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