Revert "[client] Drop signaling-side ICE candidate filter, drop overlay STUN at mux read-side instead (#6142)"

This reverts commit b57f714350.

Restores the signaling-side ICE candidate filter and the engine/worker_ice
STUN gating that #6142 removed, re-adding the mux read-side changes in
udpmux/universal.go to their pre-#6142 form.

client/internal/engine.go

@@ -54,6 +54,7 @@ import (
 	"github.com/netbirdio/netbird/client/internal/relay"
 	"github.com/netbirdio/netbird/client/internal/rosenpass"
 	"github.com/netbirdio/netbird/client/internal/routemanager"
+	"github.com/netbirdio/netbird/client/internal/routemanager/systemops"
 	"github.com/netbirdio/netbird/client/internal/statemanager"
 	"github.com/netbirdio/netbird/client/internal/syncstore"
 	"github.com/netbirdio/netbird/client/internal/updater"
@@ -1994,6 +1995,7 @@ func (e *Engine) newWgIface() (*iface.WGIface, error) {
 		WGPrivKey:    e.config.WgPrivateKey.String(),
 		MTU:          e.config.MTU,
 		TransportNet: transportNet,
+		FilterFn:     e.addrViaRoutes,
 		DisableDNS:   e.config.DisableDNS,
 	}
 
@@ -2248,6 +2250,21 @@ func (e *Engine) startNetworkMonitor() {
 	}()
 }
 
+func (e *Engine) addrViaRoutes(addr netip.Addr) (bool, netip.Prefix, error) {
+	var vpnRoutes []netip.Prefix
+	for _, routes := range e.routeManager.GetClientRoutes() {
+		if len(routes) > 0 && routes[0] != nil {
+			vpnRoutes = append(vpnRoutes, routes[0].Network)
+		}
+	}
+
+	if isVpn, prefix := systemops.IsAddrRouted(addr, vpnRoutes); isVpn {
+		return true, prefix, nil
+	}
+
+	return false, netip.Prefix{}, nil
+}
+
 func (e *Engine) stopDNSServer() {
 	if e.dnsServer == nil {
 		return

client/internal/peer/worker_ice.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"net/netip"
 	"strconv"
 	"sync"
 	"time"
@@ -164,6 +165,10 @@ func (w *WorkerICE) OnRemoteCandidate(candidate ice.Candidate, haRoutes route.HA
 		return
 	}
 
+	if candidateViaRoutes(candidate, haRoutes) {
+		return
+	}
+
 	if err := w.agent.AddRemoteCandidate(candidate); err != nil {
 		w.log.Errorf("error while handling remote candidate")
 		return
@@ -584,6 +589,34 @@ func extraSrflxCandidate(candidate ice.Candidate) (*ice.CandidateServerReflexive
 	return ec, nil
 }
 
+func candidateViaRoutes(candidate ice.Candidate, clientRoutes route.HAMap) bool {
+	addr, err := netip.ParseAddr(candidate.Address())
+	if err != nil {
+		log.Errorf("Failed to parse IP address %s: %v", candidate.Address(), err)
+		return false
+	}
+
+	var routePrefixes []netip.Prefix
+	for _, routes := range clientRoutes {
+		if len(routes) > 0 && routes[0] != nil {
+			routePrefixes = append(routePrefixes, routes[0].Network)
+		}
+	}
+
+	for _, prefix := range routePrefixes {
+		// default route is handled by route exclusion / ip rules
+		if prefix.Bits() == 0 {
+			continue
+		}
+
+		if prefix.Contains(addr) {
+			log.Debugf("Ignoring candidate [%s], its address is part of routed network %s", candidate.String(), prefix)
+			return true
+		}
+	}
+	return false
+}
+
 func isRelayCandidate(candidate ice.Candidate) bool {
 	return candidate.Type() == ice.CandidateTypeRelay
 }

client/internal/routemanager/systemops/systemops_generic.go

@@ -121,12 +121,9 @@ func (r *SysOps) addRouteToNonVPNIntf(prefix netip.Prefix, vpnIntf wgIface, init
 		return Nexthop{}, vars.ErrRouteNotAllowed
 	}
 
-	// BSDs blackhole a /32 added inside a directly-connected subnet; Linux/Windows need it to beat the wt0 route.
-	switch runtime.GOOS {
-	case "darwin", "freebsd", "netbsd", "openbsd", "dragonfly":
-		if isLocal, subnet := r.isPrefixInLocalSubnets(prefix); isLocal {
-			return Nexthop{}, fmt.Errorf("prefix %s is part of local subnet %s: %w", prefix, subnet, 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