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4.3 HTTP 1.1 Request Headers

103

Proxy Authorization

This header lets clients identify themselves to proxies that require it. 

Servlets typically ignore this header, using 

Authorization

 instead.

Range

This rarely used header lets a client that has a partial copy of a docu 

ment ask for only the parts it is missing.

Referer

This header indicates the URL of the referring Web page. For example, 

if you are at Web page 1 and click on a link to Web page 2, the URL of 

Web page 1 is included in the 

Referer

 header when the browser 

requests Web page 2. All major browsers set this header, so it is a useful 

way of tracking where requests came from. This capability is helpful for 

tracking advertisers who refer people to your site, for changing content 

slightly depending on the referring site, or simply for keeping track of 

where your traffic comes from. In the last case, most people simply rely 

on Web server log files, since the 

Referer

 is typically recorded there. 

Although it's useful, don't rely too heavily on the 

Referer

 header since 

it can be easily spoofed by a custom client. Finally, note that this header 

is 

Referer

, not the expected 

Referrer

, due to a spelling mistake by one 

of the original HTTP authors.

Upgrade

The 

Upgrade

 header lets the browser or other client specify a commu 

nication protocol it prefers over HTTP 1.1. If the server also supports 

that protocol, both the client and the server can switch protocols. This 

type of protocol negotiation is almost always performed before the serv 

let is invoked. Thus, servlets rarely care about this header.

User Agent

This header identifies the browser or other client making the request 

and can be used to return different content to different types of 

browsers. Be wary of this usage, however; relying on a hard coded list 

of browser versions and associated features can make for unreliable 

and hard to modify servlet code. Whenever possible, use something 

specific in the HTTP headers instead. For example, instead of trying 

to remember which browsers support gzip on which platforms, simply 

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