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

       packet - packet interface on device level

       #include <sys/socket.h>
       #include <linux/if_packet.h>
       #include <net/ethernet.h> /* the L2 protocols */

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

       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 structure.  protocol is the
       IEEE 802.3 protocol number in network byte 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 pro-
       tocols 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.

       In  order to create a packet socket, a process must have the CAP_NET_RAW capability in the
       user namespace that governs its network namespace.

       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
       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 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
       get packets only from a specific interface use bind(2) specifying an address in  a  struct
       sockaddr_ll  to  bind  the  packet  socket  to  an interface.  Fields used for binding are
       sll_family (should be AF_PACKET), sll_protocol, and sll_ifindex.

       The connect(2) operation is not supported on packet sockets.

       When the MSG_TRUNC flag is passed to recvmsg(2), recv(2), or 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 structure 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;   /* ARP hardware type */
               unsigned char  sll_pkttype;  /* Packet type */
               unsigned char  sll_halen;    /* Length of address */
               unsigned char  sll_addr[8];  /* Physical-layer address */

       The fields of this structure are as follows:

       *  sll_protocol is the standard ethernet protocol type in network byte 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 an 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  ad-
          dressed  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_OTHER-
          HOST for a packet to some other host that has been caught by a device driver in promis-
          cuous mode, and PACKET_OUTGOING for a packet originating from the local  host  that  is
          looped back to a packet socket.  These types make sense only 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,  and sll_protocol.  The other fields should be 0.  sll_hatype and sll_pkttype
       are set on received packets for your information.

   Socket options
       Packet socket options are configured by calling setsockopt(2) with level SOL_PACKET.

              Packet sockets can be used to configure physical-layer multicasting and promiscuous
              mode.   PACKET_ADD_MEMBERSHIP  adds  a binding and PACKET_DROP_MEMBERSHIP drops 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 field 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.

       PACKET_AUXDATA (since Linux 2.6.21)
              If this binary option is enabled, the packet socket  passes  a  metadata  structure
              along  with each packet in the recvmsg(2) control field.  The structure can be read
              with cmsg(3).  It is defined as

                  struct tpacket_auxdata {
                      __u32 tp_status;
                      __u32 tp_len;      /* packet length */
                      __u32 tp_snaplen;  /* captured length */
                      __u16 tp_mac;
                      __u16 tp_net;
                      __u16 tp_vlan_tci;
                      __u16 tp_vlan_tpid; /* Since Linux 3.14; earlier, these
                                             were unused padding bytes */

       PACKET_FANOUT (since Linux 3.1)
              To scale processing across threads, packet sockets can form  a  fanout  group.   In
              this  mode,  each matching packet is enqueued onto only one socket in the group.  A
              socket joins a fanout group by calling setsockopt(2) with level SOL_PACKET and  op-
              tion  PACKET_FANOUT.   Each  network  namespace  can  have  up to 65536 independent
              groups.  A socket selects a group by encoding the ID in the first 16  bits  of  the
              integer  option  value.  The first packet socket to join a group implicitly creates
              it.  To successfully join an existing group, subsequent packet  sockets  must  have
              the  same  protocol,  device  settings,  fanout mode and flags (see below).  Packet
              sockets can leave a fanout group only by closing the socket.  The group is  deleted
              when the last socket is closed.

              Fanout supports multiple algorithms to spread traffic between sockets, as follows:

              *  The  default  mode,  PACKET_FANOUT_HASH, sends packets from the same flow to the
                 same socket to maintain per-flow ordering.  For each packet, it chooses a socket
                 by  taking the packet flow hash modulo the number of sockets in the group, where
                 a flow hash is a hash over network-layer address  and  optional  transport-layer
                 port fields.

              *  The load-balance mode PACKET_FANOUT_LB implements a round-robin algorithm.

              *  PACKET_FANOUT_CPU  selects  the  socket based on the CPU that the packet arrived

              *  PACKET_FANOUT_ROLLOVER processes all data on a single socket, moving to the next
                 when one becomes backlogged.

              *  PACKET_FANOUT_RND selects the socket using a pseudo-random number generator.

              *  PACKET_FANOUT_QM  (available  since  Linux  3.14)  selects  the socket using the
                 recorded queue_mapping of the received skb.

              Fanout modes can take additional options.  IP fragmentation causes packets from the
              same  flow  to  have different flow hashes.  The flag PACKET_FANOUT_FLAG_DEFRAG, if
              set, causes packets to be defragmented before fanout is applied, to preserve  order
              even  in this case.  Fanout mode and options are communicated in the second 16 bits
              of the integer option value.  The flag PACKET_FANOUT_FLAG_ROLLOVER enables the roll
              over  mechanism  as  a  backup strategy: if the original fanout algorithm selects a
              backlogged socket, the packet rolls over to the next available one.

              When a malformed packet is encountered on a transmit ring, the default is to  reset
              its  tp_status  to  TP_STATUS_WRONG_FORMAT  and abort the transmission immediately.
              The malformed packet blocks itself and subsequently  enqueued  packets  from  being
              sent.   The  format  error must be fixed, the associated tp_status reset to TP_STA-
              TUS_SEND_REQUEST, and the transmission process restarted via send(2).  However,  if
              PACKET_LOSS  is  set,  any malformed packet will be skipped, its tp_status reset to
              TP_STATUS_AVAILABLE, and the transmission process continued.

