eventdns.c 46 KB
Newer Older
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
/* Async DNS Library
 * Adam Langley <agl@imperialviolet.org>
 * http://www.imperialviolet.org/eventdns.html
 * Public Domain codenext
 *
 * This software is Public Domain. To view a copy of the public domain dedication,
 * visit http://creativecommons.org/licenses/publicdomain/ or send a letter to
 * Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
 *
 * I ask and expect, but do not require, that all derivative works contain an
 * attribution similar to:
 * 	Parts developed by Adam Langley <agl@imperialviolet.org>
 *
 * You may wish to replace the word "Parts" with something else depending on
 * the amount of original code.
 *
 * (Derivative works does not include programs which link against, run or include
 * the source verbatim in their source distributions)
 *
 * Version: 0.1b
 *
 *
 * Welcome, gentle reader
 *
 * Async DNS lookups are really a whole lot harder than they should be,
 * mostly stemming from the fact that the libc resolver has never been
 * very good at them. Before you use this library you should see if libc
 * can do the job for you with the modern async call getaddrinfo_r
 * (Google for it). Otherwise, please continue.
 *
 * This code is based on libevent and you must call event_init before
 * any of the APIs in this file. You must also seed the OpenSSL random
 * source if you are using OpenSSL for ids (see below).
 *
 * This library is designed to be included and shipped with your source
 * code. You statically link with it. You should also test for the
 * existance of strtok_r and define HAVE_STRTOK_R if you have it.
 *
 * The DNS protocol requires a good source of id numbers and these
 * numbers should be unpredictable for spoofing reasons. There are
 * three methods for generating them here and you must define exactly
 * one of them. In increasing order of preference:
 *
 * DNS_USE_GETTIMEOFDAY_FOR_ID:
 *   Using the bottom 16 bits of the usec result from gettimeofday. This
 *   is a pretty poor solution but should work anywhere
 * DNS_USE_CPU_CLOCK_FOR_ID:
 *   Using the bottom 16 bits of the nsec result from the CPU's time
 *   counter. This is better, but may not work everywhere. Requires
 *   POSIX realtime support and you'll need to link against -lrt on
 *   glibc systems at least
 * DNS_USE_OPENSSL_FOR_ID:
 *   Uses the OpenSSL RAND_bytes call to generate the data. You must
 *   have seeded the pool before making any calls to this library.
 *
 * The library keeps track of the state of nameservers and will avoid
 * them when they go down. Otherwise it will round robin between them.
 *
 * Quick start guide:
 *   #include "eventdns.h"
 *   void callback(int result, char type, int count, int ttl,
 *		 void *addresses, void *arg);
 *   eventdns_resolv_conf_parse(DNS_OPTIONS_ALL, "/etc/resolv.conf");
 *   eventdns_resolve("www.hostname.com", 0, callback, NULL);
 *
 * When the lookup is complete the callback function is called. The
 * first argument will be one of the DNS_ERR_* defines in eventdns.h.
 * Hopefully it will be DNS_ERR_NONE, in which case type will be
 * DNS_IPv4_A, count will be the number of IP addresses, ttl is the time
 * which the data can be cached for (in seconds), addresses will point
 * to an array of uint32_t's and arg will be whatever you passed to
 * eventdns_resolve.
 *
 * Searching:
 *
 * In order for this library to be a good replacment for glibc's resolver it
 * supports searching. This involves setting a list of default domains, in
 * which names will be queried for. The number of nots in the query name
 * determines the order in which this list is used.
 *
 * Searching appears to be a single lookup from the point of view of the API,
 * although many DNS queries may be generated from a single call to
 * eventdns_resolve. Searching can also drastically slow down the resolution of
 * names.
 *
 * To disable searching:
