Merge remote-tracking branch 'tor-github/pr/425'

changes/bug21900

@@ -0,0 +1,4 @@
+  o Minor bugfixes (DNS):
+    - Gracefully handle empty or absent resolve.conf file by falling
+      back to using localhost DNS service and hoping it works. Fixes
+      bug 21900; bugfix on 0.2.1.10-alpha.

configure.ac

@@ -812,6 +812,8 @@ fi
 dnl Now check for particular libevent functions.
 AC_CHECK_FUNCS([evutil_secure_rng_set_urandom_device_file \
                 evutil_secure_rng_add_bytes \
+                evdns_base_get_nameserver_addr \
+
 ])
 
 LIBS="$save_LIBS"

doc/tor.1.txt

@@ -2211,7 +2211,8 @@ is non-zero):
     __filename__. The file format is the same as the standard Unix
     "**resolv.conf**" file (7). This option, like all other ServerDNS options,
     only affects name lookups that your server does on behalf of clients.
-    (Defaults to use the system DNS configuration.)
+    (Defaults to use the system DNS configuration or a localhost DNS service
+    in case no nameservers are found in a given configuration.)
 
 [[ServerDNSAllowBrokenConfig]] **ServerDNSAllowBrokenConfig** **0**|**1**::
     If this option is false, Tor exits immediately if there are problems

src/feature/relay/dns.c

@@ -1357,6 +1357,41 @@ evdns_err_is_transient(int err)
   }
 }
 
+/**
+ * Return number of configured nameservers in <b>the_evdns_base</b>.
+ */
+size_t
+number_of_configured_nameservers(void)
+{
+  return evdns_base_count_nameservers(the_evdns_base);
+}
+
+#ifdef HAVE_EVDNS_BASE_GET_NAMESERVER_ADDR
+/**
+ * Return address of configured nameserver in <b>the_evdns_base</b>
+ * at index <b>idx</b>.
+ */
+tor_addr_t *
+configured_nameserver_address(const size_t idx)
+{
+ struct sockaddr_storage sa;
+ ev_socklen_t sa_len = sizeof(sa);
+
+ if (evdns_base_get_nameserver_addr(the_evdns_base, (int)idx,
+                                    (struct sockaddr *)&sa,
+                                    sa_len) > 0) {
+   tor_addr_t *tor_addr = tor_malloc(sizeof(tor_addr_t));
+   if (tor_addr_from_sockaddr(tor_addr,
+                              (const struct sockaddr *)&sa,
+                              NULL) == 0) {
+     return tor_addr;
+   }
+ }
+
+ return NULL;
+}
+#endif
+
 /** Configure eventdns nameservers if force is true, or if the configuration
  * has changed since the last time we called this function, or if we failed on
  * our last attempt.  On Unix, this reads from /etc/resolv.conf or
@@ -1388,16 +1423,23 @@ configure_nameservers(int force)
   evdns_set_log_fn(evdns_log_cb);
   if (conf_fname) {
     log_debug(LD_FS, "stat()ing %s", conf_fname);
-    if (stat(sandbox_intern_string(conf_fname), &st)) {
+    int missing_resolv_conf = 0;
+    int stat_res = stat(sandbox_intern_string(conf_fname), &st);
+
+    if (stat_res) {
       log_warn(LD_EXIT, "Unable to stat resolver configuration in '%s': %s",
                conf_fname, strerror(errno));
-      goto err;
-    }
-    if (!force && resolv_conf_fname && !strcmp(conf_fname,resolv_conf_fname)
+      missing_resolv_conf = 1;
+    } else if (!force && resolv_conf_fname &&
+               !strcmp(conf_fname,resolv_conf_fname)
         && st.st_mtime == resolv_conf_mtime) {
       log_info(LD_EXIT, "No change to '%s'", conf_fname);
       return 0;
     }
+
+    if (stat_res == 0 && st.st_size == 0)
+      missing_resolv_conf = 1;
+
     if (nameservers_configured) {
       evdns_base_search_clear(the_evdns_base);
       evdns_base_clear_nameservers_and_suspend(the_evdns_base);
@@ -1410,20 +1452,34 @@ configure_nameservers(int force)
           sandbox_intern_string("/etc/hosts"));
     }
 #endif /* defined(DNS_OPTION_HOSTSFILE) && defined(USE_LIBSECCOMP) */
-    log_info(LD_EXIT, "Parsing resolver configuration in '%s'", conf_fname);
-    if ((r = evdns_base_resolv_conf_parse(the_evdns_base, flags,
-        sandbox_intern_string(conf_fname)))) {
-      log_warn(LD_EXIT, "Unable to parse '%s', or no nameservers in '%s' (%d)",
-               conf_fname, conf_fname, r);
-      goto err;
+
+    if (!missing_resolv_conf) {
+      log_info(LD_EXIT, "Parsing resolver configuration in '%s'", conf_fname);
+      if ((r = evdns_base_resolv_conf_parse(the_evdns_base, flags,
+          sandbox_intern_string(conf_fname)))) {
+        log_warn(LD_EXIT, "Unable to parse '%s', or no nameservers "
+                          "in '%s' (%d)", conf_fname, conf_fname, r);
+
+        if (r != 6) // "r = 6" means "no DNS servers were in resolv.conf" -
+          goto err; // in which case we expect libevent to add 127.0.0.1 as
+                    // fallback.
+      }
+      if (evdns_base_count_nameservers(the_evdns_base) == 0) {
+        log_warn(LD_EXIT, "Unable to find any nameservers in '%s'.",
+                 conf_fname);
+      }
+
+      tor_free(resolv_conf_fname);
+      resolv_conf_fname = tor_strdup(conf_fname);
+      resolv_conf_mtime = st.st_mtime;
+    } else {
+      log_warn(LD_EXIT, "Could not read your DNS config from '%s' - "
+                        "please investigate your DNS configuration. "
+                        "This is possibly a problem. Meanwhile, falling"
+                        " back to local DNS at 127.0.0.1.", conf_fname);
+      evdns_base_nameserver_ip_add(the_evdns_base, "127.0.0.1");
     }
-    if (evdns_base_count_nameservers(the_evdns_base) == 0) {
-      log_warn(LD_EXIT, "Unable to find any nameservers in '%s'.", conf_fname);
-      goto err;
-    }
-    tor_free(resolv_conf_fname);
-    resolv_conf_fname = tor_strdup(conf_fname);
-    resolv_conf_mtime = st.st_mtime;
+
     if (nameservers_configured)
       evdns_base_resume(the_evdns_base);
   }

