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 "stat-util.h"
  18 #include "string-util.h"
  19 #include "strv.h"
  20 #include "tmpfile-util-label.h"
  21
  22 int manager_check_resolv_conf(const Manager *m) {
  23         struct stat st, own;
  24
  25         assert(m);
  26
  27         /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
  28          * PRIVATE_STATIC_RESOLV_CONF. */
  29
  30         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  31                 return 0;
  32
  33         if (stat("/etc/resolv.conf", &st) < 0) {
  34                 if (errno == ENOENT)
  35                         return 0;
  36
  37                 return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  38         }
  39
  40         /* Is it symlinked to our own uplink file? */
  41         if (stat(PRIVATE_STATIC_RESOLV_CONF, &own) >= 0 &&
  42             st.st_dev == own.st_dev &&
  43             st.st_ino == own.st_ino)
  44                 return log_warning_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
  45                                          "DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to "
  46                                          PRIVATE_STATIC_RESOLV_CONF " which expects DNSStubListener= to be enabled.");
  47
  48         return 0;
  49 }
  50
  51 static bool file_is_our_own(const struct stat *st) {
  52         const char *path;
  53
  54         assert(st);
  55
  56         FOREACH_STRING(path,
  57                        PRIVATE_UPLINK_RESOLV_CONF,
  58                        PRIVATE_STUB_RESOLV_CONF,
  59                        PRIVATE_STATIC_RESOLV_CONF) {
  60
  61                 struct stat own;
  62
  63                 /* Is it symlinked to our own uplink file? */
  64                 if (stat(path, &own) >= 0 &&
  65                     st->st_dev == own.st_dev &&
  66                     st->st_ino == own.st_ino)
  67                         return true;
  68         }
  69
  70         return false;
  71 }
  72
  73 int manager_read_resolv_conf(Manager *m) {
  74         _cleanup_fclose_ FILE *f = NULL;
  75         struct stat st;
  76         unsigned n = 0;
  77         int r;
  78
  79         assert(m);
  80
  81         /* Reads the system /etc/resolv.conf, if it exists and is not
  82          * symlinked to our own resolv.conf instance */
  83
  84         if (!m->read_resolv_conf)
  85                 return 0;
  86
  87         r = stat("/etc/resolv.conf", &st);
  88         if (r < 0) {
  89                 if (errno == ENOENT)
  90                         return 0;
  91
  92                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  93                 goto clear;
  94         }
  95
  96         /* Have we already seen the file? */
  97         if (stat_inode_unmodified(&st, &m->resolv_conf_stat))
  98                 return 0;
  99
 100         if (file_is_our_own(&st))
 101                 return 0;
 102
 103         f = fopen("/etc/resolv.conf", "re");
 104         if (!f) {
 105                 if (errno == ENOENT)
 106                         return 0;
 107
 108                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 109                 goto clear;
 110         }
 111
 112         if (fstat(fileno(f), &st) < 0) {
 113                 r = log_error_errno(errno, "Failed to stat open file: %m");
 114                 goto clear;
 115         }
 116
 117         if (file_is_our_own(&st))
 118                 return 0;
 119
 120         dns_server_mark_all(m->dns_servers);
 121         dns_search_domain_mark_all(m->search_domains);
 122
 123         for (;;) {
 124                 _cleanup_free_ char *line = NULL;
 125                 const char *a;
 126                 char *l;
 127
 128                 r = read_line(f, LONG_LINE_MAX, &line);
 129                 if (r < 0) {
 130                         log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
 131                         goto clear;
 132                 }
 133                 if (r == 0)
 134                         break;
 135
 136                 n++;
 137
 138                 l = strstrip(line);
 139                 if (IN_SET(*l, '#', ';', 0))
 140                         continue;
 141
 142                 a = first_word(l, "nameserver");
 143                 if (a) {
 144                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 145                         if (r < 0)
 146                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 147
 148                         continue;
 149                 }
 150
 151                 a = first_word(l, "domain");
 152                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 153                         a = first_word(l, "search");
 154                 if (a) {
 155                         r = manager_parse_search_domains_and_warn(m, a);
 156                         if (r < 0)