 *   1. Never set it up. If you never call eventdns_resolv_conf_parse or
 *   eventdns_search_add then no searching will occur.
 *
 *   2. If you do call eventdns_resolv_conf_parse then don't pass
 *   DNS_OPTION_SEARCH (or DNS_OPTIONS_ALL, which implies it)
 *
 *   3. When calling eventdns_resolve, pass the DNS_QUERY_NO_SEARCH flag
 *
 * The order of searches depends on the number of dots in the name. If the
 * number is greater than the ndots setting then the names is first tried
 * globally. Otherwise each search domain is appended in turn.
 *
 * The ndots setting can either be set from a resolv.conf, or by calling
 * eventdns_search_ndots_set.
 *
 * For example, with ndots set to 1 (the default) and a search domain list of
 * ["myhome.net"]:
 *  Query: www
 *  Order: www.myhome.net, www.
 *
 *  Query: www.abc
 *  Order: www.abc., www.abc.myhome.net
 *
 * API reference:
 *
 * int eventdns_nameserver_add(unsigned long int addresss)
 *   Add a nameserver. The address should be an IP address in
 *   network byte order. The type of address is chosen so that
 *   it match in_addr.s_addr.
 *   Returns non-zero on error.
 *
 * int eventdns_nameserer_ip_add(const char *ip_as_string)
 *   This wraps the above function by parsing a string as an IP
 *   address and add it as a nameserver.
 *   Returns non-zero on error
 *
 * int eventdns_resolve(const char *name, int flags,
 *		      eventdns_callback_type callback,
 *		      void *ptr)
 *   Resolve a name. The name parameter should be a DNS name.
 *   The flags parameter should be 0, or DNS_QUERY_NO_SEARCH
 *   which disables searching for this query. (see defn of
 *   searching above).
 *
 *   The callback argument is a function which is called when
 *   this query completes and ptr is an argument which is passed
 *   to that callback function.
 *
 *   Returns non-zero on error
 *
 * void eventdns_search_clear()
 *   Clears the list of search domains
 *
 * void eventdns_search_add(const char *domain)
 *   Add a domain to the list of search domains
 *
 * void eventdns_search_ndots_set(int ndots)
 *   Set the number of dots which, when found in a name, causes
 *   the first query to be without any search domain.
 *
 * int eventdns_resolv_conf_parse(int flags, const char *filename)
 *   Parse a resolv.conf like file from the given filename.
 *
 *   See the manpage for resolv.conf for the format of this file.
 *   The flags argument determines what information is parsed from
 *   this file:
 *     DNS_OPTION_SEARCH - domain, search and ndots options
 *     DNS_OPTION_NAMESERVERS - nameserver lines
 *     DNS_OPTION_MISC - timeout and attempts options
 *     DNS_OPTIONS_ALL - all of the above
 *   The following directives are not parsed from the file:
 *     sortlist, rotate, no-check-names, inet6, debug
 *
 *   Returns non-zero on error:
 *    0 no errors
 *    1 failed to open file
 *    2 failed to stat file
 *    3 file too large
 *    4 out of memory
 *    5 short read from file
 *
 * Internals:
 *
 * Requests are kept in two queues. The first is the inflight queue. In
 * this queue requests have an allocated transaction id and nameserver.
 * They will soon be transmitted if they haven't already been.
 *
 * The second is the waiting queue. The size of the inflight ring is
 * limited and all other requests wait in waiting queue for space. This
 * bounds the number of concurrent requests so that we don't flood the
 * nameserver. Several algorithms require a full walk of the inflight
 * queue and so bounding its size keeps thing going nicly under huge
 * (many thousands of requests) loads.
 *
 * If a nameserver looses too many requests it is considered down and we
 * try not to use it. After a while we send a probe to that nameserver
 * (a lookup for google.com) and, if it replies, we consider it working
 * again. If the nameserver fails a probe we wait longer to try again
 * with the next probe.
 */

