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
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.
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
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_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 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 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_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 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.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/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
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.
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.
