xref: /freebsd/share/man/man4/inet.4 (revision b43b8f81578cbb7bddbd6f7b8ebe06a219c88140)
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28.\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
29.\" $FreeBSD$
30.\"
31.Dd July 20, 2022
32.Dt INET 4
33.Os
34.Sh NAME
35.Nm inet
36.Nd Internet protocol family
37.Sh SYNOPSIS
38.In sys/types.h
39.In netinet/in.h
40.Sh DESCRIPTION
41The Internet protocol family is a collection of protocols
42layered atop the
43.Em Internet Protocol
44.Pq Tn IP
45transport layer, and utilizing the Internet address format.
46The Internet family provides protocol support for the
47.Dv SOCK_STREAM , SOCK_DGRAM ,
48and
49.Dv SOCK_RAW
50socket types; the
51.Dv SOCK_RAW
52interface provides access to the
53.Tn IP
54protocol.
55.Sh ADDRESSING
56Internet addresses are four byte quantities, stored in
57network standard format (on little endian machines, such as the
58.Tn alpha ,
59.Tn amd64
60and
61.Tn i386
62these are word and byte reversed).
63The include file
64.In netinet/in.h
65defines this address
66as a discriminated union.
67.Pp
68Sockets bound to the Internet protocol family utilize
69the following addressing structure,
70.Bd -literal -offset indent
71struct sockaddr_in {
72	uint8_t		sin_len;
73	sa_family_t	sin_family;
74	in_port_t	sin_port;
75	struct in_addr	sin_addr;
76	char		sin_zero[8];
77};
78.Ed
79.Pp
80Sockets may be created with the local address
81.Dv INADDR_ANY
82to affect
83.Dq wildcard
84matching on incoming messages.
85The address in a
86.Xr connect 2
87or
88.Xr sendto 2
89call may be given as
90.Dv INADDR_ANY
91to mean
92.Dq this host .
93The distinguished address
94.Dv INADDR_BROADCAST
95is allowed as a shorthand for the broadcast address on the primary
96network if the first network configured supports broadcast.
97.Sh PROTOCOLS
98The Internet protocol family is comprised of
99the
100.Tn IP
101network protocol, Internet Control
102Message Protocol
103.Pq Tn ICMP ,
104Internet Group Management Protocol
105.Pq Tn IGMP ,
106Transmission Control
107Protocol
108.Pq Tn TCP ,
109and User Datagram Protocol
110.Pq Tn UDP .
111.Tn TCP
112is used to support the
113.Dv SOCK_STREAM
114abstraction while
115.Tn UDP
116is used to support the
117.Dv SOCK_DGRAM
118abstraction.
119A raw interface to
120.Tn IP
121is available
122by creating an Internet socket of type
123.Dv SOCK_RAW .
124The
125.Tn ICMP
126message protocol is accessible from a raw socket.
127.Pp
128The
129.Nm
130address on an interface consist of the address itself, the
131netmask, either broadcast address in case of a broadcast
132interface or peers address in case of point-to-point interface.
133The following
134.Xr ioctl 2
135commands are provided for a datagram socket in the Internet domain:
136.Pp
137.Bl -tag -width ".Dv SIOCGIFBRDADDR" -offset indent -compact
138.It Dv SIOCAIFADDR
139Add address to an interface.
140The command requires
141.Ft struct in_aliasreq
142as argument.
143.It Dv SIOCDIFADDR
144Delete address from an interface.
145The command requires
146.Ft struct ifreq
147as argument.
148.It Dv SIOCGIFADDR
149.It Dv SIOCGIFBRDADDR
150.It Dv SIOCGIFDSTADDR
151.It Dv SIOCGIFNETMASK
152Return address information from interface.
153The returned value is in
154.Ft struct ifreq .
155This way of address information retrieval is obsoleted, a
156preferred way is to use
157.Xr getifaddrs 3
158API.
159.El
160.Ss MIB Variables
161In addition to the variables supported by the transport protocols in
162.Va net.inet
163(for which the respective manual pages may be consulted),
164there are a number of general variables implemented in the
165.Va net.inet.ip
166branch of the
167.Xr sysctl 3
168MIB.
169The following general variables are defined:
170.Bl -tag -width ".Va accept_sourceroute"
171.It Va accept_sourceroute
172Boolean: enable/disable accepting of source-routed IP packets (default false).
173.It Va allow_net0
174Boolean: allow experimental use of addresses in 0.0.0.0/8 as endpoints,
175and allow forwarding of packets with these addresses.
176.It Va allow_net240
177Boolean: allow experimental use of addresses in 240.0.0.0/4 as endpoints,
178and allow forwarding of packets with these addresses.
179.It Va forwarding
180Boolean: enable/disable forwarding of IP packets.
181Defaults to off.
182.It Va loopback_prefixlen
183Integer: prefix length of the address space reserved for loopback purposes.
184The default is 8, meaning that 127.0.0.0/8 is reserved for loopback,
185and cannot be sent, received, or forwarded on a non-loopback interface.
186Use of other values is experimental.
187.It Va maxfragbucketsize
188Integer: maximum number of reassembly queues per bucket.
