resolv/res_hconf.c

   1 /* Copyright (C) 1993-2023 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 /* This file provides a Linux /etc/host.conf compatible front end to
  19    the various name resolvers (/etc/hosts, named, NIS server, etc.).
  20    Though mostly compatibly, the following differences exist compared
  21    to the original implementation:
  22
  23         - line comments can appear anywhere (not just at the beginning of
  24           a line)
  25 */
  26
  27 #include <assert.h>
  28 #include <errno.h>
  29 #include <ctype.h>
  30 #include <libintl.h>
  31 #include <memory.h>
  32 #include <stdio.h>
  33 #include <stdio_ext.h>
  34 #include <stdlib.h>
  35 #include <string.h>
  36 #include <net/if.h>
  37 #include <sys/ioctl.h>
  38 #include <unistd.h>
  39 #include <netinet/in.h>
  40 #include <libc-lock.h>
  41 #include "ifreq.h"
  42 #include "res_hconf.h"
  43 #include <wchar.h>
  44 #include <atomic.h>
  45
  46 #if IS_IN (libc)
  47 # define fgets_unlocked __fgets_unlocked
  48 #endif
  49
  50 #define _PATH_HOSTCONF  "/etc/host.conf"
  51
  52 /* Environment vars that all user to override default behavior:  */
  53 #define ENV_HOSTCONF    "RESOLV_HOST_CONF"
  54 #define ENV_TRIM_OVERR  "RESOLV_OVERRIDE_TRIM_DOMAINS"
  55 #define ENV_TRIM_ADD    "RESOLV_ADD_TRIM_DOMAINS"
  56 #define ENV_MULTI       "RESOLV_MULTI"
  57 #define ENV_REORDER     "RESOLV_REORDER"
  58
  59 enum parse_cbs
  60   {
  61     CB_none,
  62     CB_arg_trimdomain_list,
  63     CB_arg_bool
  64   };
  65
  66 static const struct cmd
  67 {
  68   const char name[11];
  69   uint8_t cb;
  70   unsigned int arg;
  71 } cmd[] =
  72 {
  73   {"order",             CB_none,                0},
  74   {"trim",              CB_arg_trimdomain_list, 0},
  75   {"multi",             CB_arg_bool,            HCONF_FLAG_MULTI},
  76   {"reorder",           CB_arg_bool,            HCONF_FLAG_REORDER}
  77 };
  78
  79 /* Structure containing the state.  */
  80 struct hconf _res_hconf;
  81
  82 /* Skip white space.  */
  83 static const char *
  84 skip_ws (const char *str)
  85 {
  86   while (isspace (*str)) ++str;
  87   return str;
  88 }
  89
  90
  91 /* Skip until whitespace, comma, end of line, or comment character.  */
  92 static const char *
  93 skip_string (const char *str)
  94 {
  95   while (*str && !isspace (*str) && *str != '#' && *str != ',')
  96     ++str;
  97   return str;
  98 }
  99
 100
 101 static const char *
 102 arg_trimdomain_list (const char *fname, int line_num, const char *args)
 103 {
 104   const char * start;
 105   size_t len;
 106
 107   do
 108     {
 109       start = args;
 110       args = skip_string (args);
 111       len = args - start;
 112
 113       if (_res_hconf.num_trimdomains >= TRIMDOMAINS_MAX)
 114         {
 115           char *buf;
 116
 117           if (__asprintf (&buf, _("\
 118 %s: line %d: cannot specify more than %d trim domains"),
 119                           fname, line_num, TRIMDOMAINS_MAX) < 0)
 120             return 0;
 121
 122           __fxprintf (NULL, "%s", buf);
 123
 124           free (buf);
 125           return 0;
 126         }
 127       _res_hconf.trimdomain[_res_hconf.num_trimdomains++] =
 128         __strndup (start, len);
 129       args = skip_ws (args);
 130       switch (*args)
 131         {
 132         case ',': case ';': case ':':
 133           args = skip_ws (++args);
 134           if (!*args || *args == '#')
 135             {
 136               char *buf;
 137
 138               if (__asprintf (&buf, _("\
 139 %s: line %d: list delimiter not followed by domain"),
 140                               fname, line_num) < 0)
 141                 return 0;
 142
 143               __fxprintf (NULL, "%s", buf);
 144
 145               free (buf);
 146               return 0;
 147             }
 148         default:
 149           break;
 150         }
 151     }
 152   while (*args && *args != '#');
 153   return args;
 154 }
 155
 156
 157 static const char *
 158 arg_bool (const char *fname, int line_num, const char *args, unsigned flag)
 159 {
 160   if (__strncasecmp (args, "on", 2) == 0)
 161     {
 162       args += 2;
 163       _res_hconf.flags |= flag;
 164     }
 165   else if (__strncasecmp (args, "off", 3) == 0)
 166     {
 167       args += 3;
 168       _res_hconf.flags &= ~flag;
 169     }
 170   else
 171     {
 172       char *buf;
 173
 174       if (__asprintf (&buf,
