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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 physi‐
cal 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 in‐
formation 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 be‐
fore 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 sock‐
addr_ll address structure. When transmitting a packet, the user-supplied buffer should con‐
tain 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 sock‐
addr_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 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
sll_halen
contain the physical-layer (e.g., IEEE 802.3) address and its length. The exact in‐
terpretation 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 re‐
ceived 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 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 en‐
ables 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_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 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 success‐
fully 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 tak‐
ing 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 in‐
teger option value. The flag PACKET_FANOUT_FLAG_ROLLOVER enables the roll over mecha‐
nism 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 mal‐
formed 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_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 to tpacket_auxdata. The pro‐
tocol 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 re‐
ceived 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 de‐
tails are described in Documentation/networking/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 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/timestamp‐
ing.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 changing tp_status to TP_STATUS_SEND_REQUEST. When packets are ready to be trans‐
mitted, 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 un‐
less 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 (traf‐
fic 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 net‐
work—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 net‐
work 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.

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 Eth‐
ernet 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 be‐
tween the two methods is that SOCK_PACKET uses the old struct sockaddr_pkt to specify an in‐
terface, 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 de‐
fined 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
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 replaced by a control mes‐
sage. There is currently no way to get the original 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 trun‐
cated 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 interface names used by the
Predictable Network Interface Names feature enabled by default in many modern Linux distribu‐
tions.

The preferred solution is to rewrite code to avoid SOCK_PACKET. Possible 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 encapsu‐
lation.

The <linux/if_ether.h> include file for physical-layer protocols.

The Linux kernel source tree. Documentation/networking/filter.rst describes how to apply
Berkeley Packet Filters to packet sockets. tools/testing/selftests/net/psock_tpacket.c con‐
tains example source code for all available versions of PACKET_RX_RING and PACKET_TX_RING.

Linux man-pages 6.7 2023-10-31 packet(7)