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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 32-bit Internet address contains both network and host parts. However, direct examination of addresses
is discouraged. For those programs which absolutely need to break addresses into their component parts,
the following ioctl(2) commands are provided for a datagram socket in the Internet domain; they have the
same form as the SIOCIFADDR command (see intro(4)).
SIOCSIFNETMASK Set interface network mask. The network mask defines the network part of the address; if
it contains more of the address than the address type would indicate, then subnets are in
use.
SIOCGIFNETMASK Get interface network mask.
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.
SEE ALSO
ioctl(2), socket(2), 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.
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.