#include "eventdns.h"
#include "eventdns_tor.h"
//#define NDEBUG

#ifndef DNS_USE_CPU_CLOCK_FOR_ID
#ifndef DNS_USE_GETTIMEOFDAY_FOR_ID
#ifndef DNS_USE_OPENSSL_FOR_ID
#error Must configure at least one id generation method.
#error Please see the documentation
#endif
#endif
#endif

// #define _POSIX_C_SOURCE 200507
#define _GNU_SOURCE

#ifdef DNS_USE_CPU_CLOCK_FOR_ID
#ifdef DNS_USE_OPENSSL_FOR_ID
#error Multiple id options selected
#endif
#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
#error Multiple id options selected
#endif
#include <time.h>
#endif

#ifdef DNS_USE_OPENSSL_FOR_ID
#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
#error Multiple id options selected
#endif
#include <openssl/rand.h>
#endif

#define _FORTIFY_SOURCE 3

#include <string.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <alloca.h>
#include <assert.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <limits.h>
#include <sys/stat.h>

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 255
#endif

#ifndef NDEBUG
#include <stdio.h>
#endif

#undef MIN
#define MIN(a,b) ((a)<(b)?(a):(b))

#ifdef __USE_ISOC99B
// libevent doesn't work without this
typedef uint8_t u_char;
typedef unsigned int uint;
#endif
#include <event.h>

#define u64 uint64_t
#define u32 uint32_t
#define u16 uint16_t
#define u8  uint8_t

#ifndef NDEBUG
#include <stdio.h>
#define log printf
#else
#define log(x,...)
#endif

#include "eventdns.h"

#define MAX_ADDRS 4  // maximum number of addresses from a single packet
// which we bother recording

#define TYPE_A 1
#define CLASS_INET 1

struct request {
	u8 *request;  // the dns packet data
	uint request_len;
	int reissue_count;
	int tx_count;  // the number of times that this packet has been sent
	void *user_pointer;  // the pointer given to us for this request
	eventdns_callback_type user_callback;
	struct nameserver *ns;  // the server which we last sent it

	// elements used by the searching code
	int search_index;
	struct search_state *search_state;
	char *search_origname;  // needs to be free()ed
	int search_flags;

	// these objects are kept in a circular list
	struct request *next, *prev;

	struct event timeout_event;

	u16 trans_id;  // the transaction id
	char request_appended;  // true if the request pointer is data which follows this struct
	char transmit_me;  // needs to be transmitted
};

struct nameserver {
	int socket;  // a connected UDP socket
	u32 address;
	int failed_times;  // number of times which we have given this server a chance
	int timedout;  // number of times in a row a request has timed out
	struct event event;
	// these objects are kept in a circular list
	struct nameserver *next, *prev;
	struct event timeout_event;  // used to keep the timeout for
				     // when we next probe this server.
				     // Valid if state == 0
	char state;  // zero if we think that this server is down
	char choaked;  // true if we have an EAGAIN from this server's socket
	char write_waiting;  // true if we are waiting for EV_WRITE events
};

static struct request *req_head = NULL, *req_waiting_head = NULL;
static struct nameserver *server_head = NULL;

// The number of good nameservers that we have
static int global_good_nameservers = 0;

// inflight requests are contained in the req_head list
// and are actually going out across the network
static int global_requests_inflight = 0;
// requests which aren't inflight are in the waiting list
// and are counted here
static int global_requests_waiting = 0;

static int global_max_requests_inflight = 64;

static struct timeval global_timeout = {3, 0};  // 3 seconds
static int global_max_reissues = 1;  // a reissue occurs when we get some errors from the server
static int global_max_retransmits = 3;  // number of times we'll retransmit a request which timed out
// number of timeouts in a row before we consider this server to be down
static int global_max_nameserver_timeout = 3;

