1inet_res(3) Erlang Module Definition inet_res(3)
2
6 inet_res - A rudimentary DNS client.
7
9 This module performs DNS name resolving to recursive name servers.
10 See also ERTS User's Guide: Inet Configuration for more information
12 about how to configure an Erlang runtime system for IP communication,
13 and how to enable this DNS client by defining 'dns' as a lookup method.
14 The DNS client then acts as a backend for the resolving functions in
15 inet.
16 This DNS client can resolve DNS records even if it is not used for nor‐
18 mal name resolving in the node.
19 This is not a full-fledged resolver, only a DNS client that relies on
21 asking trusted recursive name servers.
22
24 UDP queries are used unless resolver option usevc is true, which forces
25 TCP queries. If the query is too large for UDP, TCP is used instead.
26 For regular DNS queries, 512 bytes is the size limit.
27 When EDNS is enabled (resolver option edns is set to the EDNS version
29 (that is, 0 instead of false), resolver option udp_payload_size sets
30 the limit. If a name server replies with the TC bit set (truncation),
31 indicating that the answer is incomplete, the query is retried to that
32 name server using TCP. Resolver option udp_payload_size also sets the
33 advertised size for the maximum allowed reply size, if EDNS is enabled,
34 otherwise the name server uses the limit 512 bytes. If the reply is
35 larger, it gets truncated, forcing a TCP requery.
36 For UDP queries, resolver options timeout and retry control retransmis‐
38 sion. Each name server in the nameservers list is tried with a time-out
39 of timeout/retry. Then all name servers are tried again, doubling the
40 time-out, for a total of retry times.
41 But before all name servers are tried again, there is a (user config‐
43 urable) timeout, servfail_retry_timeout. The point of this is to pre‐
44 vent the new query to be handled by a server's servfail cache (a client
45 that is to eager will actually only get what is in the servfail cache).
46 If there is too little time left of the resolver call's timeout to do a
47 retry, the resolver call may return before the call's timeout has ex‐
48 pired.
49 For queries not using the search list, if the query to all nameservers
51 results in {error,nxdomain} or an empty answer, the same query is tried
52 for alt_nameservers.
53
55 The following data types concern the resolver:
56
58 res_option() =
59 {alt_nameservers, [nameserver()]} |
60 {edns, 0 | false} |
61 {inet6, boolean()} |
62 {nameservers, [nameserver()]} |
63 {recurse, boolean()} |
64 {retry, integer()} |
65 {timeout, integer()} |
66 {udp_payload_size, integer()} |
67 {usevc, boolean()} |
68 {nxdomain_reply, boolean()}
69 nameserver() = {inet:ip_address(), Port :: 1..65535}
71 res_error() =
73 formerr | qfmterror | servfail | nxdomain | notimp | refused |
74 badvers | timeout
75
77 The following data types concern the DNS client:
78
80 dns_name() = string()
81 A string with no adjacent dots.
83 rr_type() =
85 a | aaaa | caa | cname | gid | hinfo | ns | mb | md | mg |
86 mf | minfo | mx | naptr | null | ptr | soa | spf | srv | txt |
87 uid | uinfo | unspec | uri | wks
88 dns_class() = in | chaos | hs | any
90 dns_msg() = term()
92 This is the start of a hiearchy of opaque data structures that
94 can be examined with access functions in inet_dns, which return
95 lists of {Field,Value} tuples. The arity 2 functions only return
96 the value for a specified field.
97 dns_msg() = DnsMsg
99 inet_dns:msg(DnsMsg) ->
100 [ {header, dns_header()}
101 | {qdlist, dns_query()}
102 | {anlist, dns_rr()}
103 | {nslist, dns_rr()}
104 | {arlist, dns_rr()} ]
105 inet_dns:msg(DnsMsg, header) -> dns_header() % for example
106 inet_dns:msg(DnsMsg, Field) -> Value
107 dns_header() = DnsHeader
109 inet_dns:header(DnsHeader) ->
110 [ {id, integer()}
111 | {qr, boolean()}
112 | {opcode, query | iquery | status | integer()}
113 | {aa, boolean()}
114 | {tc, boolean()}
115 | {rd, boolean()}
116 | {ra, boolean()}
117 | {pr, boolean()}
118 | {rcode, integer(0..16)} ]
119 inet_dns:header(DnsHeader, Field) -> Value
120 query_type() = axfr | mailb | maila | any | rr_type()
122 dns_query() = DnsQuery
124 inet_dns:dns_query(DnsQuery) ->
125 [ {domain, dns_name()}
126 | {type, query_type()}
127 | {class, dns_class()} ]
128 inet_dns:dns_query(DnsQuery, Field) -> Value
129 dns_rr() = DnsRr
131 inet_dns:rr(DnsRr) -> DnsRrFields | DnsRrOptFields
132 DnsRrFields = [ {domain, dns_name()}
133 | {type, rr_type()}
134 | {class, dns_class()}
135 | {ttl, integer()}
136 | {data, dns_data()} ]
137 DnsRrOptFields = [ {domain, dns_name()}
138 | {type, opt}
139 | {udp_payload_size, integer()}
140 | {ext_rcode, integer()}
141 | {version, integer()}
142 | {z, integer()}
143 | {data, dns_data()} ]
144 inet_dns:rr(DnsRr, Field) -> Value
145 There is an information function for the types above:
147 inet_dns:record_type(dns_msg()) -> msg;
149 inet_dns:record_type(dns_header()) -> header;
150 inet_dns:record_type(dns_query()) -> dns_query;
151 inet_dns:record_type(dns_rr()) -> rr;
152 inet_dns:record_type(_) -> undefined.
