src/resolve/resolved-resolv-conf.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <resolv.h>
   4 #include <stdio_ext.h>
   5
   6 #include "alloc-util.h"
   7 #include "def.h"
   8 #include "dns-domain.h"
   9 #include "fd-util.h"
  10 #include "fileio-label.h"
  11 #include "fileio.h"
  12 #include "ordered-set.h"
  13 #include "resolved-conf.h"
  14 #include "resolved-dns-server.h"
  15 #include "resolved-resolv-conf.h"
  16 #include "string-util.h"
  17 #include "strv.h"
  18
  19 /* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
  20 #define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"
  21
  22 /* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
  23 #define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"
  24
  25 /* A static resolv.conf file containing no domains, but only our own DNS server address */
  26 #define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"
  27
  28 int manager_check_resolv_conf(const Manager *m) {
  29         const char *path;
  30         struct stat st;
  31         int r;
  32
  33         assert(m);
  34
  35         /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
  36          * PRIVATE_STATIC_RESOLV_CONF or PRIVATE_STUB_RESOLV_CONF. */
  37
  38         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  39                 return 0;
  40
  41         r = stat("/etc/resolv.conf", &st);
  42         if (r < 0) {
  43                 if (errno == ENOENT)
  44                         return 0;
  45
  46                 return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  47         }
  48
  49         FOREACH_STRING(path,
  50                        PRIVATE_STUB_RESOLV_CONF,
  51                        PRIVATE_STATIC_RESOLV_CONF) {
  52
  53                 struct stat own;
  54
  55                 /* Is it symlinked to our own uplink file? */
  56                 if (stat(path, &own) >= 0 &&
  57                     st.st_dev == own.st_dev &&
  58                     st.st_ino == own.st_ino) {
  59                         log_warning("DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to %s "
  60                                     "which expects DNSStubListener= to be enabled.", path);
  61                         return -EOPNOTSUPP;
  62                 }
  63         }
  64
  65         return 0;
  66 }
  67
  68 static bool file_is_our_own(const struct stat *st) {
  69         const char *path;
  70
  71         assert(st);
  72
  73         FOREACH_STRING(path,
  74                        PRIVATE_UPLINK_RESOLV_CONF,
  75                        PRIVATE_STUB_RESOLV_CONF,
  76                        PRIVATE_STATIC_RESOLV_CONF) {
  77
  78                 struct stat own;
  79
  80                 /* Is it symlinked to our own uplink file? */
  81                 if (stat(path, &own) >= 0 &&
  82                     st->st_dev == own.st_dev &&
  83                     st->st_ino == own.st_ino)
  84                         return true;
  85         }
  86
  87         return false;
  88 }
  89
  90 int manager_read_resolv_conf(Manager *m) {
  91         _cleanup_fclose_ FILE *f = NULL;
  92         struct stat st;
  93         unsigned n = 0;
  94         int r;
  95
  96         assert(m);
  97
  98         /* Reads the system /etc/resolv.conf, if it exists and is not
  99          * symlinked to our own resolv.conf instance */
 100
 101         if (!m->read_resolv_conf)
 102                 return 0;
 103
 104         r = stat("/etc/resolv.conf", &st);
 105         if (r < 0) {
 106                 if (errno == ENOENT)
 107                         return 0;
 108
 109                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
 110                 goto clear;
 111         }
 112
 113         /* Have we already seen the file? */
 114         if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
 115                 return 0;
 116
 117         if (file_is_our_own(&st))
 118                 return 0;
 119
 120         f = fopen("/etc/resolv.conf", "re");
 121         if (!f) {
 122                 if (errno == ENOENT)
 123                         return 0;
 124
 125                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 126                 goto clear;
 127         }
 128
 129         if (fstat(fileno(f), &st) < 0) {
 130                 r = log_error_errno(errno, "Failed to stat open file: %m");
 131                 goto clear;
 132         }
 133
 134         if (file_is_our_own(&st))
 135                 return 0;
 136
 137         dns_server_mark_all(m->dns_servers);
 138         dns_search_domain_mark_all(m->search_domains);
 139
 140         for (;;) {
 141                 _cleanup_free_ char *line = NULL;
 142                 const char *a;
 143                 char *l;
 144
 145                 r = read_line(f, LONG_LINE_MAX, &line);
 146                 if (r < 0) {
 147                         log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
 148                         goto clear;
 149                 }
 150                 if (r == 0)
 151                         break;
 152
 153                 n++;
 154
 155                 l = strstrip(line);
