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1 /*
2  * Copyright (c) 2006-2007 Niels Provos <provos@citi.umich.edu>
3  * Copyright (c) 2007-2012 Niels Provos and Nick Mathewson
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  * 1. Redistributions of source code must retain the above copyright
9  *    notice, this list of conditions and the following disclaimer.
10  * 2. Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in the
12  *    documentation and/or other materials provided with the distribution.
13  * 3. The name of the author may not be used to endorse or promote products
14  *    derived from this software without specific prior written permission.
15  *
16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
17  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
18  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
19  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
21  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
25  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
26  */
27 
28 /*
29  * The original DNS code is due to Adam Langley with heavy
30  * modifications by Nick Mathewson.  Adam put his DNS software in the
31  * public domain.  You can find his original copyright below.  Please,
32  * aware that the code as part of Libevent is governed by the 3-clause
33  * BSD license above.
34  *
35  * This software is Public Domain. To view a copy of the public domain dedication,
36  * visit http://creativecommons.org/licenses/publicdomain/ or send a letter to
37  * Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
38  *
39  * I ask and expect, but do not require, that all derivative works contain an
40  * attribution similar to:
41  *     Parts developed by Adam Langley <agl@imperialviolet.org>
42  *
43  * You may wish to replace the word "Parts" with something else depending on
44  * the amount of original code.
45  *
46  * (Derivative works does not include programs which link against, run or include
47  * the source verbatim in their source distributions)
48  */
49 
50 /** @file event2/dns.h
51  *
52  * Welcome, gentle reader
53  *
54  * Async DNS lookups are really a whole lot harder than they should be,
55  * mostly stemming from the fact that the libc resolver has never been
56  * very good at them. Before you use this library you should see if libc
57  * can do the job for you with the modern async call getaddrinfo_a
58  * (see http://www.imperialviolet.org/page25.html#e498). Otherwise,
59  * please continue.
60  *
61  * The library keeps track of the state of nameservers and will avoid
62  * them when they go down. Otherwise it will round robin between them.
63  *
64  * Quick start guide:
65  *   #include "evdns.h"
66  *   void callback(int result, char type, int count, int ttl,
67  *		 void *addresses, void *arg);
68  *   evdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
69  *   evdns_resolve("www.hostname.com", 0, callback, NULL);
70  *
71  * When the lookup is complete the callback function is called. The
72  * first argument will be one of the DNS_ERR_* defines in evdns.h.
73  * Hopefully it will be DNS_ERR_NONE, in which case type will be
74  * DNS_IPv4_A, count will be the number of IP addresses, ttl is the time
75  * which the data can be cached for (in seconds), addresses will point
76  * to an array of uint32_t's and arg will be whatever you passed to
77  * evdns_resolve.
78  *
79  * Searching:
80  *
81  * In order for this library to be a good replacement for glibc's resolver it
82  * supports searching. This involves setting a list of default domains, in
83  * which names will be queried for. The number of dots in the query name
84  * determines the order in which this list is used.
85  *
86  * Searching appears to be a single lookup from the point of view of the API,
87  * although many DNS queries may be generated from a single call to
88  * evdns_resolve. Searching can also drastically slow down the resolution
89  * of names.
90  *
91  * To disable searching:
92  *   1. Never set it up. If you never call evdns_resolv_conf_parse or
93  *   evdns_search_add then no searching will occur.
94  *
95  *   2. If you do call evdns_resolv_conf_parse then don't pass
96  *   DNS_OPTION_SEARCH (or DNS_OPTIONS_ALL, which implies it).
97  *
98  *   3. When calling evdns_resolve, pass the DNS_QUERY_NO_SEARCH flag.
99  *
100  * The order of searches depends on the number of dots in the name. If the
101  * number is greater than the ndots setting then the names is first tried
102  * globally. Otherwise each search domain is appended in turn.
103  *
104  * The ndots setting can either be set from a resolv.conf, or by calling
105  * evdns_search_ndots_set.
106  *
107  * For example, with ndots set to 1 (the default) and a search domain list of
108  * ["myhome.net"]:
109  *  Query: www
110  *  Order: www.myhome.net, www.
111  *
112  *  Query: www.abc
113  *  Order: www.abc., www.abc.myhome.net
114  *
115  * Internals:
116  *
117  * Requests are kept in two queues. The first is the inflight queue. In
118  * this queue requests have an allocated transaction id and nameserver.
