收发链路层包：packet(7)

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



NAME
       packet, AF_PACKET - packet interface on device level.

SYNOPSIS
       #include
       #include
       #include /* 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
       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 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 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  inter‐
       face.

       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 is present since glibc 2.1.  Older systems need:

           #include
           #include
           #include   /* 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.  Incom‐
       ing  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 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 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  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 include file for physical layer protocols.

COLOPHON
       This  page  is  part  of  release  3.35  of  the Linux man-pages project.  A description of the project, and information about reporting bugs, can be found at
      



Linux                                                                         2008-08-08                                                                    PACKET(7)