packet(7)
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 includ
ing 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 sock
addr_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 stan
dard sockaddr_ll address structure. When transmitting a
packet, the user supplied buffer should contain the physi
cal layer header. That packet is then queued unmodified
to the network driver of the interface defined by the des
tination 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 physi
cal 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 spe
cific interface use bind(2) specifying an address in a
struct sockaddr_ll to bind the packet socket to an inter
face. 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 sock
ets.
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 physi
cal layer broadcast packet, PACKET_MULTICAST for a packet
sent to a physical layer multicast address, PACKET_OTHER
HOST for a packet to some other host that has been caught
by a device driver in promiscuous mode, and PACKET_OUTGO
ING 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, SIOCADD
MULTI, 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 differ
ence 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 proto
col 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 sug
gested 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.
SEE ALSO
ip(7), socket(7), socket(2), raw(7), pcap(3).
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.
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