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packet — packet interface on device level.
#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); |
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.
The sockaddr_ll is a device independent physical layer address.
struct sockaddr_ll { unsigned short sll_family;unsigned short sll_protocol;int sll_ifindex;unsigned short sll_hatype;unsigned char sll_pkttype;unsigned char sll_halen;unsigned char sll_addr[8];};
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.
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;unsigned short mr_type;unsigned short mr_alen;unsigned char mr_address[8];};
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 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.
Unknown multicast group address passed.
User passed invalid memory address.
Invalid argument.
Packet is bigger than interface MTU.
Interface is not up.
Not enough memory to allocate the packet.
Unknown device name or interface index specified in interface address.
No packet received.
No interface address passed.
Interface address contained an invalid interface index.
User has insufficient privileges to carry out this operation.
In addition other errors may be generated by the low-level driver.
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 */
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.
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.
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.
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.
This page is part of release 3.52 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/.
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This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. %%%LICENSE_START(VERBATIM_ONE_PARA) Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. %%%LICENSE_END $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $ |