net: classify site-local unicast in address selection

This change adds site-local unicast classification for users still using
the deprecated addresses internally.

Change-Id: If50870c6d4a85fe471c002b161eec59efcebe2f4
Reviewed-on: https://go-review.googlesource.com/12344
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/addrselect.go

@@ -328,11 +328,15 @@ func classifyScope(ip IP) scope {
 	if ip.IsLoopback() || ip.IsLinkLocalUnicast() {
 		return scopeLinkLocal
 	}
-	if len(ip) == IPv6len && ip.To4() == nil && ip.IsMulticast() {
+	ipv6 := len(ip) == IPv6len && ip.To4() == nil
+	if ipv6 && ip.IsMulticast() {
 		return scope(ip[1] & 0xf)
 	}
-	// TODO: are there unicast scopeAdminLocal, scopeSiteLocal,
-	// scopeOrgLocal? Better question: are those even used?
+	// Site-local addresses are defined in RFC 3513 section 2.5.6
+	// (and deprecated in RFC 3879).
+	if ipv6 && ip[0] == 0xfe && ip[1]&0xc0 == 0xc0 {
+		return scopeSiteLocal
+	}
 	return scopeGlobal
 }
 

src/net/addrselect_test.go

@@ -169,6 +169,7 @@ func TestRFC6724ClassifyScope(t *testing.T) {
 		{ParseIP("127.0.0.1"), scopeLinkLocal},   // rfc6724#section-3.2
 		{ParseIP("::1"), scopeLinkLocal},         // rfc4007#section-4
 		{ParseIP("169.254.1.2"), scopeLinkLocal}, // rfc6724#section-3.2
+		{ParseIP("fec0::1"), scopeSiteLocal},
 		{ParseIP("8.8.8.8"), scopeGlobal},
 
 		{ParseIP("ff02::"), scopeLinkLocal},  // IPv6 multicast