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