fix: reuse existing client endpoint when configuring server interface

pkg/gateway/tunnel/wireguard/device.go

@@ -42,6 +42,11 @@ func configureDevice(wgcl *wgctrl.Client, options *Options, peerPubKey wgtypes.K
 	switch options.GwOptions.Mode {
 	case gateway.ModeServer:
 		confdev.ListenPort = &options.ListenPort
+
+		endpoint := getExistingEndpoint(wgcl, peerPubKey)
+		if endpoint != nil {
+			confdev.Peers[0].Endpoint = endpoint
+		}
 	case gateway.ModeClient:
 		confdev.Peers[0].Endpoint = &net.UDPAddr{
 			IP:   options.EndpointIP,
@@ -56,3 +61,48 @@ func configureDevice(wgcl *wgctrl.Client, options *Options, peerPubKey wgtypes.K
 	}
 	return nil
 }
+
+func getExistingEndpoint(wgcl *wgctrl.Client, peerPubKey wgtypes.Key) *net.UDPAddr {
+	peer := getExistingPeer(wgcl, peerPubKey)
+
+	if peer == nil {
+		return nil
+	}
+
+	if peer.Endpoint != nil {
+		klog.Infof("Discovered endpoint %s for peer %s", peer.Endpoint, peerPubKey.String())
+		return peer.Endpoint
+	}
+
+	return nil
+}
+
+func getExistingPeer(wgcl *wgctrl.Client, peerPubKey wgtypes.Key) *wgtypes.Peer {
+	dev := getExistingDevice(wgcl)
+
+	if dev == nil {
+		return nil
+	}
+
+	for i := range dev.Peers {
+		if dev.Peers[i].PublicKey == peerPubKey {
+			klog.Infof("Found existing peer for key %s", peerPubKey.String())
+			return &dev.Peers[i]
+		}
+	}
+
+	klog.Infof("No existing peer %s found", peerPubKey.String())
+	return nil
+}
+
+func getExistingDevice(wgcl *wgctrl.Client) *wgtypes.Device {
+	dev, err := wgcl.Device(tunnel.TunnelInterfaceName)
+
+	if err == nil {
+		klog.Infof("Found existing device %s", tunnel.TunnelInterfaceName)
+		return dev
+	}
+
+	klog.Infof("No existing device %s found", tunnel.TunnelInterfaceName)
+	return nil
+}