packet — packet interface on device level
#include <sys/socket.h> #include <linux/if_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 structure.
protocol is the IEEE
802.3 protocol number in network byte 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 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), or 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 structure 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 */};
The fields of this structure are as follows:
sll_protocol is the
standard ethernet protocol type in network byte 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
originating 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, and
sll_protocol. 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 socket options are configured by calling
setsockopt(2) with level
SOL_PACKET.
PACKET_ADD_MEMBERSHIPPACKET_DROP_MEMBERSHIPPacket sockets can be used to configure
physical-layer multicasting and promiscuous mode.
PACKET_ADD_MEMBERSHIP
adds a binding and PACKET_DROP_MEMBERSHIP drops 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 field 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.
PACKET_AUXDATA (since Linux
2.6.21)If this binary option is enabled, the packet socket passes a metadata structure along with each packet in the recvmsg(2) control field. The structure can be read with cmsg(3). It is defined as
struct tpacket_auxdata { __u32 tp_status;__u32 tp_len; /* packet length */__u32 tp_snaplen; /* captured length */__u16 tp_mac;__u16 tp_net;__u16 tp_vlan_tci;__u16 tp_padding;};
PACKET_FANOUT (since Linux
3.1)To scale processing across threads, packet sockets
can form a fanout group. In this mode, each matching
packet is enqueued onto only one socket in the group.
A socket joins a fanout group by calling setsockopt(2) with
level SOL_PACKET and
option PACKET_FANOUT.
Each network namespace can have up to 65536
independent groups. A socket selects a group by
encoding the ID in the first 16 bits of the integer
option value. The first packet socket to join a group
implicitly creates it. To successfully join an
existing group, subsequent packet sockets must have
the same protocol, device settings, fanout mode and
flags (see below). Packet sockets can leave a fanout
group only by closing the socket. The group is
deleted when the last socket is closed.
Fanout supports multiple algorithms to spread traffic between sockets, as follows:
The default mode,
PACKET_FANOUT_HASH, sends packets from the same flow to the same socket to maintain per-flow ordering. For each packet, it chooses a socket by taking the packet flow hash modulo the number of sockets in the group, where a flow hash is a hash over network-layer address and optional transport-layer port fields.The load-balance mode
PACKET_FANOUT_LBimplements a round-robin algorithm.
PACKET_FANOUT_CPUselects the socket based on the CPU that the packet arrived on.
PACKET_FANOUT_ROLLOVERprocesses all data on a single socket, moving to the next when one becomes backlogged.
PACKET_FANOUT_RNDselects the socket using a pseudo-random number generator.
PACKET_FANOUT_QM(available since Linux 3.14) selects the socket using the recorded queue_mapping of the received skb.
Fanout modes can take additional options. IP
fragmentation causes packets from the same flow to
have different flow hashes. The flag PACKET_FANOUT_FLAG_DEFRAG, if set,
causes packets to be defragmented before fanout is
applied, to preserve order even in this case. Fanout
mode and options are communicated in the second 16
bits of the integer option value. The flag
PACKET_FANOUT_FLAG_ROLLOVER enables
the roll over mechanism as a backup strategy: if the
original fanout algorithm selects a backlogged
socket, the packet rolls over to the next available
one.
PACKET_LOSS (with PACKET_TX_RING)When a malformed packet is encountered on a
transmit ring, the default is to reset its tp_status to
TP_STATUS_WRONG_FORMAT
and abort the transmission immediately. The malformed
packet blocks itself and subsequently enqueued
packets from being sent. The format error must be
fixed, the associated tp_status reset to
TP_STATUS_SEND_REQUEST,
and the transmission process restarted via send(2). However,
if PACKET_LOSS is set,
any malformed packet will be skipped, its tp_status reset to
TP_STATUS_AVAILABLE,
and the transmission process continued.
PACKET_RESERVE (with PACKET_RX_RING)By default, a packet receive ring writes packets immediately following the metadata structure and alignment padding. This integer option reserves additional headroom.
