src/or/dns.c

   1 /* Copyright 2003 Roger Dingledine. */
   2 /* See LICENSE for licensing information */
   3 /* $Id$ */
   4
   5 /*****
   6  * dns.c: Resolve hostnames in separate processes.
   7  *****/
   8
   9 /* See http://elvin.dstc.com/ListArchive/elvin-dev/archive/2001/09/msg00027.html
  10  * for some approaches to asynchronous dns. We will want to switch once one of
  11  * them becomes more commonly available.
  12  */
  13
  14 #include "or.h"
  15 #include "tree.h"
  16
  17 extern or_options_t options; /* command-line and config-file options */
  18
  19 /* Longest hostname we're willing to resolve. */
  20 #define MAX_ADDRESSLEN 256
  21
  22 /* Maximum DNS processes to spawn. */
  23 #define MAX_DNSWORKERS 50
  24 /* Minimum DNS processes to spawn. */
  25 #define MIN_DNSWORKERS 3
  26
  27 /* If more than this many processes are idle, shut down the extras. */
  28 #define MAX_IDLE_DNSWORKERS 10
  29
  30 /* Possible outcomes from hostname lookup: permanent failure,
  31  * transient (retryable) failure, and success */
  32 #define DNS_RESOLVE_FAILED_TRANSIENT 1
  33 #define DNS_RESOLVE_FAILED_PERMANENT 2
  34 #define DNS_RESOLVE_SUCCEEDED 3
  35
  36 int num_dnsworkers=0;
  37 int num_dnsworkers_busy=0;
  38
  39 /* Linked list of connections waiting for a DNS answer. */
  40 struct pending_connection_t {
  41   struct connection_t *conn;
  42   struct pending_connection_t *next;
  43 };
  44
  45 /* A DNS request: possibly completed, possibly pending; cached_resolve
  46  * structs are stored at the OR side in a splay tree, and as a linked
  47  * list from oldest to newest.
  48  */
  49 struct cached_resolve {
  50   SPLAY_ENTRY(cached_resolve) node;
  51   char address[MAX_ADDRESSLEN]; /* the hostname to be resolved */
  52   uint32_t addr; /* in host order. I know I'm horrible for assuming ipv4 */
  53   char state; /* 0 is pending; 1 means answer is valid; 2 means resolve failed */
  54 #define CACHE_STATE_PENDING 0
  55 #define CACHE_STATE_VALID 1
  56 #define CACHE_STATE_FAILED 2
  57   uint32_t expire; /* remove items from cache after this time */
  58   struct pending_connection_t *pending_connections;
  59   struct cached_resolve *next;
  60 };
  61
  62 static void purge_expired_resolves(uint32_t now);
  63 static int assign_to_dnsworker(connection_t *exitconn);
  64 static void dns_purge_resolve(struct cached_resolve *resolve);
  65 static void dns_found_answer(char *address, uint32_t addr, char outcome);
  66 int dnsworker_main(void *data);
  67 static int spawn_dnsworker(void);
  68 static void spawn_enough_dnsworkers(void);
  69
  70 /* Splay tree of cached_resolve objects */
  71 static SPLAY_HEAD(cache_tree, cached_resolve) cache_root;
  72
  73 /* Function to compare hashed resolves on their addresses; used to
  74  * implement splay trees. */
  75 static int compare_cached_resolves(struct cached_resolve *a,
  76                                    struct cached_resolve *b) {
  77   /* make this smarter one day? */
  78   return strncasecmp(a->address, b->address, MAX_ADDRESSLEN);
  79 }
  80
  81 SPLAY_PROTOTYPE(cache_tree, cached_resolve, node, compare_cached_resolves);
  82 SPLAY_GENERATE(cache_tree, cached_resolve, node, compare_cached_resolves);
  83
  84 /* Initialize the DNS cache */
  85 static void init_cache_tree(void) {
  86   SPLAY_INIT(&cache_root);
  87 }
  88
  89 /* Initialize the DNS subsystem; called by the OR process. */
  90 void dns_init(void) {
  91   init_cache_tree();
  92   spawn_enough_dnsworkers();
  93 }
  94
  95 static struct cached_resolve *oldest_cached_resolve = NULL; /* linked list, */
  96 static struct cached_resolve *newest_cached_resolve = NULL; /* oldest to newest */
  97
  98 /* Remove every cached_resolve whose 'expire' time is before 'now'
  99  * from the cache. */
 100 static void purge_expired_resolves(uint32_t now) {
 101   struct cached_resolve *resolve;
 102
 103   /* this is fast because the linked list
 104    * oldest_cached_resolve is ordered by when they came in.
