add config option for node hostname

this overrides the name used to refer to the node in the caddy config,
and is mostly useful because it can include environment variables.

Closes #18

Co-authored-by: kdevan <kaidevan@gmail.com>
Signed-off-by: Will Norris <will@tailscale.com>

Author: willnorris

app.go

@@ -41,6 +41,9 @@ type TSNode struct {
 	// Ephemeral specifies whether the node should be registered as ephemeral.
 	Ephemeral bool `json:"ephemeral,omitempty" caddy:"namespace=tailscale.ephemeral"`
 
+	// Hostname is the hostname to use when registering the node.
+	Hostname string `json:"hostname,omitempty" caddy:"namespace=tailscale.hostname"`
+
 	name string
 }
 
@@ -121,6 +124,11 @@ func parseTSNode(d *caddyfile.Dispenser) (TSNode, error) {
 			node.AuthKey = segment.Val()
 		case "ephemeral":
 			node.Ephemeral = true
+		case "hostname":
+			if !segment.NextArg() {
+				return node, segment.ArgErr()
+			}
+			node.Hostname = segment.Val()
 		default:
 			return node, segment.Errf("unrecognized subdirective: %s", segment.Val())
 		}

module.go

@@ -103,13 +103,15 @@ func getNode(ctx caddy.Context, name string) (*tailscaleNode, error) {
 
 	s, _, err := nodes.LoadOrNew(name, func() (caddy.Destructor, error) {
 		s := &tsnet.Server{
-			Hostname: name,
 			Logf: func(format string, args ...any) {
 				app.logger.Sugar().Debugf(format, args...)
 			},
 			Ephemeral: getEphemeral(name, app),
 		}
 
+		if s.Hostname, err = getHostname(name, app); err != nil {
+			app.logger.Warn("error parsing hostname", zap.Error(err))
+		}
 		if s.AuthKey, err = getAuthKey(name, app); err != nil {
 			app.logger.Warn("error parsing auth key", zap.Error(err))
 		}
@@ -178,6 +180,19 @@ func getEphemeral(name string, app *TSApp) bool {
 	return app.Ephemeral
 }
 
+func getHostname(name string, app *TSApp) (string, error) {
+	if app == nil {
+		return name, nil
+	}
+	if node, ok := app.Nodes[name]; ok {
+		if node.Hostname != "" {
+			return repl.ReplaceOrErr(node.Hostname, true, true)
+		}
+	}
+
+	return name, nil
+}
+
 // tailscaleNode is a wrapper around a tsnet.Server that provides a fully self-contained Tailscale node.
 // This node can listen on the tailscale network interface, or be used to connect to other nodes in the tailnet.
 type tailscaleNode struct {