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 "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 #include "tmpfile-util-label.h"
  19
  20 /* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
  21 #define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"
  22
  23 /* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
  24 #define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"
  25
  26 /* A static resolv.conf file containing no domains, but only our own DNS server address */
  27 #define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"
  28
  29 int manager_check_resolv_conf(const Manager *m) {
  30         const char *path;
  31         struct stat st;
  32         int r;
  33
  34         assert(m);
  35
  36         /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
  37          * PRIVATE_STATIC_RESOLV_CONF or PRIVATE_STUB_RESOLV_CONF. */
  38
  39         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  40                 return 0;
  41
  42         r = stat("/etc/resolv.conf", &st);
  43         if (r < 0) {
  44                 if (errno == ENOENT)
  45                         return 0;
  46
  47                 return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  48         }
  49
  50         FOREACH_STRING(path,
  51                        PRIVATE_STUB_RESOLV_CONF,
  52                        PRIVATE_STATIC_RESOLV_CONF) {
  53
  54                 struct stat own;
  55
  56                 /* Is it symlinked to our own uplink file? */
  57                 if (stat(path, &own) >= 0 &&
  58                     st.st_dev == own.st_dev &&
  59                     st.st_ino == own.st_ino) {
  60                         log_warning("DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to %s "
  61                                     "which expects DNSStubListener= to be enabled.", path);
  62                         return -EOPNOTSUPP;
  63                 }
  64         }
  65
  66         return 0;
  67 }
  68
  69 static bool file_is_our_own(const struct stat *st) {
  70         const char *path;
  71
  72         assert(st);
  73
  74         FOREACH_STRING(path,
  75                        PRIVATE_UPLINK_RESOLV_CONF,
  76                        PRIVATE_STUB_RESOLV_CONF,
  77                        PRIVATE_STATIC_RESOLV_CONF) {
  78
  79                 struct stat own;
  80
  81                 /* Is it symlinked to our own uplink file? */
  82                 if (stat(path, &own) >= 0 &&
  83                     st->st_dev == own.st_dev &&
  84                     st->st_ino == own.st_ino)
  85                         return true;
  86         }
  87
  88         return false;
  89 }
  90
  91 int manager_read_resolv_conf(Manager *m) {
  92         _cleanup_fclose_ FILE *f = NULL;
  93         struct stat st;
  94         unsigned n = 0;
  95         int r;
  96
  97         assert(m);
  98
  99         /* Reads the system /etc/resolv.conf, if it exists and is not
 100          * symlinked to our own resolv.conf instance */
 101
 102         if (!m->read_resolv_conf)
 103                 return 0;
 104
 105         r = stat("/etc/resolv.conf", &st);
 106         if (r < 0) {
 107                 if (errno == ENOENT)
 108                         return 0;
 109
 110                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
 111                 goto clear;
 112         }
 113
 114         /* Have we already seen the file? */
 115         if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
 116                 return 0;
 117
 118         if (file_is_our_own(&st))
 119                 return 0;
 120
 121         f = fopen("/etc/resolv.conf", "re");
 122         if (!f) {
 123                 if (errno == ENOENT)
 124                         return 0;
 125
 126                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 127                 goto clear;
 128         }
 129
 130         if (fstat(fileno(f), &st) < 0) {
 131                 r = log_error_errno(errno, "Failed to stat open file: %m");
 132                 goto clear;
 133         }
 134
 135         if (file_is_our_own(&st))
 136                 return 0;
 137
 138         dns_server_mark_all(m->dns_servers);
 139         dns_search_domain_mark_all(m->search_domains);
 140
 141         for (;;) {
 142                 _cleanup_free_ char *line = NULL;
 143                 const char *a;
 144                 char *l;
 145
 146                 r = read_line(f, LONG_LINE_MAX, &line);
 147                 if (r < 0) {
 148                         log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
 149                         goto clear;
 150                 }
 151                 if (r == 0)
 152                         break;
 153
 154                 n++;
 155
 156                 l = strstrip(line);
 157                 if (IN_SET(*l, '#', ';', 0))
 158                         continue;
 159
 160                 a = first_word(l, "nameserver");
 161                 if (a) {
 162                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 163                         if (r < 0)
 164                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 165
 166                         continue;
 167                 }
 168
 169                 a = first_word(l, "domain");
 170                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 171                         a = first_word(l, "search");
 172                 if (a) {
 173                         r = manager_parse_search_domains_and_warn(m, a);
 174                         if (r < 0)
 175                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 176                 }
 177
 178                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 179         }
 180
 181         m->resolv_conf_mtime = timespec_load(&st.st_mtim);
 182
 183         /* Flush out all servers and search domains that are still
 184          * marked. Those are then ones that didn't appear in the new
 185          * /etc/resolv.conf */
 186         dns_server_unlink_marked(m->dns_servers);
 187         dns_search_domain_unlink_marked(m->search_domains);
 188
 189         /* Whenever /etc/resolv.conf changes, start using the first
 190          * DNS server of it. This is useful to deal with broken
 191          * network managing implementations (like NetworkManager),
 192          * that when connecting to a VPN place both the VPN DNS
 193          * servers and the local ones in /etc/resolv.conf. Without
 194          * resetting the DNS server to use back to the first entry we
 195          * will continue to use the local one thus being unable to
 196          * resolve VPN domains. */
 197         manager_set_dns_server(m, m->dns_servers);
 198
 199         /* Unconditionally flush the cache when /etc/resolv.conf is
