resolv/res_hconf.c

   1 /* Copyright (C) 1993-2019 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by David Mosberger (davidm@azstarnet.com).
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 /* This file provides a Linux /etc/host.conf compatible front end to
  20    the various name resolvers (/etc/hosts, named, NIS server, etc.).
  21    Though mostly compatibly, the following differences exist compared
  22    to the original implementation:
  23
  24         - line comments can appear anywhere (not just at the beginning of
  25           a line)
  26 */
  27
  28 #include <assert.h>
  29 #include <errno.h>
  30 #include <ctype.h>
  31 #include <libintl.h>
  32 #include <memory.h>
  33 #include <stdio.h>
  34 #include <stdio_ext.h>
  35 #include <stdlib.h>
  36 #include <string.h>
  37 #include <net/if.h>
  38 #include <sys/ioctl.h>
  39 #include <unistd.h>
  40 #include <netinet/in.h>
  41 #include <libc-lock.h>
  42 #include "ifreq.h"
  43 #include "res_hconf.h"
  44 #include <wchar.h>
  45 #include <atomic.h>
  46
  47 #if IS_IN (libc)
  48 # define fgets_unlocked __fgets_unlocked
  49 #endif
  50
  51 #define _PATH_HOSTCONF  "/etc/host.conf"
  52
  53 /* Environment vars that all user to override default behavior:  */
  54 #define ENV_HOSTCONF    "RESOLV_HOST_CONF"
  55 #define ENV_TRIM_OVERR  "RESOLV_OVERRIDE_TRIM_DOMAINS"
  56 #define ENV_TRIM_ADD    "RESOLV_ADD_TRIM_DOMAINS"
  57 #define ENV_MULTI       "RESOLV_MULTI"
  58 #define ENV_REORDER     "RESOLV_REORDER"
  59
  60 enum parse_cbs
  61   {
  62     CB_none,
  63     CB_arg_trimdomain_list,
  64     CB_arg_bool
  65   };
  66
  67 static const struct cmd
  68 {
  69   const char name[11];
  70   uint8_t cb;
  71   unsigned int arg;
  72 } cmd[] =
  73 {
  74   {"order",             CB_none,                0},
  75   {"trim",              CB_arg_trimdomain_list, 0},
  76   {"multi",             CB_arg_bool,            HCONF_FLAG_MULTI},
  77   {"reorder",           CB_arg_bool,            HCONF_FLAG_REORDER}
  78 };
  79
  80 /* Structure containing the state.  */
  81 struct hconf _res_hconf;
  82
  83 /* Skip white space.  */
  84 static const char *
  85 skip_ws (const char *str)
  86 {
  87   while (isspace (*str)) ++str;
  88   return str;
  89 }
  90
  91
  92 /* Skip until whitespace, comma, end of line, or comment character.  */
  93 static const char *
  94 skip_string (const char *str)
  95 {
  96   while (*str && !isspace (*str) && *str != '#' && *str != ',')
  97     ++str;
  98   return str;
  99 }
 100
 101
 102 static const char *
 103 arg_trimdomain_list (const char *fname, int line_num, const char *args)
 104 {
 105   const char * start;
 106   size_t len;
 107
 108   do
 109     {
 110       start = args;
 111       args = skip_string (args);
 112       len = args - start;
 113
 114       if (_res_hconf.num_trimdomains >= TRIMDOMAINS_MAX)
 115         {
 116           char *buf;
 117
 118           if (__asprintf (&buf, _("\
 119 %s: line %d: cannot specify more than %d trim domains"),
 120                           fname, line_num, TRIMDOMAINS_MAX) < 0)
 121             return 0;
 122
 123           __fxprintf (NULL, "%s", buf);
 124
 125           free (buf);
 126           return 0;
 127         }
 128       _res_hconf.trimdomain[_res_hconf.num_trimdomains++] =
 129         __strndup (start, len);
 130       args = skip_ws (args);
 131       switch (*args)
 132         {
 133         case ',': case ';': case ':':
 134           args = skip_ws (++args);
 135           if (!*args || *args == '#')
 136             {
 137               char *buf;
 138
 139               if (__asprintf (&buf, _("\
 140 %s: line %d: list delimiter not followed by domain"),
 141                               fname, line_num) < 0)
 142                 return 0;
 143
 144               __fxprintf (NULL, "%s", buf);
 145
 146               free (buf);
 147               return 0;
 148             }
 149         default:
 150           break;
 151         }
 152     }
 153   while (*args && *args != '#');
 154   return args;
 155 }
 156
 157
 158 static const char *
 159 arg_bool (const char *fname, int line_num, const char *args, unsigned flag)
 160 {
 161   if (__strncasecmp (args, "on", 2) == 0)
 162     {
 163       args += 2;
 164       _res_hconf.flags |= flag;
 165     }
 166   else if (__strncasecmp (args, "off", 3) == 0)
 167     {
 168       args += 3;
 169       _res_hconf.flags &= ~flag;
 170     }
 171   else
 172     {
 173       char *buf;
 174
