hotfix(pmtud): set mss on all VPN routes

- fix behavior for OpenVPN splitting default route in multiple routes
- fix behavior for Wireguard if user specifies AllowedIPs

internal/routing/vpn.go

@@ -50,23 +50,28 @@ func (r *Routing) VPNLocalGatewayIP(vpnIntf string) (ip netip.Addr, err error) {
 
 var ErrVPNRouteNotFound = errors.New("VPN route not found")
 
-func (r *Routing) VPNRoute(vpnIntf string) (route netlink.Route, err error) {
+func (r *Routing) VPNRoutes(vpnIntf string) (routes []netlink.Route, err error) {
 	vpnLink, err := r.netLinker.LinkByName(vpnIntf)
 	if err != nil {
-		return route, fmt.Errorf("finding link %s: %w", vpnIntf, err)
+		return nil, fmt.Errorf("finding link %s: %w", vpnIntf, err)
 	}
 	vpnLinkIndex := vpnLink.Index
 
-	routes, err := r.netLinker.RouteList(netlink.FamilyAll)
+	allRoutes, err := r.netLinker.RouteList(netlink.FamilyAll)
 	if err != nil {
-		return route, fmt.Errorf("listing routes: %w", err)
+		return nil, fmt.Errorf("listing routes: %w", err)
 	}
-	for _, route := range routes {
-		if route.LinkIndex == vpnLinkIndex &&
-			!route.Dst.IsValid() {
-			return route, nil
+	routes = make([]netlink.Route, 0, len(allRoutes))
+	for _, route := range allRoutes {
+		if route.LinkIndex == vpnLinkIndex {
+			routes = append(routes, route)
 		}
 	}
-	return route, fmt.Errorf("%w: for interface %s in %d routes",
-		ErrVPNRouteNotFound, vpnIntf, len(routes))
+
+	if len(routes) == 0 {
+		return nil, fmt.Errorf("%w: for interface %s in %d routes",
+			ErrVPNRouteNotFound, vpnIntf, len(allRoutes))
+	}
+
+	return routes, nil
 }