Io tempo fà ho trovato questo codice
codice:
<% @Page Language="C#" ResponseEncoding="utf-8" debug="true"%>
<% @Import Namespace="System.Net.Sockets" %>
<% @Import Namespace="System.Text" %>
<% @Import Namespace="System.IO" %>
<% @Import Namespace="System.Collections" %>
<script language="C#" runat="server">
void doQuery()
{
  String strDomain = Request["dominio"] + Request["tld"];
  txtDomain.Text=strDomain;
  char[] chSplit = {'.'};
  string[] arrDomain = strDomain.Split(chSplit);
  // es darf genau ein domain name + ein suffix sein
  if (arrDomain.Length > 2)
  {
    arrDomain[1]+= '.' + arrDomain[2];
    
  }

  // das suffic darf nur 2 oder 3 zeichen lang sein
  int nLength = arrDomain[1].Length;
  if (nLength >7) 
  {
    return;
  }

  Hashtable table = new Hashtable();
  table.Add("at", "whois.nic.at");
  table.Add("de", "whois.denic.de");
  table.Add("be", "whois.dns.be");
  table.Add("gov", "whois.nic.gov");
  table.Add("mil", "whois.nic.mil");
  table.Add("it", "whois.nic.it");
  table.Add("com", "whois.internic.net");
  table.Add("info", "whois.nic.info");
  table.Add("co.uk", "whois.nic.uk");
  table.Add("org.uk", "whois.nic.uk");
  table.Add("cc", "whois.nic.cc");

  String strServer = "whois.networksolutions.com";
  if (table.ContainsKey(arrDomain[1]))
  {
    strServer = table[arrDomain[1]].ToString();
  }
  else if (nLength == 2)
  {
    // 2-letter TLD's always default to RIPE in Europe
    strServer = "whois.ripe.net";
  }
  
  String strResponse;
  bool bSuccess = DoWhoisLookup(strDomain, strServer, out strResponse);
  if (bSuccess)
  {
    txtResult.Text = strResponse;
  }
  else
  {
    txtResult.Text = "Lookup failed";
  }
}

bool DoWhoisLookup(String strDomain, String strServer, out String strResponse)
{
  strResponse = "none";
  bool bSuccess = false;

  TcpClient tcpc = new TcpClient();
  try
  {
    tcpc.Connect(strServer, 43);
  }
  catch(SocketException ex)
  {
    strResponse = "Could not connect to Whois server";
    return false;
  }

  strDomain += "\r\n";
  Byte[] arrDomain = Encoding.ASCII.GetBytes(strDomain.ToCharArray());
  try
  {
	Stream s = tcpc.GetStream();
	s.Write(arrDomain, 0, strDomain.Length);
	
	StreamReader sr = new StreamReader(tcpc.GetStream(), Encoding.ASCII);
	StringBuilder strBuilder = new StringBuilder();
	string strLine = null;

	while (null != (strLine = sr.ReadLine()))
	{
		strBuilder.Append(strLine+"
");
	}
	tcpc.Close();
		
	bSuccess = true;
	string tmp=strBuilder.ToString();
	if (tmp.IndexOf("No match")>-1 || tmp.IndexOf("No entries")>-1)
	{
	strResponse = "Il Dominio è disponibile.
Sei interessato alla registrazione di questo dominio? Contattaci all'indirizzo <a href=mailto:info@omnia.it>info@omnia.it</a> o telefona al numero 095.7796006.";
	}
	else
	{
	strResponse = "Dominio già  registrato ";
	dettagli.Text=tmp;
	showlink.Visible=true;
	}
  }
  catch(Exception e)
  {
	strResponse = e.ToString();
  }
    
    return bSuccess;
}
void Page_Load()
{
if (!IsPostBack){
doQuery();
}
}

</script>
che funziona egregiamente