189Fragmented packets are hashed to buckets.
190Each bucket has a list of reassembly queues.
191The system must compare the incoming packets to the existing reassembly queues
192in the bucket to find a matching reassembly queue.
193To preserve system resources, the system limits the number of reassembly
194queues allowed in each bucket.
195This limit is recalculated when the number of mbuf clusters is changed or
196when the value of
197.Va maxfragpackets
198changes.
199This is a per-VNET limit.
200.It Va maxfragpackets
201Integer: maximum number of fragmented packets the host will accept and
202simultaneously hold in the reassembly queue for a particular VNET.
2030 means that the host will not accept any fragmented packets for that VNET.
204\-1 means that the host will not apply this limit for that VNET.
205This limit is recalculated when the number of mbuf clusters is changed.
206This is a per-VNET limit.
207.It Va maxfrags
208Integer: maximum number of fragments the host will accept and simultaneously
209hold across all reassembly queues in all VNETs.
210If set to 0, reassembly is disabled.
211If set to -1, this limit is not applied.
212This limit is recalculated when the number of mbuf clusters is changed.
213This is a global limit.
214.It Va maxfragsperpacket
215Integer: maximum number of fragments the host will accept and hold
216in the reassembly queue for a packet.
2170 means that the host will not accept any fragmented packets for the VNET.
218This is a per-VNET limit.
219.It Va process_options
220Integer: control IP options processing.
221By setting this variable to 0, all IP options in the incoming packets
222will be ignored, and the packets will be passed unmodified.
223By setting to 1, IP options in the incoming packets will be processed
224accordingly.
225By setting to 2, an
226.Tn ICMP
227.Dq "prohibited by filter"
228message will be sent back in response to incoming packets with IP options.
229Default is 1.
230This
231.Xr sysctl 8
232variable affects packets destined for a local host as well as packets
233forwarded to some other host.
234.It Va random_id
235Boolean: control IP IDs generation behavior.
236Setting this
237.Xr sysctl 8
238to 1 causes the ID field in
239.Em non-atomic
240IP datagrams (or all IP datagrams, if
241.Va rfc6864
242is disabled) to be randomized instead of incremented by 1 with each packet
243generated.
244This closes a minor information leak which allows remote observers to
245determine the rate of packet generation on the machine by watching the
246counter.
247At the same time, on high-speed links, it can decrease the ID reuse
248cycle greatly.
249Default is 0 (sequential IP IDs).
250IPv6 flow IDs and fragment IDs are always random.
251.It Va redirect
252Boolean: enable/disable sending of ICMP redirects in response to
253.Tn IP
254packets for which a better, and for the sender directly reachable, route
255and next hop is known.
256Defaults to on.
257.It Va rfc1122_strong_es
258Boolean: in non-forwarding mode
259.Pq forwarding is disabled
260partially implement the Strong End System model per RFC1122.
261If a packet with destination address that is local arrives on a different
262interface than the interface the address belongs to, the packet would be
263silently dropped.
264Enabling this option may break certain setups, e.g. having an alias address(es)
265on loopback that are expected to be reachable by outside traffic.
266Enabling some other network features, e.g.
267.Xr carp 4
268or destination address rewriting
269.Xr pfil 4
270filters may override and bypass this check.
271Disabled by default.
272.It Va rfc6864
273Boolean: control IP IDs generation behaviour.
274True value enables RFC6864 support, which specifies that IP ID field of
275.Em atomic
276datagrams can be set to any value.
277The
278.Fx implementation sets it to zero.
279Enabled by default.
280.It Va source_address_validation
281Boolean: perform source address validation for packets destined for the local
282host.
283Consider this as following Section 3.2 of RFC3704/BCP84, where we treat local
284host as our own infrastructure.
285Forwarded packets are unaffected by this
286and it should not be considered an anti-spoof feature for a router.
287Enabled by default.
288.It Va sourceroute
289Boolean: enable/disable forwarding of source-routed IP packets (default false).
290.It Va ttl
291Integer: default time-to-live
292.Pq Dq TTL
293to use for outgoing
294.Tn IP
295packets.
296.El
297.Sh SEE ALSO
298.Xr ioctl 2 ,
299.Xr socket 2 ,
300.Xr getifaddrs 3 ,
301.Xr sysctl 3 ,
302.Xr icmp 4 ,
303.Xr intro 4 ,
304.Xr ip 4 ,
305.Xr ipfirewall 4 ,
306.Xr route 4 ,
307.Xr tcp 4 ,
308.Xr udp 4 ,
309.Xr pfil 9
310.Rs
311.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
312.%B PS1
313.%N 7
314.Re
315.Rs
316.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
317.%B PS1
318.%N 8
319.Re
320.Sh HISTORY
321The
322.Nm
323protocol interface appeared in
324.Bx 4.2 .
325The
326.Dq protocol cloning
327code appeared in
328.Fx 2.1 .
329.Sh CAVEATS
330The Internet protocol support is subject to change as
331the Internet protocols develop.
332Users should not depend
333on details of the current implementation, but rather
334the services exported.
335