119  * They will soon be transmitted if they haven't already been.
120  *
121  * The second is the waiting queue. The size of the inflight ring is
122  * limited and all other requests wait in waiting queue for space. This
123  * bounds the number of concurrent requests so that we don't flood the
124  * nameserver. Several algorithms require a full walk of the inflight
125  * queue and so bounding its size keeps thing going nicely under huge
126  * (many thousands of requests) loads.
127  *
128  * If a nameserver loses too many requests it is considered down and we
129  * try not to use it. After a while we send a probe to that nameserver
130  * (a lookup for google.com) and, if it replies, we consider it working
131  * again. If the nameserver fails a probe we wait longer to try again
132  * with the next probe.
133  */
134 
135 #ifndef _EVENT2_DNS_H_
136 #define _EVENT2_DNS_H_
137 
138 #ifdef __cplusplus
139 extern "C" {
140 #endif
141 
142 /* For integer types. */
143 #include <event2/util.h>
144 
145 /** Error codes 0-5 are as described in RFC 1035. */
146 #define DNS_ERR_NONE 0
147 /** The name server was unable to interpret the query */
148 #define DNS_ERR_FORMAT 1
149 /** The name server was unable to process this query due to a problem with the
150  * name server */
151 #define DNS_ERR_SERVERFAILED 2
152 /** The domain name does not exist */
153 #define DNS_ERR_NOTEXIST 3
154 /** The name server does not support the requested kind of query */
155 #define DNS_ERR_NOTIMPL 4
156 /** The name server refuses to reform the specified operation for policy
157  * reasons */
158 #define DNS_ERR_REFUSED 5
159 /** The reply was truncated or ill-formatted */
160 #define DNS_ERR_TRUNCATED 65
161 /** An unknown error occurred */
162 #define DNS_ERR_UNKNOWN 66
163 /** Communication with the server timed out */
164 #define DNS_ERR_TIMEOUT 67
165 /** The request was canceled because the DNS subsystem was shut down. */
166 #define DNS_ERR_SHUTDOWN 68
167 /** The request was canceled via a call to evdns_cancel_request */
168 #define DNS_ERR_CANCEL 69
169 /** There were no answers and no error condition in the DNS packet.
170  * This can happen when you ask for an address that exists, but a record
171  * type that doesn't. */
172 #define DNS_ERR_NODATA 70
173 
174 #define DNS_IPv4_A 1
175 #define DNS_PTR 2
176 #define DNS_IPv6_AAAA 3
177 
178 #define DNS_QUERY_NO_SEARCH 1
179 
180 #define DNS_OPTION_SEARCH 1
181 #define DNS_OPTION_NAMESERVERS 2
182 #define DNS_OPTION_MISC 4
183 #define DNS_OPTION_HOSTSFILE 8
184 #define DNS_OPTIONS_ALL 15
185 
186 /* Obsolete name for DNS_QUERY_NO_SEARCH */
187 #define DNS_NO_SEARCH DNS_QUERY_NO_SEARCH
188 
189 /**
190  * The callback that contains the results from a lookup.
191  * - result is one of the DNS_ERR_* values (DNS_ERR_NONE for success)
192  * - type is either DNS_IPv4_A or DNS_PTR or DNS_IPv6_AAAA
193  * - count contains the number of addresses of form type
194  * - ttl is the number of seconds the resolution may be cached for.
195  * - addresses needs to be cast according to type.  It will be an array of
196  *   4-byte sequences for ipv4, or an array of 16-byte sequences for ipv6,
197  *   or a nul-terminated string for PTR.
198  */
199 typedef void (*evdns_callback_type) (int result, char type, int count, int ttl, void *addresses, void *arg);
200 
201 struct evdns_base;
202 struct event_base;
203 
204 /**
205   Initialize the asynchronous DNS library.
206 
207   This function initializes support for non-blocking name resolution by
208   calling evdns_resolv_conf_parse() on UNIX and
209   evdns_config_windows_nameservers() on Windows.
210 
211   @param event_base the event base to associate the dns client with
212   @param initialize_nameservers 1 if resolve.conf processing should occur
213   @return evdns_base object if successful, or NULL if an error occurred.