 105    */
 106   while(oldest_cached_resolve && (oldest_cached_resolve->expire < now)) {
 107     resolve = oldest_cached_resolve;
 108     log(LOG_DEBUG,"Forgetting old cached resolve (expires %lu)", (unsigned long)resolve->expire);
 109     if(resolve->state == CACHE_STATE_PENDING) {
 110       log_fn(LOG_WARN,"Expiring a dns resolve that's still pending. Forgot to cull it?");
 111       /* XXX if resolve->pending_connections is used, then we're probably
 112        * introducing bugs by closing resolve without notifying those streams.
 113        */
 114     }
 115     oldest_cached_resolve = resolve->next;
 116     if(!oldest_cached_resolve) /* if there are no more, */
 117       newest_cached_resolve = NULL; /* then make sure the list's tail knows that too */
 118     SPLAY_REMOVE(cache_tree, &cache_root, resolve);
 119     tor_free(resolve);
 120   }
 121 }
 122
 123 /* See if we have a cache entry for 'exitconn->address'. if so,
 124  * if resolve valid, put it into exitconn->addr and return 1.
 125  * If resolve failed, return -1.
 126  *
 127  * Else, if seen before and pending, add conn to the pending list,
 128  * and return 0.
 129  *
 130  * Else, if not seen before, add conn to pending list, hand to
 131  * dns farm, and return 0.
 132  */
 133 int dns_resolve(connection_t *exitconn) {
 134   struct cached_resolve *resolve;
 135   struct cached_resolve search;
 136   struct pending_connection_t *pending_connection;
 137   struct in_addr in;
 138   uint32_t now = time(NULL);
 139   assert_connection_ok(exitconn, 0);
 140
 141   /* first check if exitconn->address is an IP. If so, we already
 142    * know the answer. */
 143   if (tor_inet_aton(exitconn->address, &in) != 0) {
 144     exitconn->addr = ntohl(in.s_addr);
 145     return 1;
 146   }
 147
 148   /* then take this opportunity to see if there are any expired
 149    * resolves in the tree. */
 150   purge_expired_resolves(now);
 151
 152   /* now check the tree to see if 'address' is already there. */
 153   strncpy(search.address, exitconn->address, MAX_ADDRESSLEN);
 154   search.address[MAX_ADDRESSLEN-1] = 0;
 155   resolve = SPLAY_FIND(cache_tree, &cache_root, &search);
 156   if(resolve) { /* already there */
 157     switch(resolve->state) {
 158       case CACHE_STATE_PENDING:
 159         /* add us to the pending list */
 160         pending_connection = tor_malloc(sizeof(struct pending_connection_t));
 161         pending_connection->conn = exitconn;
 162         pending_connection->next = resolve->pending_connections;
 163         resolve->pending_connections = pending_connection;
 164         log_fn(LOG_DEBUG,"Connection (fd %d) waiting for pending DNS resolve of '%s'",
 165                exitconn->s, exitconn->address);
 166         exitconn->state = EXIT_CONN_STATE_RESOLVING;
 167         return 0;
 168       case CACHE_STATE_VALID:
 169         exitconn->addr = resolve->addr;
 170         log_fn(LOG_DEBUG,"Connection (fd %d) found cached answer for '%s'",
 171                exitconn->s, exitconn->address);
 172         return 1;
 173       case CACHE_STATE_FAILED:
 174         return -1;
 175     }
 176     tor_assert(0);
 177   }
 178   /* not there, need to add it */
 179   resolve = tor_malloc_zero(sizeof(struct cached_resolve));
 180   resolve->state = CACHE_STATE_PENDING;
 181   resolve->expire = now + MAX_DNS_ENTRY_AGE;
 182   strncpy(resolve->address, exitconn->address, MAX_ADDRESSLEN);
 183   resolve->address[MAX_ADDRESSLEN-1] = 0;
 184
 185   /* add us to the pending list */
 186   pending_connection = tor_malloc(sizeof(struct pending_connection_t));
 187   pending_connection->conn = exitconn;
 188   pending_connection->next = NULL;
 189   resolve->pending_connections = pending_connection;
 190   exitconn->state = EXIT_CONN_STATE_RESOLVING;
 191
 192   /* add us to the linked list of resolves */
 193   if (!oldest_cached_resolve) {
 194     oldest_cached_resolve = resolve;
 195   } else {
 196     newest_cached_resolve->next = resolve;
 197   }
 198   newest_cached_resolve = resolve;
 199
 200   SPLAY_INSERT(cache_tree, &cache_root, resolve);
 201   return assign_to_dnsworker(exitconn);
 202 }
 203
 204 /* Find or spawn a dns worker process to handle resolving
 205  * exitconn->address; tell that dns worker to begin resolving.