153 So, inet_dns:(inet_dns:record_type(X))(X) converts any of these
155 data structures into a {Field,Value} list.
156 dns_data() =
158 dns_name() |
159 inet:ip4_address() |
160 inet:ip6_address() |
161 {MName :: dns_name(),
162 RName :: dns_name(),
163 Serial :: integer(),
164 Refresh :: integer(),
165 Retry :: integer(),
166 Expiry :: integer(),
167 Minimum :: integer()} |
168 {inet:ip4_address(), Proto :: integer(), BitMap :: binary()} |
169 {CpuString :: string(), OsString :: string()} |
170 {RM :: dns_name(), EM :: dns_name()} |
171 {Prio :: integer(), dns_name()} |
172 {Prio :: integer(),
173 Weight :: integer(),
174 Port :: integer(),
175 dns_name()} |
176 {Order :: integer(),
177 Preference :: integer(),
178 Flags :: string(),
179 Services :: string(),
180 Regexp :: string(),
181 dns_name()} |
182 [string()] |
183 binary()
184 Regexp is a string with characters encoded in the UTF-8 coding
186 standard.
187
189 getbyname(Name, Type) -> {ok, Hostent} | {error, Reason}
190 getbyname(Name, Type, Timeout) -> {ok, Hostent} | {error, Reason}
192 Types:
194 Name = dns_name()
196 Type = rr_type()
197 Timeout = timeout()
198 Hostent = inet:hostent()
199 Reason = inet:posix() | res_error()
200 Resolves a DNS record of the specified type for the specified
202 host, of class in. Returns, on success, a hostent() record with
203 dns_data() elements in the address list field.
204 This function uses resolver option search that is a list of do‐
206 main names. If the name to resolve contains no dots, it is
207 prepended to each domain name in the search list, and they are
208 tried in order. If the name contains dots, it is first tried as
209 an absolute name and if that fails, the search list is used. If
210 the name has a trailing dot, it is supposed to be an absolute
211 name and the search list is not used.
212 gethostbyaddr(Address) -> {ok, Hostent} | {error, Reason}
214 gethostbyaddr(Address, Timeout) -> {ok, Hostent} | {error, Reason}
216 Types:
218 Address = inet:ip_address()
220 Timeout = timeout()
221 Hostent = inet:hostent()
222 Reason = inet:posix() | res_error()
223 Backend functions used by inet:gethostbyaddr/1.
225 gethostbyname(Name) -> {ok, Hostent} | {error, Reason}
227 gethostbyname(Name, Family) -> {ok, Hostent} | {error, Reason}
229 gethostbyname(Name, Family, Timeout) ->
231 {ok, Hostent} | {error, Reason}
232 Types:
234 Name = dns_name()
236 Hostent = inet:hostent()
237 Timeout = timeout()
238 Family = inet:address_family()
239 Reason = inet:posix() | res_error()
240 Backend functions used by inet:gethostbyname/1,2.
242 This function uses resolver option search just like getby‐
244 name/2,3.
245 If resolver option inet6 is true, an IPv6 address is looked up.
247 lookup(Name, Class, Type) -> [dns_data()]
249 lookup(Name, Class, Type, Opts) -> [dns_data()]
251 lookup(Name, Class, Type, Opts, Timeout) -> [dns_data()]
253 Types:
255 Name = dns_name() | inet:ip_address()
257 Class = dns_class()
258 Type = rr_type()
259 Opts = [res_option() | verbose]
260 Timeout = timeout()
261 Resolves the DNS data for the record of the specified type and
263 class for the specified name. On success, filters out the answer
264 records with the correct Class and Type, and returns a list of
265 their data fields. So, a lookup for type any gives an empty an‐
266 swer, as the answer records have specific types that are not
267 any. An empty answer or a failed lookup returns an empty list.
