packet(7) Miscellaneous Information Manual packet(7)
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. If protocol is
set to zero, no packets are received. bind(2) can optionally be called
with a nonzero sll_protocol to start receiving packets for the protocols
specified.
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 re-
turned, even when it is longer than the buffer.
Address types
The sockaddr_ll structure is a device-independent physical-layer ad-
dress.
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 physi-
cal-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
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 informa-
tion.
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 multicast-
ing 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 ac-
tion 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 multi-
cast 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) con-
trol field. The structure can be read with cmsg(3). It is de-
fined 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_vlan_tpid; /* Since Linux 3.14; earlier, these
were unused padding bytes */
};
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 inde-
pendent 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 be-
low). 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 trans-
port-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 sec-
ond 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 back-
logged 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_AVAIL-
ABLE, and the transmission process continued.
PACKET_RESERVE (with PACKET_RX_RING)
By default, a packet receive ring writes packets immediately fol-
lowing the metadata structure and alignment padding. This inte-
ger option reserves additional headroom.
PACKET_RX_RING
Create a memory-mapped ring buffer for asynchronous packet recep-
tion. The packet socket reserves a contiguous region of applica-
tion 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 fin-
ished processing a packet, it transfers ownership of the slot
back to the socket by setting tp_status equal to TP_STATUS_KER-
NEL.
Packet sockets implement multiple variants of the packet ring.
The implementation details are described in Documentation/net-
working/packet_mmap.rst 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 statis-
tics 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 gen-
erated when the packet is copied into the ring. This integer op-
tion selects the type of timestamp. Besides the default, it sup-
port the two hardware formats described in Documentation/network-
ing/timestamping.rst 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 chang-
ing 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 over-
rides 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 vari-
ant 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 us-
ing 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 de-
vice 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.
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 ad-
dress.
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 sup-
ported only SOCK_PACKET.
NOTES
For portable programs it is suggested to use AF_PACKET via pcap(3); al-
though 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 spec-
ified 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 phys-
ical-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 de-
vice name as a null-terminated string, for example, eth0.
This structure is obsolete and should not be used in new code.
BUGS
LLC header handling
The IEEE 802.2/803.3 LLC handling could be considered as a bug.
MSG_TRUNC issues
The MSG_TRUNC recvmsg(2) extension is an ugly hack and should be re-
placed by a control message. There is currently no way to get the orig-
inal destination address of packets via SOCK_DGRAM.
spkt_device device name truncation
The spkt_device field of sockaddr_pkt has a size of 14 bytes, which is
less than the constant IFNAMSIZ defined in <net/if.h> which is 16 bytes
and describes the system limit for a network interface name. This means
the names of network devices longer than 14 bytes will be truncated to
fit into spkt_device. All these lengths include the terminating null
byte ('\0')).
Issues from this with old code typically show up with very long inter-
face names used by the Predictable Network Interface Names feature en-
abled by default in many modern Linux distributions.
The preferred solution is to rewrite code to avoid SOCK_PACKET. Possi-
ble user solutions are to disable Predictable Network Interface Names or
to rename the interface to a name of at most 13 bytes, for example using
the ip(8) tool.
Documentation issues
Socket filters are not documented.
SEE ALSO
socket(2), pcap(3), capabilities(7), ip(7), raw(7), socket(7), ip(8),
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.rst de-
scribes 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.
Linux man-pages 6.9.1 2024-06-15 packet(7)
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