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''.

   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
              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().

       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
              types.

              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().

       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 cap-
                     ture

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

              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_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 read timeout 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_name_to_val(3)
                     get time stamp type corresponding to a name

              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 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.

       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

   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
                     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 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
       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  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.

       Routines

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

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

              pcap_next_ex(3)
                     read  the next packet from a pcap_t with an error indica-
                     tion on an error

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

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

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

              pcap_get_selectable_fd(3)
                     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 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().

       Routines

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

              pcap_freecode(3)
                     free a filter program

              pcap_setfilter(3)
                     set filter for a pcap_t

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

              pcap_offline_filter(3)
                     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(3)
                     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(3)
                     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
       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  ``save-
                     file''

   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
       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

   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.)

       Routines

              pcap_inject(3)
              pcap_sendpacket(3)
                     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(3)
                     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(3)
                     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 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(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(3)

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