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 #include "tmpfile-util-label.h"
  17
  18 /* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
  19 #define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"
  20
  21 /* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
  22 #define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"
  23
  24 /* A static resolv.conf file containing no domains, but only our own DNS server address */
  25 #define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"
  26
  27 int manager_check_resolv_conf(const Manager *m) {
  28         const char *path;
  29         struct stat st;
  30         int r;
  31
  32         assert(m);
  33
  34         /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
  35          * PRIVATE_STATIC_RESOLV_CONF or PRIVATE_STUB_RESOLV_CONF. */
  36
  37         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  38                 return 0;
  39
  40         r = stat("/etc/resolv.conf", &st);
  41         if (r < 0) {
  42                 if (errno == ENOENT)
  43                         return 0;
  44
  45                 return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  46         }
  47
  48         FOREACH_STRING(path,
  49                        PRIVATE_STUB_RESOLV_CONF,
  50                        PRIVATE_STATIC_RESOLV_CONF) {
  51
  52                 struct stat own;
  53
  54                 /* Is it symlinked to our own uplink file? */
  55                 if (stat(path, &own) >= 0 &&
  56                     st.st_dev == own.st_dev &&
  57                     st.st_ino == own.st_ino) {
  58                         log_warning("DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to %s "
  59                                     "which expects DNSStubListener= to be enabled.", path);
  60                         return -EOPNOTSUPP;
  61                 }
  62         }
  63
  64         return 0;
  65 }
  66
  67 static bool file_is_our_own(const struct stat *st) {
  68         const char *path;
  69
  70         assert(st);
  71
  72         FOREACH_STRING(path,
  73                        PRIVATE_UPLINK_RESOLV_CONF,
  74                        PRIVATE_STUB_RESOLV_CONF,
  75                        PRIVATE_STATIC_RESOLV_CONF) {
  76
  77                 struct stat own;
  78
  79                 /* Is it symlinked to our own uplink file? */
  80                 if (stat(path, &own) >= 0 &&
  81                     st->st_dev == own.st_dev &&
  82                     st->st_ino == own.st_ino)
  83                         return true;
  84         }
  85
  86         return false;
  87 }
  88
  89 int manager_read_resolv_conf(Manager *m) {
  90         _cleanup_fclose_ FILE *f = NULL;
  91         struct stat st;
  92         unsigned n = 0;
  93         int r;
  94
  95         assert(m);
  96
  97         /* Reads the system /etc/resolv.conf, if it exists and is not
  98          * symlinked to our own resolv.conf instance */
  99
 100         if (!m->read_resolv_conf)
 101                 return 0;
 102
 103         r = stat("/etc/resolv.conf", &st);
 104         if (r < 0) {
 105                 if (errno == ENOENT)
 106                         return 0;
 107
 108                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
 109                 goto clear;
 110         }
 111
 112         /* Have we already seen the file? */
 113         if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
 114                 return 0;
 115
 116         if (file_is_our_own(&st))
 117                 return 0;
 118
 119         f = fopen("/etc/resolv.conf", "re");
 120         if (!f) {
 121                 if (errno == ENOENT)
 122                         return 0;
 123
 124                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 125                 goto clear;
 126         }
 127
 128         if (fstat(fileno(f), &st) < 0) {
 129                 r = log_error_errno(errno, "Failed to stat open file: %m");
 130                 goto clear;
 131         }
 132
 133         if (file_is_our_own(&st))
 134                 return 0;
 135
 136         dns_server_mark_all(m->dns_servers);
 137         dns_search_domain_mark_all(m->search_domains);
 138
 139         for (;;) {
 140                 _cleanup_free_ char *line = NULL;
 141                 const char *a;
 142                 char *l;
 143
 144                 r = read_line(f, LONG_LINE_MAX, &line);
 145                 if (r < 0) {
 146                         log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
 147                         goto clear;
 148                 }
 149                 if (r == 0)
 150                         break;
 151
 152                 n++;
 153
 154                 l = strstrip(line);
 155                 if (IN_SET(*l, '#', ';', 0))
 156                         continue;
 157
 158                 a = first_word(l, "nameserver");
 159                 if (a) {
 160                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 161                         if (r < 0)
 162                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 163
 164                         continue;
 165                 }
 166
 167                 a = first_word(l, "domain");
 168                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 169                         a = first_word(l, "search");
 170                 if (a) {
 171                         r = manager_parse_search_domains_and_warn(m, a);
 172                         if (r < 0)
 173                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 174                 }
 175
 176                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 177         }
 178