 175                       _("%s: line %d: expected `on' or `off', found `%s'\n"),
 176                       fname, line_num, args) < 0)
 177         return 0;
 178
 179       __fxprintf (NULL, "%s", buf);
 180
 181       free (buf);
 182       return 0;
 183     }
 184   return args;
 185 }
 186
 187
 188 static void
 189 parse_line (const char *fname, int line_num, const char *str)
 190 {
 191   const char *start;
 192   const struct cmd *c = 0;
 193   size_t len;
 194   size_t i;
 195
 196   str = skip_ws (str);
 197
 198   /* skip line comment and empty lines: */
 199   if (*str == '\0' || *str == '#') return;
 200
 201   start = str;
 202   str = skip_string (str);
 203   len = str - start;
 204
 205   for (i = 0; i < sizeof (cmd) / sizeof (cmd[0]); ++i)
 206     {
 207       if (__strncasecmp (start, cmd[i].name, len) == 0
 208           && strlen (cmd[i].name) == len)
 209         {
 210           c = &cmd[i];
 211           break;
 212         }
 213     }
 214   if (c == NULL)
 215     {
 216       char *buf;
 217
 218       if (__asprintf (&buf, _("%s: line %d: bad command `%s'\n"),
 219                       fname, line_num, start) < 0)
 220         return;
 221
 222       __fxprintf (NULL, "%s", buf);
 223
 224       free (buf);
 225       return;
 226     }
 227
 228   /* process args: */
 229   str = skip_ws (str);
 230
 231   if (c->cb == CB_arg_trimdomain_list)
 232     str = arg_trimdomain_list (fname, line_num, str);
 233   else if (c->cb == CB_arg_bool)
 234     str = arg_bool (fname, line_num, str, c->arg);
 235   else
 236     /* Ignore the line.  */
 237     return;
 238
 239   if (!str)
 240     return;
 241
 242   /* rest of line must contain white space or comment only: */
 243   while (*str)
 244     {
 245       if (!isspace (*str)) {
 246         if (*str != '#')
 247           {
 248             char *buf;
 249
 250             if (__asprintf (&buf,
 251                             _("%s: line %d: ignoring trailing garbage `%s'\n"),
 252                             fname, line_num, str) < 0)
 253               break;
 254
 255             __fxprintf (NULL, "%s", buf);
 256
 257             free (buf);
 258           }
 259         break;
 260       }
 261       ++str;
 262     }
 263 }
 264
 265
 266 static void
 267 do_init (void)
 268 {
 269   const char *hconf_name;
 270   int line_num = 0;
 271   char buf[256], *envval;
 272   FILE *fp;
 273
 274   memset (&_res_hconf, '\0', sizeof (_res_hconf));
 275
 276   hconf_name = getenv (ENV_HOSTCONF);
 277   if (hconf_name == NULL)
 278     hconf_name = _PATH_HOSTCONF;
 279
 280   fp = fopen (hconf_name, "rce");
 281   if (fp)
 282     {
 283       /* No threads using this stream.  */
 284       __fsetlocking (fp, FSETLOCKING_BYCALLER);
 285
 286       while (fgets_unlocked (buf, sizeof (buf), fp))
 287         {
 288           ++line_num;
 289           *__strchrnul (buf, '\n') = '\0';
 290           parse_line (hconf_name, line_num, buf);
 291         }
 292       fclose (fp);
 293     }
 294
 295   envval = getenv (ENV_MULTI);
 296   if (envval)
 297     arg_bool (ENV_MULTI, 1, envval, HCONF_FLAG_MULTI);
 298
 299   envval = getenv (ENV_REORDER);
 300   if (envval)
 301     arg_bool (ENV_REORDER, 1, envval, HCONF_FLAG_REORDER);
 302
 303   envval = getenv (ENV_TRIM_ADD);
 304   if (envval)
 305     arg_trimdomain_list (ENV_TRIM_ADD, 1, envval);
 306
 307   envval = getenv (ENV_TRIM_OVERR);
 308   if (envval)
 309     {
 310       _res_hconf.num_trimdomains = 0;
 311       arg_trimdomain_list (ENV_TRIM_OVERR, 1, envval);
 312     }
 313
 314   /* See comments on the declaration of _res_hconf.  */
 315   atomic_store_release (&_res_hconf.initialized, 1);
 316 }
 317
 318
 319 /* Initialize hconf datastructure by reading host.conf file and
 320    environment variables.  */
 321 void
 322 _res_hconf_init (void)
 323 {
 324   __libc_once_define (static, once);
 325
 326   __libc_once (once, do_init);
 327 }
 328
 329
 330 #if IS_IN (libc)
 331 # if defined SIOCGIFCONF && defined SIOCGIFNETMASK
 332 /* List of known interfaces.  */
 333 libc_freeres_ptr (
 334 static struct netaddr
 335 {
 336   int addrtype;
 337   union
 338   {
 339     struct
 340     {
 341       uint32_t  addr;
 342       uint32_t  mask;
 343     } ipv4;
 344   } u;
 345 } *ifaddrs);
 346 # endif
 347
 348 /* Reorder addresses returned in a hostent such that the first address
 349    is an address on the local subnet, if there is such an address.