214   @see evdns_base_free()
215  */
216 struct evdns_base * evdns_base_new(struct event_base *event_base, int initialize_nameservers);
217 
218 
219 /**
220   Shut down the asynchronous DNS resolver and terminate all active requests.
221 
222   If the 'fail_requests' option is enabled, all active requests will return
223   an empty result with the error flag set to DNS_ERR_SHUTDOWN. Otherwise,
224   the requests will be silently discarded.
225 
226   @param evdns_base the evdns base to free
227   @param fail_requests if zero, active requests will be aborted; if non-zero,
228 		active requests will return DNS_ERR_SHUTDOWN.
229   @see evdns_base_new()
230  */
231 void evdns_base_free(struct evdns_base *base, int fail_requests);
232 
233 /**
234   Convert a DNS error code to a string.
235 
236   @param err the DNS error code
237   @return a string containing an explanation of the error code
238 */
239 const char *evdns_err_to_string(int err);
240 
241 
242 /**
243   Add a nameserver.
244 
245   The address should be an IPv4 address in network byte order.
246   The type of address is chosen so that it matches in_addr.s_addr.
247 
248   @param base the evdns_base to which to add the name server
249   @param address an IP address in network byte order
250   @return 0 if successful, or -1 if an error occurred
251   @see evdns_base_nameserver_ip_add()
252  */
253 int evdns_base_nameserver_add(struct evdns_base *base,
254     unsigned long int address);
255 
256 /**
257   Get the number of configured nameservers.
258 
259   This returns the number of configured nameservers (not necessarily the
260   number of running nameservers).  This is useful for double-checking
261   whether our calls to the various nameserver configuration functions
262   have been successful.
263 
264   @param base the evdns_base to which to apply this operation
265   @return the number of configured nameservers
266   @see evdns_base_nameserver_add()
267  */
268 int evdns_base_count_nameservers(struct evdns_base *base);
269 
270 /**
271   Remove all configured nameservers, and suspend all pending resolves.
272 
273   Resolves will not necessarily be re-attempted until evdns_base_resume() is called.
274 
275   @param base the evdns_base to which to apply this operation
276   @return 0 if successful, or -1 if an error occurred
277   @see evdns_base_resume()
278  */
279 int evdns_base_clear_nameservers_and_suspend(struct evdns_base *base);
280 
281 
282 /**
283   Resume normal operation and continue any suspended resolve requests.
284 
285   Re-attempt resolves left in limbo after an earlier call to
286   evdns_base_clear_nameservers_and_suspend().
287 
288   @param base the evdns_base to which to apply this operation
289   @return 0 if successful, or -1 if an error occurred
290   @see evdns_base_clear_nameservers_and_suspend()
291  */
292 int evdns_base_resume(struct evdns_base *base);
293 
294 /**
295   Add a nameserver by string address.
296 
297   This function parses a n IPv4 or IPv6 address from a string and adds it as a
298   nameserver.  It supports the following formats:
299   - [IPv6Address]:port
300   - [IPv6Address]
301   - IPv6Address
302   - IPv4Address:port
303   - IPv4Address
304 
305   If no port is specified, it defaults to 53.
306 
307   @param base the evdns_base to which to apply this operation
308   @return 0 if successful, or -1 if an error occurred
309   @see evdns_base_nameserver_add()
310  */
311 int evdns_base_nameserver_ip_add(struct evdns_base *base,
312     const char *ip_as_string);
313 
314 /**
315    Add a nameserver by sockaddr.
316  **/
317 int
318 evdns_base_nameserver_sockaddr_add(struct evdns_base *base,
319     const struct sockaddr *sa, ev_socklen_t len, unsigned flags);
320 
321 struct evdns_request;
322 
323 /**
324   Lookup an A record for a given name.
325 
326   @param base the evdns_base to which to apply this operation
327   @param name a DNS hostname
328   @param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
329   @param callback a callback function to invoke when the request is completed
330   @param ptr an argument to pass to the callback function
331   @return an evdns_request object if successful, or NULL if an error occurred.
332   @see evdns_resolve_ipv6(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()
333  */
334 struct evdns_request *evdns_base_resolve_ipv4(struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr);
335 
336 /**
337   Lookup an AAAA record for a given name.
338 
339   @param base the evdns_base to which to apply this operation
340   @param name a DNS hostname
341   @param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
342   @param callback a callback function to invoke when the request is completed
343   @param ptr an argument to pass to the callback function
344   @return an evdns_request object if successful, or NULL if an error occurred.
