通过外部网站返回得到内网外部IP

static string GetIP()
{
    Uri             uri = new Uri("http://www.ikaka.com/ip/index.asp";);
    System.Net.HttpWebRequest  req = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(uri);
    req.Method          = "POST";
    req.ContentType     = "application/x-www-form-urlencoded";
    req.ContentLength   = 0;
    req.CookieContainer = new System.Net.CookieContainer();
    req.GetRequestStream().Write(new byte [0], 0, 0);
    System.Net.HttpWebResponse res = (System.Net.HttpWebResponse)(req.GetResponse());
    StreamReader    rs  = new StreamReader(res.GetResponseStream(), System.Text.Encoding.GetEncoding("GB18030"));
    string          s   = rs.ReadToEnd();
    rs.Close();
    req.Abort();
    res.Close();
    System.Text.RegularExpressions.Match m = System.Text.RegularExpressions.Regex.Match(s, @"IP：/[(?<IP>[0-9/.]*)/]");
    if (m.Success) return m.Groups["IP"].Value;
    return string.Empty;
}