 206  */
 207 static int assign_to_dnsworker(connection_t *exitconn) {
 208   connection_t *dnsconn;
 209   unsigned char len;
 210
 211   tor_assert(exitconn->state == EXIT_CONN_STATE_RESOLVING);
 212
 213   spawn_enough_dnsworkers(); /* respawn here, to be sure there are enough */
 214
 215   dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER, DNSWORKER_STATE_IDLE);
 216
 217   if(!dnsconn) {
 218     log_fn(LOG_WARN,"no idle dns workers. Failing.");
 219     dns_cancel_pending_resolve(exitconn->address);
 220     return -1;
 221   }
 222
 223   log_fn(LOG_DEBUG, "Connection (fd %d) needs to resolve '%s'; assigning to DNSWorker (fd %d)",
 224          exitconn->s, exitconn->address, dnsconn->s);
 225
 226   tor_free(dnsconn->address);
 227   dnsconn->address = tor_strdup(exitconn->address);
 228   dnsconn->state = DNSWORKER_STATE_BUSY;
 229   num_dnsworkers_busy++;
 230
 231   len = strlen(dnsconn->address);
 232   connection_write_to_buf(&len, 1, dnsconn);
 233   connection_write_to_buf(dnsconn->address, len, dnsconn);
 234
 235 //  log_fn(LOG_DEBUG,"submitted '%s'", exitconn->address);
 236   return 0;
 237 }
 238
 239 /* Remove 'conn' from the list of connections waiting for conn->address.
 240  */
 241 void connection_dns_remove(connection_t *conn)
 242 {
 243   struct pending_connection_t *pend, *victim;
 244   struct cached_resolve search;
 245   struct cached_resolve *resolve;
 246
 247   strncpy(search.address, conn->address, MAX_ADDRESSLEN);
 248   search.address[MAX_ADDRESSLEN-1] = 0;
 249
 250   resolve = SPLAY_FIND(cache_tree, &cache_root, &search);
 251   if(!resolve) {
 252     log_fn(LOG_WARN,"Address '%s' is not pending. Dropping.", conn->address);
 253     return;
 254   }
 255
 256   tor_assert(resolve->pending_connections);
 257   assert_connection_ok(conn,0);
 258
 259   pend = resolve->pending_connections;
 260
 261   if(pend->conn == conn) {
 262     resolve->pending_connections = pend->next;
 263     tor_free(pend);
 264     log_fn(LOG_DEBUG, "First connection (fd %d) no longer waiting for resolve of '%s'",
 265            conn->s, conn->address);
 266     return;
 267   } else {
 268     for( ; pend->next; pend = pend->next) {
 269       if(pend->next->conn == conn) {
 270         victim = pend->next;
 271         pend->next = victim->next;
 272         tor_free(victim);
 273         log_fn(LOG_DEBUG, "Connection (fd %d) no longer waiting for resolve of '%s'",
 274                conn->s, conn->address);
 275         return; /* more are pending */
 276       }
 277     }
 278     tor_assert(0); /* not reachable unless onlyconn not in pending list */
 279   }
 280 }
 281
 282 /* Log an error and abort if conn is waiting for a DNS resolve.