// These are the timeout values for nameservers. If we find a nameserver is down
// we try to probe it at intervals as given below. Values are in seconds.
static const struct timeval global_nameserver_timeouts[] = {{10, 0}, {60, 0}, {300, 0}, {900, 0}, {3600, 0}};
static const int global_nameserver_timeouts_length = sizeof(global_nameserver_timeouts)/sizeof(struct timeval);

const char *const eventdns_error_strings[] = {"no error", "The name server was unable to interpret the query", "The name server suffered an internal error", "The requested domain name does not exist", "The name server refused to reply to the request"};

static struct nameserver *nameserver_pick();
static void eventdns_request_insert(struct request *req, struct request **head);
static void nameserver_ready_callback(int fd, short events, void *arg);
static int eventdns_transmit();
static int eventdns_request_transmit(struct request *req);
static void nameserver_send_probe(struct nameserver *const ns);
static void search_request_finished(struct request *const);
static int search_try_next(struct request *const req);
static int search_request_new(const char *const name, int flags, eventdns_callback_type user_callback, void *user_arg);
static void eventdns_requests_pump_waiting_queue();
static u16 transaction_id_pick();
static struct request *request_new(const char *name, int flags, eventdns_callback_type callback, void *ptr);
static void request_submit(struct request *req);

// This walks the list of inflight requests to find the
// one with a matching transaction id. Returns NULL on
// failure
static struct request *
request_find_from_trans_id(u16 trans_id) {
	struct request *req = req_head, *const started_at = req_head;

	if (req) {
		do {
			if (req->trans_id == trans_id) return req;
			req = req->next;
		} while (req != started_at);
	}

	return NULL;
}

// a libevent callback function which is called when a nameserver
// has gone down and we want to test if it has came back to life yet
static void
nameserver_prod_callback(int fd, short events, void *arg) {
	struct nameserver *const ns = (struct nameserver *) arg;

	nameserver_send_probe(ns);
}

// a libevent callback which is called when a nameserver probe (to see if
// it has come back to life) times out. We increment the count of failed_times
// and wait longer to send the next probe packet.
static void
nameserver_probe_failed(struct nameserver *const ns) {
	assert(ns->state == 0);
	evtimer_del(&ns->timeout_event);
	const struct timeval *const timeout =
	  &global_nameserver_timeouts[MIN(ns->failed_times,
					  global_nameserver_timeouts_length - 1)];
	ns->failed_times++;

	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
	evtimer_add(&ns->timeout_event, (struct timeval *) timeout);
}

// called when a nameserver has been deemed to have failed. For example, too
// many packets have timed out etc
static void
nameserver_failed(struct nameserver *const ns) {
	// if this nameserver has already been marked as failed
	// then don't do anything
	if (!ns->state) return;

	log("Nameserver %lx has failed\n", (unsigned long)ns->address);
	global_good_nameservers--;
	assert(global_good_nameservers >= 0);
	if (global_good_nameservers == 0) {
		log("All nameservers have failed\n");
	}

	ns->state = 0;
	ns->failed_times = 1;

	evtimer_set(&ns->timeout_event, nameserver_prod_callback, ns);
	evtimer_add(&ns->timeout_event, (struct timeval *) &global_nameserver_timeouts[0]);

	// walk the list of inflight requests to see if any can be reassigned to
	// a different server. Requests in the waiting queue don't have a
	// nameserver assigned yet
	
	// if we don't have *any* good nameservers then there's no point 
	// trying to reassign requests to one
	if (!global_good_nameservers) return;

	struct request *req = req_head, *const started_at = req_head;
	if (req) {
		do {
			if (req->tx_count == 0 && req->ns == ns) {
				// still waiting to go out, can be moved
				// to another server
				req->ns = nameserver_pick();
			}
			req = req->next;
		} while (req != started_at);
	}
}

static void
nameserver_up(struct nameserver *const ns) {
	if (ns->state) return;
	log("Nameserver %lx is back up\n", (unsigned long)ns->address);
	evtimer_del(&ns->timeout_event);
	ns->state = 1;
	ns->failed_times = 0;
	global_good_nameservers++;
}

static void
request_trans_id_set(struct request *const req, const u16 trans_id) {
	req->trans_id = trans_id;
	*((u16 *) req->request) = htons(trans_id);
}

