices on the system at the time the "any" device is opened
|
|
have some other data link type, such as
|
|
.B DLT_EN10MB
|
|
for Ethernet.
|
|
.PP
|
|
To obtain the
|
|
.B "FILE\ *"
|
|
corresponding to a
|
|
.B pcap_t
|
|
opened for a ``savefile'', call
|
|
.BR pcap_file ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_create (3PCAP)
|
|
get a
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_activate (3PCAP)
|
|
activate a
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_findalldevs (3PCAP)
|
|
get a list of devices that can be opened for a live capture
|
|
.TP
|
|
.BR pcap_freealldevs (3PCAP)
|
|
free list of devices
|
|
.TP
|
|
.BR pcap_lookupdev (3PCAP)
|
|
get first non-loopback device on that list
|
|
.TP
|
|
.BR pcap_open_offline (3PCAP)
|
|
open a
|
|
.B pcap_t
|
|
for a ``savefile'', given a pathname
|
|
.TP
|
|
.BR pcap_fopen_offline (3PCAP)
|
|
open a
|
|
.B pcap_t
|
|
for a ``savefile'', given a
|
|
.B "FILE\ *"
|
|
.TP
|
|
.BR pcap_open_dead (3PCAP)
|
|
create a ``fake''
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_close (3PCAP)
|
|
close a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_set_snaplen (3PCAP)
|
|
set the snapshot length for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_snapshot (3PCAP)
|
|
get the snapshot length for a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_set_promisc (3PCAP)
|
|
set promiscuous mode for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_set_rfmon (3PCAP)
|
|
set monitor mode for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_can_set_rfmon (3PCAP)
|
|
determine whether monitor mode can be set for a
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_set_timeout (3PCAP)
|
|
set read timeout for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_set_buffer_size (3PCAP)
|
|
set buffer size for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_set_tstamp_type (3PCAP)
|
|
set time stamp type for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_list_tstamp_types (3PCAP)
|
|
get list of available time stamp types for a not-yet-activated
|
|
.B pcap_t
|
|
for live capture
|
|
.TP
|
|
.BR pcap_free_tstamp_types (3PCAP)
|
|
free list of available time stamp types
|
|
.TP
|
|
.BR pcap_tstamp_type_val_to_name (3PCAP)
|
|
get name for a time stamp type
|
|
.TP
|
|
.BR pcap_tstamp_type_val_to_description (3PCAP)
|
|
get description for a time stamp type
|
|
.TP
|
|
.BR pcap_tstamp_type_name_to_val (3PCAP)
|
|
get time stamp type corresponding to a name
|
|
.TP
|
|
.BR pcap_datalink (3PCAP)
|
|
get link-layer header type for a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_file (3PCAP)
|
|
get the
|
|
.B "FILE\ *"
|
|
for a
|
|
.B pcap_t
|
|
opened for a ``savefile''
|
|
.TP
|
|
.BR pcap_is_swapped (3PCAP)
|
|
determine whether a ``savefile'' being read came from a machine with the
|
|
opposite byte order
|
|
.TP
|
|
.BR pcap_major_version (3PCAP)
|
|
.PD 0
|
|
.TP
|
|
.BR pcap_minor_version (3PCAP)
|
|
get the major and minor version of the file format version for a
|
|
``savefile''
|
|
.PD
|
|
.RE
|
|
.SS Selecting a link-layer header type for a live capture
|
|
Some devices may provide more than one link-layer header type. To
|
|
obtain a list of all link-layer header types provided by a device, call
|
|
.BR pcap_list_datalinks ()
|
|
on an activated
|
|
.B pcap_t
|
|
for the device.
|
|
To free a list of link-layer header types, call
|
|
.BR pcap_free_datalinks ().
|
|
To set the link-layer header type for a device, call
|
|
.BR pcap_set_datalink ().
|
|
This should be done after the device has been activated but before any
|
|
packets are read and before any filters are compiled or installed.