345   @see evdns_resolve_ipv4(), evdns_resolve_reverse(), evdns_resolve_reverse_ipv6(), evdns_cancel_request()
346  */
347 struct evdns_request *evdns_base_resolve_ipv6(struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *ptr);
348 
349 struct in_addr;
350 struct in6_addr;
351 
352 /**
353   Lookup a PTR record for a given IP address.
354 
355   @param base the evdns_base to which to apply this operation
356   @param in an IPv4 address
357   @param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
358   @param callback a callback function to invoke when the request is completed
359   @param ptr an argument to pass to the callback function
360   @return an evdns_request object if successful, or NULL if an error occurred.
361   @see evdns_resolve_reverse_ipv6(), evdns_cancel_request()
362  */
363 struct evdns_request *evdns_base_resolve_reverse(struct evdns_base *base, const struct in_addr *in, int flags, evdns_callback_type callback, void *ptr);
364 
365 
366 /**
367   Lookup a PTR record for a given IPv6 address.
368 
369   @param base the evdns_base to which to apply this operation
370   @param in an IPv6 address
371   @param flags either 0, or DNS_QUERY_NO_SEARCH to disable searching for this query.
372   @param callback a callback function to invoke when the request is completed
373   @param ptr an argument to pass to the callback function
374   @return an evdns_request object if successful, or NULL if an error occurred.
375   @see evdns_resolve_reverse_ipv6(), evdns_cancel_request()
376  */
377 struct evdns_request *evdns_base_resolve_reverse_ipv6(struct evdns_base *base, const struct in6_addr *in, int flags, evdns_callback_type callback, void *ptr);
378 
379 /**
380   Cancels a pending DNS resolution request.
381 
382   @param base the evdns_base that was used to make the request
383   @param req the evdns_request that was returned by calling a resolve function
384   @see evdns_base_resolve_ipv4(), evdns_base_resolve_ipv6, evdns_base_resolve_reverse
385 */
386 void evdns_cancel_request(struct evdns_base *base, struct evdns_request *req);
387 
388 /**
389   Set the value of a configuration option.
390 
391   The currently available configuration options are:
392 
393     ndots, timeout, max-timeouts, max-inflight, attempts, randomize-case,
394     bind-to, initial-probe-timeout, getaddrinfo-allow-skew.
395 
396   In versions before Libevent 2.0.3-alpha, the option name needed to end with
397   a colon.
398 
399   @param base the evdns_base to which to apply this operation
400   @param option the name of the configuration option to be modified
401   @param val the value to be set
402   @return 0 if successful, or -1 if an error occurred
403  */
404 int evdns_base_set_option(struct evdns_base *base, const char *option, const char *val);
405 
406 
407 /**
408   Parse a resolv.conf file.
409 
410   The 'flags' parameter determines what information is parsed from the
411   resolv.conf file. See the man page for resolv.conf for the format of this
412   file.
413 
414   The following directives are not parsed from the file: sortlist, rotate,
415   no-check-names, inet6, debug.
416 
417   If this function encounters an error, the possible return values are: 1 =
418   failed to open file, 2 = failed to stat file, 3 = file too large, 4 = out of
419   memory, 5 = short read from file, 6 = no nameservers listed in the file
420 
421   @param base the evdns_base to which to apply this operation
422   @param flags any of DNS_OPTION_NAMESERVERS|DNS_OPTION_SEARCH|DNS_OPTION_MISC|
423     DNS_OPTION_HOSTSFILE|DNS_OPTIONS_ALL
424   @param filename the path to the resolv.conf file
425   @return 0 if successful, or various positive error codes if an error
426     occurred (see above)
427   @see resolv.conf(3), evdns_config_windows_nameservers()
428  */
429 int evdns_base_resolv_conf_parse(struct evdns_base *base, int flags, const char *const filename);
430 
431 /**
432    Load an /etc/hosts-style file from 'hosts_fname' into 'base'.
433 
434    If hosts_fname is NULL, add minimal entries for localhost, and nothing
435    else.
