lib/api: Treat *.localhost as valid localhost addresses (#7412) (ref #4815)

This loosens the ‘is this localhost?’ check to include *.localhost host
names.

This allows for clearer (hence better) names to be used in browsers,
e.g. when accessing a remote syncthing instance ‘foo’ using a ssh port
forward, one can use foo.localhost to remind oneself which one is which.
💡 Without these changes, Syncthing shows a ‘Host check error’ when
pointing a browser at http://foo.localhost/, and with these changes, the
interface loads as usual.

The .localhost top level domain is a reserved top-level domain (RFC 2606):

> The ".localhost" TLD has traditionally been statically defined in
> host DNS implementations as having an A record pointing to the
> loop back IP address and is reserved for such use.  Any other use
> would conflict with widely deployed code which assumes this use.
> – https://tools.ietf.org/html/rfc2606

As Wikipedia puts it:

> This allows the use of these names for either documentation purposes
or in local testing scenarios. – https://en.wikipedia.org/wiki/.localhost

On Linux systems, systemd-resolved resolves *.localhost, on purpose:
https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html

See also #4815, #4816.

lib/api/api.go

@@ -1772,8 +1772,13 @@ func addressIsLocalhost(addr string) bool {
 		// There was no port, so we assume the address was just a hostname
 		host = addr
 	}
-	switch strings.ToLower(host) {
-	case "localhost", "localhost.":
+	host = strings.ToLower(host)
+	switch {
+	case host == "localhost":
+		return true
+	case host == "localhost.":
+		return true
+	case strings.HasSuffix(host, ".localhost"):
 		return true
 	default:
 		ip := net.ParseIP(host)

lib/api/api_test.go

@@ -995,14 +995,14 @@ func TestAddressIsLocalhost(t *testing.T) {
 		{"[::1]:8080", true},
 		{"127.0.0.1:8080", true},
 		{"127.23.45.56:8080", true},
+		{"www.localhost", true},
+		{"www.localhost:8080", true},
 
 		// These are all non-localhost addresses
 		{"example.com", false},
 		{"example.com:8080", false},
 		{"localhost.com", false},
 		{"localhost.com:8080", false},
-		{"www.localhost", false},
-		{"www.localhost:8080", false},
 		{"192.0.2.10", false},
 		{"192.0.2.10:8080", false},
 		{"0.0.0.0", false},