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



NAME
       packet, AF_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 receiv-
       ing a packet, the address is still parsed and passed in a standard sockaddr_ll address structure.  When  trans-
       mitting  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
       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;   /* Header 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 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 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  parame-
       ter  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 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 pack-
       ets  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 encapsu-
       lation 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 difference 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 cur-
       rently 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-08                         PACKET(7)