 200          * modified, even if the data it contained was completely
 201          * identical to the previous version we used. We do this
 202          * because altering /etc/resolv.conf is typically done when
 203          * the network configuration changes, and that should be
 204          * enough to flush the global unicast DNS cache. */
 205         if (m->unicast_scope)
 206                 dns_cache_flush(&m->unicast_scope->cache);
 207
 208         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 209          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 210          * (think 192.168.1.1). */
 211         dns_server_reset_features_all(m->dns_servers);
 212
 213         return 0;
 214
 215 clear:
 216         dns_server_unlink_all(m->dns_servers);
 217         dns_search_domain_unlink_all(m->search_domains);
 218         return r;
 219 }
 220
 221 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 222         DnsScope *scope;
 223
 224         assert(s);
 225         assert(f);
 226         assert(count);
 227
 228         if (!dns_server_string(s)) {
 229                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 230                 return;
 231         }
 232
 233         /* Check if the scope this DNS server belongs to is suitable as 'default' route for lookups; resolv.conf does
 234          * not have a syntax to express that, so it must not appear as a global name server to avoid routing unrelated
 235          * domains to it (which is a privacy violation, will most probably fail anyway, and adds unnecessary load) */
 236         scope = dns_server_scope(s);
 237         if (scope && !dns_scope_is_default_route(scope)) {
 238                 log_debug("Scope of DNS server %s has only route-only domains, not using as global name server", dns_server_string(s));
 239                 return;
 240         }
 241
 242         if (*count == MAXNS)
 243                 fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 244         (*count)++;
 245
 246         fprintf(f, "nameserver %s\n", dns_server_string(s));
 247 }
 248
 249 static void write_resolv_conf_search(
 250                 OrderedSet *domains,
 251                 FILE *f) {
 252         unsigned length = 0, count = 0;
 253         Iterator i;
 254         char *domain;
 255
 256         assert(domains);
 257         assert(f);
 258
 259         fputs("search", f);
 260
 261         ORDERED_SET_FOREACH(domain, domains, i) {
 262                 if (++count > MAXDNSRCH) {
 263                         fputs("\n# Too many search domains configured, remaining ones ignored.", f);
 264                         break;
 265                 }
 266                 length += strlen(domain) + 1;
 267                 if (length > 256) {
 268                         fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 269                         break;
 270                 }
 271                 fputc(' ', f);
 272                 fputs(domain, f);
 273         }
 274
 275         fputs("\n", f);
 276 }
 277
 278 static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 279         Iterator i;
 280
 281         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 282               "#\n"
 283               "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 284               "# all known uplink DNS servers. This file lists all configured search domains.\n"
 285               "#\n"
 286               "# Third party programs must not access this file directly, but only through the\n"
 287               "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 288               "# replace this symlink by a static file or a different symlink.\n"
 289               "#\n"
 290               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 291               "# operation for /etc/resolv.conf.\n"
 292               "\n", f);
 293
 294         if (ordered_set_isempty(dns))
 295                 fputs("# No DNS servers known.\n", f);
 296         else {
 297                 unsigned count = 0;
 298                 DnsServer *s;
 299
 300                 ORDERED_SET_FOREACH(s, dns, i)
 301                         write_resolv_conf_server(s, f, &count);
 302         }
 303
 304         if (!ordered_set_isempty(domains))
 305                 write_resolv_conf_search(domains, f);
 306
 307         return fflush_and_check(f);
 308 }
 309
 310 static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 311         fputs_unlocked("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 312                        "#\n"
 313                        "# This is a dynamic resolv.conf file for connecting local clients to the\n"
 314                        "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
 315                        "# configured search domains.\n"
 316                        "#\n"
 317                        "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
 318                        "# currently in use.\n"
 319                        "#\n"
 320                        "# Third party programs must not access this file directly, but only through the\n"
 321                        "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 322                        "# replace this symlink by a static file or a different symlink.\n"
 323                        "#\n"
 324                        "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 325                        "# operation for /etc/resolv.conf.\n"
 326                        "\n"
 327                        "nameserver 127.0.0.53\n"
 328                        "options edns0\n", f);
 329
 330         if (!ordered_set_isempty(domains))
 331                 write_resolv_conf_search(domains, f);
 332
 333         return fflush_and_check(f);
 334 }
 335
 336 int manager_write_resolv_conf(Manager *m) {
 337         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 338         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 339         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 340         int r;
 341
 342         assert(m);
 343
 344         /* Read the system /etc/resolv.conf first */
 345         (void) manager_read_resolv_conf(m);
 346
 347         /* Add the full list to a set, to filter out duplicates */
 348         r = manager_compile_dns_servers(m, &dns);
 349         if (r < 0)
 350                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 351
 352         r = manager_compile_search_domains(m, &domains, false);
 353         if (r < 0)
 354                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 355
 356         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 357         if (r < 0)
 358                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 359
 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) 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 }