packet


SYNOPSIS
       #include <sys/socket.h>
       #include <netpacket/packet.h>
       #include <net/ethernet.h> /* the L2 protocols */

       packet_socket = socket(AF_PACKET, int socket_type, int protocol);

DESCRIPTION
       Packet  sockets  are  used to receive or send raw packets at the device
       driver (OSI Layer 2) level.  They allow the user to implement  protocol
       modules in user space on top of the physical layer.

       The  socket_type  is either SOCK_RAW for raw packets including the link
       level header or SOCK_DGRAM for  cooked  packets  with  the  link  level
       header  removed.   The  link level header information is available in a
       common format in a sockaddr_ll.  protocol is the  IEEE  802.3  protocol
       number in network order.  See the <linux/if_ether.h> include file for a
       list of allowed protocols.  When protocol is  set  to  htons(ETH_P_ALL)
       then all protocols are received.  All incoming packets of that protocol
       type will be passed to the packet socket before they are passed to  the
       protocols implemented in the kernel.

       Only  processes  with effective UID 0 or the CAP_NET_RAW capability may
       open packet sockets.

       SOCK_RAW packets are passed to and from the device driver  without  any
       changes  in  the  packet data.  When receiving a packet, the address is
       still parsed and passed in a standard  sockaddr_ll  address  structure.
       When transmitting a packet, the user supplied buffer should contain the
       physical layer header.  That packet is then queued  unmodified  to  the
       network  driver  of  the  interface defined by the destination address.
       Some device drivers always add other headers.  SOCK_RAW is  similar  to
       but not compatible with the obsolete AF_INET/SOCK_PACKET of Linux 2.0.

       SOCK_DGRAM operates on a slightly higher level.  The physical header is
       removed before the packet is passed to the user.  Packets sent  through
       a  SOCK_DGRAM  packet socket get a suitable physical layer header based
       on the information in the sockaddr_ll destination address  before  they
       are queued.

       By  default  all packets of the specified protocol type are passed to a
       packet socket.  To only get  packets  from  a  specific  interface  use
       bind(2)  specifying  an  address  in  a  struct sockaddr_ll to bind the
       packet  socket  to  an  interface.   Only  the  sll_protocol  and   the
       sll_ifindex address fields are used for purposes of binding.

       The connect(2) operation is not supported on packet sockets.

       When  the  MSG_TRUNC flag is passed to recvmsg(2), recv(2), recvfrom(2)
       the real length of the packet on the wire is always returned, even when
       it is longer than the buffer.

   Address Types
       sll_protocol is the standard ethernet protocol type in network order as
       defined in the <linux/if_ether.h> include file.   It  defaults  to  the
       socket's protocol.  sll_ifindex is the interface index of the interface
       (see netdevice(7)); 0 matches any interface (only permitted  for  bind-
       ing).   sll_hatype  is  a  ARP  type as defined in the <linux/if_arp.h>
       include file.  sll_pkttype contains the packet type.  Valid  types  are
       PACKET_HOST  for a packet addressed to the local host, PACKET_BROADCAST
       for a physical layer broadcast packet, PACKET_MULTICAST  for  a  packet
       sent  to  a  physical  layer  multicast address, PACKET_OTHERHOST for a
       packet to some other host that has been caught by a  device  driver  in
       promiscuous  mode, and PACKET_OUTGOING for a packet originated from the
       local host that is looped back to a packet socket.   These  types  make
       only  sense for receiving.  sll_addr and sll_halen contain the physical
       layer (e.g., IEEE 802.3) address and its length.  The exact interpreta-
       tion depends on the device.

       When  you  send  packets  it is enough to specify sll_family, sll_addr,
       sll_halen, sll_ifindex.  The other fields should be 0.  sll_hatype  and
       sll_pkttype are set on received packets for your information.  For bind
       only sll_protocol and sll_ifindex are used.

