Ubuntu Manpage: inet — Internet protocol family

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NAME

     inet — Internet protocol family

SYNOPSIS

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

DESCRIPTION

     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.

ADDRESSING

     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.

PROTOCOLS

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:

SIOCAIFADDR Add address to an interface. The command requires struct in_aliasreq
as argument.
SIOCDIFADDR Delete address from an interface. The command requires struct ifreq
as argument.
SIOCGIFADDR
SIOCGIFBRDADDR
SIOCGIFDSTADDR
SIOCGIFNETMASK 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:

IPCTL_FORWARDING (ip.forwarding) Boolean: enable/disable forwarding of IP packets.
Defaults to off.

IPCTL_SENDREDIRECTS (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.

IPCTL_DEFTTL (ip.ttl) Integer: default time-to-live (“TTL”) to use for outgoing
IP packets.

IPCTL_ACCEPTSOURCEROUTE (ip.accept_sourceroute) Boolean: enable/disable accepting of
source-routed IP packets (default false).

IPCTL_SOURCEROUTE (ip.sourceroute) Boolean: enable/disable forwarding of source-
routed IP packets (default false).

ip.process_options 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.

ip.rfc6864 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.

ip.random_id 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.

ip.maxfragpackets 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.

ip.maxfragsperpacket 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.

HISTORY

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

CAVEATS

     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.