src/resolve/resolved-resolv-conf.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <resolv.h>
   4 #include <sys/stat.h>
   5 #include <sys/types.h>
   6 #include <unistd.h>
   7
   8 #include "alloc-util.h"
   9 #include "dns-domain.h"
  10 #include "fd-util.h"
  11 #include "fileio.h"
  12 #include "fs-util.h"
  13 #include "ordered-set.h"
  14 #include "resolved-conf.h"
  15 #include "resolved-dns-server.h"
  16 #include "resolved-resolv-conf.h"
  17 #include "string-util.h"
  18 #include "strv.h"
  19 #include "tmpfile-util-label.h"
  20
  21 /* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
  22 #define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"
  23
  24 /* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
  25 #define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"
  26
  27 /* A static resolv.conf file containing no domains, but only our own DNS server address */
  28 #define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"
  29
  30 int manager_check_resolv_conf(const Manager *m) {
  31         struct stat st, own;
  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. */
  37
  38         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  39                 return 0;
  40
  41         if (stat("/etc/resolv.conf", &st) < 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         /* Is it symlinked to our own uplink file? */
  49         if (stat(PRIVATE_STATIC_RESOLV_CONF, &own) >= 0 &&
  50             st.st_dev == own.st_dev &&
  51             st.st_ino == own.st_ino)
  52                 return log_warning_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
  53                                          "DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to "
  54                                          PRIVATE_STATIC_RESOLV_CONF " which expects DNSStubListener= to be enabled.");
  55
  56         return 0;
  57 }
  58
  59 static bool file_is_our_own(const struct stat *st) {
  60         const char *path;
  61
  62         assert(st);
  63
  64         FOREACH_STRING(path,
  65                        PRIVATE_UPLINK_RESOLV_CONF,
  66                        PRIVATE_STUB_RESOLV_CONF,
  67                        PRIVATE_STATIC_RESOLV_CONF) {
  68
  69                 struct stat own;
  70
  71                 /* Is it symlinked to our own uplink file? */
  72                 if (stat(path, &own) >= 0 &&
  73                     st->st_dev == own.st_dev &&
  74                     st->st_ino == own.st_ino)
  75                         return true;
  76         }
  77
  78         return false;
  79 }
  80
  81 int manager_read_resolv_conf(Manager *m) {
  82         _cleanup_fclose_ FILE *f = NULL;
  83         struct stat st;
  84         unsigned n = 0;
  85         int r;
  86
  87         assert(m);
  88
  89         /* Reads the system /etc/resolv.conf, if it exists and is not
  90          * symlinked to our own resolv.conf instance */
  91
  92         if (!m->read_resolv_conf)
  93                 return 0;
  94
  95         r = stat("/etc/resolv.conf", &st);
  96         if (r < 0) {
  97                 if (errno == ENOENT)
  98                         return 0;
  99
 100                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
 101                 goto clear;
 102         }
 103
 104         /* Have we already seen the file? */
 105         if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
 106                 return 0;
 107
 108         if (file_is_our_own(&st))
 109                 return 0;
 110
 111         f = fopen("/etc/resolv.conf", "re");
 112         if (!f) {
 113                 if (errno == ENOENT)
 114                         return 0;
 115
 116                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 117                 goto clear;
 118         }
 119
 120         if (fstat(fileno(f), &st) < 0) {
 121                 r = log_error_errno(errno, "Failed to stat open file: %m");
 122                 goto clear;
 123         }
 124
 125         if (file_is_our_own(&st))
 126                 return 0;
 127
 128         dns_server_mark_all(m->dns_servers);
 129         dns_search_domain_mark_all(m->search_domains);
 130
 131         for (;;) {
 132                 _cleanup_free_ char *line = NULL;
 133                 const char *a;
 134                 char *l;
 135
 136                 r = read_line(f, LONG_LINE_MAX, &line);
 137                 if (r < 0) {
 138                         log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
 139                         goto clear;
 140                 }
 141                 if (r == 0)
 142                         break;
 143
 144                 n++;
 145
 146                 l = strstrip(line);
 147                 if (IN_SET(*l, '#', ';', 0))
 148                         continue;
 149
 150                 a = first_word(l, "nameserver");
 151                 if (a) {
 152                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 153                         if (r < 0)
 154                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 155
 156                         continue;
