Packet sockets

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

Linux packet sockets

The SOCK_PACKET socket type is strongly deprecated (see packet(7)), and thus not discussed here.

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 packet(7), this is a packet interface at the device level (Layer 2). The protocol field is either an IEEE 802.3 protocol number (found in linux/if_ether.h or ETH_P_ALL in network byte order. The CAP_NET_RAW capability or UID 0 are requisite to open a packet socket. bind(2) 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 sockaddr_ll).

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 support 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