 179         m->resolv_conf_mtime = timespec_load(&st.st_mtim);
 180
 181         /* Flush out all servers and search domains that are still
 182          * marked. Those are then ones that didn't appear in the new
 183          * /etc/resolv.conf */
 184         dns_server_unlink_marked(m->dns_servers);
 185         dns_search_domain_unlink_marked(m->search_domains);
 186
 187         /* Whenever /etc/resolv.conf changes, start using the first
 188          * DNS server of it. This is useful to deal with broken
 189          * network managing implementations (like NetworkManager),
 190          * that when connecting to a VPN place both the VPN DNS
 191          * servers and the local ones in /etc/resolv.conf. Without
 192          * resetting the DNS server to use back to the first entry we
 193          * will continue to use the local one thus being unable to
 194          * resolve VPN domains. */
 195         manager_set_dns_server(m, m->dns_servers);
 196
 197         /* Unconditionally flush the cache when /etc/resolv.conf is
 198          * modified, even if the data it contained was completely
 199          * identical to the previous version we used. We do this
 200          * because altering /etc/resolv.conf is typically done when
 201          * the network configuration changes, and that should be
 202          * enough to flush the global unicast DNS cache. */
 203         if (m->unicast_scope)
 204                 dns_cache_flush(&m->unicast_scope->cache);
 205
 206         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 207          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 208          * (think 192.168.1.1). */
 209         dns_server_reset_features_all(m->dns_servers);
 210
 211         return 0;
 212
 213 clear:
 214         dns_server_unlink_all(m->dns_servers);
 215         dns_search_domain_unlink_all(m->search_domains);
 216         return r;
 217 }
 218
 219 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 220         DnsScope *scope;
 221
 222         assert(s);
 223         assert(f);
 224         assert(count);
 225
 226         if (!dns_server_string(s)) {
 227                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 228                 return;
 229         }
 230
 231         /* Check if the scope this DNS server belongs to is suitable as 'default' route for lookups; resolv.conf does
 232          * not have a syntax to express that, so it must not appear as a global name server to avoid routing unrelated
 233          * domains to it (which is a privacy violation, will most probably fail anyway, and adds unnecessary load) */
 234         scope = dns_server_scope(s);
 235         if (scope && !dns_scope_is_default_route(scope)) {
 236                 log_debug("Scope of DNS server %s has only 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"
 326                        "options edns0\n", f);
 327
 328         if (!ordered_set_isempty(domains))
 329                 write_resolv_conf_search(domains, f);
 330
 331         return fflush_and_check(f);
 332 }
 333
 334 int manager_write_resolv_conf(Manager *m) {
 335
 336         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 337         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 338         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 339         int r;
 340
 341         assert(m);
 342
 343         /* Read the system /etc/resolv.conf first */
 344         (void) manager_read_resolv_conf(m);
 345
 346         /* Add the full list to a set, to filter out duplicates */
 347         r = manager_compile_dns_servers(m, &dns);
 348         if (r < 0)
 349                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 350
 351         r = manager_compile_search_domains(m, &domains, false);
 352         if (r < 0)
 353                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 354
 355         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 356         if (r < 0)
 357                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 358
 359         (void) __fsetlocking(f_uplink, FSETLOCKING_BYCALLER);
 360         (void) fchmod(fileno(f_uplink), 0644);
 361
 362         r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
 363         if (r < 0)
 364                 return log_warning_errno(r, "Failed to open private stub-resolv.conf file for writing: %m");
 365
 366         (void) __fsetlocking(f_stub, FSETLOCKING_BYCALLER);
 367         (void) fchmod(fileno(f_stub), 0644);
 368
 369         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
 370         if (r < 0) {
 371                 log_error_errno(r, "Failed to write private resolv.conf contents: %m");
 372                 goto fail;
 373         }
 374
 375         if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0) {
 376                 r = log_error_errno(errno, "Failed to move private resolv.conf file into place: %m");
 377                 goto fail;
 378         }
 379
 380         r = write_stub_resolv_conf_contents(f_stub, dns, domains);
 381         if (r < 0) {
 382                 log_error_errno(r, "Failed to write private stub-resolv.conf contents: %m");
 383                 goto fail;
 384         }
 385
 386         if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0) {
 387                 r = log_error_errno(errno, "Failed to move private stub-resolv.conf file into place: %m");
 388                 goto fail;
 389         }
 390
 391         return 0;
 392
 393 fail:
 394         (void) unlink(PRIVATE_UPLINK_RESOLV_CONF);
 395         (void) unlink(temp_path_uplink);
 396         (void) unlink(PRIVATE_STUB_RESOLV_CONF);
 397         (void) unlink(temp_path_stub);
 398
 399         return r;
 400 }