util/linuxfw, wgengine: allow ingress to magicsock UDP port on Linux (#10370)

* util/linuxfw, wgengine: allow ingress to magicsock UDP port on Linux

Updates #9084.

Currently, we have to tell users to manually open UDP ports on Linux when
certain firewalls (like ufw) are enabled. This change automates the process of
adding and updating those firewall rules as magicsock changes what port it
listens on.

Signed-off-by: Naman Sood <mail@nsood.in>

wgengine/router/router_userspace_bsd.go

@@ -196,6 +196,13 @@ func (r *userspaceBSDRouter) Set(cfg *Config) (reterr error) {
 	return reterr
 }
 
+// UpdateMagicsockPort implements the Router interface. This implementation
+// does nothing and returns nil because this router does not currently need
+// to know what the magicsock UDP port is.
+func (r *userspaceBSDRouter) UpdateMagicsockPort(_ uint16, _ string) error {
+	return nil
+}
+
 func (r *userspaceBSDRouter) Close() error {
 	return nil
 }