 350    Otherwise, nothing is changed.
 351
 352    Note that this function currently only handles IPv4 addresses.  */
 353
 354 void
 355 _res_hconf_reorder_addrs (struct hostent *hp)
 356 {
 357 #if defined SIOCGIFCONF && defined SIOCGIFNETMASK
 358   int i, j;
 359   /* Number of interfaces.  Also serves as a flag for the
 360      double-checked locking idiom.  */
 361   static int num_ifs = -1;
 362   /* Local copy of num_ifs, for non-atomic access.  */
 363   int num_ifs_local;
 364   /* We need to protect the dynamic buffer handling.  The lock is only
 365      acquired during initialization.  Afterwards, a positive num_ifs
 366      value indicates completed initialization.  */
 367   __libc_lock_define_initialized (static, lock);
 368
 369   /* Only reorder if we're supposed to.  */
 370   if ((_res_hconf.flags & HCONF_FLAG_REORDER) == 0)
 371     return;
 372
 373   /* Can't deal with anything but IPv4 for now...  */
 374   if (hp->h_addrtype != AF_INET)
 375     return;
 376
 377   /* This load synchronizes with the release MO store in the
 378      initialization block below.  */
 379   num_ifs_local = atomic_load_acquire (&num_ifs);
 380   if (num_ifs_local <= 0)
 381     {
 382       struct ifreq *ifr, *cur_ifr;
 383       int sd, num, i;
 384       /* Save errno.  */
 385       int save = errno;
 386
 387       /* Initialize interface table.  */
 388
 389       /* The SIOCGIFNETMASK ioctl will only work on an AF_INET socket.  */
 390       sd = __socket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
 391       if (sd < 0)
 392         return;
 393
 394       /* Get lock.  */
 395       __libc_lock_lock (lock);
 396
 397       /* Recheck, somebody else might have done the work by now.  No
 398          ordering is required for the load because we have the lock,
 399          and num_ifs is only updated under the lock.  Also see (3) in
 400          the analysis below.  */
 401       num_ifs_local = atomic_load_relaxed (&num_ifs);
 402       if (num_ifs_local <= 0)
 403         {
 404           /* This is the only block which writes to num_ifs.  It can
 405              be executed several times (sequentially) if
 406              initialization does not yield any interfaces, and num_ifs
 407              remains zero.  However, once we stored a positive value
 408              in num_ifs below, this block cannot be entered again due
 409              to the condition above.  */
 410           int new_num_ifs = 0;
 411
 412           /* Get a list of interfaces.  */
 413           __ifreq (&ifr, &num, sd);
 414           if (!ifr)
 415             goto cleanup;
 416
 417           ifaddrs = malloc (num * sizeof (ifaddrs[0]));
 418           if (!ifaddrs)
 419             goto cleanup1;
 420
 421           /* Copy usable interfaces in ifaddrs structure.  */
 422           for (cur_ifr = ifr, i = 0; i < num;
 423                cur_ifr = __if_nextreq (cur_ifr), ++i)
 424             {
 425               union
 426               {
 427                 struct sockaddr sa;
 428                 struct sockaddr_in sin;
 429               } ss;
 430
 431               if (cur_ifr->ifr_addr.sa_family != AF_INET)
 432                 continue;
 433
 434               ifaddrs[new_num_ifs].addrtype = AF_INET;
 435               ss.sa = cur_ifr->ifr_addr;
 436               ifaddrs[new_num_ifs].u.ipv4.addr = ss.sin.sin_addr.s_addr;
 437
 438               if (__ioctl (sd, SIOCGIFNETMASK, cur_ifr) < 0)
 439                 continue;
 440
 441               ss.sa = cur_ifr->ifr_netmask;
 442               ifaddrs[new_num_ifs].u.ipv4.mask = ss.sin.sin_addr.s_addr;
 443
 444               /* Now we're committed to this entry.  */
 445               ++new_num_ifs;
 446             }
 447           /* Just keep enough memory to hold all the interfaces we want.  */
 448           ifaddrs = realloc (ifaddrs, new_num_ifs * sizeof (ifaddrs[0]));
 449           assert (ifaddrs != NULL);
 450
 451         cleanup1:
 452           __if_freereq (ifr, num);
 453
 454         cleanup:
 455           /* Release lock, preserve error value, and close socket.  */
 456           errno = save;
 457
 458           /* Advertise successful initialization if new_num_ifs is
 459              positive (and no updates to ifaddrs are permitted after
 460              that).  Otherwise, num_ifs remains unchanged, at zero.
