Validate the client IP allocation

util/util.go

@@ -268,3 +268,43 @@ func GetAvailableIP(cidr string, allocatedList []string) (string, error) {
 
 	return "", errors.New("No more available ip address")
 }
+
+// ValidateIPAllocation to validate the list of client's ip allocation
+// They must have a correct format and available in serverAddresses space
+func ValidateIPAllocation(serverAddresses []string, ipAllocatedList []string, ipAllocationList []string) (bool, error) {
+	for _, clientCIDR := range ipAllocationList {
+		ip, _, _ := net.ParseCIDR(clientCIDR)
+
+		// clientCIDR must be in CIDR format
+		if ip == nil {
+			return false, fmt.Errorf("Invalid ip allocation input %s. Must be in CIDR format", clientCIDR)
+		}
+
+		// return false immediately if the ip is already in use (in ipAllocatedList)
+		for _, item := range ipAllocatedList {
+			if item == ip.String() {
+				return false, fmt.Errorf("IP %s already allocated", ip)
+			}
+		}
+
+		// even if it is not in use, we still need to check if it
+		// belongs to a network of the server.
+		var isValid bool = false
+		for _, serverCIDR := range serverAddresses {
+			_, serverNet, _ := net.ParseCIDR(serverCIDR)
+			if serverNet.Contains(ip) {
+				isValid = true
+				break
+			}
+		}
+
+		// current ip allocation is valid, check the next one
+		if isValid {
+			continue
+		} else {
+			return false, fmt.Errorf("IP %s does not belong to any network addresses of WireGuard server", ip)
+		}
+	}
+
+	return true, nil
+}