# packet

## 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.  If _protocol_ is set to zero,
           no packets are received.  bind(2) can optionally be called with a
           nonzero _sll_protocol_ to start receiving packets for the protocols
           specified.
    
           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 size 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;    /* Size 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_
           _sll_halen_
                  contain the physical-layer (e.g., IEEE 802.3) address and
                  its size.  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 size */
                          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 size */
                          __u32 tp_snaplen;  /* captured size */
                          __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 size 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



       **LLC header handling**
           The IEEE 802.2/803.3 LLC handling could be considered as a bug.
    
       **MSG_TRUNC issues**
           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**.
    
       **spkt_device device name truncation**
           The _spkt_device_ field of _sockaddr_pkt_ has a size of 14 bytes,
           which is less than the constant **IFNAMSIZ** defined in _< net/if.h>_
           which is 16 bytes and describes the system limit for a network
           interface name.  This means the names of network devices longer
           than 14 bytes will be truncated to fit into _spkt_device_.  All
           these sizes include the terminating null byte ('\0')).
    
           Issues from this with old code typically show up with very long
           interface names used by the **Predictable Network Interface Names**
           feature enabled by default in many modern Linux distributions.
    
           The preferred solution is to rewrite code to avoid **SOCK_PACKET**.
           Possible user solutions are to disable **Predictable Network**
           **Interface Names** or to rename the interface to a name of at most 13
           bytes, for example using the ip(8) tool.
    
       **Documentation issues**
           Socket filters are not documented.
    


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