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, i386 and ia64 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_FASTFORWARDING (ip.fastforwarding) Boolean: enable/disable the use of fast IP
forwarding code. Defaults to off. When fast IP forwarding is
enabled, IP packets are forwarded directly to the appropriate network
interface with direct processing to completion, which greatly improves
the throughput. All packets for local IP addresses, non-unicast, or
with IP options are handled by the normal IP input processing path.
All features of the normal (slow) IP forwarding path are supported
including firewall (through pfil(9) hooks) checking, except ipsec(4)
tunnel brokering. The IP fastforwarding path does not generate ICMP
redirect or source quench messages.

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).

IPCTL_RTEXPIRE (ip.rtexpire) Integer: lifetime in seconds of protocol-cloned IP
routes after the last reference drops (default one hour). This value
varies dynamically as described above.

IPCTL_RTMINEXPIRE (ip.rtminexpire) Integer: minimum value of ip.rtexpire (default ten
seconds). This value has no effect on user modifications, but
restricts the dynamic adaptation described above.

IPCTL_RTMAXCACHE (ip.rtmaxcache) Integer: trigger level of cached, unreferenced,
protocol-cloned routes which initiates dynamic adaptation (default
128).

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.random_id Boolean: control IP IDs generation behaviour. Setting this sysctl(8)
to non-zero causes the ID field in IP packets 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. In 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.