transport: add TLS support

Implement reverseproxy.TLSTransport.  We go ahead and store the provided
TLSConfig object, but for now we only use it to indicate that we should
use TLS.  We don't actually use any of the provided values to configure
the client.

Fixes #25

Signed-off-by: Will Norris <will@tailscale.com>

module.go

@@ -42,11 +42,7 @@ func getPlainListener(c context.Context, _ string, addr string, _ net.ListenConf
 	if network == "" {
 		network = "tcp"
 	}
-
-	ln := &tailscaleNode{
-		Server: s.Server,
-	}
-	return ln.Listen(network, ":"+port)
+	return s.Listen(network, ":"+port)
 }
 
 func getTLSListener(c context.Context, _ string, addr string, _ net.ListenConfig) (any, error) {

transport.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
+	"github.com/caddyserver/caddy/v2/modules/caddyhttp/reverseproxy"
 )
 
 func init() {
@@ -18,6 +19,10 @@ type Transport struct {
 	Name string `json:"name,omitempty"`
 
 	node *tailscaleNode
+
+	// A non-nil TLS config enables TLS.
+	// We do not currently use the config values for anything.
+	TLS *reverseproxy.TLSConfig `json:"tls,omitempty"`
 }
 
 func (t *Transport) CaddyModule() caddy.ModuleInfo {
@@ -64,14 +69,30 @@ func (t *Transport) Cleanup() error {
 
 func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
 	if req.URL.Scheme == "" {
-		req.URL.Scheme = "http"
+		if t.TLSEnabled() {
+			req.URL.Scheme = "https"
+		} else {
+			req.URL.Scheme = "http"
+		}
 	}
 	return t.node.HTTPClient().Transport.RoundTrip(req)
 }
 
+// TLSEnabled returns true if TLS is enabled.
+func (h Transport) TLSEnabled() bool {
+	return h.TLS != nil
+}
+
+// EnableTLS enables TLS on the transport.
+func (h *Transport) EnableTLS(config *reverseproxy.TLSConfig) error {
+	h.TLS = config
+	return nil
+}
+
 var (
-	_ http.RoundTripper     = (*Transport)(nil)
-	_ caddy.Provisioner     = (*Transport)(nil)
-	_ caddy.CleanerUpper    = (*Transport)(nil)
-	_ caddyfile.Unmarshaler = (*Transport)(nil)
+	_ http.RoundTripper         = (*Transport)(nil)
+	_ caddy.Provisioner         = (*Transport)(nil)
+	_ caddy.CleanerUpper        = (*Transport)(nil)
+	_ caddyfile.Unmarshaler     = (*Transport)(nil)
+	_ reverseproxy.TLSTransport = (*Transport)(nil)
 )