INET(4) FreeBSD Kernel Interfaces Manual INET(4)
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 (sysctl) Variables
In addition to the variables supported by the transport protocols in
net.inet (for which the respective manual pages may be consulted), there
are a number of general variables implemented in the net.inet.ip branch
of the sysctl(3) MIB, which can be also read or modified with sysctl(8).
The following general variables are defined:
accept_sourceroute Boolean: enable/disable accepting of source-routed IP
packets (default false).
allow_net0 Boolean: allow forwarding of, and ICMP responses to,
packets with addresses in 0.0.0.0/8.
allow_net240 Boolean: allow forwarding of, and ICMP responses to,
packets with addresses in 240.0.0.0/4.
curfrags Integer: Current number of IPv4 fragments across all
reassembly queues in all VNETs (read-only).
forwarding Boolean: enable/disable forwarding of IP packets.
Defaults to off.
fragpackets Integer: Current number of IPv4 fragment reassembly
queue entries for the VNET (read-only).
fragttl Integer: time to live for IPv4 packet fragments in
the per-VNET reassemby queue.
loopback_prefixlen Integer: prefix length of the address space reserved
for loopback purposes. The default is 8, meaning
that 127.0.0.0/8 is reserved for loopback, and cannot
be sent, received, or forwarded on a non-loopback
interface. Use of other values is experimental.
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 maxfragpackets changes.
This is a per-VNET limit.
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.
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.
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.
mcast Variables under the net.inet.ip.mcast node are
documented in ip(4).
no_same_prefix Boolean: Refuse to create same prefixes on different
interfaces. This is a per-VNET value.
portrange Variables under the net.inet.ip.portrange node
control port ranges used by transport protocols; see
ip(4) for details.
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.
random_id Boolean: control IP IDs generation behavior. Setting
this sysctl(8) to 1 causes the ID field in non-atomic
IP datagrams (or all IP datagrams, if 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.
random_id_collisions
Integer: count of IP ID collisions (read-only, per-
VNET).
random_id_period Integer: size of the IP ID array, which is the number
of previous packets for which the IDs are recorded.
The number must be between 512 and 32768 inclusive.
This is a per-VNET value.
random_id_total Integer: count of IP IDs created (read-only, per-
VNET).
reass_hashsize Number of hash slots in the IPv4 reassembly queue
(loader tunable).
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.
rfc1122_strong_es Boolean: in non-forwarding mode (forwarding is
disabled) partially implement the Strong End System
model per RFC1122. If a packet with destination
address that is local arrives on a different
interface than the interface the address belongs to,
the packet would be silently dropped. Enabling this
option may break certain setups, e.g. having an alias
address(es) on loopback that are expected to be
reachable by outside traffic. Enabling some other
network features, e.g. carp(4) or destination
address rewriting pfil(4) filters may override and
bypass this check. Disabled by default.
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.
source_address_validation
Boolean: perform source address validation for
packets destined for the local host. Consider this
as following Section 3.2 of RFC3704/BCP84, where we
treat local host as our own infrastructure.
Forwarded packets are unaffected by this and it
should not be considered an anti-spoof feature for a
router. Enabled by default.
sourceroute Boolean: enable/disable forwarding of source-routed
IP packets (default false).
ttl Integer: default time-to-live ("TTL") to use for
outgoing IP packets.
SEE ALSO
ioctl(2), socket(2), getifaddrs(3), sysctl(3), icmp(4), intro(4), ip(4),
ipfirewall(4), route(4), tcp(4), udp(4), sysctl(8), 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.
FreeBSD 15.1-STABLE-HBSD December 31, 2024 INET(4)