// Called to remove a request from a list and dealloc it.
// head is a pointer to the head of the list it should be
// removed from or NULL if the request isn't in a list.
static void
request_finished(struct request *const req, struct request **head) {
	if (head) {
		if (req->next == req) {
			// only item in the list
			*head = NULL;
		} else {
			req->next->prev = req->prev;
			req->prev->next = req->next;
			if (*head == req) *head = req->next;
		}
	}

	log("Removing timeout for %x\n", (unsigned) req);
	evtimer_del(&req->timeout_event);

	search_request_finished(req);
	global_requests_inflight--;

	if (!req->request_appended) {
		// need to free the request data on it's own
		free(req->request);
	} else {
		// the request data is appended onto the header
		// so everything gets free()ed when we:
	}

	free(req);

	eventdns_requests_pump_waiting_queue();
}

// This is called when a server returns a funny error code.
// We try the request again with another server.
//
// return:
//   0 ok
//   1 failed/reissue is pointless
static int
request_reissue(struct request *req) {
	const struct nameserver *const last_ns = req->ns;
	// the last nameserver should have been marked as failing
	// by the caller of this function, therefore pick will try
	// not to return it
	req->ns = nameserver_pick();
	if (req->ns == last_ns) {
		// ... but pick did return it
		// not a lot of point in trying again with the
		// same server
		return 1;
	}

	req->reissue_count++;
	req->tx_count = 0;
	req->transmit_me = 1;

	return 0;
}

// this function looks for space on the inflight queue and promotes
// requests from the waiting queue if it can.
static void
eventdns_requests_pump_waiting_queue() {
	while (global_requests_inflight < global_max_requests_inflight &&
	    global_requests_waiting) {
		// move a request from the waiting queue to the inflight queue
		assert(req_waiting_head);
		struct request *req;
		if (req_waiting_head->next == req_waiting_head) {
			// only one item in the queue
			req = req_waiting_head;
			req_waiting_head = NULL;
		} else {
			req = req_waiting_head;
			req->next->prev = req->prev;
			req->prev->next = req->next;
			req_waiting_head = req->next;
		}

		global_requests_waiting--;
		global_requests_inflight++;
		
		req->ns = nameserver_pick();
		request_trans_id_set(req, transaction_id_pick());

		eventdns_request_insert(req, &req_head);
		eventdns_request_transmit(req);
		eventdns_transmit();
	}
}

// this processes a parsed reply packet
static void
reply_handle(u16 trans_id, u16 flags, u32 ttl, u32 addrcount, u32 *addresses) {
	int error;
	static const int error_codes[] = {DNS_ERR_FORMAT, DNS_ERR_SERVERFAILED, DNS_ERR_NOTEXIST, DNS_ERR_NOTIMPL, DNS_ERR_REFUSED};

	struct request *const req = request_find_from_trans_id(trans_id);
	if (!req) return;
	
	if (flags & 0x020f || !addrcount) {
		// there was an error
		if (flags & 0x0200) {
			error = DNS_ERR_TRUNCATED;
		} else {
			u16 error_code = (flags & 0x000f) - 1;
			if (error_code > 4) {
				error = DNS_ERR_UNKNOWN;
			} else {
				error = error_codes[error_code];
			}
		}

		switch(error) {
		case DNS_ERR_SERVERFAILED:
		case DNS_ERR_NOTIMPL:
		case DNS_ERR_REFUSED:
			// we regard these errors as marking a bad nameserver
			if (req->reissue_count < global_max_reissues) {
				nameserver_failed(req->ns);
				if (!request_reissue(req)) return;
			}
			break;
		default:
			// we got a good reply from the nameserver
			nameserver_up(req->ns);
		}

		if (req->search_state) {
			// if we have a list of domains to search in, try the next one
			if (!search_try_next(req)) {
				// a new request was issued so this request is finished and
				// the user callback will be made when that request (or a
				// child of it) finishes.
				request_finished(req, &req_head);
				return;
			}
		}
		