 157                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 158                 }
 159
 160                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 161         }
 162
 163         m->resolv_conf_stat = st;
 164
 165         /* Flush out all servers and search domains that are still
 166          * marked. Those are then ones that didn't appear in the new
 167          * /etc/resolv.conf */
 168         dns_server_unlink_marked(m->dns_servers);
 169         dns_search_domain_unlink_marked(m->search_domains);
 170
 171         /* Whenever /etc/resolv.conf changes, start using the first
 172          * DNS server of it. This is useful to deal with broken
 173          * network managing implementations (like NetworkManager),
 174          * that when connecting to a VPN place both the VPN DNS
 175          * servers and the local ones in /etc/resolv.conf. Without
 176          * resetting the DNS server to use back to the first entry we
 177          * will continue to use the local one thus being unable to
 178          * resolve VPN domains. */
 179         manager_set_dns_server(m, m->dns_servers);
 180
 181         /* Unconditionally flush the cache when /etc/resolv.conf is
 182          * modified, even if the data it contained was completely
 183          * identical to the previous version we used. We do this
 184          * because altering /etc/resolv.conf is typically done when
 185          * the network configuration changes, and that should be
 186          * enough to flush the global unicast DNS cache. */
 187         if (m->unicast_scope)
 188                 dns_cache_flush(&m->unicast_scope->cache);
 189
 190         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 191          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 192          * (think 192.168.1.1). */
 193         dns_server_reset_features_all(m->dns_servers);
 194
 195         return 0;
 196
 197 clear:
 198         dns_server_unlink_all(m->dns_servers);
 199         dns_search_domain_unlink_all(m->search_domains);
 200         return r;
 201 }
 202
 203 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 204         DnsScope *scope;
 205
 206         assert(s);
 207         assert(f);
 208         assert(count);
 209
 210         if (!dns_server_string(s)) {
 211                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 212                 return;
 213         }
 214
 215         /* Check if the scope this DNS server belongs to is suitable as 'default' route for lookups; resolv.conf does
 216          * not have a syntax to express that, so it must not appear as a global name server to avoid routing unrelated
 217          * domains to it (which is a privacy violation, will most probably fail anyway, and adds unnecessary load) */
 218         scope = dns_server_scope(s);
 219         if (scope && !dns_scope_is_default_route(scope)) {
 220                 log_debug("Scope of DNS server %s has only route-only domains, not using as global name server", dns_server_string(s));
 221                 return;
 222         }
 223
 224         if (*count == MAXNS)
 225                 fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 226         (*count)++;
 227
 228         fprintf(f, "nameserver %s\n", dns_server_string(s));
 229 }
 230
 231 static void write_resolv_conf_search(
 232                 OrderedSet *domains,
 233                 FILE *f) {
 234         unsigned length = 0, count = 0;
 235         Iterator i;
 236         char *domain;
 237
 238         assert(domains);
 239         assert(f);
 240
 241         fputs("search", f);
 242
 243         ORDERED_SET_FOREACH(domain, domains, i) {
 244                 if (++count > MAXDNSRCH) {
 245                         fputs("\n# Too many search domains configured, remaining ones ignored.", f);
 246                         break;
 247                 }
 248                 length += strlen(domain) + 1;
 249                 if (length > 256) {
 250                         fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 251                         break;
 252                 }
 253                 fputc(' ', f);
 254                 fputs(domain, f);
 255         }
 256
 257         fputs("\n", f);
 258 }
 259
 260 static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 261         Iterator i;
 262
 263         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 264               "#\n"
 265               "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 266               "# all known uplink DNS servers. This file lists all configured search domains.\n"
 267               "#\n"
 268               "# Third party programs should typically not access this file directly, but only\n"
 269               "# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a\n"
 270               "# different way, replace this symlink by a static file or a different symlink.\n"
 271               "#\n"
