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PACKET(7) Linux Programmer's Manual PACKET(7)
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.
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's protocol.
* sll_ifindex is the interface index of the interface (see
netdevice(7)); 0 matches any interface (only permitted for bind‐
ing). 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.
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 multi‐
casting 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, SIOCADD‐
MULTI, 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 encod‐
ing 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 pre‐
serve 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_sta‐
tus 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 contin‐
ued.
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_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 simi‐
lar 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_STA‐
TUS_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/net‐
working/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 sta‐
tistics 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 meta‐
data header. By default, this is a software generated time‐
stamp 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 Doc‐
umentation/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 appli‐
ances using packet sockets that intend to brute-force flood
the network—for example, to test devices under load in a simi‐
lar 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.
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.
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.
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.
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'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 4.15 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/.
Linux 2017-09-15 PACKET(7)
Pages that refer to this page: getsockopt(2), socket(2), getifaddrs(3), arp(7), ip(7), netdevice(7), raw(7), socket(7)
Copyright and license for this manual page
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