PCAP(3) PCAP(3)
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''.
To open a handle for a live capture, call pcap_create(), set the appro-
priate 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
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 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
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 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
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 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-
out.
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().
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().
Reading packets from a network interface may require that you have spe-
cial 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 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.
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 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.
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 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
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).
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 net-
works). When reading from a ``savefile'', the snapshot length speci-
fied when the capture was performed will limit the amount of packet
data available. pcap_next() returns that pointer; pcap_next_ex() sup-
plies that pointer through a pointer argument.
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 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
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, University 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(3)
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