268 Calls resolve/* with the same arguments and filters the result,
270 so Opts is described for those functions.
271 resolve(Name, Class, Type) -> {ok, dns_msg()} | Error
273 resolve(Name, Class, Type, Opts) -> {ok, dns_msg()} | Error
275 resolve(Name, Class, Type, Opts, Timeout) ->
277 {ok, dns_msg()} | Error
278 Types:
280 Name = dns_name() | inet:ip_address()
282 Class = dns_class()
283 Type = rr_type()
284 Opts = [Opt]
285 Opt = res_option() | verbose | atom()
286 Timeout = timeout()
287 Error = {error, Reason} | {error, {Reason, dns_msg()}}
288 Reason = inet:posix() | res_error()
289 Resolves a DNS record of the specified type and class for the
291 specified name. The returned dns_msg() can be examined using ac‐
292 cess functions in inet_db, as described in section in DNS Types.
293 If Name is an ip_address(), the domain name to query for is gen‐
295 erated as the standard reverse ".IN-ADDR.ARPA." name for an IPv4
296 address, or the ".IP6.ARPA." name for an IPv6 address. In this
297 case, you most probably want to use Class = in and Type = ptr,
298 but it is not done automatically.
299 Opts overrides the corresponding resolver options. If option
301 nameservers is specified, it is assumed that it is the complete
302 list of name serves, so resolver option alt_nameserves is ig‐
303 nored. However, if option alt_nameserves is also specified to
304 this function, it is used.
305 Option verbose (or rather {verbose,true}) causes diagnostics
307 printout through io:format/2 of queries, replies retransmis‐
308 sions, and so on, similar to from utilities, such as dig and
309 nslookup.
310 Option nxdomain_reply (or rather {nxdomain_reply,true}) causes
312 nxdomain errors from DNS servers to be returned as {error, {nx‐
313 domain, dns_msg()}}. dns_msg() contains the additional sections
314 that where included by the answering server. This is mainly use‐
315 ful to inspect the SOA record to get the TTL for negative
316 caching.
317 If Opt is any atom, it is interpreted as {Opt,true} unless the
319 atom string starts with "no", making the interpretation
320 {Opt,false}. For example, usevc is an alias for {usevc,true} and
321 nousevc is an alias for {usevc,false}.
322 Option inet6 has no effect on this function. You probably want
324 to use Type = a | aaaa instead.
325
327 This access functions example shows how lookup/3 can be implemented us‐
328 ing resolve/3 from outside the module:
329 example_lookup(Name, Class, Type) ->
331 case inet_res:resolve(Name, Class, Type) of
332 {ok,Msg} ->
333 [inet_dns:rr(RR, data)
334 || RR <- inet_dns:msg(Msg, anlist),
335 inet_dns:rr(RR, type) =:= Type,
336 inet_dns:rr(RR, class) =:= Class];
337 {error,_} ->
338 []
339 end.
340
342 These are deprecated because the annoying double meaning of the name
343 servers/time-out argument, and because they have no decent place for a
344 resolver options list.
345
347 nslookup(Name, Class, Type) -> {ok, dns_msg()} | {error, Reason}
348 nslookup(Name, Class, Type, Timeout) ->
350 {ok, dns_msg()} | {error, Reason}
351 nslookup(Name, Class, Type, Nameservers) ->
353 {ok, dns_msg()} | {error, Reason}
354 Types:
356 Name = dns_name() | inet:ip_address()
358 Class = dns_class()
359 Type = rr_type()
360 Timeout = timeout()
361 Nameservers = [nameserver()]
362 Reason = inet:posix() | res_error()
363 Resolves a DNS record of the specified type and class for the
365 specified name.
366 nnslookup(Name, Class, Type, Nameservers) ->
368 {ok, dns_msg()} | {error, Reason}
369 nnslookup(Name, Class, Type, Nameservers, Timeout) ->
371 {ok, dns_msg()} | {error, Reason}
372 Types:
374 Name = dns_name() | inet:ip_address()
376 Class = dns_class()
377 Type = rr_type()
378 Timeout = timeout()
379 Nameservers = [nameserver()]
380 Reason = inet:posix()
381 Resolves a DNS record of the specified type and class for the
383 specified name.
384Ericsson AB kernel 8.1.3 inet_res(3)