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30 .Dd October 7, 2006
31 .Dt EVDNS 3
32 .Os
33 .Sh NAME
34 .Nm evdns_init
35 .Nm evdns_shutdown
36 .Nm evdns_err_to_string
37 .Nm evdns_nameserver_add
38 .Nm evdns_count_nameservers
39 .Nm evdns_clear_nameservers_and_suspend
40 .Nm evdns_resume
41 .Nm evdns_nameserver_ip_add
42 .Nm evdns_resolve_ipv4
43 .Nm evdns_resolve_reverse
44 .Nm evdns_resolv_conf_parse
45 .Nm evdns_config_windows_nameservers
46 .Nm evdns_search_clear
47 .Nm evdns_search_add
48 .Nm evdns_search_ndots_set
49 .Nm evdns_set_log_fn
50 .Nd asynchronous functions for DNS resolution.
51 .Sh SYNOPSIS
52 .Fd #include <sys/time.h>
53 .Fd #include <event.h>
54 .Fd #include <evdns.h>
55 .Ft int
56 .Fn evdns_init
57 .Ft void
58 .Fn evdns_shutdown "int fail_requests"
59 .Ft "const char *"
60 .Fn evdns_err_to_string "int err"
61 .Ft int
62 .Fn evdns_nameserver_add "unsigned long int address"
63 .Ft int
64 .Fn evdns_count_nameservers
65 .Ft int
66 .Fn evdns_clear_nameservers_and_suspend
67 .Ft int
68 .Fn evdns_resume
69 .Ft int
70 .Fn evdns_nameserver_ip_add(const char *ip_as_string);
71 .Ft int
72 .Fn evdns_resolve_ipv4 "const char *name" "int flags" "evdns_callback_type callback" "void *ptr"
73 .Ft int
74 .Fn evdns_resolve_reverse "struct in_addr *in" "int flags" "evdns_callback_type callback" "void *ptr"
75 .Ft int
76 .Fn evdns_resolv_conf_parse "int flags" "const char *"
77 .Ft void
78 .Fn evdns_search_clear
79 .Ft void
80 .Fn evdns_search_add "const char *domain"
81 .Ft void
82 .Fn evdns_search_ndots_set "const int ndots"
83 .Ft void
84 .Fn evdns_set_log_fn "evdns_debug_log_fn_type fn"
85 .Ft int
86 .Fn evdns_config_windows_nameservers
87 .Sh DESCRIPTION
88 Welcome, gentle reader
89 .Pp
90 Async DNS lookups are really a whole lot harder than they should be,
91 mostly stemming from the fact that the libc resolver has never been
92 very good at them. Before you use this library you should see if libc
93 can do the job for you with the modern async call getaddrinfo_a
94 (see http://www.imperialviolet.org/page25.html#e498). Otherwise,
95 please continue.
96 .Pp
97 This code is based on libevent and you must call event_init before
98 any of the APIs in this file. You must also seed the OpenSSL random
99 source if you are using OpenSSL for ids (see below).
100 .Pp
101 This library is designed to be included and shipped with your source
102 code. You statically link with it. You should also test for the
103 existence of strtok_r and define HAVE_STRTOK_R if you have it.
104 .Pp
105 The DNS protocol requires a good source of id numbers and these
106 numbers should be unpredictable for spoofing reasons. There are
107 three methods for generating them here and you must define exactly
108 one of them. In increasing order of preference:
109 .Pp
110 .Bl -tag -width "DNS_USE_GETTIMEOFDAY_FOR_ID" -compact -offset indent
111 .It DNS_USE_GETTIMEOFDAY_FOR_ID
112 Using the bottom 16 bits of the usec result from gettimeofday. This
113 is a pretty poor solution but should work anywhere.
114 .It DNS_USE_CPU_CLOCK_FOR_ID
115 Using the bottom 16 bits of the nsec result from the CPU's time
116 counter. This is better, but may not work everywhere. Requires
117 POSIX realtime support and you'll need to link against -lrt on
118 glibc systems at least.
