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INET(4) Device Drivers Manual INET(4)


Internet protocol family


#include <sys/types.h>
#include <netinet/in.h>


The Internet protocol family is a collection of protocols layered atop the Internet Protocol (IP) transport layer, and utilizing the Internet address format. The Internet family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IP protocol.


Internet addresses are four byte quantities, stored in network standard format (on little endian machines, such as the alpha, amd64 and i386 these are word and byte reversed). The include file <netinet/in.h> defines this address as a discriminated union.

Sockets bound to the Internet protocol family utilize the following addressing structure,

struct sockaddr_in {
	uint8_t		sin_len;
	sa_family_t	sin_family;
	in_port_t	sin_port;
	struct in_addr	sin_addr;
	char		sin_zero[8];

Sockets may be created with the local address INADDR_ANY to affect “wildcard” matching on incoming messages. The address in a connect(2) or sendto(2) call may be given as INADDR_ANY to mean “this host”. The distinguished address INADDR_BROADCAST is allowed as a shorthand for the broadcast address on the primary network if the first network configured supports broadcast.


The Internet protocol family is comprised of the IP network protocol, Internet Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM abstraction. A raw interface to IP is available by creating an Internet socket of type SOCK_RAW. The ICMP message protocol is accessible from a raw socket.

The inet address on an interface consist of the address itself, the netmask, either broadcast address in case of a broadcast interface or peers address in case of point-to-point interface. The following ioctl(2) commands are provided for a datagram socket in the Internet domain:

Add address to an interface. The command requires struct in_aliasreq as argument.
Delete address from an interface. The command requires struct ifreq as argument.
Return address information from interface. The returned value is in struct ifreq. This way of address information retrieval is obsoleted, a preferred way is to use getifaddrs(3) API.

MIB Variables

A number of variables are implemented in the net.inet branch of the sysctl(3) MIB. In addition to the variables supported by the transport protocols (for which the respective manual pages may be consulted), the following general variables are defined:
(ip.forwarding) Boolean: enable/disable forwarding of IP packets. Defaults to off.
(ip.redirect) Boolean: enable/disable sending of ICMP redirects in response to IP packets for which a better, and for the sender directly reachable, route and next hop is known. Defaults to on.
(ip.ttl) Integer: default time-to-live (“TTL”) to use for outgoing IP packets.
(ip.accept_sourceroute) Boolean: enable/disable accepting of source-routed IP packets (default false).
(ip.sourceroute) Boolean: enable/disable forwarding of source-routed IP packets (default false).
Integer: control IP options processing. By setting this variable to 0, all IP options in the incoming packets will be ignored, and the packets will be passed unmodified. By setting to 1, IP options in the incoming packets will be processed accordingly. By setting to 2, an ICMP “prohibited by filter” message will be sent back in response to incoming packets with IP options. Default is 1. This sysctl(8) variable affects packets destined for a local host as well as packets forwarded to some other host.
Boolean: control IP IDs generation behaviour. True value enables RFC6864 support, which specifies that IP ID field of atomic datagrams can be set to any value. The FreeBSD implementation sets it to zero. Enabled by default.
Boolean: control IP IDs generation behaviour. Setting this sysctl(8) to 1 causes the ID field in non-atomic IP datagrams (or all IP datagrams, if ip.rfc6864 is disabled) to be randomized instead of incremented by 1 with each packet generated. This closes a minor information leak which allows remote observers to determine the rate of packet generation on the machine by watching the counter. At the same time, on high-speed links, it can decrease the ID reuse cycle greatly. Default is 0 (sequential IP IDs). IPv6 flow IDs and fragment IDs are always random.
Integer: maximum number of fragmented packets the host will accept and hold in the reassembling queue simultaneously. 0 means that the host will not accept any fragmented packets. -1 means that the host will accept as many fragmented packets as it receives.
Integer: maximum number of fragments the host will accept and hold in the reassembling queue for a packet. 0 means that the host will not accept any fragmented packets.


ioctl(2), socket(2), getifaddrs(3), sysctl(3), icmp(4), intro(4), ip(4), ipfirewall(4), route(4), tcp(4), udp(4), pfil(9)

An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.

An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.


The inet protocol interface appeared in 4.2BSD. The “protocol cloning” code appeared in FreeBSD 2.1.


The Internet protocol support is subject to change as the Internet protocols develop. Users should not depend on details of the current implementation, but rather the services exported.
February 4, 2016 Linux 4.9.0-9-amd64