Ubuntu Manpage: packet - packet interface on device level.

name
synopsis
description
errors
versions
notes
bugs
see also
colophon

NAME

       packet - packet interface on device level.

SYNOPSIS

       #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

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 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 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 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 timestamp 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.

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.

       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

       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 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.

   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

       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.

COLOPHON

       This  page  is  part  of  release 3.54 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/.