		// all else failed. Pass the failure up
		req->user_callback(error, 0, 0, 0, NULL, req->user_pointer);
		request_finished(req, &req_head);
	} else {
		// all ok, tell the user
		req->user_callback(DNS_ERR_NONE, DNS_IPv4_A, addrcount, ttl, addresses, req->user_pointer);
		nameserver_up(req->ns);
		request_finished(req, &req_head);
	}
}

// parses a raw packet from the wire
static void
reply_parse(u8 *packet, int length) {
	int j = 0;  // index into packet
	u16 _t;  // used by the macros
	u32 _t32;  // used by the macros

#define GET32(x) do { if (j + 4 > length) return; memcpy(&_t32, packet + j, 4); j += 4; x = ntohl(_t32); } while(0);
#define GET16(x) do { if (j + 2 > length) return; memcpy(&_t, packet + j, 2); j += 2; x = ntohs(_t); } while(0);
#define GET8(x) do { if (j >= length) return; x = packet[j++]; } while(0);
	u16 trans_id, flags, questions, answers, authority, additional, datalength;
	u32 ttl, ttl_r = 0xffffffff;
	u32 addresses[MAX_ADDRS];
	int addresses_done = 0;

	GET16(trans_id);
	GET16(flags);
	GET16(questions);
	GET16(answers);
	GET16(authority);
	GET16(additional);

	if (!(flags & 0x8000)) return;  // must be an answer
	if (flags & 0x020f) {
		// there was an error
		reply_handle(trans_id, flags, 0, 0, NULL);
		return;
	}
	// if (!answers) return;  // must have an answer of some form
	
	// This macro skips a name in the DNS reply. Normally the 
	// names are a series of length prefixed strings terminated with
	// a length of 0 (the lengths are u8's < 63).
	// However, the length can start with a pair of 1 bits and that
	// means that the next 14 bits are a pointer within the current
	// packet. The name stops after a pointer like that.
#define SKIP_NAME \
	for(;;) { \
		u8 label_len; \
		GET8(label_len); \
		if (!label_len) break; \
		if (label_len & 0xc0) { \
			GET8(label_len); \
			break; \
		} \
		if (label_len > 63) return; \
		j += label_len; \
	}

	uint i;
	// skip over each question in the reply
	for (i = 0; i < questions; ++i) {
		// the question looks like
		//   <label:name><u16:type><u16:class>
		SKIP_NAME;
		j += 4;
	}

	// now we have the answer section which looks like
	// <label:name><u16:type><u16:class><u32:ttl><u16:len><data...>
	
	for (i = 0; i < answers; ++i) {
		u16 type, class;

		SKIP_NAME;
		GET16(type);
		GET16(class);
		GET32(ttl);
		GET16(datalength);

		if (type == TYPE_A && class == CLASS_INET) {
			const uint addrcount = datalength >> 2;  // each IP address is 4 bytes
			const uint addrtocopy = MIN(MAX_ADDRS - addresses_done, addrcount);

			ttl_r = MIN(ttl_r, ttl);
			// we only bother with the first four addresses.
			if (j + 4*addrtocopy > length) return;
			memcpy(&addresses[addresses_done], packet + j, 4*addrtocopy);
			j += 4*addrtocopy;
			addresses_done += addrtocopy;
			if (addresses_done == MAX_ADDRS) break;
		} else {
			// skip over any other type of resource
			j += datalength;
		}
	}

	reply_handle(trans_id, flags, ttl_r, addresses_done, addresses);
#undef SKIP_NAME
#undef GET32
#undef GET16
#undef GET8
}

// Try to choose a strong transaction id which isn't already in flight
static u16
transaction_id_pick() {
	for (;;) {
#ifdef DNS_USE_CPU_CLOCK_FOR_ID
		struct timespec ts;
		if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts)) abort();
		const u16 trans_id = ts.tv_nsec & 0xffff;
#endif

#ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
		struct timeval tv;
		gettimeofday(&tv, NULL);
		const u16 trans_id = tv.tv_usec & 0xffff;
#endif

#ifdef DNS_USE_OPENSSL_FOR_ID
		u16 trans_id;
		if (RAND_pseudo_bytes((u8 *) &trans_id, 2) == -1) {
			/* // in the case that the RAND call fails we back
			// down to using gettimeofday.
			struct timeval tv;
			gettimeofday(&tv, NULL);
			trans_id = tv.tv_usec & 0xffff; */
			abort();
		}
#endif

		if (trans_id == 0xffff) continue;
		// now check to see if that id is already inflight
		const struct request *req = req_head, *const started_at = req_head;
		if (req) {
			do {
				if (req->trans_id == trans_id) break;
				req = req->next;
			} while (req != started_at);
		}
		// we didn't find it, so this is a good id
		if (req == started_at) return trans_id;
	}
}

// choose a namesever to use. This function will try to ignore
// nameservers which we think are down and load balence across the rest
// by updating the server_head global each time.
static struct nameserver *
nameserver_pick() {
	if (!server_head) return NULL;

	// if we don't have any good nameservers then there's no
	// point in trying to find one.
	if (!global_good_nameservers) {
		server_head = server_head->next;
		return server_head;
	}
	
	// remember that nameservers are in a circular list
	struct nameserver *started_at = server_head, *picked;
	for (;;) {
		if (server_head->state) {
			// we think this server is currently good
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}

		server_head = server_head->next;
		if (server_head == started_at) {
			// all the nameservers seem to be down
			// so we just return this one and hope for the
			// best
			assert(global_good_nameservers == 0);
			picked = server_head;
			server_head = server_head->next;
			return picked;
		}
	}
}

// this is called when a namesever socket is ready for reading
static void
nameserver_read(struct nameserver *ns) {
	u8 packet[1500];

	for (;;) {
          	const int r = recv(ns->socket, packet, sizeof(packet), 0);
		if (r < 0) {
			if (errno == EAGAIN) return;
			nameserver_failed(ns);
			return;
		}
		reply_parse(packet, r);
	}
}

// set if we are waiting for the ability to write to this server.
// if waiting is true then we ask libevent for EV_WRITE events, otherwise
// we stop these events.
static void
nameserver_write_waiting(struct nameserver *ns, char waiting) {
	if (ns->write_waiting == waiting) return;
	
	ns->write_waiting = waiting;
	event_del(&ns->event);
	event_set(&ns->event, ns->socket, EV_READ | (waiting ? EV_WRITE : 0) | EV_PERSIST,
			nameserver_ready_callback, ns);
	event_add(&ns->event, NULL);
}

// a callback function. Called by libevent when the kernel says that
// a nameserver socket is ready for writing or reading
static void
nameserver_ready_callback(int fd, short events, void *arg) {
	struct nameserver *ns = (struct nameserver *) arg;
	
	if (events & EV_WRITE) {
		ns->choaked = 0;
		if (!eventdns_transmit()) {
			nameserver_write_waiting(ns, 0);
		}
	}
	if (events & EV_READ) {
		nameserver_read(ns);
	}
}

// Converts a string to a length-prefixed set of DNS labels.
// @buf must be strlen(name)+2 or longer. name and buf must
// not overlap. name_len should be the length of name
//
// Input: abc.def
// Output: <3>abc<3>def<0>
//
// Returns the length of the data. negative on error
//   -1  label was > 63 bytes
//   -2  name was > 255 bytes
static int
dnsname_to_labels(u8 *const buf, const char *name, const int name_len) { \
	const char *end = name + name_len; \
	int j = 0;  // current offset into buf

	if (name_len > 255) return -2;
	
	for (;;) {
		const char *const start = name;
		name = strchr(name, '.');
		if (!name) {
			const uint label_len = end - start;
			if (label_len > 63) return -1;
			buf[j++] = label_len;

			memcpy(buf + j, start, end - start);
			j += end - start;
			break;
		} else {
			// append length of the label.
			const uint label_len = name - start;
			if (label_len > 63) return -1;
			buf[j++] = label_len;

			memcpy(buf + j, start, name - start);
			j += name - start;
			// hop over the '.'
			name++;
		}
	}