 175       if (__asprintf (&buf,
 176                       _("%s: line %d: expected `on' or `off', found `%s'\n"),
 177                       fname, line_num, args) < 0)
 178         return 0;
 179
 180       __fxprintf (NULL, "%s", buf);
 181
 182       free (buf);
 183       return 0;
 184     }
 185   return args;
 186 }
 187
 188
 189 static void
 190 parse_line (const char *fname, int line_num, const char *str)
 191 {
 192   const char *start;
 193   const struct cmd *c = 0;
 194   size_t len;
 195   size_t i;
 196
 197   str = skip_ws (str);
 198
 199   /* skip line comment and empty lines: */
 200   if (*str == '\0' || *str == '#') return;
 201
 202   start = str;
 203   str = skip_string (str);
 204   len = str - start;
 205
 206   for (i = 0; i < sizeof (cmd) / sizeof (cmd[0]); ++i)
 207     {
 208       if (__strncasecmp (start, cmd[i].name, len) == 0
 209           && strlen (cmd[i].name) == len)
 210         {
 211           c = &cmd[i];
 212           break;
 213         }
 214     }
 215   if (c == NULL)
 216     {
 217       char *buf;
 218
 219       if (__asprintf (&buf, _("%s: line %d: bad command `%s'\n"),
 220                       fname, line_num, start) < 0)
 221         return;
 222
 223       __fxprintf (NULL, "%s", buf);
 224
 225       free (buf);
 226       return;
 227     }
 228
 229   /* process args: */
 230   str = skip_ws (str);
 231
 232   if (c->cb == CB_arg_trimdomain_list)
 233     str = arg_trimdomain_list (fname, line_num, str);
 234   else if (c->cb == CB_arg_bool)
 235     str = arg_bool (fname, line_num, str, c->arg);
 236   else
 237     /* Ignore the line.  */
 238     return;
 239
 240   if (!str)
 241     return;
 242
 243   /* rest of line must contain white space or comment only: */
 244   while (*str)
 245     {
 246       if (!isspace (*str)) {
 247         if (*str != '#')
 248           {
 249             char *buf;
 250
 251             if (__asprintf (&buf,
 252                             _("%s: line %d: ignoring trailing garbage `%s'\n"),
 253                             fname, line_num, str) < 0)
 254               break;
 255
 256             __fxprintf (NULL, "%s", buf);
 257
 258             free (buf);
 259           }
 260         break;
 261       }
 262       ++str;
 263     }
 264 }
 265
 266
 267 static void
 268 do_init (void)
 269 {
 270   const char *hconf_name;
 271   int line_num = 0;
 272   char buf[256], *envval;
 273   FILE *fp;
 274
 275   memset (&_res_hconf, '\0', sizeof (_res_hconf));
 276
 277   hconf_name = getenv (ENV_HOSTCONF);
 278   if (hconf_name == NULL)
 279     hconf_name = _PATH_HOSTCONF;
 280
 281   fp = fopen (hconf_name, "rce");
 282   if (fp)
 283     {
 284       /* No threads using this stream.  */
 285       __fsetlocking (fp, FSETLOCKING_BYCALLER);
 286
 287       while (fgets_unlocked (buf, sizeof (buf), fp))
 288         {
 289           ++line_num;
 290           *__strchrnul (buf, '\n') = '\0';
 291           parse_line (hconf_name, line_num, buf);
 292         }
 293       fclose (fp);
 294     }
 295
 296   envval = getenv (ENV_MULTI);
 297   if (envval)
 298     arg_bool (ENV_MULTI, 1, envval, HCONF_FLAG_MULTI);
 299
 300   envval = getenv (ENV_REORDER);
 301   if (envval)
 302     arg_bool (ENV_REORDER, 1, envval, HCONF_FLAG_REORDER);
 303
 304   envval = getenv (ENV_TRIM_ADD);
 305   if (envval)
 306     arg_trimdomain_list (ENV_TRIM_ADD, 1, envval);
 307
 308   envval = getenv (ENV_TRIM_OVERR);
 309   if (envval)
 310     {
 311       _res_hconf.num_trimdomains = 0;
 312       arg_trimdomain_list (ENV_TRIM_OVERR, 1, envval);
 313     }
 314
 315   /* See comments on the declaration of _res_hconf.  */
 316   atomic_store_release (&_res_hconf.initialized, 1);
 317 }
 318
 319
 320 /* Initialize hconf datastructure by reading host.conf file and
 321    environment variables.  */
 322 void
 323 _res_hconf_init (void)
 324 {
 325   __libc_once_define (static, once);
 326
 327   __libc_once (once, do_init);
 328 }
 329
 330
 331 #if IS_IN (libc)
 332 # if defined SIOCGIFCONF && defined SIOCGIFNETMASK
 333 /* List of known interfaces.  */
 334 libc_freeres_ptr (
 335 static struct netaddr
 336 {
 337   int addrtype;
 338   union
 339   {
 340     struct
 341     {
 342       uint32_t  addr;
 343       uint32_t  mask;
 344     } ipv4;
 345   } u;
 346 } *ifaddrs);
 347 # endif
 348
 349 /* Reorder addresses returned in a hostent such that the first address
 350    is an address on the local subnet, if there is such an address.