436 
437    Note that only evdns_getaddrinfo uses the /etc/hosts entries.
438 
439    Return 0 on success, negative on failure.
440 */
441 int evdns_base_load_hosts(struct evdns_base *base, const char *hosts_fname);
442 
443 /**
444   Obtain nameserver information using the Windows API.
445 
446   Attempt to configure a set of nameservers based on platform settings on
447   a win32 host.  Preferentially tries to use GetNetworkParams; if that fails,
448   looks in the registry.
449 
450   @return 0 if successful, or -1 if an error occurred
451   @see evdns_resolv_conf_parse()
452  */
453 #ifdef WIN32
454 int evdns_base_config_windows_nameservers(struct evdns_base *);
455 #define EVDNS_BASE_CONFIG_WINDOWS_NAMESERVERS_IMPLEMENTED
456 #endif
457 
458 
459 /**
460   Clear the list of search domains.
461  */
462 void evdns_base_search_clear(struct evdns_base *base);
463 
464 
465 /**
466   Add a domain to the list of search domains
467 
468   @param domain the domain to be added to the search list
469  */
470 void evdns_base_search_add(struct evdns_base *base, const char *domain);
471 
472 
473 /**
474   Set the 'ndots' parameter for searches.
475 
476   Sets the number of dots which, when found in a name, causes
477   the first query to be without any search domain.
478 
479   @param ndots the new ndots parameter
480  */
481 void evdns_base_search_ndots_set(struct evdns_base *base, const int ndots);
482 
483 /**
484   A callback that is invoked when a log message is generated
485 
486   @param is_warning indicates if the log message is a 'warning'
487   @param msg the content of the log message
488  */
489 typedef void (*evdns_debug_log_fn_type)(int is_warning, const char *msg);
490 
491 
492 /**
493   Set the callback function to handle DNS log messages.  If this
494   callback is not set, evdns log messages are handled with the regular
495   Libevent logging system.
496 
497   @param fn the callback to be invoked when a log message is generated
498  */
499 void evdns_set_log_fn(evdns_debug_log_fn_type fn);
500 
501 /**
502    Set a callback that will be invoked to generate transaction IDs.  By
503    default, we pick transaction IDs based on the current clock time, which
504    is bad for security.
505 
506    @param fn the new callback, or NULL to use the default.
507 
508    NOTE: This function has no effect in Libevent 2.0.4-alpha and later,
509    since Libevent now provides its own secure RNG.
510  */
511 void evdns_set_transaction_id_fn(ev_uint16_t (*fn)(void));
512 
513 /**
514    Set a callback used to generate random bytes.  By default, we use
515    the same function as passed to evdns_set_transaction_id_fn to generate
516    bytes two at a time.  If a function is provided here, it's also used
517    to generate transaction IDs.
518 
519    NOTE: This function has no effect in Libevent 2.0.4-alpha and later,
520    since Libevent now provides its own secure RNG.
521 */
522 void evdns_set_random_bytes_fn(void (*fn)(char *, size_t));
523 
524 /*
525  * Functions used to implement a DNS server.
526  */
527 
528 struct evdns_server_request;
529 struct evdns_server_question;
530 
531 /**
532    A callback to implement a DNS server.  The callback function receives a DNS
533    request.  It should then optionally add a number of answers to the reply
534    using the evdns_server_request_add_*_reply functions, before calling either
535    evdns_server_request_respond to send the reply back, or
536    evdns_server_request_drop to decline to answer the request.
537 
538    @param req A newly received request
539    @param user_data A pointer that was passed to
540       evdns_add_server_port_with_base().
541  */
542 typedef void (*evdns_request_callback_fn_type)(struct evdns_server_request *, void *);
543 #define EVDNS_ANSWER_SECTION 0
544 #define EVDNS_AUTHORITY_SECTION 1
545 #define EVDNS_ADDITIONAL_SECTION 2
546 
547 #define EVDNS_TYPE_A	   1
548 #define EVDNS_TYPE_NS	   2
549 #define EVDNS_TYPE_CNAME   5
550 #define EVDNS_TYPE_SOA	   6
551 #define EVDNS_TYPE_PTR	  12
552 #define EVDNS_TYPE_MX	  15
553 #define EVDNS_TYPE_TXT	  16
554 #define EVDNS_TYPE_AAAA	  28
555 
556 #define EVDNS_QTYPE_AXFR 252
557 #define EVDNS_QTYPE_ALL	 255
558 
559 #define EVDNS_CLASS_INET   1
560 
561 /* flags that can be set in answers; as part of the err parameter */
562 #define EVDNS_FLAGS_AA	0x400
563 #define EVDNS_FLAGS_RD	0x080
564 
565 /** Create a new DNS server port.
