From 577ac5c91a9a95969c0dc8eb6ca68943f3557504 Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Wed, 21 Mar 2018 09:05:30 +0100
Subject: [PATCH] When using a fallback resolver, favor IPv6 for DoH servers if
 use_ipv6 is set

Fixes #153
---
 dnscrypt-proxy/config.go     |  2 ++
 dnscrypt-proxy/xtransport.go | 52 +++++++++++++++++++++++++-----------
 2 files changed, 38 insertions(+), 16 deletions(-)

diff --git a/dnscrypt-proxy/config.go b/dnscrypt-proxy/config.go
index c9048810..6fc057de 100644
--- a/dnscrypt-proxy/config.go
+++ b/dnscrypt-proxy/config.go
@@ -182,6 +182,8 @@ func ConfigLoad(proxy *Proxy, svcFlag *string) error {
 	if len(config.FallbackResolver) > 0 {
 		proxy.xTransport.ignoreSystemDNS = config.IgnoreSystemDNS
 	}
+	proxy.xTransport.useIPv4 = config.SourceIPv4
+	proxy.xTransport.useIPv6 = config.SourceIPv6
 	proxy.timeout = time.Duration(config.Timeout) * time.Millisecond
 	proxy.maxClients = config.MaxClients
 	proxy.mainProto = "udp"
diff --git a/dnscrypt-proxy/xtransport.go b/dnscrypt-proxy/xtransport.go
index 232edeba..24876df9 100644
--- a/dnscrypt-proxy/xtransport.go
+++ b/dnscrypt-proxy/xtransport.go
@@ -135,30 +135,50 @@ func (xTransport *XTransport) Fetch(method string, url *url.URL, accept string,
 		dlog.Debugf("IP for [%s] was cached to [%s], but connection failed: [%s]", host, cachedIP, err)
 		return nil, 0, err
 	}
-	if !xTransport.useIPv4 {
-		return nil, 0, fmt.Errorf("IPv4 connectivity would be required to use [%s]", host)
-	}
-	dnsClient := new(dns.Client)
-	msg := new(dns.Msg)
-	msg.SetQuestion(dns.Fqdn(host), dns.TypeA)
-	msg.SetEdns0(4096, true)
 	if !xTransport.ignoreSystemDNS {
 		dlog.Noticef("System DNS configuration not usable yet, exceptionally resolving [%s] using fallback resolver [%s]", host, xTransport.fallbackResolver)
 	} else {
 		dlog.Debugf("Resolving [%s] using fallback resolver [%s]", host, xTransport.fallbackResolver)
 	}
-	in, _, err := dnsClient.Exchange(msg, xTransport.fallbackResolver)
+	dnsClient := new(dns.Client)
+
+	var foundIP *string
+	err = nil
+	if xTransport.useIPv4 {
+		msg := new(dns.Msg)
+		msg.SetQuestion(dns.Fqdn(host), dns.TypeA)
+		msg.SetEdns0(4096, true)
+		var in *dns.Msg
+		in, _, err = dnsClient.Exchange(msg, xTransport.fallbackResolver)
+		if err == nil {
+			for _, answer := range in.Answer {
+				if answer.Header().Rrtype == dns.TypeA {
+					foundIPx := answer.(*dns.A).A.String()
+					foundIP = &foundIPx
+					break
+				}
+			}
+		}
+	}
+	if xTransport.useIPv6 && foundIP == nil {
+		msg := new(dns.Msg)
+		msg.SetQuestion(dns.Fqdn(host), dns.TypeAAAA)
+		msg.SetEdns0(4096, true)
+		var in *dns.Msg
+		in, _, err = dnsClient.Exchange(msg, xTransport.fallbackResolver)
+		if err == nil {
+			for _, answer := range in.Answer {
+				if answer.Header().Rrtype == dns.TypeAAAA {
+					foundIPx := "[" + answer.(*dns.AAAA).AAAA.String() + "]"
+					foundIP = &foundIPx
+					break
+				}
+			}
+		}
+	}
 	if err != nil {
 		return nil, 0, err
 	}
-	var foundIP *string
-	for _, answer := range in.Answer {
-		if answer.Header().Rrtype == dns.TypeA {
-			foundIPx := answer.(*dns.A).A.String()
-			foundIP = &foundIPx
-			break
-		}
-	}
 	if foundIP == nil {
 		return nil, 0, fmt.Errorf("No IP found for [%s]", host)
 	}