 351    Otherwise, nothing is changed.
 352
 353    Note that this function currently only handles IPv4 addresses.  */
 354
 355 void
 356 _res_hconf_reorder_addrs (struct hostent *hp)
 357 {
 358 #if defined SIOCGIFCONF && defined SIOCGIFNETMASK
 359   int i, j;
 360   /* Number of interfaces.  Also serves as a flag for the
 361      double-checked locking idiom.  */
 362   static int num_ifs = -1;
 363   /* Local copy of num_ifs, for non-atomic access.  */
 364   int num_ifs_local;
 365   /* We need to protect the dynamic buffer handling.  The lock is only
 366      acquired during initialization.  Afterwards, a positive num_ifs
 367      value indicates completed initialization.  */
 368   __libc_lock_define_initialized (static, lock);
 369
 370   /* Only reorder if we're supposed to.  */
 371   if ((_res_hconf.flags & HCONF_FLAG_REORDER) == 0)
 372     return;
 373
 374   /* Can't deal with anything but IPv4 for now...  */
 375   if (hp->h_addrtype != AF_INET)
 376     return;
 377
 378   /* This load synchronizes with the release MO store in the
 379      initialization block below.  */
 380   num_ifs_local = atomic_load_acquire (&num_ifs);
 381   if (num_ifs_local <= 0)
 382     {
 383       struct ifreq *ifr, *cur_ifr;
 384       int sd, num, i;
 385       /* Save errno.  */
 386       int save = errno;
 387
 388       /* Initialize interface table.  */
 389
 390       /* The SIOCGIFNETMASK ioctl will only work on an AF_INET socket.  */
 391       sd = __socket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
 392       if (sd < 0)
 393         return;
 394
 395       /* Get lock.  */
 396       __libc_lock_lock (lock);
 397
 398       /* Recheck, somebody else might have done the work by now.  No
 399          ordering is required for the load because we have the lock,
 400          and num_ifs is only updated under the lock.  Also see (3) in
 401          the analysis below.  */
 402       num_ifs_local = atomic_load_relaxed (&num_ifs);
 403       if (num_ifs_local <= 0)
 404         {
 405           /* This is the only block which writes to num_ifs.  It can
 406              be executed several times (sequentially) if
 407              initialization does not yield any interfaces, and num_ifs
 408              remains zero.  However, once we stored a positive value
 409              in num_ifs below, this block cannot be entered again due
 410              to the condition above.  */
 411           int new_num_ifs = 0;
 412
 413           /* Get a list of interfaces.  */
 414           __ifreq (&ifr, &num, sd);
 415           if (!ifr)
 416             goto cleanup;
 417
 418           ifaddrs = malloc (num * sizeof (ifaddrs[0]));
 419           if (!ifaddrs)
 420             goto cleanup1;
 421
 422           /* Copy usable interfaces in ifaddrs structure.  */
 423           for (cur_ifr = ifr, i = 0; i < num;
 424                cur_ifr = __if_nextreq (cur_ifr), ++i)
 425             {
 426               union
 427               {
 428                 struct sockaddr sa;
 429                 struct sockaddr_in sin;
 430               } ss;
 431
 432               if (cur_ifr->ifr_addr.sa_family != AF_INET)
 433                 continue;
 434
 435               ifaddrs[new_num_ifs].addrtype = AF_INET;
 436               ss.sa = cur_ifr->ifr_addr;
 437               ifaddrs[new_num_ifs].u.ipv4.addr = ss.sin.sin_addr.s_addr;
 438
 439               if (__ioctl (sd, SIOCGIFNETMASK, cur_ifr) < 0)
 440                 continue;
 441
 442               ss.sa = cur_ifr->ifr_netmask;
 443               ifaddrs[new_num_ifs].u.ipv4.mask = ss.sin.sin_addr.s_addr;
 444
 445               /* Now we're committed to this entry.  */
 446               ++new_num_ifs;
 447             }
 448           /* Just keep enough memory to hold all the interfaces we want.  */
 449           ifaddrs = realloc (ifaddrs, new_num_ifs * sizeof (ifaddrs[0]));
 450           assert (ifaddrs != NULL);
 451
 452         cleanup1:
 453           __if_freereq (ifr, num);
 454
 455         cleanup:
 456           /* Release lock, preserve error value, and close socket.  */
 457           errno = save;
 458
 459           /* Advertise successful initialization if new_num_ifs is
 460              positive (and no updates to ifaddrs are permitted after
 461              that).  Otherwise, num_ifs remains unchanged, at zero.