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_list_datalinks (3PCAP)
|
|
get a list of link-layer header types for a device
|
|
.TP
|
|
.BR pcap_free_datalinks (3PCAP)
|
|
free list of link-layer header types
|
|
.TP
|
|
.BR pcap_set_datalink (3PCAP)
|
|
set link-layer header type for a device
|
|
.TP
|
|
.BR pcap_datalink_val_to_name (3PCAP)
|
|
get name for a link-layer header type
|
|
.TP
|
|
.BR pcap_datalink_val_to_description (3PCAP)
|
|
get description for a link-layer header type
|
|
.TP
|
|
.BR pcap_datalink_name_to_val (3PCAP)
|
|
get link-layer header type corresponding to a name
|
|
.RE
|
|
.SS Reading packets
|
|
Packets are read with
|
|
.BR pcap_dispatch ()
|
|
or
|
|
.BR pcap_loop (),
|
|
which process one or more packets, calling a callback routine for each
|
|
packet, or with
|
|
.BR pcap_next ()
|
|
or
|
|
.BR pcap_next_ex (),
|
|
which return the next packet.
|
|
The callback for
|
|
.BR pcap_dispatch ()
|
|
and
|
|
.BR pcap_loop ()
|
|
is supplied a pointer to a
|
|
.IR "struct pcap_pkthdr" ,
|
|
which includes 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
|
|
The callback is also supplied a
|
|
.I const u_char
|
|
pointer to the first
|
|
.B caplen
|
|
(as given in the
|
|
.I struct pcap_pkthdr
|
|
mentioned above)
|
|
bytes of data from the packet. This 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
|
|
.BR pcap_set_snaplen ()
|
|
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). When reading
|
|
from a ``savefile'', the snapshot length specified when the capture was
|
|
performed will limit the amount of packet data available.
|
|
.PP
|
|
.BR pcap_next ()
|
|
is passed an argument that points to a
|
|
.I struct pcap_pkthdr
|
|
structure, and fills it in with the time stamp and length values for the
|
|
packet. It returns a
|
|
.I const u_char
|
|
to the first
|
|
.B caplen
|
|
bytes of the packet on success, and NULL on error.
|
|
.PP
|
|
.BR pcap_next_ex ()
|
|
is passed two pointer arguments, one of which points to a
|
|
.IR struct pcap_pkthdr *
|
|
and one of which points to a
|
|
.IR "const u_char" *.
|
|
It sets the first pointer to point to a
|
|
.I struct pcap_pkthdr
|
|
structure with the time stamp and length values for the packet, and sets
|
|
the second pointer to point to the first
|
|
.B caplen
|
|
bytes of the packet.
|
|
.PP
|
|
To force the loop in
|
|
.BR pcap_dispatch ()
|
|
or
|
|
.BR pcap_loop ()
|
|
to terminate, call
|
|
.BR pcap_breakloop ().
|
|
.PP
|
|
By default, when reading packets from an interface opened for a live
|
|
capture,
|
|
.BR pcap_dispatch (),
|
|
.BR pcap_next (),
|
|
and
|
|
.BR pcap_next_ex ()
|
|
will, if no packets are currently available to be read, block waiting
|
|
for packets to become available. On some, but
|
|
.I not
|
|
all, platforms, if a read timeout was specified, the wait will terminate
|
|
after the read timeout expires; applications should be prepared for
|
|
this, as it happens on some platforms, but should not rely on it, as it
|
|
does not happen on other platforms.
|
|
.PP
|
|
A handle can be put into ``non-blocking mode'', so that those routines
|
|
will, rather than blocking, return an indication that no packets are
|
|
available to read. Call
|
|
.BR pcap_setnonblock ()
|
|
to put a handle into non-blocking mode or to take it out of non-blocking
|
|
mode; call
|
|
.BR pcap_getnonblock ()
|
|
to determine whether a handle is in non-blocking mode. Note that
|
|
non-blocking mode does not work correctly in Mac OS X 10.6.
|
|
.PP
|
|
Non-blocking mode is often combined with routines such as
|
|
.BR select (2)
|
|
or
|
|
.BR poll (2)
|
|
or other routines a platform offers to wait for the availability of data
|
|
on any of a set of descriptors. To obtain, for a handle, a descriptor
|
|
that can be used in those routines, call
|
|
.BR pcap_get_selectable_fd ().
|
|
Not all handles have such a descriptor available;
|
|
.BR pcap_get_selectable_fd ()
|
|
will return \-1 if no such descriptor exists. In addition, for various
|
|
reasons, one or more of those routines will not work properly with the
|
|
descriptor; the documentation for
|
|
.BR pcap_get_selectable_fd ()
|
|
gives details.
