xref: /freebsd/share/man/man4/inet.4 (revision 1dfcff294e44d4b45813288ef4095c36abb22f0e)
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28.\"     From: @(#)inet.4	8.1 (Berkeley) 6/5/93
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31.Dd November 12, 2021
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
161A number of variables are implemented in the net.inet branch of the
162.Xr sysctl 3
163MIB.
164In addition to the variables supported by the transport protocols
165(for which the respective manual pages may be consulted),
166the following general variables are defined:
167.Bl -tag -width IPCTL_ACCEPTSOURCEROUTE
168.It Dv IPCTL_FORWARDING
169.Pq ip.forwarding
170Boolean: enable/disable forwarding of IP packets.
171Defaults to off.
172.It Dv IPCTL_SENDREDIRECTS
173.Pq ip.redirect
174Boolean: enable/disable sending of ICMP redirects in response to
175.Tn IP
176packets for which a better, and for the sender directly reachable, route
177and next hop is known.
178Defaults to on.
179.It Dv IPCTL_DEFTTL
180.Pq ip.ttl
181Integer: default time-to-live
182.Pq Dq TTL
183to use for outgoing
184.Tn IP
185packets.
186.It Dv IPCTL_ACCEPTSOURCEROUTE
187.Pq ip.accept_sourceroute
188Boolean: enable/disable accepting of source-routed IP packets (default false).
189.It Dv IPCTL_SOURCEROUTE
190.Pq ip.sourceroute
191Boolean: enable/disable forwarding of source-routed IP packets (default false).
192.It Va ip.process_options
193Integer: control IP options processing.
194By setting this variable to 0, all IP options in the incoming packets
195will be ignored, and the packets will be passed unmodified.
196By setting to 1, IP options in the incoming packets will be processed
197accordingly.
198By setting to 2, an
199.Tn ICMP
200.Dq "prohibited by filter"
201message will be sent back in response to incoming packets with IP options.
202Default is 1.
203This
204.Xr sysctl 8
205variable affects packets destined for a local host as well as packets
206forwarded to some other host.
207.It Va ip.rfc1122_strong_es
208Boolean: in non-forwarding mode
209.Pq ip.forwarding is disabled
210partially implement the Strong End System model per RFC1122.
211If a packet with destination address that is local arrives on a different
212interface than the interface the address belongs to, the packet would be
213silently dropped.
214Enabling this option may break certain setups, e.g. having an alias address(es)
215on loopback that are expected to be reachable by outside traffic.
216Enabling some other network features, e.g.
217.Xr carp 4
218or destination address rewriting
219.Xr pfil 4
220filters may override and bypass this check.
221Disabled by default.
222.It Va ip.source_address_validation
223Boolean: perform source address validation for packets destined for the local
224host.
225Consider this as following Section 3.2 of RFC3704/BCP84, where we treat local
226host as our own infrastructure.
227This has no effect on packets to be forwarded, so don't consider it as
228anti-spoof feature for a router.
229Enabled by default.
230.It Va ip.rfc6864
231Boolean: control IP IDs generation behaviour.
232True value enables RFC6864 support, which specifies that IP ID field of
233.Em atomic
234datagrams can be set to any value.
235The
236.Fx implementation sets it to zero.
237Enabled by default.
238.It Va ip.random_id
239Boolean: control IP IDs generation behaviour.
240Setting this
241.Xr sysctl 8
242to 1 causes the ID field in
243.Em non-atomic
244IP datagrams (or all IP datagrams, if
245.Va ip.rfc6864
246is disabled) to be randomized instead of incremented by 1 with each packet
247generated.
248This closes a minor information leak which allows remote observers to
249determine the rate of packet generation on the machine by watching the
250counter.
251At the same time, on high-speed links, it can decrease the ID reuse
252cycle greatly.
253Default is 0 (sequential IP IDs).
254IPv6 flow IDs and fragment IDs are always random.
255.It Va ip.maxfrags
256Integer: maximum number of fragments the host will accept and simultaneously
257hold across all reassembly queues in all VNETs.
258If set to 0, reassembly is disabled.
259If set to -1, this limit is not applied.
260This limit is recalculated when the number of mbuf clusters is changed.
261This is a global limit.
262.It Va ip.maxfragpackets
263Integer: maximum number of fragmented packets the host will accept and
264simultaneously hold in the reassembly queue for a particular VNET.
2650 means that the host will not accept any fragmented packets for that VNET.
266\-1 means that the host will not apply this limit for that VNET.
267This limit is recalculated when the number of mbuf clusters is changed.
268This is a per-VNET limit.
269.It Va ip.maxfragbucketsize
270Integer: maximum number of reassembly queues per bucket.
271Fragmented packets are hashed to buckets.
272Each bucket has a list of reassembly queues.
273The system must compare the incoming packets to the existing reassembly queues
274in the bucket to find a matching reassembly queue.
275To preserve system resources, the system limits the number of reassembly
276queues allowed in each bucket.
277This limit is recalculated when the number of mbuf clusters is changed or
278when the value of
279.Va ip.maxfragpackets
280changes.
281This is a per-VNET limit.
282.It Va ip.maxfragsperpacket
283Integer: maximum number of fragments the host will accept and hold
284in the reassembly queue for a packet.
2850 means that the host will not accept any fragmented packets for the VNET.
286This is a per-VNET limit.
287.It Va ip.allow_net0
288Boolean: allow experimental use of addresses in 0.0.0.0/8 as endpoints,
289and allow forwarding of packets with these addresses.
290.It Va ip.allow_net240
291Boolean: allow experimental use of addresses in 240.0.0.0/4 as endpoints,
292and allow forwarding of packets with these addresses.
293.It Va ip.loopback_prefixlen
294Integer: prefix length of the address space reserved for loopback purposes.
295The default is 8, meaning that 127.0.0.0/8 is reserved for loopback,
296and cannot be sent, received, or forwarded on a non-loopback interface.
297Use of other values is experimental.
298.El
299.Sh SEE ALSO
300.Xr ioctl 2 ,
301.Xr socket 2 ,
302.Xr getifaddrs 3 ,
303.Xr sysctl 3 ,
304.Xr icmp 4 ,
305.Xr intro 4 ,
306.Xr ip 4 ,
307.Xr ipfirewall 4 ,
308.Xr route 4 ,
309.Xr tcp 4 ,
310.Xr udp 4 ,
311.Xr pfil 9
312.Rs
313.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
314.%B PS1
315.%N 7
316.Re
317.Rs
318.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
319.%B PS1
320.%N 8
321.Re
322.Sh HISTORY
323The
324.Nm
325protocol interface appeared in
326.Bx 4.2 .
327The
328.Dq protocol cloning
329code appeared in
330.Fx 2.1 .
331.Sh CAVEATS
332The Internet protocol support is subject to change as
333the Internet protocols develop.
334Users should not depend
335on details of the current implementation, but rather
336the services exported.
337