csdn登录跳转

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><link href="/Content/style.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="/scripts/jquery-1.4.1.min.js"></script>
    <script type="text/javascript" src="/scripts/csdn.js?v=1.1"></script>
    
 
<title>
会员登录 - 会员中心 - CSDN.NET
</title></head>
 
<body>
<script src="http://csdnimg.cn/pubnav/js/pub_topnav_2011.js"type="text/javascript"></script>
 
<div class="full">
 
 
<div class="logo_login02">
<img src="/images/logo_login01.gif" />
</div>
<div class="content_login">
<div class="top_bg"></div>
<div style="padding:40px;">
<div class="left_login">
<iframe id="logfrm" src="/account/loginbox?callback=logined&hidethird=1&from=http%3a%2f%2fforum.csdn.net%2fPointForum%2fForum%2fUserTopicList.aspx%3fsectionid%3d3036657c-277c-476c-982d-75f154e09050" style="width:420px;height:220px;" frameborder="0" scrolling="no"></iframe>
</div>
<div class="center_login"></div>
<div class="right_login">
<p><img src="/images/pic_text.gif" alt="" /></p>
<p class="button"><a class="btn_loginr" href="/account/register"><span>立即注册</span></a></p>
</div>
<div class="clear"></div>
<table id="tb_third" cellpadding="4" cellspacing="8">
<tr>
    <td width="70"><b style="font-size:12px; color:#777;">第三方登录</b></td>
    <td id="td_third">
        <a href="/auth/sinat"><img src="/images/button/third_sinat.png" /></a>
        <a href="/auth/qq"><img src="/images/button/third_qq.png" /></a>
        
        <a href="/auth/google"><img src="/images/button/third_google.png" /></a>
        <a href="/auth/aliyun"><img src="/images/button/third_aliyun.png" /></a>
    </td>
</tr>
</table>
</div>
<div class="btm_bg"></div>
</div>
<script type="text/javascript"> 
    var f = decodeURIComponent('http%3a%2f%2fforum.csdn.net%2fPointForum%2fForum%2fUserTopicList.aspx%3fsectionid%3d3036657c-277c-476c-982d-75f154e09050');
</script>
<script type="text/javascript"> 
    if (f.indexOf("about:") > -1 || f.indexOf("javascript:") > -1)
        f = "http://www.csdn.net/";
    var plow = false;
    function logined(data) {
        if (data.status) {
            if (plow) location = "/account/pwdnote?from=" + escape(f);
            else location = f;
        } else {
            if (data.error.indexOf("激活") > -1) {
                location = "/account/active";
            } else {
                alert(data.error);
            }
        }
    }
    $(function () {
        if (f != 'http://www.csdn.net/') {
            $('#td_third a').attr('href', function () {
                return this.href += '?from=' + encodeURIComponent(f);
            });
        }
    });
</script>
 
 
<div style="text-align: center;">
<script type="text/javascript" src="http://csdnimg.cn/pubfooter/js/publib_footernew.js"></script>
</div>
 
</div>
<script type="text/javascript" src="/scripts/csdn.loading.js"></script>
<script type="text/javascript">document.write("<img src=http://counter.csdn.net/pv.aspx?id=20 border=0 width=0 height=0>");</script>
</body>
</html>
这是CSDN登录源码，其中iframe的src属性中form参数是怎样得到的？急需，谢谢还有变量 f 的值（url地址）是如何实现的？