packet(7) - Linux manual page (original) (raw)
packet(7) Miscellaneous Information Manual packet(7)
NAME top
packet - packet interface on device levelSYNOPSIS top
**#include <sys/socket.h>**
**#include <linux/if_packet.h>**
**#include <net/ethernet.h> /* the L2 protocols */**
**packet_socket = socket(AF_PACKET, int** _sockettype_**, int** _protocol_**);**DESCRIPTION top
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 _sockettype_ 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 _sockaddrll_ structure. _protocol_
is the IEEE 802.3 protocol number in network byte order. See the
_<linux/ifether.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)](../man2/bind.2.html) can optionally be called with a
nonzero _sllprotocol_ 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 _sockaddrll_
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 _sockaddrll_
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)](../man2/bind.2.html) specifying an address in a _struct sockaddrll_ to bind
the packet socket to an interface. Fields used for binding are
_sllfamily_ (should be **AF_PACKET**), _sllprotocol_, and _sllifindex_.
The [connect(2)](../man2/connect.2.html) operation is not supported on packet sockets.
When the **MSG_TRUNC** flag is passed to [recvmsg(2)](../man2/recvmsg.2.html), [recv(2)](../man2/recv.2.html), or
[recvfrom(2)](../man2/recvfrom.2.html), the real size of the packet on the wire is always
returned, even when it is longer than the buffer.Address types The sockaddrll 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; /* Size of address */
unsigned char sll_addr[8]; /* Physical-layer address */
};
The fields of this structure are as follows:
_sllprotocol_
is the standard ethernet protocol type in network byte
order as defined in the _<linux/ifether.h>_ include file.
It defaults to the socket's protocol.
_sllifindex_
is the interface index of the interface (see [netdevice(7)](../man7/netdevice.7.html));
0 matches any interface (only permitted for binding).
_sllhatype_ is an ARP type as defined in the
_<linux/ifarp.h>_ include file.
_sllpkttype_
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.
_slladdr_
_sllhalen_
contain the physical-layer (e.g., IEEE 802.3) address and
its size. The exact interpretation depends on the device.
When you send packets, it is enough to specify _sllfamily_,
_slladdr_, _sllhalen_, _sllifindex_, and _sllprotocol_. The other
fields should be 0. _sllhatype_ and _sllpkttype_ 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 _packetmreq_ structure as argument:
struct packet_mreq {
int mr_ifindex; /* interface index */
unsigned short mr_type; /* action */
unsigned short mr_alen; /* address size */
unsigned char mr_address[8]; /* physical-layer address */
};
_mrifindex_ contains the interface index for the interface
whose status should be changed. The _mrtype_ 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
_mraddress_ and _mralen_, 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)](../man2/recvmsg.2.html) control field. The structure can be read with
[cmsg(3)](../man3/cmsg.3.html). It is defined as
struct tpacket_auxdata {
__u32 tp_status;
__u32 tp_len; /* packet size */
__u32 tp_snaplen; /* captured size */
__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)](../man2/setsockopt.2.html) 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 _tpstatus_ 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 _tpstatus_ reset to
**TP_STATUS_SEND_REQUEST**, and the transmission process
restarted via [send(2)](../man2/send.2.html). However, if **PACKET_LOSS** is set, any
malformed packet will be skipped, its _tpstatus_ 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 _tpsnaplen_) into
subsequent slots. Each packet is preceded by a metadata
structure similar to _tpacketauxdata_. The protocol fields
encode the offset to the data from the start of the
metadata header. _tpnet_ stores the offset to the network
layer. If the packet socket is of type **SOCK_DGRAM**, then
_tpmac_ 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 _tpstatus_ field. The packet socket owns
all slots with _tpstatus_ 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 _tpstatus_ 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 _tpstatus_ equal to **TP_STATUS_KERNEL**.
Packet sockets implement multiple variants of the packet
ring. The implementation details are described in
_Documentation/networking/packetmmap.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
_tpstatus_ equal to **TP_STATUS_AVAILABLE** and schedules them
for transmission by changing _tpstatus_ to
**TP_STATUS_SEND_REQUEST**. When packets are ready to be
transmitted, the application calls [send(2)](../man2/send.2.html) or a variant
thereof. The _buf_ and _len_ fields of this call are ignored.
If an address is passed using [sendto(2)](../man2/sendto.2.html) or [sendmsg(2)](../man2/sendmsg.2.html), then
that overrides the socket default. On successful
transmission, the socket resets _tpstatus_ 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)](../man7/netdevice.7.html) and
[socket(7)](../man7/socket.7.html) 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 top
**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 top
**AF_PACKET** is a new feature in Linux 2.2. Earlier Linux versions
supported only **SOCK_PACKET**.NOTES top
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 size 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 sockaddrpkt_ 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;
};
_spktfamily_ contains the device type, _spktprotocol_ is the IEEE
802.3 protocol type as defined in _<sys/ifether.h>_ and _spktdevice_
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 top
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 message. There is currently no way to get the original destination address of packets via SOCK_DGRAM.
spkt_device device name truncation The spktdevice field of sockaddrpkt 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 spktdevice. All these sizes 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 distributions.
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)](../man8/ip.8.html) tool.Documentation issues Socket filters are not documented.
SEE ALSO top
[socket(2)](../man2/socket.2.html), **pcap**(3), [capabilities(7)](../man7/capabilities.7.html), [ip(7)](../man7/ip.7.html), [raw(7)](../man7/raw.7.html), [socket(7)](../man7/socket.7.html),
[ip(8)](../man8/ip.8.html),
RFC 894 for the standard IP Ethernet encapsulation. RFC 1700 for
the IEEE 802.3 IP encapsulation.
The _<linux/ifether.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/psocktpacket.c_ contains example
source code for all available versions of **PACKET_RX_RING** and
**PACKET_TX_RING**.COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 packet(7)
Pages that refer to this page:bind(2), getsockopt(2), socket(2), getifaddrs(3), pcap_findalldevs(3pcap), address_families(7), arp(7), ip(7), netdevice(7), raw(7), socket(7), bpfc(8), netsniff-ng(8), trafgen(8)