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PCAP(3PCAP) PCAP(3PCAP)
NAME
pcap - Packet Capture library
SYNOPSIS
#include <pcap/pcap.h>
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''.
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 pcap_create(), set the appropriate options on the handle, and then acti-vate activate
vate it with pcap_activate().
To obtain a list of devices that can be opened for a live capture, call pcap_findalldevs(); to free
the list returned by pcap_findalldevs(), call pcap_freealldevs(). pcap_lookupdev() will return the
first device on that list that is not a ``loopback`` network interface.
To open a handle for a ``savefile'' from which to read packets, given the pathname of the ``save-file'', ``savefile'',
file'', call pcap_open_offline(); to set up a handle for a ``savefile'', given a FILE * referring to
a file already opened for reading, call pcap_fopen_offline().
In order to get a ``fake'' pcap_t for use in routines that require a pcap_t as an argument, such as
routines to open a ``savefile'' for writing and to compile a filter expression, call
pcap_open_dead().
pcap_create(), pcap_open_offline(), pcap_fopen_offline(), and pcap_open_dead() return a pointer to a
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
pcap_close().
The options that can be set on a capture handle include
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 snaplen, and snaplen 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 snapshot length of 65535 should be sufficient, on most if not all networks, to capture all
the data available from the packet.
The snapshot length is set with pcap_set_snaplen().
promiscuous mode
On broadcast LANs such as Ethernet, if the network isn't switched, or if the adapter is con-nected connected
nected 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 pack-ets packets
ets not sent to a network address that the network adapter isn't configured to recognize.
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.
Note that even if an application does not set promiscuous mode, the adapter 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 setting of promiscuous mode is ignored.
Promiscuous mode is set with pcap_set_promisc().
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.
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.
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.
Monitor mode is set with pcap_set_rfmon(), and pcap_can_set_rfmon() can be used to determine
whether an adapter can be put into monitor mode.
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.
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.
The read timeout is required so that an application won't wait for the operating system's cap-ture capture
ture buffer to fill up before packets are delivered; if packets are arriving slowly, that wait
could take an arbitrarily long period of time.
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.
NOTE: the read timeout cannot be used to cause calls that read packets to return within a lim-ited limited
ited 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 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.
The read timeout is set with pcap_set_timeout().
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.
The buffer size is set with pcap_set_buffer_size().
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
pcap-tstamp(7) for a list of time stamp types.
The time stamp type is set with pcap_set_tstamp_type().
Reading packets from a network interface may require that you have special privileges:
Under SunOS 3.x or 4.x with NIT or BPF:
You must have read access to /dev/nit or /dev/bpf*.
Under Solaris with DLPI:
You must have read/write access to the network pseudo device, e.g. /dev/le. On at least some
versions of Solaris, however, this is not sufficient to allow tcpdump to capture in promiscu-ous promiscuous
ous 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 out-going outgoing
going packets, so a capture not done in promiscuous mode may not be very useful.
In newer versions of Solaris, you must have been given the 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 defaultpriv key with the usermod (1M) command.
Under HP-UX with DLPI:
You must be root or the application capturing packets must be installed setuid to root.
Under IRIX with snoop:
You must be root or the application capturing packets must be installed setuid to root.
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 -D
flag).
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 opera-tion operation
tion on that interface using 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 pfconfig, so 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.
Under BSD (this includes Mac OS X):
You must have read access to /dev/bpf* on systems that don't have a cloning BPF device, or to
/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 permis-sions permissions
sions on the BPF devices - it might involve configuring devfs to set the ownership or permis-sions permissions
sions 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.
Reading a saved packet file doesn't require special privileges.
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 pcap_datalink();
http://www.tcpdump.org/linktypes.html lists the values it returns and describes the packet formats
that correspond to those values.
To obtain the FILE * corresponding to a pcap_t opened for a ``savefile'', call pcap_file().