 461              This store synchronizes with the initial acquire MO
 462              load.  */
 463           atomic_store_release (&num_ifs, new_num_ifs);
 464           /* Keep the local copy current, to save another load.  */
 465           num_ifs_local = new_num_ifs;
 466         }
 467
 468       __libc_lock_unlock (lock);
 469
 470       __close (sd);
 471     }
 472
 473   /* num_ifs_local cannot be negative because the if statement above
 474      covered this case.  It can still be zero if we just performed
 475      initialization, but could not find any interfaces.  */
 476   if (num_ifs_local == 0)
 477     return;
 478
 479   /* The code below accesses ifaddrs, so we need to ensure that the
 480      initialization happens-before this point.
 481
 482      The actual initialization is sequenced-before the release store
 483      to num_ifs, and sequenced-before the end of the critical section.
 484
 485      This means there are three possible executions:
 486
 487      (1) The thread that initialized the data also uses it, so
 488          sequenced-before is sufficient to ensure happens-before.
 489
 490      (2) The release MO store of num_ifs synchronizes-with the acquire
 491          MO load, and the acquire MO load is sequenced before the use
 492          of the initialized data below.
 493
 494      (3) We enter the critical section, and the relaxed MO load of
 495          num_ifs yields a positive value.  The write to ifaddrs is
 496          sequenced-before leaving the critical section.  Leaving the
 497          critical section happens-before we entered the critical
 498          section ourselves, which means that the write to ifaddrs
 499          happens-before this point.
 500
 501      Consequently, all potential writes to ifaddrs (and the data it
 502      points to) happens-before this point.  */
 503
 504   /* Find an address for which we have a direct connection.  */
 505   for (i = 0; hp->h_addr_list[i]; ++i)
 506     {
 507       struct in_addr *haddr = (struct in_addr *) hp->h_addr_list[i];
 508
 509       for (j = 0; j < num_ifs_local; ++j)
 510         {
 511           uint32_t if_addr    = ifaddrs[j].u.ipv4.addr;
 512           uint32_t if_netmask = ifaddrs[j].u.ipv4.mask;
 513
 514           if (((haddr->s_addr ^ if_addr) & if_netmask) == 0)
 515             {
 516               void *tmp;
 517
 518               tmp = hp->h_addr_list[i];
 519               hp->h_addr_list[i] = hp->h_addr_list[0];
 520               hp->h_addr_list[0] = tmp;
 521               return;
 522             }
 523         }
 524     }
 525 #endif /* defined(SIOCGIFCONF) && ... */
 526 }
 527
 528
 529 /* If HOSTNAME has a postfix matching any of the trimdomains, trim away
 530    that postfix.  Notice that HOSTNAME is modified inplace.  Also, the
 531    original code applied all trimdomains in order, meaning that the
 532    same domainname could be trimmed multiple times.  I believe this
 533    was unintentional.  */
 534 void
 535 _res_hconf_trim_domain (char *hostname)
 536 {
 537   size_t hostname_len, trim_len;
 538   int i;
 539
 540   hostname_len = strlen (hostname);
 541
 542   for (i = 0; i < _res_hconf.num_trimdomains; ++i)
 543     {
 544       const char *trim = _res_hconf.trimdomain[i];
 545
 546       trim_len = strlen (trim);
 547       if (hostname_len > trim_len
 548           && __strcasecmp (&hostname[hostname_len - trim_len], trim) == 0)
 549         {
 550           hostname[hostname_len - trim_len] = '\0';
 551           break;
 552         }
 553     }
 554 }
 555
 556
 557 /* Trim all hostnames/aliases in HP according to the trimdomain list.
 558    Notice that HP is modified inplace!  */
 559 void
 560 _res_hconf_trim_domains (struct hostent *hp)
 561 {
 562   int i;
 563
 564   if (_res_hconf.num_trimdomains == 0)
 565     return;
 566
 567   _res_hconf_trim_domain (hp->h_name);
 568   for (i = 0; hp->h_aliases[i]; ++i)
 569     _res_hconf_trim_domain (hp->h_aliases[i]);
 570 }
 571 #endif