566 
567     @param base The event base to handle events for the server port.
568     @param socket A UDP socket to accept DNS requests.
569     @param flags Always 0 for now.
570     @param callback A function to invoke whenever we get a DNS request
571       on the socket.
572     @param user_data Data to pass to the callback.
573     @return an evdns_server_port structure for this server port.
574  */
575 struct evdns_server_port *evdns_add_server_port_with_base(struct event_base *base, evutil_socket_t socket, int flags, evdns_request_callback_fn_type callback, void *user_data);
576 /** Close down a DNS server port, and free associated structures. */
577 void evdns_close_server_port(struct evdns_server_port *port);
578 
579 /** Sets some flags in a reply we're building.
580     Allows setting of the AA or RD flags
581  */
582 void evdns_server_request_set_flags(struct evdns_server_request *req, int flags);
583 
584 /* Functions to add an answer to an in-progress DNS reply.
585  */
586 int evdns_server_request_add_reply(struct evdns_server_request *req, int section, const char *name, int type, int dns_class, int ttl, int datalen, int is_name, const char *data);
587 int evdns_server_request_add_a_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl);
588 int evdns_server_request_add_aaaa_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl);
589 int evdns_server_request_add_ptr_reply(struct evdns_server_request *req, struct in_addr *in, const char *inaddr_name, const char *hostname, int ttl);
590 int evdns_server_request_add_cname_reply(struct evdns_server_request *req, const char *name, const char *cname, int ttl);
591 
592 /**
593    Send back a response to a DNS request, and free the request structure.
594 */
595 int evdns_server_request_respond(struct evdns_server_request *req, int err);
596 /**
597    Free a DNS request without sending back a reply.
598 */
599 int evdns_server_request_drop(struct evdns_server_request *req);
600 struct sockaddr;
601 /**
602     Get the address that made a DNS request.
603  */
604 int evdns_server_request_get_requesting_addr(struct evdns_server_request *_req, struct sockaddr *sa, int addr_len);
605 
606 /** Callback for evdns_getaddrinfo. */
607 typedef void (*evdns_getaddrinfo_cb)(int result, struct evutil_addrinfo *res, void *arg);
608 
609 struct evdns_base;
610 struct evdns_getaddrinfo_request;
611 /** Make a non-blocking getaddrinfo request using the dns_base in 'dns_base'.
612  *
613  * If we can answer the request immediately (with an error or not!), then we
614  * invoke cb immediately and return NULL.  Otherwise we return
615  * an evdns_getaddrinfo_request and invoke cb later.
616  *
617  * When the callback is invoked, we pass as its first argument the error code
618  * that getaddrinfo would return (or 0 for no error).  As its second argument,
619  * we pass the evutil_addrinfo structures we found (or NULL on error).  We
620  * pass 'arg' as the third argument.
621  *
622  * Limitations:
623  *
624  * - The AI_V4MAPPED and AI_ALL flags are not currently implemented.
625  * - For ai_socktype, we only handle SOCKTYPE_STREAM, SOCKTYPE_UDP, and 0.
626  * - For ai_protocol, we only handle IPPROTO_TCP, IPPROTO_UDP, and 0.
627  */
628 struct evdns_getaddrinfo_request *evdns_getaddrinfo(
629     struct evdns_base *dns_base,
630     const char *nodename, const char *servname,
631     const struct evutil_addrinfo *hints_in,
632     evdns_getaddrinfo_cb cb, void *arg);
633 
634 /* Cancel an in-progress evdns_getaddrinfo.  This MUST NOT be called after the
635  * getaddrinfo's callback has been invoked.  The resolves will be canceled,
636  * and the callback will be invoked with the error EVUTIL_EAI_CANCEL. */
637 void evdns_getaddrinfo_cancel(struct evdns_getaddrinfo_request *req);
638 
639 #ifdef __cplusplus
640 }
641 #endif
642 
643 #endif  /* !_EVENT2_DNS_H_ */
644