net: don't avoid resolving .local addresses

.local addresses are used by things like Kubernetes and Weave DNS; Go
should not avoid resolving them.

This is a partial revert of https://golang.org/cl/21328 which was too
strict of an interpretation of RFC 6762.

Fixes #16739

Change-Id: I349415b4eab5d61240dd18217bd95dc7d2105cd5
Reviewed-on: https://go-review.googlesource.com/27250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient_unix.go

@@ -338,8 +338,9 @@ func lookup(ctx context.Context, name string, qtype uint16) (cname string, rrs [
 }
 
 // avoidDNS reports whether this is a hostname for which we should not
-// use DNS. Currently this includes only .onion and .local names,
-// per RFC 7686 and RFC 6762, respectively. See golang.org/issue/13705.
+// use DNS. Currently this includes only .onion, per RFC 7686. See
+// golang.org/issue/13705. Does not cover .local names (RFC 6762),
+// see golang.org/issue/16739.
 func avoidDNS(name string) bool {
 	if name == "" {
 		return true
@@ -347,7 +348,7 @@ func avoidDNS(name string) bool {
 	if name[len(name)-1] == '.' {
 		name = name[:len(name)-1]
 	}
-	return stringsHasSuffixFold(name, ".onion") || stringsHasSuffixFold(name, ".local")
+	return stringsHasSuffixFold(name, ".onion")
 }
 
 // nameList returns a list of names for sequential DNS queries.

src/net/dnsclient_unix_test.go

@@ -112,10 +112,11 @@ func TestAvoidDNSName(t *testing.T) {
 		{"foo.ONION", true},
 		{"foo.ONION.", true},
 
-		{"foo.local.", true},
-		{"foo.local", true},
-		{"foo.LOCAL", true},
-		{"foo.LOCAL.", true},
+		// But do resolve *.local address; Issue 16739
+		{"foo.local.", false},
+		{"foo.local", false},
+		{"foo.LOCAL", false},
+		{"foo.LOCAL.", false},
 
 		{"", true}, // will be rejected earlier too