 283  */
 284 void assert_connection_edge_not_dns_pending(connection_t *conn) {
 285   struct pending_connection_t *pend;
 286   struct cached_resolve *resolve;
 287
 288   SPLAY_FOREACH(resolve, cache_tree, &cache_root) {
 289     for(pend = resolve->pending_connections;
 290         pend;
 291         pend = pend->next) {
 292       tor_assert(pend->conn != conn);
 293     }
 294   }
 295 }
 296
 297 /* Log an error and abort if any connection waiting for a DNS resolve is
 298  * corrupted. */
 299 void assert_all_pending_dns_resolves_ok(void) {
 300   struct pending_connection_t *pend;
 301   struct cached_resolve *resolve;
 302
 303   SPLAY_FOREACH(resolve, cache_tree, &cache_root) {
 304     for(pend = resolve->pending_connections;
 305         pend;
 306         pend = pend->next) {
 307       assert_connection_ok(pend->conn, 0);
 308     }
 309   }
 310 }
 311
 312 /* Mark all connections waiting for 'address' for close.  Then cancel
 313  * the resolve for 'address' itself, and remove any cached results for
 314  * 'address' from the cache.
 315  */
 316 void dns_cancel_pending_resolve(char *address) {
 317   struct pending_connection_t *pend;
 318   struct cached_resolve search;
 319   struct cached_resolve *resolve;
 320   connection_t *pendconn;
 321
 322   strncpy(search.address, address, MAX_ADDRESSLEN);
 323   search.address[MAX_ADDRESSLEN-1] = 0;
 324
 325   resolve = SPLAY_FIND(cache_tree, &cache_root, &search);
 326   if(!resolve) {
 327     log_fn(LOG_WARN,"Address '%s' is not pending. Dropping.", address);
 328     return;
 329   }
 330
 331   tor_assert(resolve->pending_connections);
 332
 333   /* mark all pending connections to fail */
 334   log_fn(LOG_DEBUG, "Failing all connections waiting on DNS resolve of '%s'",
 335          address);
 336   while(resolve->pending_connections) {
 337     pend = resolve->pending_connections;
 338     /* So that mark_for_close doesn't double-remove the connection. */
 339     pend->conn->state = EXIT_CONN_STATE_RESOLVEFAILED;
 340     pendconn = pend->conn; /* don't pass complex things to the
 341                               connection_mark_for_close macro */
 342     connection_mark_for_close(pendconn, END_STREAM_REASON_MISC);
 343     resolve->pending_connections = pend->next;
 344     tor_free(pend);
 345   }
 346
 347   dns_purge_resolve(resolve);
 348 }
 349
 350 /* Remove 'resolve' from the cache.
 351  */
 352 static void dns_purge_resolve(struct cached_resolve *resolve) {
 353   struct cached_resolve *tmp;
 354
 355   /* remove resolve from the linked list */
 356   if(resolve == oldest_cached_resolve) {
 357     oldest_cached_resolve = resolve->next;
 358     if(oldest_cached_resolve == NULL)
 359       newest_cached_resolve = NULL;
 360   } else {
 361     /* FFFF make it a doubly linked list if this becomes too slow */
 362     for(tmp=oldest_cached_resolve; tmp && tmp->next != resolve; tmp=tmp->next) ;
 363     tor_assert(tmp); /* it's got to be in the list, or we screwed up somewhere else */
 364     tmp->next = resolve->next; /* unlink it */
 365
 366     if(newest_cached_resolve == resolve)
 367       newest_cached_resolve = tmp;
 368   }
 369
 370   /* remove resolve from the tree */
 371   SPLAY_REMOVE(cache_tree, &cache_root, resolve);
 372
 373   tor_free(resolve);
 374 }
 375
 376 /* Called on the OR side when a DNS worker tells us the outcome of a DNS
 377  * resolve: tell all pending connections about the result of the lookup, and
 378  * cache the value.  ('address' is a NUL-terminated string containing the
 379  * address to look up; 'addr' is an IPv4 address in host order; 'outcome' is
 380  * one of DNS_RESOLVE_{FAILED_TRANSIENT|FAILED_PERMANENT|SUCCEEDED}.
