Manual Pages  — PCAP

pcap - Packet Capture library

SYNOPSIS DESCRIPTION Opening a capture handle for reading Selecting a link-layer header type for a live capture Reading packets Filters Incoming and outgoing packets Capture statistics Opening a handle for writing captured packets Writing packets Injecting packets Reporting errors Getting library version information BACKWARD COMPATIBILITY SEE ALSO AUTHORS BUGS

#include <pcap/pcap.h>

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, 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(3)   get a pcap_t for live capture pcap_activate(3)   activate a pcap_t for live capture pcap_findalldevs(3)   get a list of devices that can be opened for a live capture pcap_freealldevs(3)   free list of devices pcap_lookupdev(3)   get first non-loopback device on that list pcap_open_offline(3)   open a pcap_t for a ``savefile'', given a pathname pcap_open_offline_with_tstamp_precision(3)   open a pcap_t for a ``savefile'', given a pathname, and specify the precision to provide for packet time stamps pcap_fopen_offline(3)   open a pcap_t for a ``savefile'', given a FILE * pcap_fopen_offline_with_tstamp_precision(3)   open a pcap_t for a ``savefile'', given a FILE *, and specify the precision to provide for packet time stamps pcap_open_dead(3)   create a ``fake'' pcap_t pcap_close(3)   close a pcap_t pcap_set_snaplen(3)   set the snapshot length for a not-yet-activated pcap_t for live capture pcap_snapshot(3)   get the snapshot length for a pcap_t pcap_set_promisc(3)   set promiscuous mode for a not-yet-activated pcap_t for live capture pcap_set_protocol_linux(3)   set capture protocol for a not-yet-activated pcap_t for live capture (Linux only) pcap_set_rfmon(3)   set monitor mode for a not-yet-activated pcap_t for live capture pcap_can_set_rfmon(3)   determine whether monitor mode can be set for a pcap_t for live capture pcap_set_timeout(3)   set packet buffer timeout for a not-yet-activated pcap_t for live capture pcap_set_immediate_mode(3)   set immediate mode for a not-yet-activated pcap_t for live capture pcap_set_buffer_size(3)   set buffer size for a not-yet-activated pcap_t for live capture pcap_set_tstamp_type(3)   set time stamp type for a not-yet-activated pcap_t for live capture pcap_list_tstamp_types(3)   get list of available time stamp types for a not-yet-activated pcap_t for live capture pcap_free_tstamp_types(3)   free list of available time stamp types pcap_tstamp_type_val_to_name(3)   get name for a time stamp type pcap_tstamp_type_val_to_description(3)   get description for a time stamp type pcap_tstamp_type_name_to_val(3)   get time stamp type corresponding to a name pcap_set_tstamp_precision(3)   set time stamp precision for a not-yet-activated pcap_t for live capture pcap_get_tstamp_precision(3)   get the time stamp precision of a pcap_t for live capture pcap_datalink(3)   get link-layer header type for a pcap_t pcap_file(3)   get the FILE * for a pcap_t opened for a ``savefile'' pcap_is_swapped(3)   determine whether a ``savefile'' being read came from a machine with the opposite byte order pcap_major_version(3) pcap_minor_version(3)   get the major and minor version of the file format version for a ``savefile''

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(3)   get a list of link-layer header types for a device pcap_free_datalinks(3)   free list of link-layer header types pcap_set_datalink(3)   set link-layer header type for a device pcap_datalink_val_to_name(3)   get name for a link-layer header type pcap_datalink_val_to_description(3)   get description for a link-layer header type pcap_datalink_name_to_val(3)   get link-layer header type corresponding to a name

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(3)   specify whether to capture incoming packets, outgoing packets, or both

To get statistics about packets received and dropped in a live capture, call pcap_stats().

Routines
	pcap_stats(3)   get capture statistics

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(3)   open a pcap_dumper_t for a ``savefile``, given a pathname pcap_dump_fopen(3)   open a pcap_dumper_t for a ``savefile``, given a FILE * pcap_dump_close(3)   close a pcap_dumper_t pcap_dump_file(3)   get the FILE * for a pcap_dumper_t opened for a ``savefile''

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(3)   write packet to a pcap_dumper_t pcap_dump_flush(3)   flush buffered packets written to a pcap_dumper_t to the ``savefile'' pcap_dump_ftell(3)   get current file position for a pcap_dumper_t

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(3) pcap_sendpacket(3)   transmit a packet

Some routines return error or warning status codes; to convert them to a string, use pcap_statustostr().

Routines
	pcap_statustostr(3)   get a string for an error or warning status code

To get a string giving version information about libpcap, call pcap_lib_version().

Routines
	pcap_lib_version(3)   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/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.

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.