 156                 if (IN_SET(*l, '#', ';', 0))
 157                         continue;
 158
 159                 a = first_word(l, "nameserver");
 160                 if (a) {
 161                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 162                         if (r < 0)
 163                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 164
 165                         continue;
 166                 }
 167
 168                 a = first_word(l, "domain");
 169                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 170                         a = first_word(l, "search");
 171                 if (a) {
 172                         r = manager_parse_search_domains_and_warn(m, a);
 173                         if (r < 0)
 174                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 175                 }
 176
 177                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 178         }
 179
 180         m->resolv_conf_mtime = timespec_load(&st.st_mtim);
 181
 182         /* Flush out all servers and search domains that are still
 183          * marked. Those are then ones that didn't appear in the new
 184          * /etc/resolv.conf */
 185         dns_server_unlink_marked(m->dns_servers);
 186         dns_search_domain_unlink_marked(m->search_domains);
 187
 188         /* Whenever /etc/resolv.conf changes, start using the first
 189          * DNS server of it. This is useful to deal with broken
 190          * network managing implementations (like NetworkManager),
 191          * that when connecting to a VPN place both the VPN DNS
 192          * servers and the local ones in /etc/resolv.conf. Without
 193          * resetting the DNS server to use back to the first entry we
 194          * will continue to use the local one thus being unable to
 195          * resolve VPN domains. */
 196         manager_set_dns_server(m, m->dns_servers);
 197
 198         /* Unconditionally flush the cache when /etc/resolv.conf is
 199          * modified, even if the data it contained was completely
 200          * identical to the previous version we used. We do this
 201          * because altering /etc/resolv.conf is typically done when
 202          * the network configuration changes, and that should be
 203          * enough to flush the global unicast DNS cache. */
 204         if (m->unicast_scope)
 205                 dns_cache_flush(&m->unicast_scope->cache);
 206
 207         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 208          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 209          * (think 192.168.1.1). */
 210         dns_server_reset_features_all(m->dns_servers);
 211
 212         return 0;
 213
 214 clear:
 215         dns_server_unlink_all(m->dns_servers);
 216         dns_search_domain_unlink_all(m->search_domains);
 217         return r;
 218 }
 219
 220 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 221         assert(s);
 222         assert(f);
 223         assert(count);
 224
 225         if (!dns_server_string(s)) {
 226                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 227                 return;
 228         }
 229
 230         /* Check if the DNS server is limited to particular domains;
 231          * resolv.conf does not have a syntax to express that, so it must not
 232          * appear as a global name server to avoid routing unrelated domains to
 233          * it (which is a privacy violation, will most probably fail anyway,
 234          * and adds unnecessary load) */
 235         if (dns_server_limited_domains(s)) {
 236                 log_debug("DNS server %s has route-only domains, not using as global name server", dns_server_string(s));
 237                 return;
 238         }
 239
 240         if (*count == MAXNS)
 241                 fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 242         (*count)++;
 243
 244         fprintf(f, "nameserver %s\n", dns_server_string(s));
 245 }
 246
 247 static void write_resolv_conf_search(
 248                 OrderedSet *domains,
 249                 FILE *f) {
 250         unsigned length = 0, count = 0;
 251         Iterator i;
 252         char *domain;
 253
 254         assert(domains);
 255         assert(f);
 256
 257         fputs("search", f);
 258
 259         ORDERED_SET_FOREACH(domain, domains, i) {
 260                 if (++count > MAXDNSRCH) {
 261                         fputs("\n# Too many search domains configured, remaining ones ignored.", f);
 262                         break;
 263                 }
 264                 length += strlen(domain) + 1;
 265                 if (length > 256) {
 266                         fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 267                         break;
 268                 }
 269                 fputc(' ', f);
 270                 fputs(domain, f);
 271         }
 272
 273         fputs("\n", f);
 274 }
 275
 276 static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 277         Iterator i;
 278
 279         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 280               "#\n"
