string stringMAC = string.Empty;
            ManagementClass MC = new ManagementClass ("Win32_NetworkAdapterConfiguration");
            ManagementObjectCollection MOC= MC.GetInstances();
            foreach(ManagementObject MO in MOC)
            {
                if ((bool)MO["IPEnabled"] == true)
                {
                    stringMAC += MO["MACAddress"].ToString();
                }
            }

------解决方案--------------------
怀疑上面的代码获得是服务器的MAC地址。

------解决方案--------------------
做一个 ActiveX 使用上面的代码 就可以获取客户机了！

------解决方案--------------------
证书认证不能实现楼主要求的功能么？

------解决方案--------------------
看看这个可以吗
//添加组件System.Management
using System.Management;
using System.Runtime.InteropServices;

[DllImport("Iphlpapi.dll")]
   private static extern int SendARP(Int32 dest, Int32 host, ref Int64 mac, ref Int32 length);
   [DllImport("Ws2_32.dll")]
   private static extern Int32 inet_addr(string ip);


protected void Page_Load(object sender, EventArgs e)
   {
       try
       {
           string userip = Request.UserHostAddress;  
           Int32 ldest = inet_addr(userip);   //目的地的ip    
           Int32 lhost = inet_addr("");   //本地服务器的ip    
           Int64 macinfo = new Int64();
           Int32 len = 6;
           int res = SendARP(ldest, 0, ref   macinfo, ref   len);
           string mac_src = macinfo.ToString("X");
           if (mac_src == "0")
           {
               if (userip == "127.0.0.1")
                   userinfo.Text = "正在访问Localhost!";
               else
                   userinfo.Text = "欢迎来自IP为" + userip + "的朋友！";
               return;
           }
           while (mac_src.Length < 12)
           {
               mac_src = mac_src.Insert(0, "0");
           }
           string mac_dest = "";
           for (int i = 0; i < 11; i++)
           {
               if (0 == (i % 2))
                   mac_dest = mac_dest.Insert(0, mac_src.Substring(i, 2));
           }
           userinfo.Text = "欢迎来自IP为" + userip + ",MAC地址为" + mac_dest + "的朋友！";
       }
       catch (Exception err)
       {
           Response.Write(err.Message);
       } 
   }