119 .It DNS_USE_OPENSSL_FOR_ID
120 Uses the OpenSSL RAND_bytes call to generate the data. You must
121 have seeded the pool before making any calls to this library.
122 .El
123 .Pp
124 The library keeps track of the state of nameservers and will avoid
125 them when they go down. Otherwise it will round robin between them.
126 .Pp
127 Quick start guide:
128 #include "evdns.h"
129 void callback(int result, char type, int count, int ttl,
130 void *addresses, void *arg);
131 evdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
132 evdns_resolve("www.hostname.com", 0, callback, NULL);
133 .Pp
134 When the lookup is complete the callback function is called. The
135 first argument will be one of the DNS_ERR_* defines in evdns.h.
136 Hopefully it will be DNS_ERR_NONE, in which case type will be
137 DNS_IPv4_A, count will be the number of IP addresses, ttl is the time
138 which the data can be cached for (in seconds), addresses will point
139 to an array of uint32_t's and arg will be whatever you passed to
140 evdns_resolve.
141 .Pp
142 Searching:
143 .Pp
144 In order for this library to be a good replacement for glibc's resolver it
145 supports searching. This involves setting a list of default domains, in
146 which names will be queried for. The number of dots in the query name
147 determines the order in which this list is used.
148 .Pp
149 Searching appears to be a single lookup from the point of view of the API,
150 although many DNS queries may be generated from a single call to
151 evdns_resolve. Searching can also drastically slow down the resolution
152 of names.
153 .Pp
154 To disable searching:
155 .Bl -enum -compact -offset indent
156 .It
157 Never set it up. If you never call
158 .Fn evdns_resolv_conf_parse,
159 .Fn evdns_init,
160 or
161 .Fn evdns_search_add
162 then no searching will occur.
163 .It
164 If you do call
165 .Fn evdns_resolv_conf_parse
166 then don't pass
167 .Va DNS_OPTION_SEARCH
168 (or
169 .Va DNS_OPTIONS_ALL,
170 which implies it).
171 .It
172 When calling
173 .Fn evdns_resolve,
174 pass the
175 .Va DNS_QUERY_NO_SEARCH
176 flag.
177 .El
178 .Pp
179 The order of searches depends on the number of dots in the name. If the
180 number is greater than the ndots setting then the names is first tried
181 globally. Otherwise each search domain is appended in turn.
182 .Pp
183 The ndots setting can either be set from a resolv.conf, or by calling
184 evdns_search_ndots_set.
185 .Pp
186 For example, with ndots set to 1 (the default) and a search domain list of
187 ["myhome.net"]:
188 Query: www
189 Order: www.myhome.net, www.
190 .Pp
191 Query: www.abc
192 Order: www.abc., www.abc.myhome.net
193 .Pp
194 .Sh API reference
195 .Pp
196 .Bl -tag -width 0123456
197 .It Ft int Fn evdns_init
198 Initializes support for non-blocking name resolution by calling
199 .Fn evdns_resolv_conf_parse
200 on UNIX and
201 .Fn evdns_config_windows_nameservers
202 on Windows.
203 .It Ft int Fn evdns_nameserver_add "unsigned long int address"
204 Add a nameserver. The address should be an IP address in
205 network byte order. The type of address is chosen so that
206 it matches in_addr.s_addr.
207 Returns non-zero on error.
208 .It Ft int Fn evdns_nameserver_ip_add "const char *ip_as_string"
209 This wraps the above function by parsing a string as an IP
210 address and adds it as a nameserver.
211 Returns non-zero on error
212 .It Ft int Fn evdns_resolve "const char *name" "int flags" "evdns_callback_type callback" "void *ptr"
213 Resolve a name. The name parameter should be a DNS name.
214 The flags parameter should be 0, or DNS_QUERY_NO_SEARCH
215 which disables searching for this query. (see defn of
216 searching above).
217 .Pp
218 The callback argument is a function which is called when
219 this query completes and ptr is an argument which is passed
220 to that callback function.
