Don't add exclusion routes for IPs that are part of connected networks (#2258)

This prevents arp/ndp issues on macOS leading to unreachability of that IP.

client/internal/routemanager/systemops/systemops_generic.go

@@ -50,7 +50,7 @@ func (r *SysOps) setupRefCounter(initAddresses []net.IP) (nbnet.AddHookFunc, nbn
 			nexthop, err := r.addRouteToNonVPNIntf(prefix, r.wgInterface, initialNexthop)
 			if errors.Is(err, vars.ErrRouteNotAllowed) || errors.Is(err, vars.ErrRouteNotFound) {
 				log.Tracef("Adding for prefix %s: %v", prefix, err)
-				// These errors are not critical but also we should not track and try to remove the routes either.
+				// These errors are not critical, but also we should not track and try to remove the routes either.
 				return nexthop, refcounter.ErrIgnore
 			}
 			return nexthop, err
@@ -135,6 +135,11 @@ func (r *SysOps) addRouteToNonVPNIntf(prefix netip.Prefix, vpnIntf *iface.WGIfac
 		return Nexthop{}, vars.ErrRouteNotAllowed
 	}
 
+	// Check if the prefix is part of any local subnets
+	if isLocal, subnet := r.isPrefixInLocalSubnets(prefix); isLocal {
+		return Nexthop{}, fmt.Errorf("prefix %s is part of local subnet %s: %w", prefix, subnet, vars.ErrRouteNotAllowed)
+	}
+
 	// Determine the exit interface and next hop for the prefix, so we can add a specific route
 	nexthop, err := GetNextHop(addr)
 	if err != nil {
@@ -167,6 +172,36 @@ func (r *SysOps) addRouteToNonVPNIntf(prefix netip.Prefix, vpnIntf *iface.WGIfac
 	return exitNextHop, nil
 }
 
+func (r *SysOps) isPrefixInLocalSubnets(prefix netip.Prefix) (bool, *net.IPNet) {
+	localInterfaces, err := net.Interfaces()
+	if err != nil {
+		log.Errorf("Failed to get local interfaces: %v", err)
+		return false, nil
+	}
+
+	for _, intf := range localInterfaces {
+		addrs, err := intf.Addrs()
+		if err != nil {
+			log.Errorf("Failed to get addresses for interface %s: %v", intf.Name, err)
+			continue
+		}
+
+		for _, addr := range addrs {
+			ipnet, ok := addr.(*net.IPNet)
+			if !ok {
+				log.Errorf("Failed to convert address to IPNet: %v", addr)
+				continue
+			}
+
+			if ipnet.Contains(prefix.Addr().AsSlice()) {
+				return true, ipnet
+			}
+		}
+	}
+
+	return false, nil
+}
+
 // genericAddVPNRoute adds a new route to the vpn interface, it splits the default prefix
 // in two /1 prefixes to avoid replacing the existing default route
 func (r *SysOps) genericAddVPNRoute(prefix netip.Prefix, intf *net.Interface) error {