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ASP ADVANCED
Both browser requests and server responses include headers. Headers provide additional information about the content of a request or a response. They can also contain information about the browser making the request or the server providing the response. Active Server Pages includes a number of collections and methods that can help you manipulate headers. Some of these methods are targeted at a particular task, such as providing a Web page with a content rating or providing an expiration date for the Web page. Other methods and collections allow you to manipulate headers in general. The following sections explain how to use the methods and collections that affect headers. Retrieving
Headers When a browser requests a Web page from a server, the request includes a number of headers. You can retrieve these headers by using the serverVariables collection of the Request object. The ServerVariables collection contains both headers and additional items of information about the server. The following Active Server Page dumps all the contents of the ServerVariables collection to the browser window (see Figure Below):- <HTML> The ServerVariables collection
The
headers (and server variables) in the ServerVariables collection hold a
wide assortment of different types of information.
Appendix A, "Quick ASP Object and Component Reference,"
contains a list of the standard headers and server variables in this
collection. The following
list explains the more useful ones:
Certain
versions of Internet Explorer (such as versions 3.0 and 4.0 in Windows 95)
contain additional headers in their requests:
You
can retrieve the contents of a particular header or server variable by
passing its name to the ServerVariables collection.
For example, the following
script (continued on the next page) allows a user to access the page only
if he or she arrived from the Active Server Page named origin.asp: <HTML> In
this script, the HTTP_REFERER header determines the page the visitor used
to link to the current page. If
the user didn't arrive from the page
located at http://mysite/origin.asp, he or she isn't allowed to view the
contents of the page. Using Headers to Control How a Page Is Cached Proxy
servers are used to reduce the amount of time needed to retrieve a Web
page over the Internet. Proxy
servers keep local copies of Web pages in a cache; when someone requests a
Web page, it can be retrieved from the proxy rather than from the original
server. From
the point of view of Active Server Pages, however, proxy servers are an
evil nuisance. The whole
point of an Active Server Page is to display dynamic content.
In other words, an Active Server Page can display content that
changes every time it's viewed. You
don't want a proxy server to respond with stale copies of your Active
Server Pages. By
default, proxy servers should not cache your Active Server Pages.
The ASP CACHE -CONTROL General header provides directives for how
proxy servers should cache Web pages.
When you use Active Server Pages, by default, this header directs
proxy servers not to cache your pages.
However, you can override this default behavior.
If for some odd reason you want proxy servers to cache your Active
Server Pages, you can modify the CacheControl property of the Response
object. Add the following
line to the top of an Active Server Page to allow the page to be cached by
proxy servers: <%
Response.CacheControl="Public" %> You
can also indicate how you want Web browsers to cache your Active Server
Pages. Browsers commonly have
memory and disk caches. Two
properties of the Response object control how browsers cache your web
pages. By using the Expires
property of the response object, you can specify the amount of time in
minutes that a browser should use a cached copy of a Web page.
If you set this property to 0, the browser won't cache the Active
Server Page at all. Here's
an example: <% Response.Expires=0 %> You
can also specify an absolute date and time when a cached copy of an Active
Server Page should expire. When
you do this, the Web browser will continue to use a cached copy of the
Active Server Page until this date and time arrives.
The following example directs browsers to cache a page until the
year 1998: <% Response.ExpiresAbsolute=#Jan 1,1998 00:00:00# %> Using
Headers to Specify Content Rating The
Platform for Internet Content Selection (PICS) provides a standard
way to label the content of a Web page or a whole Web site.
For example, you can use PICS labels to indicate the level of
violence or sexual explicitness of the content of a Website. PICS
itself is not a rating service. PICS is a specification for the format of
content labels. The PICS standard is intended to be compatible with the
decentralized culture of the Web. The
idea is that many different rating services can use PICS to label Web
sites according to their own standards.
For example, the Parent-Teacher Association might use PICS to label
Web sites in a very different way than an organization such as a Beavis
and Butt-Head fan club. The
decision of which rating service to use is left in the hands of the
individual. In theory, an
individual could select the rating service that reflects his or her
personal tastes. This is the
vision, anyway. Currently,
only one rating service is widely supported.
Internet Explorer versions 3.0 and above support the PICS standard.
However, by default, the only rating service it uses is the one
provided by the Recreational Software Advisory Council (RSAC).
This is the same organization that places ratings on computer
games. RSAC uses a rating
system that has four categories. You
can rate your site according to its level of violence, nudity, sex, and
language. For each category,
you can specify a level between I and 5. The
user can configure Internet Explorer to block Web sites that don't meet
particular content preferences. For
example, someone can indicate that he or she wants all sites blocked that
use stronger language than mild expletives.
The browser can also be configured to block all sites that
haven't been given a content rating. (Of course, at this time, this would
also block the vast majority of sites on the Internet.) To receive an RSAC rating for your Web site, register the site with RSAC. (Currently there's no fee for this service.) The RSAC Web site is located at http://www.rsac.org. Once there, answer a series of questions about the content of your web site At
the end of the process, you're supplied with a PICS label.
For example, the ASP Site Web site (the companion Web site for this
book) received the following PICS label: (PICS-1.1
"http://www.rsac.org/ratingsv01.html" l gen true After
you have a PICS label for the Web Site, you need to include the label in
the pics- label header of your home page.
There's a special property of the Response object for this
particular purpose. You can
use the PICS property to add a PICS label to the correct header.
Here's an example: <%
Response.PICS("(PICS-1.1"&CHR(34) This
method of adding a PICS label is extremely cumbersome.
You must include the whole PICS label in one unbroken line.
Furthermore, you must specify all quotation marks by using CHR
(34). Fortunately, an easier
way to add a PICS label is available.
You can simply include the following HTML tag in the head of
your HTML document: <META
http-equiv="PICS-Label" The META http-equiv HTML tag is used to specify a response header within an HTML document. This method of specifying the PICS label accomplishes the same thing as the previous example. However, when using the META tag, you don't have to worry about placing everything on a single line or not using quotation marks.
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