packet

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



NAME
       packet - packet interface on device level

SYNOPSIS
       #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);

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 <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.

       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 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 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 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 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_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 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_STATUS_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.rst in the Linux kernel
              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 differs 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/networking/timestamping.rst 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 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 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 <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.

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 encapsulation.

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

       The Linux kernel source tree.  Documentation/networking/filter.rst
       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 5.13 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                          PACKET(7)