all: support ipv6 for upstream bootstrap ip

cmd/ctrld/net.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"net"
+	"sync"
+)
+
+var (
+	stackOnce   sync.Once
+	ipv6Enabled bool
+)
+
+func probeStack() {
+	if ln, err := net.Listen("tcp6", "[::]:0"); err == nil {
+		ln.Close()
+		ipv6Enabled = true
+	}
+}
+
+func supportsIPv6() bool {
+	stackOnce.Do(probeStack)
+	return ipv6Enabled
+}

cmd/ctrld/prog.go

@@ -51,23 +51,40 @@ func (p *prog) run() {
 		if uc.BootstrapIP == "" {
 			// resolve it manually and set the bootstrap ip
 			c := new(dns.Client)
-			m := new(dns.Msg)
+			for _, dnsType := range []uint16{dns.TypeAAAA, dns.TypeA} {
-			m.SetQuestion(uc.Domain+".", dns.TypeA)
+				if !supportsIPv6() && dnsType == dns.TypeAAAA {
-			m.RecursionDesired = true
+					continue
-			r, _, err := c.Exchange(m, net.JoinHostPort(bootstrapDNS, "53"))
+				}
-			if err != nil {
+				m := new(dns.Msg)
-				proxyLog.Error().Err(err).Msgf("could not resolve domain %s for upstream.%s", uc.Domain, n)
+				m.SetQuestion(uc.Domain+".", dnsType)
-			} else {
+				m.RecursionDesired = true
+				r, _, err := c.Exchange(m, net.JoinHostPort(bootstrapDNS, "53"))
+				if err != nil {
+					proxyLog.Error().Err(err).Msgf("could not resolve domain %s for upstream.%s", uc.Domain, n)
+					continue
+				}
 				if r.Rcode != dns.RcodeSuccess {
 					proxyLog.Error().Msgf("could not resolve domain return code: %d, upstream.%s", r.Rcode, n)
-				} else {
+					continue
-					for _, a := range r.Answer {
-						if ar, ok := a.(*dns.A); ok {
-							uc.BootstrapIP = ar.A.String()
-							proxyLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Setting bootstrap IP for upstream.%s", n)
-						}
-					}
 				}
+				if len(r.Answer) == 0 {
+					continue
+				}
+				for _, a := range r.Answer {
+					switch ar := a.(type) {
+					case *dns.A:
+						uc.BootstrapIP = ar.A.String()
+					case *dns.AAAA:
+						uc.BootstrapIP = ar.AAAA.String()
+					default:
+						continue
+					}
+					proxyLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Setting bootstrap IP for upstream.%s", n)
+					// Stop if we reached here, because we got the bootstrap IP from r.Answer.
+					break
+				}
+				// If we reached here, uc.BootstrapIP was set, nothing to do anymore.
+				break
 			}
 		}
 		uc.SetupTransport()

config.go

@@ -2,7 +2,6 @@ package ctrld
 
 import (
 	"context"
-	"fmt"
 	"net"
 	"net/http"
 	"net/url"
@@ -145,8 +144,10 @@ func (uc *UpstreamConfig) SetupTransport() {
 		}
 		Log(ctx, ProxyLog.Debug(), "debug dial context %s - %s - %s", addr, network, bootstrapDNS)
 		// if we have a bootstrap ip set, use it to avoid DNS lookup
-		if uc.BootstrapIP != "" && addr == fmt.Sprintf("%s:443", uc.Domain) {
+		if uc.BootstrapIP != "" {
-			addr = fmt.Sprintf("%s:443", uc.BootstrapIP)
+			if _, port, _ := net.SplitHostPort(addr); port != "" {
+				addr = net.JoinHostPort(uc.BootstrapIP, port)
+			}
 			Log(ctx, ProxyLog.Debug(), "sending doh request to: %s", addr)
 		}
 		return dialer.DialContext(ctx, network, addr)