View source for Packet sockets

Packet sockets allow a program to more directly interface with the networking stack than standard Layer 4 Berkeley sockets (e.g. AF_INET/AF_INET6 + SOCK_STREAM/SOCK_DGRAM).

==[[Linux APIs|Linux]] packet sockets==
The SOCK_PACKET socket type is strongly deprecated (see packet(7)), and thus not discussed here.
{| class="wikitable" border="1"
! Socket type
! CAP_NET_ADMIN/root required?
! Zero-copy?
! Layers wholly directly specified
! Layers partly directly specified
|-
| PACKET_TX_MMAP-enabled PF_PACKET, SOCK_RAW
| Y
| Y
| 2+
| 2+
|-
| PF_PACKET, SOCK_RAW
| Y
| N
| 2+
| 2+
|-
| PF_PACKET, SOCK_DGRAM
| Y
| N
| 3+
| 2+
|-
| PF_INET, SOCK_RAW, IPPROTO_RAW
| Y
| N
| 3+
| 3+
|-
| IP_HDRINCL-enabled PF_INET, SOCK_RAW
(only one IP protocol per socket when protocol is other than IPPROTO_RAW,
not valid for PF_INET6 sockets)
| Y
| N
| 3+
| 3+
|-
| PF_INET, SOCK_RAW
(only one IP protocol per socket when protocol is other than IPPROTO_RAW)
| Y
| N
| 4+
| 3+
|-
| PF_INET
(only one IP protocol per socket when protocol is other than IPPROTO_RAW, and only certain IP protocols are supported at all when type is not SOCK_RAW)
| N
| N
| 5+
| 3+
|-
|}

==PF_PACKET==
As described in <tt>packet(7)</tt>, this is a packet interface at the device level (Layer 2). The protocol field is either an [[Standards|IEEE 802.3]] protocol number (found in <tt>linux/if_ether.h</tt> or ETH_P_ALL in network byte order. The CAP_NET_RAW [[POSIX Capabilities|capability]] or UID 0 are requisite to open a packet socket. <tt>bind(2)</tt> can be used to select a single interface to use with the packet socket.
===SOCK_RAW===
Raw packets including the link-level header.
===SOCK_DGRAM===
Cooked packets (common, protocol-independent link-layer header in a <tt>sockaddr_ll</tt>).
==AF_INET/AF_INET6==
The protocol field restricts the Layer 3 protocols which will be passed to the socket; IPPROTO_RAW can set arbitrary IP protocol values (for transmission, but not reception), and thus implies the IP_HDRINCL socket option. Even if IP_HDRINCL is set, raw sockets might rewrite the following fields:
* Total length
* Checksum
* Packet ID (given a random value if set to 0)
* Source address (selected via routing lookup if set to 0)
The IP_HDRINCL (IPV6_HDRINCL) option is not supported for IPv6 raw sockets.
===SOCK_RAW===
Path MTU is performed on outgoing packets unless disabled via IP_MTU_DISCOVER sockopt, in which case they might be fragmented if they exceed the outgoing device's MTU (unless IP_HDRINCL is used). Fragment reassembly is performed before delivering packets to receiving sockets. sendto() must be used with raw sockets, as opposed to a mere send(). When using an IPv6 raw socket, sin6_port must be set to 0 to avoid an EINVAL ("Invalid Argument") error.

===SOCK_PACKET===
Obsolete since Linux 2.0.

==See also==
* [[Linux APIs]] and [[FreeBSD APIs]]
* "[https://fly.io/blog/bpf-xdp-packet-filters-and-udp/ BPF, XDP, Packet Filters and UDP]" at fly.io