src/resolve/resolved-resolv-conf.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2014 Tom Gundersen <teg@jklm.no>
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18  ***/
  19
  20 #include <resolv.h>
  21
  22 #include "alloc-util.h"
  23 #include "dns-domain.h"
  24 #include "fd-util.h"
  25 #include "fileio-label.h"
  26 #include "fileio.h"
  27 #include "ordered-set.h"
  28 #include "resolved-conf.h"
  29 #include "resolved-dns-server.h"
  30 #include "resolved-resolv-conf.h"
  31 #include "string-util.h"
  32 #include "strv.h"
  33
  34 int manager_read_resolv_conf(Manager *m) {
  35         _cleanup_fclose_ FILE *f = NULL;
  36         struct stat st, own;
  37         char line[LINE_MAX];
  38         usec_t t;
  39         int r;
  40
  41         assert(m);
  42
  43         /* Reads the system /etc/resolv.conf, if it exists and is not
  44          * symlinked to our own resolv.conf instance */
  45
  46         if (!m->read_resolv_conf)
  47                 return 0;
  48
  49         r = stat("/etc/resolv.conf", &st);
  50         if (r < 0) {
  51                 if (errno == ENOENT)
  52                         return 0;
  53
  54                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  55                 goto clear;
  56         }
  57
  58         /* Have we already seen the file? */
  59         t = timespec_load(&st.st_mtim);
  60         if (t == m->resolv_conf_mtime)
  61                 return 0;
  62
  63         /* Is it symlinked to our own file? */
  64         if (stat(PRIVATE_RESOLV_CONF, &own) >= 0 &&
  65             st.st_dev == own.st_dev &&
  66             st.st_ino == own.st_ino)
  67                 return 0;
  68
  69         f = fopen("/etc/resolv.conf", "re");
  70         if (!f) {
  71                 if (errno == ENOENT)
  72                         return 0;
  73
  74                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
  75                 goto clear;
  76         }
  77
  78         if (fstat(fileno(f), &st) < 0) {
  79                 r = log_error_errno(errno, "Failed to stat open file: %m");
  80                 goto clear;
  81         }
  82
  83         dns_server_mark_all(m->dns_servers);
  84         dns_search_domain_mark_all(m->search_domains);
  85
  86         FOREACH_LINE(line, f, r = -errno; goto clear) {
  87                 const char *a;
  88                 char *l;
  89
  90                 l = strstrip(line);
  91                 if (IN_SET(*l, '#', ';'))
  92                         continue;
  93
  94                 a = first_word(l, "nameserver");
  95                 if (a) {
  96                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
  97                         if (r < 0)
  98                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
  99
 100                         continue;
 101                 }
 102
 103                 a = first_word(l, "domain");
 104                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 105                         a = first_word(l, "search");
 106                 if (a) {
 107                         r = manager_parse_search_domains_and_warn(m, a);
 108                         if (r < 0)
 109                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 110                 }
 111         }
 112
 113         m->resolv_conf_mtime = t;
 114
 115         /* Flush out all servers and search domains that are still
 116          * marked. Those are then ones that didn't appear in the new
 117          * /etc/resolv.conf */
 118         dns_server_unlink_marked(m->dns_servers);
 119         dns_search_domain_unlink_marked(m->search_domains);
 120
 121         /* Whenever /etc/resolv.conf changes, start using the first
 122          * DNS server of it. This is useful to deal with broken
 123          * network managing implementations (like NetworkManager),
 124          * that when connecting to a VPN place both the VPN DNS
 125          * servers and the local ones in /etc/resolv.conf. Without
 126          * resetting the DNS server to use back to the first entry we
 127          * will continue to use the local one thus being unable to
 128          * resolve VPN domains. */
 129         manager_set_dns_server(m, m->dns_servers);
 130
 131         /* Unconditionally flush the cache when /etc/resolv.conf is
 132          * modified, even if the data it contained was completely
 133          * identical to the previous version we used. We do this
 134          * because altering /etc/resolv.conf is typically done when
 135          * the network configuration changes, and that should be
 136          * enough to flush the global unicast DNS cache. */
 137         if (m->unicast_scope)
 138                 dns_cache_flush(&m->unicast_scope->cache);
 139
 140         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 141          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 142          * (think 192.168.1.1). */
 143         dns_server_reset_features_all(m->dns_servers);
 144
 145         return 0;
 146
 147 clear:
 148         dns_server_unlink_all(m->dns_servers);
 149         dns_search_domain_unlink_all(m->search_domains);