   Socket Options
       Packet sockets can be used to configure physical layer multicasting and
       promiscuous mode.  It works by calling setsockopt(2) on a packet socket
       for SOL_PACKET and one of the options PACKET_ADD_MEMBERSHIP  to  add  a
       binding  or  PACKET_DROP_MEMBERSHIP  to  drop  it.   They both expect a
       packet_mreq structure as argument:

           struct packet_mreq {
               int            mr_ifindex;    /* interface index */
               unsigned short mr_type;       /* action */
               unsigned short mr_alen;       /* address length */
               unsigned char  mr_address[8]; /* physical layer address */
           };

       mr_ifindex contains the interface index for the interface whose  status
       should  be  changed.   The  mr_type parameter specifies which action to
       perform.  PACKET_MR_PROMISC enables receiving all packets on  a  shared
       medium  (often  known as "promiscuous mode"), PACKET_MR_MULTICAST binds
       the  socket  to  the  physical  layer  multicast  group  specified   in
       mr_address  and  mr_alen,  and PACKET_MR_ALLMULTI sets the socket up to
       receive all multicast packets arriving at the interface.

       In addition the traditional ioctls SIOCSIFFLAGS, SIOCADDMULTI, SIOCDEL-
       MULTI can be used for the same purpose.

   Ioctls
       SIOCGSTAMP  can  be  used to receive the timestamp of the last received
       packet.  Argument is a struct timeval.

       In addition all standard ioctls defined in netdevice(7)  and  socket(7)
       are valid on packet sockets.

   Error Handling
       EMSGSIZE
              Packet is bigger than interface MTU.

       ENETDOWN
              Interface is not up.

       ENOBUFS
              Not enough memory to allocate the packet.

       ENODEV Unknown  device  name  or interface index specified in interface
              address.

       ENOENT No packet received.

       ENOTCONN
              No interface address passed.

       ENXIO  Interface address contained an invalid interface index.

       EPERM  User has insufficient privileges to carry out this operation.

              In addition other errors  may  be  generated  by  the  low-level
              driver.

VERSIONS
       AF_PACKET  is  a new feature in Linux 2.2.  Earlier Linux versions sup-
       ported only SOCK_PACKET.

       The include file  <netpacket/packet.h>  is  present  since  glibc  2.1.
       Older systems need:

           #include <asm/types.h>
           #include <linux/if_packet.h>
           #include <linux/if_ether.h>  /* The L2 protocols */

NOTES
       For  portable  programs  it  is suggested to use AF_PACKET via pcap(3);
       although this only covers a subset of the AF_PACKET features.

       The SOCK_DGRAM packet sockets make no attempt to create  or  parse  the
       IEEE  802.2  LLC  header  for  a IEEE 802.3 frame.  When ETH_P_802_3 is
       specified as protocol for sending the kernel creates  the  802.3  frame
       and  fills  out the length field; the user has to supply the LLC header
       to get a fully conforming packet.  Incoming 802.3 packets are not  mul-
       tiplexed on the DSAP/SSAP protocol fields; instead they are supplied to
       the user as protocol ETH_P_802_2 with the LLC header prepended.  It  is
       thus  not  possible to bind to ETH_P_802_3; bind to ETH_P_802_2 instead
       and do the protocol multiplex yourself.  The default for sending is the
       standard Ethernet DIX encapsulation with the protocol filled in.

       Packet sockets are not subject to the input or output firewall chains.

   Compatibility
       In  Linux  2.0,  the  only  way  to  get a packet socket was by calling
       protocol type as defined in <sys/if_ether.h>  and  spkt_device  is  the
       device name as a null-terminated string, for example, eth0.

       This structure is obsolete and should not be used in new code.

BUGS
       glibc  2.1  does not have a define for SOL_PACKET.  The suggested work-
       around is to use:

           #ifndef SOL_PACKET
           #define SOL_PACKET 263
           #endif

       This is fixed in later glibc versions and also does not occur on  libc5
       systems.

       The IEEE 802.2/803.3 LLC handling could be considered as a bug.

       Socket filters are not documented.

       The  MSG_TRUNC  recvmsg(2)  extension  is  an  ugly  hack and should be
       replaced by a control message.  There is currently no way  to  get  the
       original destination address of packets via SOCK_DGRAM.

SEE ALSO
       socket(2), pcap(3), capabilities(7), ip(7), raw(7), socket(7)

       RFC 894 for the standard IP Ethernet encapsulation.

       RFC 1700 for the IEEE 802.3 IP encapsulation.

       The <linux/if_ether.h> include file for physical layer protocols.

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2008-08-08                         PACKET(7)
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