 462              This store synchronizes with the initial acquire MO
 463              load.  */
 464           atomic_store_release (&num_ifs, new_num_ifs);
 465           /* Keep the local copy current, to save another load.  */
 466           num_ifs_local = new_num_ifs;
 467         }
 468
 469       __libc_lock_unlock (lock);
 470
 471       __close (sd);
 472     }
 473
 474   /* num_ifs_local cannot be negative because the if statement above
 475      covered this case.  It can still be zero if we just performed
 476      initialization, but could not find any interfaces.  */
 477   if (num_ifs_local == 0)
 478     return;
 479
 480   /* The code below accesses ifaddrs, so we need to ensure that the
 481      initialization happens-before this point.
 482
 483      The actual initialization is sequenced-before the release store
 484      to num_ifs, and sequenced-before the end of the critical section.
 485
 486      This means there are three possible executions:
 487
 488      (1) The thread that initialized the data also uses it, so
 489          sequenced-before is sufficient to ensure happens-before.
 490
 491      (2) The release MO store of num_ifs synchronizes-with the acquire
 492          MO load, and the acquire MO load is sequenced before the use
 493          of the initialized data below.
 494
 495      (3) We enter the critical section, and the relaxed MO load of
 496          num_ifs yields a positive value.  The write to ifaddrs is
 497          sequenced-before leaving the critical section.  Leaving the
 498          critical section happens-before we entered the critical
 499          section ourselves, which means that the write to ifaddrs
 500          happens-before this point.
 501
 502      Consequently, all potential writes to ifaddrs (and the data it
 503      points to) happens-before this point.  */
 504
 505   /* Find an address for which we have a direct connection.  */
 506   for (i = 0; hp->h_addr_list[i]; ++i)
 507     {
 508       struct in_addr *haddr = (struct in_addr *) hp->h_addr_list[i];
 509
 510       for (j = 0; j < num_ifs_local; ++j)
 511         {
 512           uint32_t if_addr    = ifaddrs[j].u.ipv4.addr;
 513           uint32_t if_netmask = ifaddrs[j].u.ipv4.mask;
 514
 515           if (((haddr->s_addr ^ if_addr) & if_netmask) == 0)
 516             {
 517               void *tmp;
 518
 519               tmp = hp->h_addr_list[i];
 520               hp->h_addr_list[i] = hp->h_addr_list[0];
 521               hp->h_addr_list[0] = tmp;
 522               return;
 523             }
 524         }
 525     }
 526 #endif /* defined(SIOCGIFCONF) && ... */
 527 }
 528
 529
 530 /* If HOSTNAME has a postfix matching any of the trimdomains, trim away
 531    that postfix.  Notice that HOSTNAME is modified inplace.  Also, the
 532    original code applied all trimdomains in order, meaning that the
 533    same domainname could be trimmed multiple times.  I believe this
 534    was unintentional.  */
 535 void
 536 _res_hconf_trim_domain (char *hostname)
 537 {
 538   size_t hostname_len, trim_len;
 539   int i;
 540
 541   hostname_len = strlen (hostname);
 542
 543   for (i = 0; i < _res_hconf.num_trimdomains; ++i)
 544     {
 545       const char *trim = _res_hconf.trimdomain[i];
 546
 547       trim_len = strlen (trim);
 548       if (hostname_len > trim_len
 549           && __strcasecmp (&hostname[hostname_len - trim_len], trim) == 0)
 550         {
 551           hostname[hostname_len - trim_len] = '\0';
 552           break;
 553         }
 554     }
 555 }
 556
 557
 558 /* Trim all hostnames/aliases in HP according to the trimdomain list.
 559    Notice that HP is modified inplace!  */
 560 void
 561 _res_hconf_trim_domains (struct hostent *hp)
 562 {
 563   int i;
 564
 565   if (_res_hconf.num_trimdomains == 0)
 566     return;
 567
 568   _res_hconf_trim_domain (hp->h_name);
 569   for (i = 0; hp->h_aliases[i]; ++i)
 570     _res_hconf_trim_domain (hp->h_aliases[i]);
 571 }
 572 #endif