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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.maxfrags Integer: maximum number of fragments the host will accept and simultaneously
hold across all reassembly queues in all VNETs. If set to 0, reassembly is
disabled. If set to -1, this limit is not applied. This limit is recalculated
when the number of mbuf clusters is changed. This is a global limit.
ip.maxfragpackets Integer: maximum number of fragmented packets the host will accept and
simultaneously hold in the reassembly queue for a particular VNET. 0 means that
the host will not accept any fragmented packets for that VNET. -1 means that
the host will not apply this limit for that VNET. This limit is recalculated
when the number of mbuf clusters is changed. This is a per-VNET limit.
ip.maxfragbucketsize Integer: maximum number of reassembly queues per bucket. Fragmented packets are
hashed to buckets. Each bucket has a list of reassembly queues. The system
must compare the incoming packets to the existing reassembly queues in the
bucket to find a matching reassembly queue. To preserve system resources, the
system limits the number of reassembly queues allowed in each bucket. This
limit is recalculated when the number of mbuf clusters is changed or when the
value of ip.maxfragpackets changes. This is a per-VNET limit.
ip.maxfragsperpacket Integer: maximum number of fragments the host will accept and hold in the
reassembly queue for a packet. 0 means that the host will not accept any
fragmented packets for the VNET. This is a per-VNET limit.
SEE ALSO
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.
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.
Debian August 14, 2018 INET(4)