PACKET_RX_RINGCreate a memory-mapped ring buffer for
asynchronous packet reception. The packet socket
reserves a contiguous region of application address
space, lays it out into an array of packet slots and
copies packets (up to tp_snaplen) into
subsequent slots. Each packet is preceded by a
metadata structure similar to tpacket_auxdata. The
protocol fields encode the offset to the data from
the start of the metadata header. tp_net stores the
offset to the network layer. If the packet socket is
of type SOCK_DGRAM,
then tp_mac
is the same. If it is of type SOCK_RAW, then that field stores
the offset to the link-layer frame. Packet socket and
application communicate the head and tail of the ring
through the tp_status field. The
packet socket owns all slots with tp_status equal to
TP_STATUS_KERNEL. After
filling a slot, it changes the status of the slot to
transfer ownership to the application. During normal
operation, the new tp_status value has at
least the TP_STATUS_USER bit set to signal
that a received packet has been stored. When the
application has finished processing a packet, it
transfers ownership of the slot back to the socket by
setting tp_status equal to
TP_STATUS_KERNEL.
Packet sockets implement multiple variants of the
packet ring. The implementation details are described
in Documentation/networking/packet_mmap.txt
in the Linux kernel source tree.
PACKET_STATISTICSRetrieve packet socket statistics in the form of a structure
struct tpacket_stats { unsigned int tp_packets; /* Total packet count */unsigned int tp_drops; /* Dropped packet count */};
Receiving statistics resets the internal counters.
The statistics structure differs when using a ring of
variant TPACKET_V3.
PACKET_TIMESTAMP (with PACKET_RX_RING; since Linux
2.6.36)The packet receive ring always stores a timestamp
in the metadata header. By default, this is a
software generated timestamp generated when the
packet is copied into the ring. This integer option
selects the type of timestamp. Besides the default,
it support the two hardware formats described in
Documentation/networking/timestamping.txt
in the Linux kernel source tree.
PACKET_TX_RING (since Linux
2.6.31)Create a memory-mapped ring buffer for packet
transmission. This option is similar to PACKET_RX_RING and takes the same
arguments. The application writes packets into slots
with tp_status equal to
TP_STATUS_AVAILABLE and
schedules them for transmission by changing
tp_status to
TP_STATUS_SEND_REQUEST.
When packets are ready to be transmitted, the
application calls send(2) or a
variant thereof. The buf and len fields of this
call are ignored. If an address is passed using
sendto(2) or
sendmsg(2), then
that overrides the socket default. On successful
transmission, the socket resets tp_status to
TP_STATUS_AVAILABLE. It
immediately aborts the transmission on error unless
PACKET_LOSS is set.
PACKET_VERSION (with PACKET_RX_RING; since Linux
2.6.27)By default, PACKET_RX_RING creates a packet
receive ring of variant TPACKET_V1. To create another
variant, configure the desired variant by setting
this integer option before creating the ring.
PACKET_QDISC_BYPASS (since Linux
3.14)By default, packets sent through packet sockets pass through the kernel's qdisc (traffic control) layer, which is fine for the vast majority of use cases. For traffic generator appliances using packet sockets that intend to brute-force flood the network—for example, to test devices under load in a similar fashion to pktgen—this layer can be bypassed by setting this integer option to 1. A side effect is that packet buffering in the qdisc layer is avoided, which will lead to increased drops when network device transmit queues are busy; therefore, use at your own risk.
SIOCGSTAMP can be used to
receive the timestamp of the last received packet. Argument
is a struct timeval
variable.
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.
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 prefixed.
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 with the call:
socket(AF_INET, SOCK_PACKET, protocol)
This is still supported, but deprecated and strongly
discouraged. 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.
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.
The Linux kernel source tree. /Documentation/networking/filter.txt
describes how to apply Berkeley Packet Filters to packet
sockets. /tools/testing/selftests/net/psock_tpacket.c
contains example source code for all available versions of
PACKET_RX_RING and PACKET_TX_RING.
This page is part of release 4.00 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
http://www.kernel.org/doc/man−pages/.
|
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 $ |