NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       packet - packet interface on device level

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netpacket/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.  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.

       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 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.  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;   /* ARP hardware 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 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 originated 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.  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 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 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 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.  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, moves 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
              packet 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)
              If set, do not silently drop a packet on transmission error,
              but return it with status set to TP_STATUS_WRONG_FORMAT.

       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 status 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 status is TP_STATUS_USER, to signal that a
              correctly 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 the status
              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 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.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
              status TP_STATUS_AVAILABLE and schedules them for transmission
              by changing the 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 the slot to
              TP_STATUS_AVAILABLE.  It discards packets silently 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.

       The include file <netpacket/packet.h> is present since glibc 2.1.
       Older systems need:

           #include <asm/types.h>
           #include <linux/if_packet.h>
           #include <linux/if_ether.h>  /* The L2 protocols */

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 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 <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

       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         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.

COLOPHON         top

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                            2014-02-26                        PACKET(7)