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 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
       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 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 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 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 difference between the two methods is that SOCK_PACKET uses the old struct sockaddr_pkt to specify an inter-
       face, 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 orig-
       inal 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 3.25 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								    2008-08-08								 PACKET(7)