Network interface error while login in asp.net

 protected void btnLogin_Click(object sender, EventArgs e)
{
            string LoginName = txtloginname.Text.Trim();
            string Password = txtpassword.Text.Trim();
            AdminLogin oLogin = new AdminLogin();
            oLogin.LoginName = LoginName;
            oLogin.Password = Password;
            oLogin.MachineIp = "";
            string sa;
            string strHostName = System.Net.Dns.GetHostName();
            sa = System.Net.Dns.GetHostAddresses(strHostName).GetValue(0).ToString();
           
            if (HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"] != null)
            {
                sa = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"].ToString();
            }
            else if (HttpContext.Current.Request.UserHostAddress.Length != 0)
            {
                sa = HttpContext.Current.Request.UserHostAddress;
                sa = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
            }
            string s = GetMAcAddress();
            
            oLogin.MachineIp = sa.ToString().Trim();

            DataSet ds = AdminLoginManager.CheckLoginInfo(oLogin, conn);

                    if (ds.Tables[0].Rows.Count > 0)
                    {
                        
                            SessionInfo.LoginId = ds.Tables[0].Rows[0]["Admin_Login_Id"].ToString().Trim();
                            SessionInfo.Login_UserName = ds.Tables[0].Rows[0]["Admin_Login_Name"].ToString().Trim();

                            SessionInfo.LoginName = ds.Tables[0].Rows[0]["FullName"].ToString().Trim();

                            SessionInfo.LoginType = ds.Tables[0].Rows[0]["UserType"].ToString().Trim();

                            if (ds.Tables[0].Rows[0]["PwdChange"].ToString() == "N")
                            {
                                Response.Redirect("/ChangePassword.aspx", false);
                            }
                            else
                            {
                                Response.Redirect("/Admin_Home.aspx", false);
                            }
                            Log logObj = new Log();
                            string servername = "CjOrder";
                            string username = txtloginname.Text.Trim();
                            string eventtype = logObj.adUserValid;
                            string eventdesc = "User Logged In Successfully";
                            LogManager.Insert_Log(conn, servername, username, eventtype, eventdesc, oLogin.MachineIp);
                       
                    }
                    
          }     

private string GetMAcAddress()
    {
        string macAddress = string.Empty;
        foreach (System.Net.NetworkInformation.NetworkInterface nic in System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces())
        {
            if (nic.OperationalStatus == System.Net.NetworkInformation.OperationalStatus.Up)
            {
                macAddress += nic.GetPhysicalAddress().ToString();
                break;
            }
        }
        return macAddress;
}