Provided by: libpcap0.8-dev_1.9.1-3_amd64 
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().
immediate mode
In immediate mode, packets are always delivered as soon as they arrive, with no
buffering. Immediate mode is set with pcap_set_immediate_mode().
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_immediate_mode(3PCAP)
set immediate mode 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(). If the routine indicates that data is available to read on the
descriptor, an attempt should be made to read from the device.
Not all handles have such a descriptor available; pcap_get_selectable_fd() will return
PCAP_ERROR if no such descriptor is available. If no such descriptor is available, this
may be because the device must be polled periodically for packets; in that case,
pcap_get_required_select_timeout() will return a pointer to a struct timeval whose value
can be used as a timeout in those routines. When the routine returns, an attmept should
be made to read packets from the device. If pcap_get_required_select_timeout() returns
NULL, no such timeout is available, and those routines cannot be used with the device.
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)
pcap_get_required_select_timeout(3PCAP)
if no descriptor usable with select(2) and poll(2) is available for the
pcap_t, attempt to get a timeout usable with those routines
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
BACKWARD 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(8), 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.
25 July 2018 PCAP(3PCAP)