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RedHat 9 (Linux i386) - man page for packet (redhat section 7)

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

NAME
       packet, PF_PACKET - packet interface on device level.

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

       packet_socket = socket(PF_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 phys-
       ical 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
       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
       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  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_MULTI-
       CAST 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 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
       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
       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 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, e.g. eth0.

       This structure is obsolete and should not be used in new code.

NOTES
       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
       ENETDOWN
	      Interface is not up.

       ENOTCONN
	      No interface address passed.

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

       EMSGSIZE
	      Packet is bigger than interface MTU.

       ENOBUFS
	      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
	      Unknown multicast group address passed.

       ENOENT No packet received.

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

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

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 extension is an ugly hack and should be replaced by a  control  mes-
       sage.   There  is  currently no way to get the original destination address of packets via
       SOCK_DGRAM.

CREDITS
       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
       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)


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