Ubuntu Manpage: packet, PF_PACKET - packet interface on device level.

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NAME

       packet, PF_PACKET - packet interface on device level.

SYNOPSIS

       #include <sys/socket.h>
       #include <features.h>    /* for the glibc version number */
       #if __GLIBC__ >= 2 && __GLIBC_MINOR >= 1
       #include <netpacket/packet.h>
       #include <net/ethernet.h>     /* the L2 protocols */
       #else
       #include <asm/types.h>
       #include <linux/if_packet.h>
       #include <linux/if_ether.h>   /* The L2 protocols */
       #endif

       packet_socket = socket(PF_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. protocol is the IEEE 802.3
protocol number in network 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 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
only get packets 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.

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; /* Header 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 order as defined in the
linux/if_ether.h include file. sll_ifindex is the interface index of the interface (see
netdevice(2) ); 0 matches any interface (only legal for binding). sll_hatype is a 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 only sense 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.

SOCKET OPTIONS

       Packet  sockets can be used to configure physical layer multicasting and promiscuous mode.
       It works by calling setsockopt(2) on a packet socket for SOL_PACKET and one of the options
       PACKET_ADD_MEMBERSHIP  to  add  a binding or PACKET_DROP_MEMBERSHIP to drop 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.

IOCTLS

       SIOCGSTAMP can be used to receive the time stamp of the last received packet. Argument  is
       a struct timeval.

       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.

COMPATIBILITY

       In  Linux  2.0,  the  only  way  to  get  a  packet  socket was by calling socket(PF_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, e.g. eth0.

       This structure is obsolete and should not be used in new code.

NOTES

       For portable programs it is suggested to use PF_PACKET via  pcap(3);  although  this  only
       covers a subset of the PF_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 prepended. 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.

ERRORS

       ENETDOWN
              Interface is not up.

       ENOTCONN
              No interface address passed.

       ENODEV Unknown device name or interface index specified in interface address.

       EMSGSIZE
              Packet is bigger than interface MTU.

       ENOBUFS
              Not enough memory to allocate the packet.

       EFAULT User passed invalid memory address.

       EINVAL Invalid argument.

       ENXIO  Interface address contained illegal interface index.

       EPERM  User has insufficient privileges to carry out this operation.

       EADDRNOTAVAIL
              Unknown multicast group address passed.

       ENOENT No packet received.

              In addition other errors may be generated by the low-level driver.

VERSIONS

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

BUGS

       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.

CREDITS

       This  man page was writen by Andi Kleen with help from Matthew Wilcox.  PF_PACKET in Linux
       2.2 was implemented by Alexey Kuznetsov, based on code by Alan Cox and others.