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

NAME
       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 structure.  protocol is the
       IEEE 802.3 protocol number in network byte order.  See the  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.

       Only processes with 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
       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  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
           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_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_ADD_MEMBERSHIP
       PACKET_DROP_MEMBERSHIP
              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_padding;
                  };

       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
              option 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
                 on.

              *  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.

       PACKET_LOSS (with PACKET_TX_RING)
              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.

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

       PACKET_RX_RING
              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.

       PACKET_STATISTICS
              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
              default, 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
              default, 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
              option 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 pktgen—
              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  increased
              drops  when  network  device  transmit  queues are busy; therefore, use at your own
              risk.

   Ioctls
       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
       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.

NOTES
       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.

   Compatibility
       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
       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
       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 encap‐
       sulation.

       The  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
       PACKET_TX_RING.

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-05-07                                  PACKET(7)

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