[client] Use native windows sock opts to avoid routing loops (#4314)

- Move `util/grpc` and `util/net` to `client` so `internal` packages can be accessed
 - Add methods to return the next best interface after the NetBird interface.
- Use `IP_UNICAST_IF` sock opt to force the outgoing interface for the NetBird `net.Dialer` and `net.ListenerConfig` to avoid routing loops. The interface is picked by the new route lookup method.
- Some refactoring to avoid import cycles
- Old behavior is available through `NB_USE_LEGACY_ROUTING=true` env var

client/net/net.go

@@ -0,0 +1,69 @@
+package net
+
+import (
+	"fmt"
+	"math/big"
+	"net"
+	"net/netip"
+)
+
+const (
+	// ControlPlaneMark is the fwmark value used to mark packets that should not be routed through the NetBird interface to
+	// avoid routing loops.
+	// This includes all control plane traffic (mgmt, signal, flows), relay, ICE/stun/turn and everything that is emitted by the wireguard socket.
+	// It doesn't collide with the other marks, as the others are used for data plane traffic only.
+	ControlPlaneMark = 0x1BD00
+
+	// Data plane marks (0x1BD10 - 0x1BDFF)
+
+	// DataPlaneMarkLower is the lowest value for the data plane range
+	DataPlaneMarkLower = 0x1BD10
+	// DataPlaneMarkUpper is the highest value for the data plane range
+	DataPlaneMarkUpper = 0x1BDFF
+
+	// DataPlaneMarkIn is the mark for inbound data plane traffic.
+	DataPlaneMarkIn = 0x1BD10
+
+	// DataPlaneMarkOut is the mark for outbound data plane traffic.
+	DataPlaneMarkOut = 0x1BD11
+
+	// PreroutingFwmarkRedirected is applied to packets that are were redirected (input -> forward, e.g. by Docker or Podman) for special handling.
+	PreroutingFwmarkRedirected = 0x1BD20
+
+	// PreroutingFwmarkMasquerade is applied to packets that arrive from the NetBird interface and should be masqueraded.
+	PreroutingFwmarkMasquerade = 0x1BD21
+
+	// PreroutingFwmarkMasqueradeReturn is applied to packets that will leave through the NetBird interface and should be masqueraded.
+	PreroutingFwmarkMasqueradeReturn = 0x1BD22
+)
+
+// IsDataPlaneMark determines if a fwmark is in the data plane range (0x1BD10-0x1BDFF)
+func IsDataPlaneMark(fwmark uint32) bool {
+	return fwmark >= DataPlaneMarkLower && fwmark <= DataPlaneMarkUpper
+}
+
+func GetLastIPFromNetwork(network netip.Prefix, fromEnd int) (netip.Addr, error) {
+	var endIP net.IP
+	addr := network.Addr().AsSlice()
+	mask := net.CIDRMask(network.Bits(), len(addr)*8)
+
+	for i := 0; i < len(addr); i++ {
+		endIP = append(endIP, addr[i]|^mask[i])
+	}
+
+	// convert to big.Int
+	endInt := big.NewInt(0)
+	endInt.SetBytes(endIP)
+
+	// subtract fromEnd from the last ip
+	fromEndBig := big.NewInt(int64(fromEnd))
+	resultInt := big.NewInt(0)
+	resultInt.Sub(endInt, fromEndBig)
+
+	ip, ok := netip.AddrFromSlice(resultInt.Bytes())
+	if !ok {
+		return netip.Addr{}, fmt.Errorf("invalid IP address from network %s", network)
+	}
+
+	return ip.Unmap(), nil
+}