DragonFly On-Line Manual Pages
pcap - Packet Capture library
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
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_cre-
ate(), set the appropriate options on the handle, and then activate it
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
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().
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 use-
ful 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
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
Promiscuous mode is set with pcap_set_promisc().
On IEEE 802.11 wireless LANs, even if an adapter is in promiscu-
ous 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
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.
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 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 read timeout; on platforms that
don't, the read timeout is ignored. A zero value for the time-
out, on platforms that support a read timeout, will cause a read
to wait forever to allow enough packets to arrive, with no time-
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 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().
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
The buffer size is set with pcap_set_buffer_size().
On some platforms, the time stamp given to packets on live cap-
tures 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
The time stamp type is set with pcap_set_tstamp_type().
Reading packets from a network interface may require that you have spe-
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 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 (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.
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 pfcon-
fig, 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 permissions on the BPF devices - it
might involve configuring devfs to set the ownership or permis-
sions every time the system is booted, if the system even sup-
ports 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'' con-
taining 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 cor-
respond to those values.
To obtain the FILE * corresponding to a pcap_t opened for a ``save-
file'', call pcap_file().
get a pcap_t for live capture
activate a pcap_t for live capture
get a list of devices that can be opened for a live cap-
free list of devices
get first non-loopback device on that list
open a pcap_t for a ``savefile'', given a pathname
open a pcap_t for a ``savefile'', given a FILE *
create a ``fake'' pcap_t
close a pcap_t
set the snapshot length for a not-yet-activated pcap_t
for live capture
get the snapshot length for a pcap_t
set promiscuous mode for a not-yet-activated pcap_t for
set monitor mode for a not-yet-activated pcap_t for live
determine whether monitor mode can be set for a pcap_t
for live capture
set read timeout for a not-yet-activated pcap_t for live
set buffer size for a not-yet-activated pcap_t for live
set time stamp type for a not-yet-activated pcap_t for
get list of available time stamp types for a not-yet-
activated pcap_t for live capture
free list of available time stamp types
get name for a time stamp type
get description for a time stamp type
get time stamp type corresponding to a name
get link-layer header type for a pcap_t
get the FILE * for a pcap_t opened for a ``savefile''
determine whether a ``savefile'' being read came from a
machine with the opposite byte order
get the major and minor version of the file format ver-
sion 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.
get a list of link-layer header types for a device
free list of link-layer header types
set link-layer header type for a device
get name for a link-layer header type
get description for a link-layer header type
get link-layer header type corresponding to a name
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 call-
back 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
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).
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
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 time-
out 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
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_getnon-
block() 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 availabil-
ity 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 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.
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
read packets from a pcap_t until an interrupt or error
read the next packet from a pcap_t without an indication
whether an error occurred
read the next packet from a pcap_t with an error indica-
tion on an error
prematurely terminate the loop in pcap_dispatch() or
set or clear non-blocking mode on a pcap_t
get the state of non-blocking mode for a pcap_t
attempt to get a descriptor for a pcap_t that can be used
in calls such as select(2) and poll(2)
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 fil-
tering 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 com-
piled 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_set-
filter(). 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().
compile filter expression to a pseudo-machine-language
free a filter program
set filter for a pcap_t
get network address and network mask for a capture device
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().
specify whether to capture incoming packets, outgoing
packets, or both
To get statistics about packets received and dropped in a live capture,
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 ``save-
file`` to which to write packets, given the pathname the ``savefile''
should have, call pcap_dump_open(); to set up a handle for a ``save-
file'', 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
open a pcap_dumper_t for a ``savefile``, given a pathname
open a pcap_dumper_t for a ``savefile``, given a FILE *
close a pcap_dumper_t
get the FILE * for a pcap_dumper_t opened for a ``save-
To write a packet to a pcap_dumper_t, call pcap_dump(). Packets writ-
ten 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
write packet to a pcap_dumper_t
flush buffered packets written to a pcap_dumper_t to the
get current file position for a pcap_dumper_t
If you have the required privileges, you can inject packets onto a net-
work 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
transmit a packet
Some routines return error or warning status codes; to convert them to
a string, use pcap_statustostr().
get a string for an error or warning status code
Getting library version information
To get a string giving version information about libpcap, call
get library version string
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_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 addi-
tional 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
autoconf(1), tcpdump(1), tcpslice(1), pcap-filter(7), pfconfig(8),
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
Please send problems, bugs, questions, desirable enhancements, etc. to:
4 April 2008 PCAP(3)
pcap_breakloop - force a pcap_dispatch() or pcap_loop() call to return
void pcap_breakloop(pcap_t *);
pcap_breakloop() sets a flag that will force pcap_dispatch() or
pcap_loop() to return rather than looping; they will return the number
of packets that have been processed so far, or -2 if no packets have
been processed so far.
This routine is safe to use inside a signal handler on UNIX or a con-
sole control handler on Windows, as it merely sets a flag that is
checked within the loop.
The flag is checked in loops reading packets from the OS - a signal by
itself will not necessarily terminate those loops - as well as in loops
processing a set of packets returned by the OS. Note that if you are
catching signals on UNIX systems that support restarting system calls
after a signal, and calling pcap_breakloop() in the signal handler, you
must specify, when catching those signals, that system calls should NOT
be restarted by that signal. Otherwise, if the signal interrupted a
call reading packets in a live capture, when your signal handler
returns after calling pcap_breakloop(), the call will be restarted, and
the loop will not terminate until more packets arrive and the call com-
Note also that, in a multi-threaded application, if one thread is
blocked in pcap_dispatch(), pcap_loop(), pcap_next(), or
pcap_next_ex(), a call to pcap_breakloop() in a different thread will
not unblock that thread; you will need to use whatever mechanism the OS
provides for breaking a thread out of blocking calls in order to
unblock the thread, such as thread cancellation in systems that support
Note that pcap_next() and pcap_next_ex() will, on some platforms, loop
reading packets from the OS; that loop will not necessarily be termi-
nated by a signal, so pcap_breakloop() should be used to terminate
packet processing even if pcap_next() or pcap_next_ex() is being used.
pcap_breakloop() does not guarantee that no further packets will be
processed by pcap_dispatch() or pcap_loop() after it is called; at most
one more packet might be processed.
If -2 is returned from pcap_dispatch() or pcap_loop(), the flag is
cleared, so a subsequent call will resume reading packets. If a posi-
tive number is returned, the flag is not cleared, so a subsequent call
will return -2 and clear the flag.
pcap(3), pcap_loop(3), pcap_next_ex(3)
5 April 2008 PCAP_BREAKLOOP(3)