 281               "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 282               "# all known uplink DNS servers. This file lists all configured search domains.\n"
 283               "#\n"
 284               "# Third party programs must not access this file directly, but only through the\n"
 285               "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 286               "# replace this symlink by a static file or a different symlink.\n"
 287               "#\n"
 288               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 289               "# operation for /etc/resolv.conf.\n"
 290               "\n", f);
 291
 292         if (ordered_set_isempty(dns))
 293                 fputs("# No DNS servers known.\n", f);
 294         else {
 295                 unsigned count = 0;
 296                 DnsServer *s;
 297
 298                 ORDERED_SET_FOREACH(s, dns, i)
 299                         write_resolv_conf_server(s, f, &count);
 300         }
 301
 302         if (!ordered_set_isempty(domains))
 303                 write_resolv_conf_search(domains, f);
 304
 305         return fflush_and_check(f);
 306 }
 307
 308 static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 309         fputs_unlocked("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 310                        "#\n"
 311                        "# This is a dynamic resolv.conf file for connecting local clients to the\n"
 312                        "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
 313                        "# configured search domains.\n"
 314                        "#\n"
 315                        "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
 316                        "# currently in use.\n"
 317                        "#\n"
 318                        "# Third party programs must not access this file directly, but only through the\n"
 319                        "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 320                        "# replace this symlink by a static file or a different symlink.\n"
 321                        "#\n"
 322                        "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 323                        "# operation for /etc/resolv.conf.\n"
 324                        "\n"
 325                        "nameserver 127.0.0.53\n", f);
 326
 327         if (!ordered_set_isempty(domains))
 328                 write_resolv_conf_search(domains, f);
 329
 330         return fflush_and_check(f);
 331 }
 332
 333 int manager_write_resolv_conf(Manager *m) {
 334
 335         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 336         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 337         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 338         int r;
 339
 340         assert(m);
 341
 342         /* Read the system /etc/resolv.conf first */
 343         (void) manager_read_resolv_conf(m);
 344
 345         /* Add the full list to a set, to filter out duplicates */
 346         r = manager_compile_dns_servers(m, &dns);
 347         if (r < 0)
 348                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 349
 350         r = manager_compile_search_domains(m, &domains, false);
 351         if (r < 0)
 352                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 353
 354         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 355         if (r < 0)
 356                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 357
 358         (void) __fsetlocking(f_uplink, FSETLOCKING_BYCALLER);
 359         (void) fchmod(fileno(f_uplink), 0644);
 360
 361         r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
 362         if (r < 0)
 363                 return log_warning_errno(r, "Failed to open private stub-resolv.conf file for writing: %m");
 364
 365         (void) __fsetlocking(f_stub, FSETLOCKING_BYCALLER);
 366         (void) fchmod(fileno(f_stub), 0644);
 367
 368         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
 369         if (r < 0) {
 370                 log_error_errno(r, "Failed to write private resolv.conf contents: %m");
 371                 goto fail;
 372         }
 373
 374         if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0) {
 375                 r = log_error_errno(errno, "Failed to move private resolv.conf file into place: %m");
 376                 goto fail;
 377         }
 378
 379         r = write_stub_resolv_conf_contents(f_stub, dns, domains);
 380         if (r < 0) {
 381                 log_error_errno(r, "Failed to write private stub-resolv.conf contents: %m");
 382                 goto fail;
 383         }
 384
 385         if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0) {
 386                 r = log_error_errno(errno, "Failed to move private stub-resolv.conf file into place: %m");
 387                 goto fail;
 388         }
 389
 390         return 0;
 391
 392 fail:
 393         (void) unlink(PRIVATE_UPLINK_RESOLV_CONF);
 394         (void) unlink(temp_path_uplink);
 395         (void) unlink(PRIVATE_STUB_RESOLV_CONF);
 396         (void) unlink(temp_path_stub);
 397
 398         return r;
 399 }