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_dispatch (3PCAP)
|
|
read a bufferful of packets from a
|
|
.B pcap_t
|
|
open for a live capture or the full set of packets from a
|
|
.B pcap_t
|
|
open for a ``savefile''
|
|
.TP
|
|
.BR pcap_loop (3PCAP)
|
|
read packets from a
|
|
.B pcap_t
|
|
until an interrupt or error occurs
|
|
.TP
|
|
.BR pcap_next (3PCAP)
|
|
read the next packet from a
|
|
.B pcap_t
|
|
without an indication whether an error occurred
|
|
.TP
|
|
.BR pcap_next_ex (3PCAP)
|
|
read the next packet from a
|
|
.B pcap_t
|
|
with an error indication on an error
|
|
.TP
|
|
.BR pcap_breakloop (3PCAP)
|
|
prematurely terminate the loop in
|
|
.BR pcap_dispatch ()
|
|
or
|
|
.BR pcap_loop ()
|
|
.TP
|
|
.BR pcap_setnonblock (3PCAP)
|
|
set or clear non-blocking mode on a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_getnonblock (3PCAP)
|
|
get the state of non-blocking mode for a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_get_selectable_fd (3PCAP)
|
|
attempt to get a descriptor for a
|
|
.B pcap_t
|
|
that can be used in calls such as
|
|
.BR select (2)
|
|
and
|
|
.BR poll (2)
|
|
.RE
|
|
.SS Filters
|
|
In order to cause only certain packets to be returned when reading
|
|
packets, a filter can be set on a handle. For a live capture, the
|
|
filtering will be performed in kernel mode, if possible, to avoid
|
|
copying ``uninteresting'' packets from the kernel to user mode.
|
|
.PP
|
|
A filter can be specified as a text string; the syntax and semantics of
|
|
the string are as described by
|
|
.BR pcap-filter (7).
|
|
A filter string is compiled into a program in a pseudo-machine-language
|
|
by
|
|
.BR pcap_compile ()
|
|
and the resulting program can be made a filter for a handle with
|
|
.BR pcap_setfilter ().
|
|
The result of
|
|
.BR pcap_compile ()
|
|
can be freed with a call to
|
|
.BR pcap_freecode ().
|
|
.BR pcap_compile ()
|
|
may require a network mask for certain expressions in the filter string;
|
|
.BR pcap_lookupnet ()
|
|
can be used to find the network address and network mask for a given
|
|
capture device.
|
|
.PP
|
|
A compiled filter can also be applied directly to a packet that has been
|
|
read using
|
|
.BR pcap_offline_filter ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_compile (3PCAP)
|
|
compile filter expression to a pseudo-machine-language code program
|
|
.TP
|
|
.BR pcap_freecode (3PCAP)
|
|
free a filter program
|
|
.TP
|
|
.BR pcap_setfilter (3PCAP)
|
|
set filter for a
|
|
.B pcap_t
|
|
.TP
|
|
.BR pcap_lookupnet (3PCAP)
|
|
get network address and network mask for a capture device
|
|
.TP
|
|
.BR pcap_offline_filter (3PCAP)
|
|
apply a filter program to a packet
|
|
.RE
|
|
.SS Incoming and outgoing packets
|
|
By default, libpcap will attempt to capture both packets sent by the
|
|
machine and packets received by the machine. To limit it to capturing
|
|
only packets received by the machine or, if possible, only packets sent
|
|
by the machine, call
|
|
.BR pcap_setdirection ().
|
|
.TP
|
|
.BR Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_setdirection (3PCAP)
|
|
specify whether to capture incoming packets, outgoing packets, or both
|
|
.RE
|
|
.SS Capture statistics
|
|
To get statistics about packets received and dropped in a live capture,
|
|
call
|
|
.BR pcap_stats ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_stats (3PCAP)
|
|
get capture statistics
|
|
.RE
|
|
.SS Opening a handle for writing captured packets
|
|
To open a ``savefile`` to which to write packets, given the pathname the
|
|
``savefile'' should have, call
|
|
.BR pcap_dump_open ().
|
|
To open a ``savefile`` to which to write packets, given the pathname the
|
|
``savefile'' should have, call
|
|
.BR pcap_dump_open ();
|
|
to set up a handle for a ``savefile'', given a
|
|
.B "FILE\ *"
|
|
referring to a file already opened for writing, call
|
|
.BR pcap_dump_fopen ().
|
|
They each return pointers to a
|
|
.BR pcap_dumper_t ,
|
|
which is the handle used for writing packets to the ``savefile''. If it
|
|
succeeds, it will have created the file if it doesn't exist and
|
|
truncated the file if it does exist.
