evdns.3 11 KB

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  1. .\"
  2. .\" Copyright (c) 2006 Niels Provos <provos@citi.umich.edu>
  3. .\" All rights reserved.
  4. .\"
  5. .\" Redistribution and use in source and binary forms, with or without
  6. .\" modification, are permitted provided that the following conditions
  7. .\" are met:
  8. .\"
  9. .\" 1. Redistributions of source code must retain the above copyright
  10. .\" notice, this list of conditions and the following disclaimer.
  11. .\" 2. Redistributions in binary form must reproduce the above copyright
  12. .\" notice, this list of conditions and the following disclaimer in the
  13. .\" documentation and/or other materials provided with the distribution.
  14. .\" 3. The name of the author may not be used to endorse or promote products
  15. .\" derived from this software without specific prior written permission.
  16. .\"
  17. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
  18. .\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
  19. .\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
  20. .\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  21. .\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  22. .\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
  23. .\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
  24. .\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  25. .\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  26. .\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27. .\"
  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.