relay: Allow RFC1918 addresses for non public relays

In other words, if PublishServerDescriptor is set to 0 and AssumeReachable to
1, then allow a relay to hold a RFC1918 address.

Reasons for this are documented in #40208

Fixes #40208

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket40208

+  o Minor bugfixes (relay):
+    - Allow relays to have a RFC1918 address if PublishServerDescriptor is set
+      to 0 and AssumeReachable is set to 1. This is to support the use case of a
+      bridge on a local network that can be used by restricted users on that
+      network to reach the Tor network. Fixes bug 40208; bugfix on
+      0.4.5.1-alpha.

src/app/config/resolve_addr.c

@@ -193,7 +193,19 @@ address_can_be_used(const tor_addr_t *addr, const or_options_t *options,
     goto allow;
   }
 
-  /* We have a private IP address. It is allowed only if we set custom
+  /* We allow internal addresses to be used if the PublishServerDescriptor is
+   * unset and AssumeReachable (or for IPv6) is set.
+   *
+   * This is to cover the case where a relay/bridge might be run behind a
+   * firewall on a local network to users can reach the network through it
+   * using Tor Browser for instance. */
+  if (options->PublishServerDescriptor_ == NO_DIRINFO &&
+      (options->AssumeReachable ||
+       (tor_addr_family(addr) == AF_INET6 && options->AssumeReachableIPv6))) {
+    goto allow;
+  }
+
+  /* We have a private IP address. This is also allowed if we set custom
    * directory authorities. */
   if (using_default_dir_authorities(options)) {
     log_fn(warn_severity, LD_CONFIG,

src/test/test_config.c

@@ -1460,6 +1460,7 @@ test_config_find_my_address(void *arg)
 
   options = options_new();
   options_init(options);
+  options->PublishServerDescriptor_ = V3_DIRINFO;
 
   /*
    * Case 0:
@@ -1782,6 +1783,22 @@ test_config_find_my_address(void *arg)
   VALIDATE_FOUND_ADDRESS(true, RESOLVED_ADDR_INTERFACE, NULL);
   CLEANUP_FOUND_ADDRESS;
 
+  /*
+   * Case 15: Address is a local address (internal) but we unset
+   * PublishServerDescriptor_ so we are allowed to hold it.
+   */
+  options->PublishServerDescriptor_ = NO_DIRINFO;
+  if (p->family == AF_INET) {
+    options->AssumeReachable = 1;
+  }
+  config_line_append(&options->Address, "Address", p->internal_ip);
+
+  tor_addr_parse(&test_addr, p->internal_ip);
+  retval = find_my_address(options, p->family, LOG_NOTICE, &resolved_addr,
+                           &method_used, &hostname_out);
+  VALIDATE_FOUND_ADDRESS(true, RESOLVED_ADDR_CONFIGURED, NULL);
+  CLEANUP_FOUND_ADDRESS;
+
   UNMOCK(get_interface_address6);
   UNMOCK(tor_gethostname);
   UNMOCK(tor_addr_lookup);