 381  */
 382 static void dns_found_answer(char *address, uint32_t addr, char outcome) {
 383   struct pending_connection_t *pend;
 384   struct cached_resolve search;
 385   struct cached_resolve *resolve;
 386   connection_t *pendconn;
 387
 388   strncpy(search.address, address, MAX_ADDRESSLEN);
 389   search.address[MAX_ADDRESSLEN-1] = 0;
 390
 391   resolve = SPLAY_FIND(cache_tree, &cache_root, &search);
 392   if(!resolve) {
 393     log_fn(LOG_INFO,"Resolved unasked address '%s'? Dropping.", address);
 394     /* XXX Why drop?  Just because we don't care now doesn't mean we shouldn't
 395      * XXX cache the result for later. */
 396     return;
 397   }
 398
 399   if (resolve->state != CACHE_STATE_PENDING) {
 400     /* XXXX Maybe update addr? or check addr for consistency? Or let
 401      * VALID replace FAILED? */
 402     log_fn(LOG_WARN, "Resolved '%s' which was already resolved; ignoring",
 403            address);
 404     tor_assert(resolve->pending_connections == NULL);
 405     return;
 406   }
 407   /* Removed this assertion: in fact, we'll sometimes get a double answer
 408    * to the same question.  This can happen when we ask one worker to resolve
 409    * X.Y.Z., then we cancel the request, and then we ask another worker to
 410    * resolve X.Y.Z. */
 411   /* tor_assert(resolve->state == CACHE_STATE_PENDING); */
 412
 413   resolve->addr = ntohl(addr);
 414   if(outcome == DNS_RESOLVE_SUCCEEDED)
 415     resolve->state = CACHE_STATE_VALID;
 416   else
 417     resolve->state = CACHE_STATE_FAILED;
 418
 419   while(resolve->pending_connections) {
 420     pend = resolve->pending_connections;
 421     assert_connection_ok(pend->conn,time(NULL));
 422     pend->conn->addr = resolve->addr;
 423
 424
 425     if(resolve->state == CACHE_STATE_FAILED) {
 426       pendconn = pend->conn; /* don't pass complex things to the
 427                                 connection_mark_for_close macro */
 428       /* prevent double-remove. */
 429       pend->conn->state = EXIT_CONN_STATE_RESOLVEFAILED;
 430       connection_mark_for_close(pendconn, END_STREAM_REASON_RESOLVEFAILED);
 431     } else {
 432       /* prevent double-remove. */
 433       pend->conn->state = EXIT_CONN_STATE_CONNECTING;
 434       connection_exit_connect(pend->conn);
 435     }
 436     resolve->pending_connections = pend->next;
 437     tor_free(pend);
 438   }
 439
 440   if(outcome == DNS_RESOLVE_FAILED_TRANSIENT) { /* remove from cache */
 441     dns_purge_resolve(resolve);
 442   }
 443 }
 444
 445 /******************************************************************/
 446
 447 /*****
 448  * Connection between OR and dnsworker
 449  *****/
 450
 451 /* Write handler: called when we've pushed a request to a dnsworker. */
 452 int connection_dns_finished_flushing(connection_t *conn) {
 453   tor_assert(conn && conn->type == CONN_TYPE_DNSWORKER);
 454   connection_stop_writing(conn);
 455   return 0;
 456 }
 457
 458 /* Read handler: called when we get data from a dnsworker.  If the
 459  * connection is closed, mark the dnsworker as dead.  Otherwise, see
 460  * if we have a complete answer.  If so, call dns_found_answer on the
 461  * result.  If not, wait.  Returns 0. */
 462 int connection_dns_process_inbuf(connection_t *conn) {
 463   char success;
 464   uint32_t addr;
 465
 466   tor_assert(conn && conn->type == CONN_TYPE_DNSWORKER);
 467
 468   if(conn->inbuf_reached_eof) {
 469     log_fn(LOG_WARN,"Read eof. Worker died unexpectedly.");
 470     if(conn->state == DNSWORKER_STATE_BUSY) {