 157                 }
 158
 159                 a = first_word(l, "domain");
 160                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 161                         a = first_word(l, "search");
 162                 if (a) {
 163                         r = manager_parse_search_domains_and_warn(m, a);
 164                         if (r < 0)
 165                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 166                 }
 167
 168                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 169         }
 170
 171         m->resolv_conf_mtime = timespec_load(&st.st_mtim);
 172
 173         /* Flush out all servers and search domains that are still
 174          * marked. Those are then ones that didn't appear in the new
 175          * /etc/resolv.conf */
 176         dns_server_unlink_marked(m->dns_servers);
 177         dns_search_domain_unlink_marked(m->search_domains);
 178
 179         /* Whenever /etc/resolv.conf changes, start using the first
 180          * DNS server of it. This is useful to deal with broken
 181          * network managing implementations (like NetworkManager),
 182          * that when connecting to a VPN place both the VPN DNS
 183          * servers and the local ones in /etc/resolv.conf. Without
 184          * resetting the DNS server to use back to the first entry we
 185          * will continue to use the local one thus being unable to
 186          * resolve VPN domains. */
 187         manager_set_dns_server(m, m->dns_servers);
 188
 189         /* Unconditionally flush the cache when /etc/resolv.conf is
 190          * modified, even if the data it contained was completely
 191          * identical to the previous version we used. We do this
 192          * because altering /etc/resolv.conf is typically done when
 193          * the network configuration changes, and that should be
 194          * enough to flush the global unicast DNS cache. */
 195         if (m->unicast_scope)
 196                 dns_cache_flush(&m->unicast_scope->cache);
 197
 198         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 199          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 200          * (think 192.168.1.1). */
 201         dns_server_reset_features_all(m->dns_servers);
 202
 203         return 0;
 204
 205 clear:
 206         dns_server_unlink_all(m->dns_servers);
 207         dns_search_domain_unlink_all(m->search_domains);
 208         return r;
 209 }
 210
 211 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 212         DnsScope *scope;
 213
 214         assert(s);
 215         assert(f);
 216         assert(count);
 217
 218         if (!dns_server_string(s)) {
 219                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 220                 return;
 221         }
 222
 223         /* Check if the scope this DNS server belongs to is suitable as 'default' route for lookups; resolv.conf does
 224          * not have a syntax to express that, so it must not appear as a global name server to avoid routing unrelated
 225          * domains to it (which is a privacy violation, will most probably fail anyway, and adds unnecessary load) */
 226         scope = dns_server_scope(s);
 227         if (scope && !dns_scope_is_default_route(scope)) {
 228                 log_debug("Scope of DNS server %s has only route-only domains, not using as global name server", dns_server_string(s));
 229                 return;
 230         }
 231
 232         if (*count == MAXNS)
 233                 fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 234         (*count)++;
 235
 236         fprintf(f, "nameserver %s\n", dns_server_string(s));
 237 }
 238
 239 static void write_resolv_conf_search(
 240                 OrderedSet *domains,
 241                 FILE *f) {
 242         unsigned length = 0, count = 0;
 243         Iterator i;
 244         char *domain;
 245
 246         assert(domains);
 247         assert(f);
 248
 249         fputs("search", f);
 250
 251         ORDERED_SET_FOREACH(domain, domains, i) {
 252                 if (++count > MAXDNSRCH) {
 253                         fputs("\n# Too many search domains configured, remaining ones ignored.", f);
 254                         break;
 255                 }
 256                 length += strlen(domain) + 1;
 257                 if (length > 256) {
 258                         fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 259                         break;
 260                 }
 261                 fputc(' ', f);
 262                 fputs(domain, f);
 263         }
 264
 265         fputs("\n", f);
 266 }
 267
 268 static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 269         Iterator i;
 270
 271         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 272               "#\n"
 273               "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 274               "# all known uplink DNS servers. This file lists all configured search domains.\n"
 275               "#\n"
 276               "# Third party programs should typically not access this file directly, but only\n"
 277               "# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a\n"
 278               "# different way, replace this symlink by a static file or a different symlink.\n"
 279               "#\n"
 280               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 281               "# operation for /etc/resolv.conf.\n"
 282               "\n", f);
 283
 284         if (ordered_set_isempty(dns))
 285                 fputs("# No DNS servers known.\n", f);
 286         else {
 287                 unsigned count = 0;
 288                 DnsServer *s;
 289
 290                 ORDERED_SET_FOREACH(s, dns, i)
 291                         write_resolv_conf_server(s, f, &count);
 292         }
 293
 294         if (!ordered_set_isempty(domains))
 295                 write_resolv_conf_search(domains, f);
 296
 297         return fflush_and_check(f);
 298 }
 299
 300 static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 301         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 302               "#\n"
 303               "# This is a dynamic resolv.conf file for connecting local clients to the\n"
 304               "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
 305               "# configured search domains.\n"
 306               "#\n"
 307               "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
 308               "# currently in use.\n"
 309               "#\n"
 310               "# Third party programs should typically not access this file directly, but only\n"
 311               "# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a\n"
 312               "# different way, replace this symlink by a static file or a different symlink.\n"
 313               "#\n"
 314               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 315               "# operation for /etc/resolv.conf.\n"
 316               "\n"
 317               "nameserver 127.0.0.53\n"
 318               "options edns0\n", f);
 319
 320         if (!ordered_set_isempty(domains))
 321                 write_resolv_conf_search(domains, f);
 322
 323         return fflush_and_check(f);
 324 }
 325
 326 int manager_write_resolv_conf(Manager *m) {
 327         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 328         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 329         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 330         int r;
 331
 332         assert(m);
 333
 334         /* Read the system /etc/resolv.conf first */
 335         (void) manager_read_resolv_conf(m);
 336
 337         /* Add the full list to a set, to filter out duplicates */
 338         r = manager_compile_dns_servers(m, &dns);
 339         if (r < 0)
 340                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 341
 342         r = manager_compile_search_domains(m, &domains, false);
 343         if (r < 0)
 344                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 345
 346         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 347         if (r < 0)
 348                 return log_warning_errno(r, "Failed to open new %s for writing: %m", PRIVATE_UPLINK_RESOLV_CONF);
 349
 350         (void) fchmod(fileno(f_uplink), 0644);
 351
 352         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
 353         if (r < 0) {
 354                 log_error_errno(r, "Failed to write new %s: %m", PRIVATE_UPLINK_RESOLV_CONF);
 355                 goto fail;
 356         }
 357
 358         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO) {
 359                 r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
 360                 if (r < 0) {
 361                         log_warning_errno(r, "Failed to open new %s for writing: %m", PRIVATE_STUB_RESOLV_CONF);
 362                         goto fail;
 363                 }
 364
 365                 (void) fchmod(fileno(f_stub), 0644);
 366
 367                 r = write_stub_resolv_conf_contents(f_stub, dns, domains);
 368                 if (r < 0) {
 369                         log_error_errno(r, "Failed to write new %s: %m", PRIVATE_STUB_RESOLV_CONF);
 370                         goto fail;
 371                 }
 372
 373                 if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0)
 374                         r = log_error_errno(errno, "Failed to move new %s into place: %m", PRIVATE_STUB_RESOLV_CONF);
 375
 376         } else {
 377                 r = symlink_atomic(basename(PRIVATE_UPLINK_RESOLV_CONF), PRIVATE_STUB_RESOLV_CONF);
 378                 if (r < 0)
 379                         log_error_errno(r, "Failed to symlink %s: %m", PRIVATE_STUB_RESOLV_CONF);
 380         }
 381
 382         if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0)
 383                 r = log_error_errno(errno, "Failed to move new %s into place: %m", PRIVATE_UPLINK_RESOLV_CONF);
 384
 385  fail:
 386         if (r < 0) {
 387                 /* Something went wrong, perform cleanup... */
 388                 (void) unlink(temp_path_uplink);
 389                 (void) unlink(temp_path_stub);
 390         }
 391
 392         return r;
 393 }