Routines
pcap_create(3PCAP)
get a pcap_t for live capture
pcap_activate(3PCAP)
activate a pcap_t for live capture
pcap_findalldevs(3PCAP)
get a list of devices that can be opened for a live capture
pcap_freealldevs(3PCAP)
free list of devices
pcap_lookupdev(3PCAP)
get first non-loopback device on that list
pcap_open_offline(3PCAP)
open a pcap_t for a ``savefile'', given a pathname
pcap_fopen_offline(3PCAP)
open a pcap_t for a ``savefile'', given a FILE *
pcap_open_dead(3PCAP)
create a ``fake'' pcap_t
pcap_close(3PCAP)
close a pcap_t
pcap_set_snaplen(3PCAP)
set the snapshot length for a not-yet-activated pcap_t for live capture
pcap_snapshot(3PCAP)
get the snapshot length for a pcap_t
pcap_set_promisc(3PCAP)
set promiscuous mode for a not-yet-activated pcap_t for live capture
pcap_set_rfmon(3PCAP)
set monitor mode for a not-yet-activated pcap_t for live capture
pcap_can_set_rfmon(3PCAP)
determine whether monitor mode can be set for a pcap_t for live capture
pcap_set_timeout(3PCAP)
set read timeout for a not-yet-activated pcap_t for live capture
pcap_set_buffer_size(3PCAP)
set buffer size for a not-yet-activated pcap_t for live capture
pcap_set_tstamp_type(3PCAP)
set time stamp type for a not-yet-activated pcap_t for live capture
pcap_list_tstamp_types(3PCAP)
get list of available time stamp types for a not-yet-activated pcap_t for live capture
pcap_free_tstamp_types(3PCAP)
free list of available time stamp types
pcap_tstamp_type_val_to_name(3PCAP)
get name for a time stamp type
pcap_tstamp_type_val_to_description(3PCAP)
get description for a time stamp type
pcap_tstamp_name_to_val(3PCAP)
get time stamp type corresponding to a name
pcap_datalink(3PCAP)
get link-layer header type for a pcap_t
pcap_file(3PCAP)
get the FILE * for a pcap_t opened for a ``savefile''
pcap_is_swapped(3PCAP)
determine whether a ``savefile'' being read came from a machine with the opposite byte
order
pcap_major_version (3PCAP)
pcap_minor_version (3PCAP)
get the major and minor version of the file format version for a ``savefile''
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 pcap_list_datalinks() on an activated pcap_t for the device.
To free a list of link-layer header types, call pcap_free_datalinks(). To set the link-layer header
type for a device, call 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.
Routines
pcap_list_datalinks(3PCAP)
get a list of link-layer header types for a device
pcap_free_datalinks(3PCAP)
free list of link-layer header types
pcap_set_datalink(3PCAP)
set link-layer header type for a device
pcap_datalink_val_to_name(3PCAP)
get name for a link-layer header type
pcap_datalink_val_to_description(3PCAP)
get description for a link-layer header type
pcap_datalink_name_to_val(3PCAP)
get link-layer header type corresponding to a name
Reading packets
Packets are read with pcap_dispatch() or pcap_loop(), which process one or more packets, calling a
callback routine for each packet, or with pcap_next() or pcap_next_ex(), which return the next
packet. The callback for pcap_dispatch() and pcap_loop() is supplied a pointer to a struct
pcap_pkthdr, which includes 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).
pcap_next_ex() supplies that pointer through a pointer argument. pcap_next() is passed an argument
that points to a struct pcap_pkthdr structure, and fills it in.
The callback is also supplied 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.
This 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_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. pcap_next() returns that pointer; pcap_next_ex() supplies that
pointer through a pointer argument.
To force the loop in pcap_dispatch() or pcap_loop() to terminate, call pcap_breakloop().
By default, when reading packets from an interface opened for a live capture, pcap_dispatch(),
pcap_next(), and pcap_next_ex() will, if no packets are currently available to be read, block waiting
for packets to become available. On some, but 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.
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 pcap_setnonblock() to put a handle
into non-blocking mode or to take it out of non-blocking mode; call 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.
Non-blocking mode is often combined with routines such as select(2) or 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 pcap_get_selectable_fd(). Not all
handles have such a descriptor available; pcap_get_selectable_fd() will return -1 if no such descrip-tor descriptor
tor exists. In addition, for various reasons, one or more of those routines will not work properly
with the descriptor; the documentation for pcap_get_selectable_fd() gives details.
Routines
pcap_dispatch(3PCAP)
read a bufferful of packets from a pcap_t open for a live capture or the full set of
packets from a pcap_t open for a ``savefile''
pcap_loop(3PCAP)
read packets from a pcap_t until an interrupt or error occurs
pcap_next(3PCAP)
read the next packet from a pcap_t without an indication whether an error occurred
pcap_next_ex(3PCAP)
read the next packet from a pcap_t with an error indication on an error
pcap_breakloop(3PCAP)
prematurely terminate the loop in pcap_dispatch() or pcap_loop()
pcap_setnonblock(3PCAP)
set or clear non-blocking mode on a pcap_t
pcap_getnonblock(3PCAP)
get the state of non-blocking mode for a pcap_t
pcap_get_selectable_fd(3PCAP)
attempt to get a descriptor for a pcap_t that can be used in calls such as select(2)
and poll(2)
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.
