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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.  proto-
       col 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 sock-
       addr_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 bind-
       ing.

       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.  Argu-
       ment 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 send-
       ing 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 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  sock-
       addr_pkt  to  specify  an  interface, which doesn't provide physical layer indepen-
       dence.

           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-termi-
       nated 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 con-
       trol 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 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 information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-08-08                         PACKET(7)

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