PCAP
Section: Misc. Reference Manual Pages (3PCAP)
Updated: 20 April 2018
Page Index
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 activate 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 ``savefile'', 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 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.
-
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.
- packet buffer 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 "packet buffer 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 packet buffer 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.
-
Not all platforms support a packet buffer timeout; on platforms that
don't, the packet buffer timeout is ignored. A zero value for the
timeout, on platforms that support a packet buffer timeout, will cause a
read to wait forever to allow enough packets to arrive, with no timeout.
A negative value is invalid; the result of setting the timeout to a
negative value is unpredictable.
-
NOTE:
the packet buffer timeout cannot be used to cause calls that read
packets to return within a limited period of time, because, on some
platforms, the packet buffer timeout isn't supported, and, on other
platforms, the timer doesn't start until at least one packet arrives.
This means that the packet buffer 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 packet buffer 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 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.
-
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 (8)
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
operation 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 macOS):
-
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 macOS), 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.
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();
https://www.tcpdump.org/linktypes.html
lists the values it returns and describes the packet formats that
correspond to those values.
Do
NOT
assume that the packets for a given capture or ``savefile`` will have
any given link-layer header type, such as
DLT_EN10MB
for Ethernet. For example, the "any" device on Linux will have a
link-layer header type of
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
DLT_EN10MB
for Ethernet.
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_open_offline_with_tstamp_precision(3PCAP)
-
open a
pcap_t
for a ``savefile'', given a pathname, and specify the precision to
provide for packet time stamps
- pcap_fopen_offline(3PCAP)
-
open a
pcap_t
for a ``savefile'', given a
FILE *
- pcap_fopen_offline_with_tstamp_precision(3PCAP)
-
open a
pcap_t
for a ``savefile'', given a
FILE *,
and specify the precision to provide for packet time stamps
- 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_protocol_linux(3PCAP)
-
set capture protocol for a not-yet-activated
pcap_t
for live capture (Linux only)
- 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 packet buffer 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_type_name_to_val(3PCAP)
-
get time stamp type corresponding to a name
- pcap_set_tstamp_precision(3PCAP)
-
set time stamp precision for a not-yet-activated
pcap_t
for live capture
- pcap_get_tstamp_precision(3PCAP)
-
get the time stamp precision of a
pcap_t
for live capture
- 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
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).
The callback is also supplied a
const u_char
pointer to the first
caplen
(as given in the
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
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()
is passed an argument that points to a
struct pcap_pkthdr
structure, and fills it in with the time stamp and length values for the
packet. It returns a
const u_char
to the first
caplen
bytes of the packet on success, and NULL on error.
pcap_next_ex()
is passed two pointer arguments, one of which points to a
structpcap_pkthdr*
and one of which points to a
const u_char*.
It sets the first pointer to point to a
struct pcap_pkthdr
structure with the time stamp and length values for the packet, and sets
the second pointer to point to the first
caplen
bytes of the packet.
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 packet buffer timeout was specified, the wait will
terminate after the packet buffer 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. Note that the
wait might, or might not, terminate even if no packets are available;
applications should be prepared for this to happen, but must not rely on
it happening.
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 any of a set of
descriptors to be ready to read. 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 descriptor 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. Note that, just as an attempt to read packets from a
pcap_t
may not return any packets if the packet buffer timeout expires, a
select(),
poll(),
or other such call may, if the packet buffer timeout expires, indicate
that a descriptor is ready to read even if there are no packets
available to read.
- 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_filter().
- 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 packets 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_statustostr().
- 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_lib_version().
- Routines
-
-
- pcap_lib_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 platforms; 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(8)
AUTHORS
The original authors of libpcap 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
-
https://www.tcpdump.org/
BUGS
To report a security issue please send an e-mail to
security@tcpdump.org.
To report bugs and other problems, contribute patches, request a
feature, provide generic feedback etc please see the file
CONTRIBUTING
in the libpcap source tree root.