Provided by: libpcap0.8-dev_1.5.3-2_amd64 bug

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''.

   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_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.

       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  timeout,  on  platforms  that  support  a  read
              timeout,  has  platform-dependent  behavior  that could cause a read to wait for an
              unlimited 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 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 (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 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  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();  http://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(3PCAP)
                     get a pcap_t for live capture

              pcap_activate(3PCAP)
                     activate a pcap_t for live capture

              pcap_findalldevs(3PCAP)
                     get a list of devices that can be opened for a live capture

              pcap_freealldevs(3PCAP)
                     free list of devices

              pcap_lookupdev(3PCAP)
                     get first non-loopback device on that list

              pcap_open_offline(3PCAP)
                     open a pcap_t for a ``savefile'', given a pathname

              pcap_fopen_offline(3PCAP)
                     open a pcap_t for a ``savefile'', given a FILE *

              pcap_open_dead(3PCAP)
                     create a ``fake'' pcap_t

              pcap_close(3PCAP)
                     close a pcap_t

              pcap_set_snaplen(3PCAP)
                     set the snapshot length for a not-yet-activated pcap_t for live capture

              pcap_snapshot(3PCAP)
                     get the snapshot length for a pcap_t

              pcap_set_promisc(3PCAP)
                     set promiscuous mode for a not-yet-activated pcap_t for live capture

              pcap_set_rfmon(3PCAP)
                     set monitor mode for a not-yet-activated pcap_t for live capture

              pcap_can_set_rfmon(3PCAP)
                     determine whether monitor mode can be set for a pcap_t for live capture

              pcap_set_timeout(3PCAP)
                     set read timeout for a not-yet-activated pcap_t for live capture

              pcap_set_buffer_size(3PCAP)
                     set buffer size for a not-yet-activated pcap_t for live capture

              pcap_set_tstamp_type(3PCAP)
                     set time stamp type for a not-yet-activated pcap_t for live capture

              pcap_list_tstamp_types(3PCAP)
                     get  list  of  available time stamp types for a not-yet-activated pcap_t for
                     live capture

              pcap_free_tstamp_types(3PCAP)
                     free list of available time stamp types

              pcap_tstamp_type_val_to_name(3PCAP)
                     get name for a time stamp type

              pcap_tstamp_type_val_to_description(3PCAP)
                     get description for a time stamp type

              pcap_tstamp_name_to_val(3PCAP)
                     get time stamp type corresponding to a name

              pcap_datalink(3PCAP)
                     get link-layer header type for a pcap_t

              pcap_file(3PCAP)
                     get the FILE * for a pcap_t opened for a ``savefile''

              pcap_is_swapped(3PCAP)
                     determine whether a ``savefile'' being read came from  a  machine  with  the
                     opposite byte order

              pcap_major_version(3PCAP)
              pcap_minor_version(3PCAP)
                     get  the  major  and  minor  version  of  the  file  format  version  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.

       Routines

              pcap_list_datalinks(3PCAP)
                     get a list of link-layer header types for a device

              pcap_free_datalinks(3PCAP)
                     free list of link-layer header types

              pcap_set_datalink(3PCAP)
                     set link-layer header type for a device

              pcap_datalink_val_to_name(3PCAP)
                     get name for a link-layer header type

              pcap_datalink_val_to_description(3PCAP)
                     get description for a link-layer header type

              pcap_datalink_name_to_val(3PCAP)
                     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 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 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 snapshot 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 pcap_breakloop().

       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 timeout 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_getnonblock() 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 availability  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 descriptor 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.

       Routines

              pcap_dispatch(3PCAP)
                     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''

              pcap_loop(3PCAP)
                     read packets from a pcap_t until an interrupt or error occurs

              pcap_next(3PCAP)
                     read the next packet from a pcap_t without an indication  whether  an  error
                     occurred

              pcap_next_ex(3PCAP)
                     read the next packet from a pcap_t with an error indication on an error

              pcap_breakloop(3PCAP)
                     prematurely terminate the loop in pcap_dispatch() or pcap_loop()

              pcap_setnonblock(3PCAP)
                     set or clear non-blocking mode on a pcap_t

              pcap_getnonblock(3PCAP)
                     get the state of non-blocking mode for a pcap_t

              pcap_get_selectable_fd(3PCAP)
                     attempt  to  get a descriptor for a pcap_t that can be used in calls such as
                     select(2) and poll(2)

   Filters
       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 filtering 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 compiled 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_setfilter().   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().

       Routines

              pcap_compile(3PCAP)
                     compile filter expression to a pseudo-machine-language code program

              pcap_freecode(3PCAP)
                     free a filter program

              pcap_setfilter(3PCAP)
                     set filter for a pcap_t

              pcap_lookupnet(3PCAP)
                     get network address and network mask for a capture device

              pcap_offline_filter(3PCAP)
                     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().

       Routines

              pcap_setdirection(3PCAP)
                     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().

       Routines

              pcap_stats(3PCAP)
                     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 ``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(3PCAP)
                     open a pcap_dumper_t for a ``savefile``, given a pathname

              pcap_dump_fopen(3PCAP)
                     open a pcap_dumper_t for a ``savefile``, given a FILE *

              pcap_dump_close(3PCAP)
                     close a pcap_dumper_t

              pcap_dump_file(3PCAP)
                     get the FILE * for a pcap_dumper_t opened for a ``savefile''

   Writing packets
       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(3PCAP)
                     write packet to a pcap_dumper_t

              pcap_dump_flush(3PCAP)
                     flush buffered packets written to a pcap_dumper_t to the ``savefile''

              pcap_dump_ftell(3PCAP)
                     get current file position for a pcap_dumper_t

   Injecting packets
       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(3PCAP)
              pcap_sendpacket(3PCAP)
                     transmit a packet

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

       Routines

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

   Getting library version information
       To get a string giving version information about libpcap, call pcap_library_version().

       Routines

              pcap_library_version(3PCAP)
                     get library version string

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 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.

SEE ALSO

       autoconf(1), tcpdump(8), 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

                                           1 July 2013                                PCAP(3PCAP)