	// the labels must be terminated by a 0.
	// It's possible that the name ended in a .
	// in which case the zero is already there
	if (!j || buf[j-1]) buf[j++] = 0;
	return j;
}

// Finds the length of a dns request for a DNS name of the given
// length. The actual request may be smaller than the value returned
// here
static int
eventdns_request_len(const int name_len) {
	return 96 + // length of the DNS standard header
		name_len + 2 + 
		4;  // space for the resource type
}

// build a dns request packet into buf. buf should be at least as long
// as eventdns_request_len told you it should be.
//
// Returns the amount of space used. Negative on error.
static int
eventdns_request_data_build(const char *const name, const int name_len, const u16 trans_id,
		const u16 type, const u16 class,
		u8 *const buf) {
	int j = 0;  // current offset into buf
	u16 _t;  // used by the macros
	
#define APPEND16(x) do { _t = htons(x); memcpy(buf + j, &_t, 2); j += 2; } while(0);
	APPEND16(trans_id);
	APPEND16(0x0100);  // standard query, recusion needed
	APPEND16(1);  // one question
	APPEND16(0);  // no answers
	APPEND16(0);  // no authority
	APPEND16(0);  // no additional

	u8 *labels = (u8 *) alloca(name_len + 2);
	const int labels_len = dnsname_to_labels(labels, name, name_len);
	if (labels_len < 0) return labels_len;
	memcpy(buf + j, labels, labels_len);
	j += labels_len;

	APPEND16(type);
	APPEND16(class);
#undef APPEND16

	return j;
}

// this is a libevent callback function which is called when a request
// has timed out.
static void
eventdns_request_timeout_callback(int fd, short events, void *arg) {
	struct request *const req = (struct request *) arg;
	
	log("Request %x timed out\n", (unsigned) arg);

	req->ns->timedout++;
	if (req->ns->timedout > global_max_nameserver_timeout) {
		nameserver_failed(req->ns);
	}

	evtimer_del(&req->timeout_event);
	if (req->tx_count >= global_max_retransmits) {
		// this request has failed
		req->user_callback(DNS_ERR_TIMEOUT, 0, 0, 0, NULL, req->user_pointer);
		request_finished(req, &req_head);
	} else {
		// retransmit it
		eventdns_request_transmit(req);
	}
}

// try to send a request to a given server.
//
// return:
//   0 ok
//   1 temporary failure
//   2 other failure
static int
eventdns_request_transmit_to(struct request *req, struct nameserver *server) {
	const int r = send(server->socket, req->request, req->request_len, 0);
	if (r < 0) {
		if (errno == EAGAIN) return 1;
		return 2;
	} else if (r != req->request_len) {
		return 1;  // short write
	} else {
		return 0;
	}
}

// try to send a request, updating the fields of the request
// as needed
//
// return:
//   0 ok
//   1 failed
static int
eventdns_request_transmit(struct request *req) {
	int retcode = 0;
	
	// if we fail to send this packet then this flag marks it
	// for eventdns_transmit
	req->transmit_me = 1;
	if (req->trans_id == 0xffff) abort();

	if (req->ns->choaked) {
		// don't bother trying to write to a socket
		// which we have had EAGAIN from
		return 1;
	}

	const int r = eventdns_request_transmit_to(req, req->ns);
	switch (r) {
	case 1:
		// temp failure
		req->ns->choaked = 1;
		nameserver_write_waiting(req->ns, 1);
		return 1;
	case 2:
		// failed in some other way
		nameserver_failed(req->ns);
		retcode = 1;
		// fall through
	default:
For faster browsing, not all history is shown. View entire blame