 150         return r;
 151 }
 152
 153 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 154         assert(s);
 155         assert(f);
 156         assert(count);
 157
 158         if (!dns_server_string(s)) {
 159                 log_warning("Our of memory, or invalid DNS address. Ignoring server.");
 160                 return;
 161         }
 162
 163         /* Check if the DNS server is limited to particular domains;
 164          * resolv.conf does not have a syntax to express that, so it must not
 165          * appear as a global name server to avoid routing unrelated domains to
 166          * it (which is a privacy violation, will most probably fail anyway,
 167          * and adds unnecessary load) */
 168         if (dns_server_limited_domains(s)) {
 169                 log_debug("DNS server %s has route-only domains, not using as global name server", dns_server_string(s));
 170                 return;
 171         }
 172
 173         if (*count == MAXNS)
 174                 fputs_unlocked("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 175         (*count)++;
 176
 177         fprintf(f, "nameserver %s\n", dns_server_string(s));
 178 }
 179
 180 static void write_resolv_conf_search(
 181                 OrderedSet *domains,
 182                 FILE *f) {
 183         unsigned length = 0, count = 0;
 184         Iterator i;
 185         char *domain;
 186
 187         assert(domains);
 188         assert(f);
 189
 190         fputs_unlocked("search", f);
 191
 192         ORDERED_SET_FOREACH(domain, domains, i) {
 193                 if (++count > MAXDNSRCH) {
 194                         fputs_unlocked("\n# Too many search domains configured, remaining ones ignored.", f);
 195                         break;
 196                 }
 197                 length += strlen(domain) + 1;
 198                 if (length > 256) {
 199                         fputs_unlocked("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 200                         break;
 201                 }
 202                 fputc_unlocked(' ', f);
 203                 fputs_unlocked(domain, f);
 204         }
 205
 206         fputs_unlocked("\n", f);
 207 }
 208
 209 static int write_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 210         Iterator i;
 211
 212         fputs_unlocked("# This file is managed by man:systemd-resolved(8). Do not edit.\n#\n"
 213                        "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 214                        "# all known DNS servers.\n#\n"
 215                        "# Third party programs must not access this file directly, but only through the\n"
 216                        "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 217                        "# replace this symlink by a static file or a different symlink.\n#\n"
 218                        "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 219                        "# operation for /etc/resolv.conf.\n\n", f);
 220
 221         if (ordered_set_isempty(dns))
 222                 fputs_unlocked("# No DNS servers known.\n", f);
 223         else {
 224                 unsigned count = 0;
 225                 DnsServer *s;
 226
 227                 ORDERED_SET_FOREACH(s, dns, i)
 228                         write_resolv_conf_server(s, f, &count);
 229         }
 230
 231         if (!ordered_set_isempty(domains))
 232                 write_resolv_conf_search(domains, f);
 233
 234         return fflush_and_check(f);
 235 }
 236
 237 int manager_write_resolv_conf(Manager *m) {
 238
 239         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 240         _cleanup_free_ char *temp_path = NULL;
 241         _cleanup_fclose_ FILE *f = NULL;
 242         int r;
 243
 244         assert(m);
 245
 246         /* Read the system /etc/resolv.conf first */
 247         (void) manager_read_resolv_conf(m);
 248
 249         /* Add the full list to a set, to filter out duplicates */
 250         r = manager_compile_dns_servers(m, &dns);
 251         if (r < 0)
 252                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 253
 254         r = manager_compile_search_domains(m, &domains, false);
 255         if (r < 0)
 256                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 257
 258         r = fopen_temporary_label(PRIVATE_RESOLV_CONF, PRIVATE_RESOLV_CONF, &f, &temp_path);
 259         if (r < 0)
 260                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 261
 262         (void) fchmod(fileno(f), 0644);
 263
 264         r = write_resolv_conf_contents(f, dns, domains);
 265         if (r < 0) {
 266                 log_error_errno(r, "Failed to write private resolv.conf contents: %m");
 267                 goto fail;
 268         }
 269
 270         if (rename(temp_path, PRIVATE_RESOLV_CONF) < 0) {
 271                 r = log_error_errno(errno, "Failed to move private resolv.conf file into place: %m");
 272                 goto fail;
 273         }
 274
 275         return 0;
 276
 277 fail:
 278         (void) unlink(PRIVATE_RESOLV_CONF);
 279         (void) unlink(temp_path);
 280
 281         return r;
 282 }