A filter can be specified as a text string; the syntax and semantics of the string are as described
by pcap-filter(7). A filter string is compiled into a program in a pseudo-machine-language by
pcap_compile() and the resulting program can be made a filter for a handle with pcap_setfilter().
The result of pcap_compile() can be freed with a call to pcap_freecode(). pcap_compile() may require
a network mask for certain expressions in the filter string; pcap_lookupnet() can be used to find the
network address and network mask for a given capture device.
A compiled filter can also be applied directly to a packet that has been read using pcap_offline_fil-ter(). pcap_offline_filter().
ter().
Routines
pcap_compile(3PCAP)
compile filter expression to a pseudo-machine-language code program
pcap_freecode(3PCAP)
free a filter program
pcap_setfilter(3PCAP)
set filter for a pcap_t
pcap_lookupnet(3PCAP)
get network address and network mask for a capture device
pcap_offline_filter(3PCAP)
apply a filter program to a packet
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 pcap_setdirection().
Routines
pcap_setdirection(3PCAP)
specify whether to capture incoming packets, outgoing packets, or both
Capture statistics
To get statistics about packets received and dropped in a live capture, call pcap_stats().
Routines
pcap_stats(3PCAP)
get capture statistics
Opening a handle for writing captured packets
To open a ``savefile`` to which to write packets, given the pathname the ``savefile'' should have,
call pcap_dump_open(). To open a ``savefile`` to which to write packets, given the pathname the
``savefile'' should have, call pcap_dump_open(); to set up a handle for a ``savefile'', given a
FILE * referring to a file already opened for writing, call pcap_dump_fopen(). They each return
pointers to a 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 pcap_dumper_t, call pcap_dump_close().
Routines
pcap_dump_open(3PCAP)
open a pcap_dumper_t for a ``savefile``, given a pathname
pcap_dump_fopen(3PCAP)
open a pcap_dumper_t for a ``savefile``, given a FILE *
pcap_dump_close(3PCAP)
close a pcap_dumper_t
pcap_dump_file(3PCAP)
get the FILE * for a pcap_dumper_t opened for a ``savefile''
Writing packets
To write a packet to a pcap_dumper_t, call pcap_dump(). Packets written with pcap_dump() may be
buffered, rather than being immediately written to the ``savefile''. Closing the pcap_dumper_t will
cause all buffered-but-not-yet-written packets to be written to the ``savefile''. To force all pack-ets packets
ets written to the pcap_dumper_t, and not yet written to the ``savefile'' because they're buffered by
the pcap_dumper_t, to be written to the ``savefile'', without closing the pcap_dumper_t, call
pcap_dump_flush().
Routines
pcap_dump(3PCAP)
write packet to a pcap_dumper_t
pcap_dump_flush(3PCAP)
flush buffered packets written to a pcap_dumper_t to the ``savefile''
pcap_dump_ftell(3PCAP)
get current file position for a pcap_dumper_t
Injecting packets
If you have the required privileges, you can inject packets onto a network with a pcap_t for a live
capture, using pcap_inject() or pcap_sendpacket(). (The two routines exist for compatibility with
both OpenBSD and WinPcap; they perform the same function, but have different return values.)
Routines
pcap_inject (3PCAP)
pcap_sendpacket (3PCAP)
transmit a packet
Reporting errors
Some routines return error or warning status codes; to convert them to a string, use pcap_status-tostr(). pcap_statustostr().
tostr().
Routines
pcap_statustostr(3PCAP)
get a string for an error or warning status code
Getting library version information
To get a string giving version information about libpcap, call pcap_library_version().
Routines
pcap_library_version(3PCAP)
get library version string
BACKWARDS COMPATIBILITY
In versions of libpcap prior to 1.0, the pcap.h header file was not in a pcap directory on most plat-forms; platforms;
forms; if you are writing an application that must work on versions of libpcap prior to 1.0, include
<pcap.h>, which will include <pcap/pcap.h> for you, rather than including <pcap/pcap.h>.
pcap_create() and 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
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 pcap_create() and pcap_activate(), use an 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.
SEE ALSO
autoconf(1), tcpdump(1), tcpslice(1), pcap-filter(7), pfconfig(8), usermod(1M)
AUTHORS
The original authors of libpcap are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, Uni-versity University
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@lists.tcpdump.org
4 April 2008 PCAP(3PCAP)
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