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.