|
|
To close a
|
|
.BR pcap_dumper_t ,
|
|
call
|
|
.BR pcap_dump_close ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_dump_open (3PCAP)
|
|
open a
|
|
.B pcap_dumper_t
|
|
for a ``savefile``, given a pathname
|
|
.TP
|
|
.BR pcap_dump_fopen (3PCAP)
|
|
open a
|
|
.B pcap_dumper_t
|
|
for a ``savefile``, given a
|
|
.B "FILE\ *"
|
|
.TP
|
|
.BR pcap_dump_close (3PCAP)
|
|
close a
|
|
.B pcap_dumper_t
|
|
.TP
|
|
.BR pcap_dump_file (3PCAP)
|
|
get the
|
|
.B "FILE\ *"
|
|
for a
|
|
.B pcap_dumper_t
|
|
opened for a ``savefile''
|
|
.RE
|
|
.SS Writing packets
|
|
To write a packet to a
|
|
.BR pcap_dumper_t ,
|
|
call
|
|
.BR pcap_dump ().
|
|
Packets written with
|
|
.BR pcap_dump ()
|
|
may be buffered, rather than being immediately written to the
|
|
``savefile''. Closing the
|
|
.B pcap_dumper_t
|
|
will cause all buffered-but-not-yet-written packets to be written to the
|
|
``savefile''.
|
|
To force all packets written to the
|
|
.BR pcap_dumper_t ,
|
|
and not yet written to the ``savefile'' because they're buffered by the
|
|
.BR pcap_dumper_t ,
|
|
to be written to the ``savefile'', without closing the
|
|
.BR pcap_dumper_t ,
|
|
call
|
|
.BR pcap_dump_flush ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_dump (3PCAP)
|
|
write packet to a
|
|
.B pcap_dumper_t
|
|
.TP
|
|
.BR pcap_dump_flush (3PCAP)
|
|
flush buffered packets written to a
|
|
.B pcap_dumper_t
|
|
to the ``savefile''
|
|
.TP
|
|
.BR pcap_dump_ftell (3PCAP)
|
|
get current file position for a
|
|
.B pcap_dumper_t
|
|
.RE
|
|
.SS Injecting packets
|
|
If you have the required privileges, you can inject packets onto a
|
|
network with a
|
|
.B pcap_t
|
|
for a live capture, using
|
|
.BR pcap_inject ()
|
|
or
|
|
.BR pcap_sendpacket ().
|
|
(The two routines exist for compatibility with both OpenBSD and WinPcap;
|
|
they perform the same function, but have different return values.)
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_inject (3PCAP)
|
|
.PD 0
|
|
.TP
|
|
.BR pcap_sendpacket (3PCAP)
|
|
transmit a packet
|
|
.PD
|
|
.RE
|
|
.SS Reporting errors
|
|
Some routines return error or warning status codes; to convert them to a
|
|
string, use
|
|
.BR pcap_statustostr ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_statustostr (3PCAP)
|
|
get a string for an error or warning status code
|
|
.RE
|
|
.SS Getting library version information
|
|
To get a string giving version information about libpcap, call
|
|
.BR pcap_lib_version ().
|
|
.TP
|
|
.B Routines
|
|
.RS
|
|
.TP
|
|
.BR pcap_lib_version (3PCAP)
|
|
get library version string
|
|
.RE
|
|
.SH BACKWARDS COMPATIBILITY
|
|
.PP
|
|
In versions of libpcap prior to 1.0, the
|
|
.B pcap.h
|
|
header file was not in a
|
|
.B pcap
|
|
directory on most platforms; if you are writing an application that must
|
|
work on versions of libpcap prior to 1.0, include
|
|
.BR <pcap.h> ,
|
|
which will include
|
|
.B <pcap/pcap.h>
|
|
for you, rather than including
|
|
.BR <pcap/pcap.h> .
|
|
.PP
|
|
.BR pcap_create ()
|
|
and
|
|
.BR pcap_activate ()
|
|
were not available in versions of libpcap prior to 1.0; if you are
|
|
writing an application that must work on versions of libpcap prior to
|
|
1.0, either use
|
|
.BR pcap_open_live ()
|
|
to get a handle for a live capture or, if you want to be able to use the
|
|
additional capabilities offered by using
|
|
.BR pcap_create ()
|
|
and
|
|
.BR pcap_activate (),
|
|
use an
|
|
.BR autoconf (1)
|
|
script or some other configuration script to check whether the libpcap
|
|
1.0 APIs are available and use them only if they are.
|
|
.SH SEE ALSO
|
|
autoconf(1), tcpdump(1), tcpslice(1), pcap-filter(7), pfconfig(8),
|
|
usermod(1M)
|
|
.SH AUTHORS
|
|
The original authors of libpcap 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
|
|
tcpdump-workers@lists.tcpdump.org
|
|
.RE
|