Question

I’m trying to check if computer on the net is online by using code which supposedly to check it by using ARP packets.

I am always getting message that host is offline even when I’m sure that it’s online. I have checked on my localhost IP and on some always working IPs such as google.

That could be wrong with this code?

[DllImport("iphlpapi.dll", ExactSpelling = true)]
public static extern int SendARP(IPAddress DestIP, int SrcIP, byte[] pMacAddr, ref uint PhyAddrLen);

private byte[] macAddr = new byte[6];
private uint macAddrLen;

private void Ping(IPAddress address)
{
    if (SendARP(address, 0, new byte[6], ref macAddrLen) == 0)
    {
        open++;
        txtDisplay.AppendText("Host " + address + " is open." + Environment.NewLine);
    }
    else
    {
        closed++;
        txtDisplay.AppendText("Host " + address + " is closed." + Environment.NewLine);
    }
}

By using previous code I’m basically trying to do something like following code. But the problem with this code is that when host is closed that it takes like 2 seconds to get the respond which I want to eliminate. Someone suggested to use ARP ping:

private void Ping(IPAddress address)
{
    Ping pingSender = new Ping();
    PingOptions options = new PingOptions();
    if (cbDontFragment.Checked) options.DontFragment = true;
    else options.DontFragment = false;
    string dataa = string.Empty;
    int dataCounter = 0;
    options.Ttl = (int)nudTTL.Value;

    for (int i = 0; i < nudData.Value; i++)
    {
        dataCounter++;
        if (dataCounter == 10) dataCounter = 0;
        dataa += dataCounter.ToString();
    }

    byte[] buffer = Encoding.ASCII.GetBytes(dataa);
    int timeout = 120;
    try
    {
        PingReply reply = pingSender.Send(address, timeout, buffer, options);

        if (reply.Status == IPStatus.Success)
        {
            open++;
            txtDisplay.AppendText("Host " + address + " is open. ");
            if (cbDontFragment.Checked) txtDisplay.AppendText(" Don't fragment. ");
            txtDisplay.AppendText(" TTL: " + options.Ttl.ToString() + " ");
            txtDisplay.AppendText(" Bytes: " + nudData.Value + " ");
            txtDisplay.AppendText(Environment.NewLine);
        }
        else
        {
            closed++;
            txtDisplay.AppendText("Host " + address + " is closed. ");
            if (cbDontFragment.Checked) txtDisplay.AppendText(" Don't fragment. ");
            txtDisplay.AppendText(" TTL: " + options.Ttl.ToString() + " ");
            txtDisplay.AppendText(" Bytes: " + nudData.Value + " ");
            txtDisplay.AppendText(Environment.NewLine);
        }
    }
    catch (Exception ex)
    {
        txtDisplay.SelectedText += Environment.NewLine + ex.Message;
    }
}

Answer 1

ARP cannot be used for what you are trying to do. It only works over a local network.

It’s purpose is to resolve an IP address (which is routed) to a MAC address (which is not). It is never sent beyond a network segment (a lan)