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.Dd September 8, 2022
.Dt INET 4
.Os
.Sh NAME
.Nm inet
.Nd Internet protocol family
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.Sh DESCRIPTION
The Internet protocol family is a collection of protocols
layered atop the
.Em Internet Protocol
.Pq Tn IP
transport layer, and utilizing the Internet address format.
The Internet family provides protocol support for the
.Dv SOCK_STREAM , SOCK_DGRAM ,
and
.Dv SOCK_RAW
socket types; the
.Dv SOCK_RAW
interface provides access to the
.Tn IP
protocol.
.Sh ADDRESSING
Internet addresses are four byte quantities, stored in
network standard format (on little endian machines, such as the
.Tn alpha ,
.Tn amd64
and
.Tn i386
these are word and byte reversed).
The include file
.In netinet/in.h
defines this address
as a discriminated union.
.Pp
Sockets bound to the Internet protocol family utilize
the following addressing structure,
.Bd -literal -offset indent
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];
};
.Ed
.Pp
Sockets may be created with the local address
.Dv INADDR_ANY
to affect
.Dq wildcard
matching on incoming messages.
The address in a
.Xr connect 2
or
.Xr sendto 2
call may be given as
.Dv INADDR_ANY
to mean
.Dq this host .
The distinguished address
.Dv INADDR_BROADCAST
is allowed as a shorthand for the broadcast address on the primary
network if the first network configured supports broadcast.
.Sh PROTOCOLS
The Internet protocol family is comprised of
the
.Tn IP
network protocol, Internet Control
Message Protocol
.Pq Tn ICMP ,
Internet Group Management Protocol
.Pq Tn IGMP ,
Transmission Control
Protocol
.Pq Tn TCP ,
and User Datagram Protocol
.Pq Tn UDP .
.Tn TCP
is used to support the
.Dv SOCK_STREAM
abstraction while
.Tn UDP
is used to support the
.Dv SOCK_DGRAM
abstraction.
A raw interface to
.Tn IP
is available
by creating an Internet socket of type
.Dv SOCK_RAW .
The
.Tn ICMP
message protocol is accessible from a raw socket.
.Pp
The
.Nm
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
.Xr ioctl 2
commands are provided for a datagram socket in the Internet domain:
.Pp
.Bl -tag -width ".Dv SIOCGIFBRDADDR" -offset indent -compact
.It Dv SIOCAIFADDR
Add address to an interface.
The command requires
.Ft struct in_aliasreq
as argument.
.It Dv SIOCDIFADDR
Delete address from an interface.
The command requires
.Ft struct ifreq
as argument.
.It Dv SIOCGIFADDR
.It Dv SIOCGIFBRDADDR
.It Dv SIOCGIFDSTADDR
.It Dv SIOCGIFNETMASK
Return address information from interface.
The returned value is in
.Ft struct ifreq .
This way of address information retrieval is obsoleted, a
preferred way is to use
.Xr getifaddrs 3
API.
.El
.Ss MIB (sysctl) Variables
In addition to the variables supported by the transport protocols in
.Va net.inet
(for which the respective manual pages may be consulted),
there are a number of general variables implemented in the
.Va net.inet.ip
branch of the
.Xr sysctl 3
MIB, which can be also read or modified with
.Xr sysctl 8 .
The following general variables are defined:
.Bl -tag -width ".Va accept_sourceroute"
.It Va accept_sourceroute
Boolean: enable/disable accepting of source-routed IP packets (default false).
.It Va allow_net0
Boolean: allow use of addresses in 0.0.0.0/8 as endpoints,
and allow forwarding of packets with these addresses.
.It Va allow_net240
Boolean: allow use of addresses in 240.0.0.0/4 as endpoints,
and allow forwarding of packets with these addresses.
.It Va curfrags
Integer: Current number of IPv4 fragments across all reassembly queues
in all VNETs (read-only).
.It Va forwarding
Boolean: enable/disable forwarding of IP packets.
Defaults to off.
.It Va fragpackets
Integer: Current number of IPv4 fragment reassembly queue entries
for the VNET (read-only).
.It Va fragttl
Integer: time to live for IPv4 packet fragments in the per-VNET reassemby queue.
.It Va 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.
.It Va 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
.Va maxfragpackets
changes.
This is a per-VNET limit.
.It Va 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.
.It Va 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.
.It Va 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.
.It Va mcast
Variables under the
.Va net.inet.ip.mcast
node are documented in
.Xr ip 4 .
.It Va no_same_prefix
Boolean: Refuse to create same prefixes on different interfaces.
This is a per-VNET value.
.It Va portrange
Variables under the
.Va net.inet.ip.portrange
node control port ranges used by transport protocols; see
.Xr ip 4
for details.
.It Va 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
.Tn ICMP
.Dq "prohibited by filter"
message will be sent back in response to incoming packets with IP options.
Default is 1.
This
.Xr sysctl 8
variable affects packets destined for a local host as well as packets
forwarded to some other host.
.It Va random_id
Boolean: control IP IDs generation behavior.
Setting this
.Xr sysctl 8
to 1 causes the ID field in
.Em non-atomic
IP datagrams (or all IP datagrams, if
.Va 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.
.It Va random_id_collisions
Integer: count of IP ID collisions (read-only, per-VNET).
.It Va 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.
.It Va random_id_total
Integer: count of IP IDs created (read-only, per-VNET).
.It Va reass_hashsize
Number of hash slots in the IPv4 reassembly queue (loader tunable).
.It Va redirect
Boolean: enable/disable sending of ICMP redirects in response to
.Tn IP
packets for which a better, and for the sender directly reachable, route
and next hop is known.
Defaults to on.
.It Va rfc1122_strong_es
Boolean: in non-forwarding mode
.Pq 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.
.Xr carp 4
or destination address rewriting
.Xr pfil 4
filters may override and bypass this check.
Disabled by default.
.It Va rfc6864
Boolean: control IP IDs generation behaviour.
True value enables RFC6864 support, which specifies that IP ID field of
.Em atomic
datagrams can be set to any value.
The
.Fx implementation sets it to zero.
Enabled by default.
.It Va 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.
.It Va sourceroute
Boolean: enable/disable forwarding of source-routed IP packets (default false).
.It Va ttl
Integer: default time-to-live
.Pq Dq TTL
to use for outgoing
.Tn IP
packets.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr getifaddrs 3 ,
.Xr sysctl 3 ,
.Xr icmp 4 ,
.Xr intro 4 ,
.Xr ip 4 ,
.Xr ipfirewall 4 ,
.Xr route 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr sysctl 8 ,
.Xr pfil 9
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re
.Sh HISTORY
The
.Nm
protocol interface appeared in
.Bx 4.2 .
The
.Dq protocol cloning
code appeared in
.Fx 2.1 .
.Sh 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.