libnetwork/resolvconf: fallback to loopback nameservers for internal resolver

When the host has no valid external nameservers (or /etc/resolv.conf is
missing/dangling), TransformForIntNS previously left ExtNameServers empty
and appended 'NO EXTERNAL NAMESERVERS DEFINED'. This caused the embedded
DNS resolver (127.0.0.11) in user-defined networks to have no upstream
nameservers, causing external queries to fail with SERVFAIL.

As already documented in TransformForIntNS:
'If no ExtNameServer addresses are found, use the defaults.'

Fallback to host loopback addresses (127.0.0.1, ::1) with HostLoopback: true,
matching host libc resolver fallback behavior when resolv.conf is missing.

Signed-off-by: arsalanyavari <arya48.yavari79@gmail.com>

daemon/libnetwork/internal/resolvconf/resolvconf.go

@@ -44,6 +44,10 @@ var (
 		netip.MustParseAddr("2001:4860:4860::8888"),
 		netip.MustParseAddr("2001:4860:4860::8844"),
 	}
+	defaultLoopbackNSs = []netip.Addr{
+		netip.MustParseAddr("127.0.0.1"),
+		netip.MustParseAddr("::1"),
+	}
 )
 
 // ResolvConf represents a resolv.conf file. It can be constructed by
@@ -286,7 +290,13 @@ func (rc *ResolvConf) TransformForIntNS(
 
 	rc.md.Transform = "internal resolver"
 	if len(rc.md.ExtNameServers) == 0 {
-		rc.md.Warnings = append(rc.md.Warnings, "NO EXTERNAL NAMESERVERS DEFINED")
+		for _, addr := range defaultLoopbackNSs {
+			rc.md.ExtNameServers = append(rc.md.ExtNameServers, ExtDNSEntry{
+				Addr:         addr,
+				HostLoopback: true,
+			})
+		}
+		rc.md.Warnings = append(rc.md.Warnings, "Used default nameservers.")
 	}
 	return slices.Clone(rc.md.ExtNameServers), nil
 }

daemon/libnetwork/internal/resolvconf/resolvconf_test.go

@@ -404,6 +404,10 @@ func TestRCTransformForIntNS(t *testing.T) {
 		{
 			name:  "No config",
 			input: "",
+			expExtServers: []ExtDNSEntry{
+				mke("127.0.0.1", true),
+				mke("::1", true),
+			},
 		},
 	}
 

daemon/libnetwork/internal/resolvconf/testdata/TestRCTransformForIntNS/No_config.golden

@@ -1,5 +1,6 @@
 nameserver 127.0.0.11
 
 # Based on host file: '/etc/resolv.conf' (internal resolver)
-# NO EXTERNAL NAMESERVERS DEFINED
+# Used default nameservers.
+# ExtServers: [host(127.0.0.1) host(::1)]
 # Overrides: []