 471       dns_cancel_pending_resolve(conn->address);
 472       num_dnsworkers_busy--;
 473     }
 474     num_dnsworkers--;
 475     connection_mark_for_close(conn,0);
 476     return 0;
 477   }
 478
 479   tor_assert(conn->state == DNSWORKER_STATE_BUSY);
 480   if(buf_datalen(conn->inbuf) < 5) /* entire answer available? */
 481     return 0; /* not yet */
 482   tor_assert(buf_datalen(conn->inbuf) == 5);
 483
 484   connection_fetch_from_buf(&success,1,conn);
 485   connection_fetch_from_buf((char *)&addr,sizeof(uint32_t),conn);
 486
 487   log_fn(LOG_DEBUG, "DNSWorker (fd %d) returned answer for '%s'",
 488          conn->s, conn->address);
 489
 490   tor_assert(success >= DNS_RESOLVE_FAILED_TRANSIENT);
 491   tor_assert(success <= DNS_RESOLVE_SUCCEEDED);
 492   dns_found_answer(conn->address, addr, success);
 493
 494   tor_free(conn->address);
 495   conn->address = tor_strdup("<idle>");
 496   conn->state = DNSWORKER_STATE_IDLE;
 497   num_dnsworkers_busy--;
 498
 499   return 0;
 500 }
 501
 502 /* Implementation for DNS workers; this code runs in a separate
 503  * execution context.  It takes as its argument an fdarray as returned
 504  * by socketpair(), and communicates via fdarray[1].  The protocol is
 505  * as follows:
 506  *     The OR says:
 507  *           ADDRESSLEN [1 byte]
 508  *           ADDRESS    [ADDRESSLEN bytes]
 509  *     The DNS worker does the lookup, and replies:
 510  *           OUTCOME    [1 byte]
 511  *           IP         [4 bytes]
 512  *
 513  * OUTCOME is one of DNS_RESOLVE_{FAILED_TRANSIENT|FAILED_PERMANENT|SUCCEEDED}.
 514  * IP is in host order.
 515  *
 516  * The dnsworker runs indefinitely, until its connection is closed or an error
 517  * occurs.
 518  */
 519 int dnsworker_main(void *data) {
 520   char address[MAX_ADDRESSLEN];
 521   unsigned char address_len;
 522   char answer[5];
 523   uint32_t ip;
 524   int *fdarray = data;
 525   int fd;
 526
 527   tor_close_socket(fdarray[0]); /* this is the side of the socketpair the parent uses */
 528   fd = fdarray[1]; /* this side is ours */
 529 #ifndef MS_WINDOWS
 530   connection_free_all(); /* so the child doesn't hold the parent's fd's open */
 531 #endif
 532
 533   for(;;) {
 534
 535     if(recv(fd, &address_len, 1, 0) != 1) {
 536       log_fn(LOG_INFO,"dnsworker exiting because tor process died.");
 537       spawn_exit();
 538     }
 539     tor_assert(address_len > 0);
 540
 541     if(read_all(fd, address, address_len, 1) != address_len) {
 542       log_fn(LOG_ERR,"read hostname failed. Child exiting.");
 543       spawn_exit();
 544     }
 545     address[address_len] = 0; /* null terminate it */
 546
 547     switch (tor_lookup_hostname(address, &ip)) {
 548       case 1:
 549         log_fn(LOG_INFO,"Could not resolve dest addr %s (transient).",address);
 550         answer[0] = DNS_RESOLVE_FAILED_TRANSIENT;
 551         break;
 552       case -1:
 553         log_fn(LOG_INFO,"Could not resolve dest addr %s (permanent).",address);
 554         answer[0] = DNS_RESOLVE_FAILED_PERMANENT;
 555         break;
 556       case 0:
 557         log_fn(LOG_INFO,"Resolved address '%s'.",address);
 558         answer[0] = DNS_RESOLVE_SUCCEEDED;
 559         break;
 560     }
 561     set_uint32(answer+1, ip);
 562     if(write_all(fd, answer, 5, 1) != 5) {
 563       log_fn(LOG_ERR,"writing answer failed. Child exiting.");
 564       spawn_exit();
 565     }
 566   }
 567   return 0; /* windows wants this function to return an int */
 568 }
 569
 570 /* Launch a new DNS worker; return 0 on success, -1 on failure.
