Using ControlD bootstrap DNS again

So on system where there's no available DNS, non-ControlD upstreams
could be bootstrapped like before.

While at it, also improving lookupIP to not initializing OS resolver
anymore, removing the un-necessary contention for accquiring/releasing
OS resolver mutex.

config.go

@@ -427,11 +427,18 @@ func (uc *UpstreamConfig) UID() string {
 
 // SetupBootstrapIP manually find all available IPs of the upstream.
 // The first usable IP will be used as bootstrap IP of the upstream.
+// The upstream domain will be looked up using following orders:
+//
+// - Current system DNS settings.
+// - Direct IPs table for ControlD upstreams.
+// - ControlD Bootstrap DNS 76.76.2.22
+//
+// The setup process will block until there's usable IPs found.
 func (uc *UpstreamConfig) SetupBootstrapIP() {
 	b := backoff.NewBackoff("setupBootstrapIP", func(format string, args ...any) {}, 10*time.Second)
 	isControlD := uc.IsControlD()
 	for {
-		uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout)
+		uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, defaultNameservers())
 		// For ControlD upstream, the bootstrap IPs could not be RFC 1918 addresses,
 		// filtering them out here to prevent weird behavior.
 		if isControlD {
@@ -449,6 +456,11 @@ func (uc *UpstreamConfig) SetupBootstrapIP() {
 				ProxyLogger.Load().Warn().Msgf("no record found for %q, lookup from direct IP table", uc.Domain)
 			}
 		}
+		if len(uc.bootstrapIPs) == 0 {
+			ProxyLogger.Load().Warn().Msgf("no record found for %q, using bootstrap server: %s", uc.Domain, PremiumDNSBoostrapIP)
+			uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, []string{net.JoinHostPort(PremiumDNSBoostrapIP, "53")})
+
+		}
 		if len(uc.bootstrapIPs) > 0 {
 			break
 		}

resolver.go

@@ -466,27 +466,26 @@ func (d dummyResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, err
 	return ans, nil
 }
 
-// LookupIP looks up host using OS resolver.
+// LookupIP looks up domain using current system nameservers settings.
 // It returns a slice of that host's IPv4 and IPv6 addresses.
 func LookupIP(domain string) []string {
-	return lookupIP(domain, -1)
+	return lookupIP(domain, -1, defaultNameservers())
 }
 
-func lookupIP(domain string, timeout int) (ips []string) {
+// lookupIP looks up domain with given timeout and bootstrapDNS.
+// If timeout is negative, default timeout 2000 ms will be used.
+// It returns nil if bootstrapDNS is nil or empty.
+func lookupIP(domain string, timeout int, bootstrapDNS []string) (ips []string) {
 	if net.ParseIP(domain) != nil {
 		return []string{domain}
 	}
-	resolverMutex.Lock()
+	if bootstrapDNS == nil {
-	if or == nil {
+		ProxyLogger.Load().Debug().Msgf("empty bootstrap DNS")
-		ProxyLogger.Load().Debug().Msgf("Initialize OS resolver in lookupIP")
+		return nil
-		or = newResolverWithNameserver(defaultNameservers())
 	}
-	nss := *or.lanServers.Load()
-	nss = append(nss, *or.publicServers.Load()...)
-	resolverMutex.Unlock()
 
-	resolver := newResolverWithNameserver(nss)
+	resolver := newResolverWithNameserver(bootstrapDNS)
-	ProxyLogger.Load().Debug().Msgf("resolving %q using bootstrap DNS %q", domain, nss)
+	ProxyLogger.Load().Debug().Msgf("resolving %q using bootstrap DNS %q", domain, bootstrapDNS)
 	timeoutMs := 2000
 	if timeout > 0 && timeout < timeoutMs {
 		timeoutMs = timeout