221 .Pp
222 Returns non-zero on error
223 .It Ft void Fn evdns_search_clear
224 Clears the list of search domains
225 .It Ft void Fn evdns_search_add "const char *domain"
226 Add a domain to the list of search domains
227 .It Ft void Fn evdns_search_ndots_set "int ndots"
228 Set the number of dots which, when found in a name, causes
229 the first query to be without any search domain.
230 .It Ft int Fn evdns_count_nameservers "void"
231 Return the number of configured nameservers (not necessarily the
232 number of running nameservers). This is useful for double-checking
233 whether our calls to the various nameserver configuration functions
234 have been successful.
235 .It Ft int Fn evdns_clear_nameservers_and_suspend "void"
236 Remove all currently configured nameservers, and suspend all pending
237 resolves. Resolves will not necessarily be re-attempted until
238 evdns_resume() is called.
239 .It Ft int Fn evdns_resume "void"
240 Re-attempt resolves left in limbo after an earlier call to
241 evdns_clear_nameservers_and_suspend().
242 .It Ft int Fn evdns_config_windows_nameservers "void"
243 Attempt to configure a set of nameservers based on platform settings on
244 a win32 host. Preferentially tries to use GetNetworkParams; if that fails,
245 looks in the registry. Returns 0 on success, nonzero on failure.
246 .It Ft int Fn evdns_resolv_conf_parse "int flags" "const char *filename"
247 Parse a resolv.conf like file from the given filename.
248 .Pp
249 See the man page for resolv.conf for the format of this file.
250 The flags argument determines what information is parsed from
251 this file:
252 .Bl -tag -width "DNS_OPTION_NAMESERVERS" -offset indent -compact -nested
253 .It DNS_OPTION_SEARCH
254 domain, search and ndots options
255 .It DNS_OPTION_NAMESERVERS
256 nameserver lines
257 .It DNS_OPTION_MISC
258 timeout and attempts options
259 .It DNS_OPTIONS_ALL
260 all of the above
261 .El
262 .Pp
263 The following directives are not parsed from the file:
264 sortlist, rotate, no-check-names, inet6, debug
265 .Pp
266 Returns non-zero on error:
267 .Bl -tag -width "0" -offset indent -compact -nested
268 .It 0
269 no errors
270 .It 1
271 failed to open file
272 .It 2
273 failed to stat file
274 .It 3
275 file too large
276 .It 4
277 out of memory
278 .It 5
279 short read from file
280 .El
281 .El
282 .Sh Internals:
283 Requests are kept in two queues. The first is the inflight queue. In
284 this queue requests have an allocated transaction id and nameserver.
285 They will soon be transmitted if they haven't already been.
286 .Pp
287 The second is the waiting queue. The size of the inflight ring is
288 limited and all other requests wait in waiting queue for space. This
289 bounds the number of concurrent requests so that we don't flood the
290 nameserver. Several algorithms require a full walk of the inflight
291 queue and so bounding its size keeps thing going nicely under huge
292 (many thousands of requests) loads.
293 .Pp
294 If a nameserver loses too many requests it is considered down and we
295 try not to use it. After a while we send a probe to that nameserver
296 (a lookup for google.com) and, if it replies, we consider it working
297 again. If the nameserver fails a probe we wait longer to try again
298 with the next probe.
299 .Sh SEE ALSO
300 .Xr event 3 ,
301 .Xr gethostbyname 3 ,
302 .Xr resolv.conf 5
303 .Sh HISTORY
304 The
305 .Nm evdns
306 API was developed by Adam Langley on top of the
307 .Nm libevent
308 API.
309 The code was integrate into
310 .Nm Tor
311 by Nick Mathewson and finally put into
312 .Nm libevent
313 itself by Niels Provos.
314 .Sh AUTHORS
315 The
316 .Nm evdns
317 API and code was written by Adam Langley with significant
318 contributions by Nick Mathewson.
319 .Sh BUGS
320 This documentation is neither complete nor authoritative.
321 If you are in doubt about the usage of this API then
322 check the source code to find out how it works, write
323 up the missing piece of documentation and send it to
324 me for inclusion in this man page.