 571  */
 572 static int spawn_dnsworker(void) {
 573   int fd[2];
 574   connection_t *conn;
 575
 576   if(tor_socketpair(AF_UNIX, SOCK_STREAM, 0, fd) < 0) {
 577     log(LOG_ERR, "Couldn't construct socketpair: %s",
 578         tor_socket_strerror(tor_socket_errno(-1)));
 579     exit(1);
 580   }
 581
 582   spawn_func(dnsworker_main, (void*)fd);
 583   log_fn(LOG_DEBUG,"just spawned a worker.");
 584   tor_close_socket(fd[1]); /* we don't need the worker's side of the pipe */
 585
 586   conn = connection_new(CONN_TYPE_DNSWORKER);
 587
 588   set_socket_nonblocking(fd[0]);
 589
 590   /* set up conn so it's got all the data we need to remember */
 591   conn->s = fd[0];
 592   conn->address = tor_strdup("<unused>");
 593
 594   if(connection_add(conn) < 0) { /* no space, forget it */
 595     log_fn(LOG_WARN,"connection_add failed. Giving up.");
 596     connection_free(conn); /* this closes fd[0] */
 597     return -1;
 598   }
 599
 600   conn->state = DNSWORKER_STATE_IDLE;
 601   connection_start_reading(conn);
 602
 603   return 0; /* success */
 604 }
 605
 606 /* If we have too many or too few DNS workers, spawn or kill some.
 607  */
 608 static void spawn_enough_dnsworkers(void) {
 609   int num_dnsworkers_needed; /* aim to have 1 more than needed,
 610                            * but no less than min and no more than max */
 611   connection_t *dnsconn;
 612
 613   /* XXX This may not be the best strategy. Maybe we should queue pending
 614    *     requests until the old ones finish or time out: otherwise, if
 615    *     the connection requests come fast enough, we never get any DNS done. -NM
 616    * XXX But if we queue them, then the adversary can pile even more
 617    *     queries onto us, blocking legitimate requests for even longer.
 618    *     Maybe we should compromise and only kill if it's been at it for
 619    *     more than, e.g., 2 seconds. -RD
 620    */
 621   if(num_dnsworkers_busy == MAX_DNSWORKERS) {
 622     /* We always want at least one worker idle.
 623      * So find the oldest busy worker and kill it.
 624      */
 625     dnsconn = connection_get_by_type_state_lastwritten(CONN_TYPE_DNSWORKER,
 626                                                        DNSWORKER_STATE_BUSY);
 627     tor_assert(dnsconn);
 628
 629     log_fn(LOG_WARN, "%d DNS workers are spawned; all are busy. Killing one.",
 630            MAX_DNSWORKERS);
 631
 632     connection_mark_for_close(dnsconn,0);
 633     num_dnsworkers_busy--;
 634     num_dnsworkers--;
 635   }
 636
 637   if(num_dnsworkers_busy >= MIN_DNSWORKERS)
 638     num_dnsworkers_needed = num_dnsworkers_busy+1;
 639   else
 640     num_dnsworkers_needed = MIN_DNSWORKERS;
 641
 642   while(num_dnsworkers < num_dnsworkers_needed) {
 643     if(spawn_dnsworker() < 0) {
 644       log(LOG_WARN,"spawn_enough_dnsworkers(): spawn failed!");
 645       return;
 646     }
 647     num_dnsworkers++;
 648   }
 649
 650   while(num_dnsworkers > num_dnsworkers_busy+MAX_IDLE_DNSWORKERS) { /* too many idle? */
 651     /* cull excess workers */
 652     log_fn(LOG_WARN,"%d of %d dnsworkers are idle. Killing one.",
 653            num_dnsworkers-num_dnsworkers_needed, num_dnsworkers);
 654     dnsconn = connection_get_by_type_state(CONN_TYPE_DNSWORKER, DNSWORKER_STATE_IDLE);
 655     tor_assert(dnsconn);
 656     connection_mark_for_close(dnsconn,0);
 657     num_dnsworkers--;
 658   }
 659 }
 660
 661 /*
 662   Local Variables:
 663   mode:c
 664   indent-tabs-mode:nil
 665   c-basic-offset:2
 666   End:
 667 */