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