Originální popis anglicky:
packet, PF_PACKET - packet interface on device level.
Návod, kniha: Linux Programmer's Manual
#include <sys/socket.h>
#include <netpacket/packet.h>
#include <net/ethernet.h> /* the L2 protocols */
packet_socket = socket(PF_PACKET, int socket_type, int protocol);
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.
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.
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. It defaults to the
socket's protocol.
sll_ifindex is the interface index of the interface
(see
netdevice(7)); 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.
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.
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.
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.
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.
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.
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.
- 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.
PF_PACKET is a new feature in Linux 2.2. Earlier Linux versions supported
only
SOCK_PACKET.
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 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.
The include file
<netpacket/packet.h> is present since glibc2.1.
Older systems need
#include <asm/types.h>
#include <linux/if_packet.h>
#include <linux/if_ether.h> /* The L2 protocols */
socket(2),
pcap(3),
capabilities(7),
ip(7),
raw(7),
socket(7)
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.
