PCAP(3) Library Functions Manual PCAP(3)
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
To open a handle for a live capture, call pcap_create(), set the appropriate options on the handle, and then activate it with pcap_acti-
vate(). To open a handle for a ``savefile'' with captured packets, call pcap_open_offline(). Both pcap_create() and pcap_open_offline()
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''.
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 applica-
tion, 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 mul-
ticast 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
Promiscuous mode is set with pcap_set_promisc().
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 mon-
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 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 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 dropped.
The buffer size is set with pcap_set_buffer_size().
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 privi-
lege 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 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 owner-
ship or permissions on the BPF devices - it might involve configuring devfs to set the ownership or permissions every time the sys-
tem 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 hap-
pen at boot time.
Reading a saved packet file doesn't require special privileges.
To open a ``savefile`` to which to write packets, call pcap_dump_open(). It returns a pointer to a pcap_dumper_t, which is the handle used
for writing packets to the ``savefile''.
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 cap-
ture, 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_open_live() 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.
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 applica-
tion 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
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.
autoconf(1), tcpdump(8), tcpslice(1), pcap-filter(7), pfconfig(8), usermod(1M)
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)