packet(7) — Linux manual page

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

NAME         top

       packet - packet interface on device level

SYNOPSIS         top

       #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         top

       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         top

       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         top

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

NOTES         top

       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         top

       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         top

       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         top

       This page is part of release 5.11 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)

Pages that refer to this page: bind(2)getsockopt(2)socket(2)getifaddrs(3)address_families(7)arp(7)ip(7)netdevice(7)raw(7)socket(7)bpfc(8)netsniff-ng(8)trafgen(8)