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NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON |
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packet(7) Miscellaneous Information 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. 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 size 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; /* Size 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 size. 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 size */
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 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) 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.
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 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 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.
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 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 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) tool.
Documentation issues
Socket filters are not documented.
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
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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux 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)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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