From b0be1f3af84b6ee82973ba9d7d175ee8e0863f18 Mon Sep 17 00:00:00 2001
From: Rob Murray <rob.murray@docker.com>
Date: Wed, 18 Dec 2024 12:22:45 +0000
Subject: [PATCH] Enable external DNS if a network has an IPv6 gateway

Signed-off-by: Rob Murray <rob.murray@docker.com>
---
 integration/network/dns_test.go | 36 +++++++++++++++++++++++++++++++++
 libnetwork/endpoint_info.go     |  2 +-
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/integration/network/dns_test.go b/integration/network/dns_test.go
index 320db1647bae6..d0798732cfbb6 100644
--- a/integration/network/dns_test.go
+++ b/integration/network/dns_test.go
@@ -90,3 +90,39 @@ func TestIntDNSAsExtDNS(t *testing.T) {
 		})
 	}
 }
+
+// TestExtDNSInIPv6OnlyNw checks that an IPv6-only bridge network has external
+// DNS access.
+func TestExtDNSInIPv6OnlyNw(t *testing.T) {
+	skip.If(t, testEnv.DaemonInfo.OSType == "windows", "No IPv6 Windows")
+	skip.If(t, testEnv.IsRootless, "Can't use resolver on host in rootless mode")
+	ctx := setupTest(t)
+
+	// Start a DNS server on the loopback interface.
+	network.StartDaftDNS(t, "127.0.0.1")
+
+	// Set up a temp resolv.conf pointing at that DNS server, and a daemon using it.
+	tmpFileName := network.WriteTempResolvConf(t, "127.0.0.1")
+	d := daemon.New(t, daemon.WithEnvVars("DOCKER_TEST_RESOLV_CONF_PATH="+tmpFileName), daemon.WithExperimental())
+	d.StartWithBusybox(ctx, t)
+	defer d.Stop(t)
+
+	c := d.NewClientT(t)
+	defer c.Close()
+
+	netName := "v6only"
+	network.CreateNoError(ctx, t, c, netName,
+		network.WithDriver("bridge"),
+		network.WithIPv4(false),
+		network.WithIPv6(),
+	)
+	defer network.RemoveNoError(ctx, t, c, netName)
+
+	ctrId := container.Run(ctx, t, c, container.WithNetworkMode(netName))
+	defer c.ContainerRemove(ctx, ctrId, containertypes.RemoveOptions{Force: true})
+
+	res, err := container.Exec(ctx, c, ctrId, []string{"nslookup", "test.example"})
+	assert.NilError(t, err)
+	assert.Check(t, is.Equal(res.ExitCode, 0))
+	assert.Check(t, is.Contains(res.Stdout(), network.DNSRespAddr))
+}
diff --git a/libnetwork/endpoint_info.go b/libnetwork/endpoint_info.go
index c75769d06885d..313f6efadacdd 100644
--- a/libnetwork/endpoint_info.go
+++ b/libnetwork/endpoint_info.go
@@ -388,7 +388,7 @@ func (ep *Endpoint) hasGatewayOrDefaultRoute() bool {
 	defer ep.mu.Unlock()
 
 	if ep.joinInfo != nil {
-		if len(ep.joinInfo.gw) > 0 {
+		if len(ep.joinInfo.gw) > 0 || len(ep.joinInfo.gw6) > 0 {
 			return true
 		}
 		for _, route := range ep.joinInfo.StaticRoutes {