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PCAP(3PCAP)							   PCAP(3PCAP)

       pcap - Packet Capture library

       #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

   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
       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 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().

       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

	      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,	has  platform-
	      dependent behavior that could cause a read to wait for an unlim‐
	      ited amount of time until the capture buffer fills up  or	 could
	      cause  a read timeout of 1 millisecond to be used.  We recommend
	      that a value of zero not be used.

	      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

	      The buffer size is set with pcap_set_buffer_size().

       timestamp type
	      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‐
       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.

       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.

       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 ``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
		     live capture

		     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
		     live capture

		     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

   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 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).

       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 snap‐
       shot  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

       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
       other platforms.

       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
		     a ``savefile''

		     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
		     code program

		     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

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


		     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‐

   Writing packets
       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

   Injecting packets
       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
       return values.)


		     transmit a packet

   Reporting errors
       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:


				  1 July 2013			   PCAP(3PCAP)

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