Fallback to the system resolver if the fallback resolver doesn't work

This is useful if fallback_resolver has been set to random junk, or
to an external resolver, but port 53 is blocked.

At least, it may allow the server to start.

dnscrypt-proxy/example-dnscrypt-proxy.toml

@@ -198,8 +198,7 @@ cert_refresh_delay = 240
 fallback_resolver = '9.9.9.9:53'
 
 
-## Never let dnscrypt-proxy try to use the system DNS settings;
-## unconditionally use the fallback resolver.
+## Always use the fallback resolver before the system DNS settings
 
 ignore_system_dns = true
 

dnscrypt-proxy/xtransport.go

@@ -279,6 +279,10 @@ func (xTransport *XTransport) resolveWithCache(host string) (err error) {
 			}
 		}
 	}
+	if err != nil && xTransport.ignoreSystemDNS {
+		dlog.Noticef("Fallback resolver [%v] didn't respond - Trying with the system resolver as a last resort", xTransport.fallbackResolver)
+		foundIP, ttl, err = xTransport.resolveUsingSystem(host)
+	}
 	if ttl < MinResolverIPTTL {
 		ttl = MinResolverIPTTL
 	}
@@ -323,6 +327,7 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 		return nil, 0, errors.New("Onion service is not reachable without Tor")
 	}
 	if err := xTransport.resolveWithCache(host); err != nil {
+		dlog.Errorf("Unable to resolve [%v] - Make sure that the system resolver works, or that `fallback_resolver` has been set to a resolver that can be reached", host)
 		return nil, 0, err
 	}
 	req := &http.Request{