 272               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 273               "# operation for /etc/resolv.conf.\n"
 274               "\n", f);
 275
 276         if (ordered_set_isempty(dns))
 277                 fputs("# No DNS servers known.\n", f);
 278         else {
 279                 unsigned count = 0;
 280                 DnsServer *s;
 281
 282                 ORDERED_SET_FOREACH(s, dns, i)
 283                         write_resolv_conf_server(s, f, &count);
 284         }
 285
 286         if (!ordered_set_isempty(domains))
 287                 write_resolv_conf_search(domains, f);
 288
 289         return fflush_and_check(f);
 290 }
 291
 292 static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 293         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 294               "#\n"
 295               "# This is a dynamic resolv.conf file for connecting local clients to the\n"
 296               "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
 297               "# configured search domains.\n"
 298               "#\n"
 299               "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
 300               "# currently in use.\n"
 301               "#\n"
 302               "# Third party programs should typically not access this file directly, but only\n"
 303               "# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a\n"
 304               "# different way, replace this symlink by a static file or a different symlink.\n"
 305               "#\n"
 306               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 307               "# operation for /etc/resolv.conf.\n"
 308               "\n"
 309               "nameserver 127.0.0.53\n"
 310               "options edns0 trust-ad\n", f);
 311
 312         if (!ordered_set_isempty(domains))
 313                 write_resolv_conf_search(domains, f);
 314
 315         return fflush_and_check(f);
 316 }
 317
 318 int manager_write_resolv_conf(Manager *m) {
 319         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 320         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 321         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 322         int r;
 323
 324         assert(m);
 325
 326         /* Read the system /etc/resolv.conf first */
 327         (void) manager_read_resolv_conf(m);
 328
 329         /* Add the full list to a set, to filter out duplicates */
 330         r = manager_compile_dns_servers(m, &dns);
 331         if (r < 0)
 332                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 333
 334         r = manager_compile_search_domains(m, &domains, false);
 335         if (r < 0)
 336                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 337
 338         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 339         if (r < 0)
 340                 return log_warning_errno(r, "Failed to open new %s for writing: %m", PRIVATE_UPLINK_RESOLV_CONF);
 341
 342         (void) fchmod(fileno(f_uplink), 0644);
 343
 344         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
 345         if (r < 0) {
 346                 log_error_errno(r, "Failed to write new %s: %m", PRIVATE_UPLINK_RESOLV_CONF);
 347                 goto fail;
 348         }
 349
 350         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO) {
 351                 r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
 352                 if (r < 0) {
 353                         log_warning_errno(r, "Failed to open new %s for writing: %m", PRIVATE_STUB_RESOLV_CONF);
 354                         goto fail;
 355                 }
 356
 357                 (void) fchmod(fileno(f_stub), 0644);
 358
 359                 r = write_stub_resolv_conf_contents(f_stub, dns, domains);
 360                 if (r < 0) {
 361                         log_error_errno(r, "Failed to write new %s: %m", PRIVATE_STUB_RESOLV_CONF);
 362                         goto fail;
 363                 }
 364
 365                 if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0)
 366                         r = log_error_errno(errno, "Failed to move new %s into place: %m", PRIVATE_STUB_RESOLV_CONF);
 367
 368         } else {
 369                 r = symlink_atomic(basename(PRIVATE_UPLINK_RESOLV_CONF), PRIVATE_STUB_RESOLV_CONF);
 370                 if (r < 0)
 371                         log_error_errno(r, "Failed to symlink %s: %m", PRIVATE_STUB_RESOLV_CONF);
 372         }
 373
 374         if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0)
 375                 r = log_error_errno(errno, "Failed to move new %s into place: %m", PRIVATE_UPLINK_RESOLV_CONF);
 376
 377  fail:
 378         if (r < 0) {
 379                 /* Something went wrong, perform cleanup... */
 380                 (void) unlink(temp_path_uplink);
 381                 (void) unlink(temp_path_stub);
 382         }
 383
 384         return r;
 385 }