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PACKET(7)                           Linux Programmer's Manual                           PACKET(7)



NAME
       packet - packet interface on device level.

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  stan-
       dard  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 suit-
       able 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
       get  packets  only 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
       The sockaddr_ll is a device independent physical layer address.

           struct sockaddr_ll {
               unsigned short sll_family;   /* Always AF_PACKET */
               unsigned short sll_protocol; /* Physical layer protocol */
               int            sll_ifindex;  /* Interface number */
               unsigned short sll_hatype;   /* ARP hardware type */
               unsigned char  sll_pkttype;  /* Packet type */
               unsigned char  sll_halen;    /* Length of address */
               unsigned char  sll_addr[8];  /* Physical layer address */
           };

       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  binding).   sll_hatype  is  an 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 sense only for receiving.  sll_addr
       and sll_halen contain the physical layer (e.g., IEEE 802.3) address and its  length.   The
       exact interpretation 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, SIOCDELMULTI  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
       Packet sockets do no error handling other than errors occurred while passing the packet to
       the device driver.  They don't have the concept of a pending error.

ERRORS
       EADDRNOTAVAIL
              Unknown multicast group address passed.

       EFAULT User passed invalid memory address.

       EINVAL Invalid argument.

       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  supported  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 covers
       only 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 multiplexed 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  socket(AF_INET,
       SOCK_PACKET,  protocol).   This is still supported but strongly deprecated.  The main dif-
       ference between the two methods is that SOCK_PACKET uses the old  struct  sockaddr_pkt  to
       specify an interface, which doesn't provide physical layer independence.

           struct sockaddr_pkt {
               unsigned short spkt_family;
               unsigned char  spkt_device[14];
               unsigned short spkt_protocol;
           };

       spkt_family  contains  the  device  type, spkt_protocol is the IEEE 802.3 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 workaround 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 encap-
       sulation.

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

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2012-05-10                                  PACKET(7)

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