Section (7) packet
Name
packet — packet interface on device level
Synopsis
#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) ; |
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
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.
In order to create a packet socket, a process must have
the CAP_NET_RAW
capability in
the user namespace that governs its network namespace.
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. Fields used for binding are sll_family
(should be
AF_PACKET
),
sll_protocol
, and
sll_ifindex
.
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.
Address types
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_zsingle_quotesz_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 arePACKET_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, andPACKET_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
andsll_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.
Socket options
Packet socket options are configured by calling
setsockopt(2) with level
SOL_PACKET
.
PACKET_ADD_MEMBERSHIP
PACKET_DROP_MEMBERSHIP
-
Packet sockets can be used to configure physical-layer multicasting and promiscuous mode.
PACKET_ADD_MEMBERSHIP
adds a binding andPACKET_DROP_MEMBERSHIP
drops it. They both expect apacket_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. Themr_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 inmr_address
andmr_alen
, andPACKET_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 optionPACKET_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_LB
implements a round-robin algorithm. -
PACKET_FANOUT_CPU
selects the socket based on the CPU that the packet arrived on. -
PACKET_FANOUT_ROLLOVER
processes all data on a single socket, moving to the next when one becomes backlogged. -
PACKET_FANOUT_RND
selects 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 flagPACKET_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
(withPACKET_TX_RING
)-
When a malformed packet is encountered on a transmit ring, the default is to reset its
tp_status
toTP_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 associatedtp_status
reset toTP_STATUS_SEND_REQUEST
, and the transmission process restarted via send(2). However, ifPACKET_LOSS
is set, any malformed packet will be skipped, itstp_status
reset toTP_STATUS_AVAILABLE
, and the transmission process continued. PACKET_RESERVE
(withPACKET_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_RING
-
Create 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 totpacket_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 typeSOCK_DGRAM
, thentp_mac
is the same. If it is of typeSOCK_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 thetp_status
field. The packet socket owns all slots withtp_status
equal toTP_STATUS_KERNEL
. After filling a slot, it changes the status of the slot to transfer ownership to the application. During normal operation, the newtp_status
value has at least theTP_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 settingtp_status
equal toTP_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_STATISTICS
-
Retrieve 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
(withPACKET_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 withtp_status
equal toTP_STATUS_AVAILABLE
and schedules them for transmission by changingtp_status
toTP_STATUS_SEND_REQUEST
. When packets are ready to be transmitted, the application calls send(2) or a variant thereof. Thebuf
andlen
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 resetstp_status
toTP_STATUS_AVAILABLE
. It immediately aborts the transmission on error unlessPACKET_LOSS
is set. PACKET_VERSION
(withPACKET_RX_RING
; since Linux 2.6.27)-
By default,
PACKET_RX_RING
creates a packet receive ring of variantTPACKET_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_zsingle_quotesz_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.
Ioctls
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.
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
.
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 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.
Compatibility
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_zsingle_quotesz_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
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
.
SEE ALSO
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
.
COLOPHON
This page is part of release 4.16 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
https://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 $ |