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NAME    [Toc]    [Back]

       packet, PF_PACKET - packet interface on device level.

SYNOPSIS    [Toc]    [Back]

       #include <sys/socket.h>
       #include <features.h>	/* for the glibc version number */
       #if __GLIBC__ >= 2 && __GLIBC_MINOR >= 1
       #include <netpacket/packet.h>
       #include <net/ethernet.h>     /* the L2 protocols */
       #include <asm/types.h>
       #include <linux/if_packet.h>
       #include <linux/if_ether.h>   /* The L2 protocols */

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

DESCRIPTION    [Toc]    [Back]

       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 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    [Toc]    [Back]

       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(2)); 0 matches any interface (only legal	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

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       SIOCGSTAMP  can	be used to receive the time stamp 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    [Toc]    [Back]

       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.

COMPATIBILITY    [Toc]    [Back]

       In  Linux  2.0,	the  only  way	to  get a packet socket was by calling
       socket(PF_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, e.g. eth0.

       This structure is obsolete and should not be used in new code.

NOTES    [Toc]    [Back]

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

ERRORS    [Toc]    [Back]

	      Interface is not up.

       ENOTCONN    [Toc]    [Back]
	      No interface address passed.

       ENODEV Unknown device name or interface index  specified  in  interface

       EMSGSIZE    [Toc]    [Back]
	      Packet is bigger than interface MTU.

       ENOBUFS    [Toc]    [Back]
	      Not enough memory to allocate the packet.

       EFAULT User passed invalid memory address.

       EINVAL Invalid argument.

       ENXIO  Interface address contained illegal interface index.

       EPERM  User has insufficient privileges to carry out this operation.

       EADDRNOTAVAIL    [Toc]    [Back]
	      Unknown multicast group address passed.

       ENOENT No packet received.

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

VERSIONS    [Toc]    [Back]

       PF_PACKET is a new feature in Linux 2.2. Earlier  Linux	versions  supported

BUGS    [Toc]    [Back]

       glibc  2.1  does not have a define for SOL_PACKET.  The suggested workaround
 is to use
	      #ifndef SOL_PACKET
	      #define SOL_PACKET 263
       This is fixed in later glibc versions and also does not occur on  libc5

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

       Socket filters are not documented.

       The  MSG_TRUNC recvmsg 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.

CREDITS    [Toc]    [Back]

       This  man page was written by Andi Kleen with help from Matthew Wilcox.
       PF_PACKET in Linux 2.2 was implemented by Alexey  Kuznetsov,  based  on
       code by Alan Cox and others.

SEE ALSO    [Toc]    [Back]

       ip(7), socket(7), socket(2), raw(7), pcap(3)

       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.

Linux Man Page			  1999-04-29			     PACKET(7)
[ Back ]
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