              By default, a packet receive ring writes packets immediately following the metadata
              structure and alignment padding.  This integer option reserves additional headroom.

              Create  a  memory-mapped ring buffer for asynchronous packet reception.  The packet
              socket reserves a contiguous region of application address space, lays it out  into
              an  array  of  packet  slots  and copies packets (up to tp_snaplen) into subsequent
              slots.  Each packet is preceded by a metadata structure similar to tpacket_auxdata.
              The  protocol  fields  encode the offset to the data from the start of the metadata
              header.  tp_net stores the offset to the network layer.  If the packet socket is of
              type  SOCK_DGRAM,  then  tp_mac  is the same.  If it is of type SOCK_RAW, then that
              field stores the offset to the link-layer frame.   Packet  socket  and  application
              communicate  the head and tail of the ring through the tp_status field.  The packet
              socket owns all slots with tp_status equal to TP_STATUS_KERNEL.   After  filling  a
              slot,  it  changes the status of the slot to transfer ownership to the application.
              During normal operation, the new tp_status value has at  least  the  TP_STATUS_USER
              bit set to signal that a received packet has been stored.  When the application has
              finished processing a packet, it transfers ownership of the slot back to the socket
              by setting tp_status equal to TP_STATUS_KERNEL.

              Packet  sockets implement multiple variants of the packet ring.  The implementation
              details are described in Documentation/networking/packet_mmap.txt in the Linux ker-
              nel source tree.

              Retrieve packet socket statistics in the form of a structure

                  struct tpacket_stats {
                      unsigned int tp_packets;  /* Total packet count */
                      unsigned int tp_drops;    /* Dropped packet count */

              Receiving  statistics  resets the internal counters.  The statistics structure dif-
              fers when using a ring of variant TPACKET_V3.

       PACKET_TIMESTAMP (with PACKET_RX_RING; since Linux 2.6.36)
              The packet receive ring always stores a timestamp in the metadata header.   By  de-
              fault,  this  is a software generated timestamp generated when the packet is copied
              into the ring.  This integer option selects the type of timestamp.  Besides the de-
              fault,  it  support  the  two  hardware formats described in Documentation/network-
              ing/timestamping.txt in the Linux kernel source tree.

       PACKET_TX_RING (since Linux 2.6.31)
              Create a memory-mapped ring buffer for packet transmission.  This option is similar
              to  PACKET_RX_RING  and  takes  the same arguments.  The application writes packets
              into slots with tp_status equal  to  TP_STATUS_AVAILABLE  and  schedules  them  for
              transmission  by  changing  tp_status  to TP_STATUS_SEND_REQUEST.  When packets are
              ready to be transmitted, the application calls send(2) or a variant  thereof.   The
              buf  and  len  fields  of  this  call  are  ignored.  If an address is passed using
              sendto(2) or sendmsg(2), then that overrides the  socket  default.   On  successful
              transmission,  the  socket resets tp_status to TP_STATUS_AVAILABLE.  It immediately
              aborts the transmission on error unless PACKET_LOSS is set.

       PACKET_VERSION (with PACKET_RX_RING; since Linux 2.6.27)
              By default, PACKET_RX_RING creates a packet receive ring of variant TPACKET_V1.  To
              create  another  variant, configure the desired variant by setting this integer op-
              tion before creating the ring.

       PACKET_QDISC_BYPASS (since Linux 3.14)
              By default, packets sent through packet sockets pass  through  the  kernel's  qdisc
              (traffic  control)  layer,  which  is fine for the vast majority of use cases.  For
              traffic generator appliances using packet sockets that intend to brute-force  flood
              the  network--for  example, to test devices under load in a similar fashion to pkt-
              gen--this layer can be bypassed by setting this integer option to 1.  A side effect
              is  that  packet  buffering  in  the qdisc layer is avoided, which will lead to in-
              creased drops when network device transmit queues are busy; therefore, use at  your
              own risk.

       SIOCGSTAMP  can be used to receive the timestamp of the last received packet.  Argument is
       a struct timeval variable.

       In addition, all standard ioctls defined in netdevice(7) and socket(7) are valid on packet

   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.

              Unknown multicast group address passed.

       EFAULT User passed invalid memory address.

       EINVAL Invalid argument.

              Packet is bigger than interface MTU.

              Interface is not up.

              Not enough memory to allocate the packet.

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

       ENOENT No packet received.

              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.

       AF_PACKET is  a  new  feature  in  Linux  2.2.   Earlier  Linux  versions  supported  only

       For  portable  programs it is suggested to use AF_PACKET via pcap(3); although this covers
       only 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 prefixed.  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 send-
       ing is the standard Ethernet DIX encapsulation with the protocol filled in.

       Packet sockets are not subject to the input or output firewall chains.

       In Linux 2.0, the only way to get a packet socket was with the call:

           socket(AF_INET, SOCK_PACKET, protocol)

       This is still supported, but deprecated and strongly discouraged.  The main difference be-
       tween 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 de-
       fined 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.

       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

       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 encap-

       The <linux/if_ether.h> include file for physical-layer protocols.

       The Linux kernel source tree.  /Documentation/networking/filter.txt describes how to apply
       Berkeley  Packet  Filters to packet sockets.  /tools/testing/selftests/net/psock_tpacket.c
       contains  example  source  code  for  all  available  versions   of   PACKET_RX_RING   and

       This  page  is  part of release 5.05 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at

Linux                                       2020-02-09                                  PACKET(7)

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