Simple dynamic DNS client for namecheap. Looks for files in \ProgramData\DynDns in form of <yourdomain>.namecheap, containing dyndns password. Can be used with csi (C# script) and Task Scheduler

DynDns.csx

using System.Diagnostics;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Security.AccessControl;

IPAddress GetMyExternalIpAddress() {

    var dnsQuery = new ProcessStartInfo("nslookup", "myip.opendns.com. resolver1.opendns.com")
    {
        RedirectStandardOutput = true,
        UseShellExecute = false,
    };

    var lookup = Process.Start(dnsQuery);
    var lookupResult = lookup.StandardOutput.ReadToEnd().Split('\n');
    bool answer = false;
    foreach(var line in lookupResult)
    {
        if (line.Contains("Name:"))
        {
            System.Console.WriteLine("Got IP!");
            answer = true;
        }
        else if (!answer)
            continue;

        if (line.StartsWith("Address:"))
            return IPAddress.Parse(line.Substring("Address:".Length).Trim());
    }

    throw new SocketException();
}

var appData = System.Environment.GetFolderPath(System.Environment.SpecialFolder.CommonApplicationData);
var dynDnsFolder = Path.Combine(appData, "DynDns");
var adminsOnly = new DirectorySecurity();
adminsOnly.AddAccessRule(new FileSystemAccessRule(@"BUILTIN\Administrators", FileSystemRights.FullControl,
    InheritanceFlags.ContainerInherit | InheritanceFlags.ObjectInherit,
    PropagationFlags.None, AccessControlType.Allow));
var hosts = Directory.CreateDirectory(dynDnsFolder, adminsOnly);

void NamecheapUpdateIp(IPAddress newIp, string hostName, string hostPassword) {
    var request = HttpWebRequest.Create($"https://dynamicdns.park-your-domain.com/update?host=@&domain={hostName}&password={hostPassword.TrimEnd('\n')}&ip={newIp}");
    request.GetResponse();
}

IPAddress myIp = GetMyExternalIpAddress();

foreach(var hostFile in hosts.EnumerateFiles())
{
    if (".namecheap".Equals(hostFile.Extension, System.StringComparison.OrdinalIgnoreCase))
    {
        var host = Path.GetFileNameWithoutExtension(hostFile.Name);
        NamecheapUpdateIp(myIp, host, File.ReadAllText(hostFile.FullName));
        System.Console.WriteLine($"Updated {host} to {myIp}");
    }
    else
        System.Console.Error.WriteLine("Unrecognized service type: " + hostFile.Extension);
}

Don't forget to restrict access to .namecheap files to administrators only, and run script either as admin, or SYSTEM

P.S. park-your-domain.com is owned by Namecheap. Reverse engineered from Namecheap official DynDns client.