src/feature/relay/dns.h

@@ -45,6 +45,11 @@ size_t dns_cache_handle_oom(time_t now, size_t min_remove_bytes);
 #ifdef DNS_PRIVATE
 #include "feature/relay/dns_structs.h"
 
+size_t number_of_configured_nameservers(void);
+#ifdef HAVE_EVDNS_BASE_GET_NAMESERVER_ADDR
+tor_addr_t *configured_nameserver_address(const size_t idx);
+#endif
+
 MOCK_DECL(STATIC int,dns_resolve_impl,(edge_connection_t *exitconn,
 int is_resolve,or_circuit_t *oncirc, char **hostname_out,
 int *made_connection_pending_out, cached_resolve_t **resolve_out));

src/test/test_dns.c

@@ -1,6 +1,7 @@
 /* Copyright (c) 2015-2018, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+#include "orconfig.h"
 #include "core/or/or.h"
 #include "test/test.h"
 
@@ -13,9 +14,71 @@
 
 #include "core/or/edge_connection_st.h"
 #include "core/or/or_circuit_st.h"
+#include "app/config/or_options_st.h"
+#include "app/config/config.h"
+
+#include <event2/event.h>
+#include <event2/dns.h>
 
 #define NS_MODULE dns
 
+#ifdef HAVE_EVDNS_BASE_GET_NAMESERVER_ADDR
+#define NS_SUBMODULE configure_nameservers_fallback
+
+static or_options_t options = {
+  .ORPort_set = 1,
+};
+
+static const or_options_t *
+mock_get_options(void)
+{
+  return &options;
+}
+
+static void
+NS(test_main)(void *arg)
+{
+  (void)arg;
+  tor_addr_t *nameserver_addr = NULL;
+
+  MOCK(get_options, mock_get_options);
+
+  options.ServerDNSResolvConfFile = (char *)"no_such_file!!!";
+
+  dns_init(); // calls configure_nameservers()
+
+  tt_int_op(number_of_configured_nameservers(), OP_EQ, 1);
+
+  nameserver_addr = configured_nameserver_address(0);
+
+  tt_assert(tor_addr_family(nameserver_addr) == AF_INET);
+  tt_assert(tor_addr_eq_ipv4h(nameserver_addr, 0x7f000001));
+
+#ifndef _WIN32
+  tor_free(nameserver_addr);
+
+  options.ServerDNSResolvConfFile = (char *)"/dev/null";
+
+  dns_init();
+
+  tt_int_op(number_of_configured_nameservers(), OP_EQ, 1);
+
+  nameserver_addr = configured_nameserver_address(0);
+
+  tt_assert(tor_addr_family(nameserver_addr) == AF_INET);
+  tt_assert(tor_addr_eq_ipv4h(nameserver_addr, 0x7f000001));
+#endif
+
+  UNMOCK(get_options);
+
+ done:
+  tor_free(nameserver_addr);
+  return;
+}
+
+#undef NS_SUBMODULE
+#endif
+
 #define NS_SUBMODULE clip_ttl
 
 static void
@@ -736,6 +799,9 @@ NS(test_main)(void *arg)
 #undef NS_SUBMODULE
 
 struct testcase_t dns_tests[] = {
+#ifdef HAVE_EVDNS_BASE_GET_NAMESERVER_ADDR
+   TEST_CASE(configure_nameservers_fallback),
+#endif
    TEST_CASE(clip_ttl),
    TEST_CASE(resolve),
    TEST_CASE_ASPECT(resolve_impl, addr_is_ip_no_need_to_resolve),