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Modification of HTTP Variables

You can see what kind of information is sent by your browser on the Browser Options tab in A4Proxy, in the window titled "Outgoing request". It lists all latest requests sent by your browser, with all the variables. A4Proxy includes two pre-set modifications: User-Agent (showing the version of your browser and operating system) and Language (default language of your system). In order to use them, you will need to enable HTTP request modification on the Browser Options tab and click the Defaults button. That will hide your actual operating system and browser type and version numbers behind the generic Mozilla 4.0; the language will be set to English.

You can also modify any variables manually, however, you must know that modifying HTTP request variables is not a trivial thing.

  1. First of all, the variables are not very strictly standardized. A particular browser might generate a variable which would not be understood by the majority of web servers. It might sound strange, but it is true. Some variables are required for the Web to work, so they are regarded as standard (these include Host, Get, Post, Connect; you will see them in the Outgoing requests window, but you must never try to modify them).

    Only a few other variables are accepted widely enough (such as User-Agent, Referer, Accept-Language; they are actually not essential for the system to work) and there is a good chance for all others to be ignored by many sites.

  2. The problem is that if the website you visit does analyze the secondary variables, and you have modified their content, you may not be able to get to that site, or its pages may be displayed incorrectly. This may be true even for some variables which are usually considered safe to modify, such as Referer. Some sites won't allow you to download files or see pictures if the Referer field is "wrong" (they try to prevent other sites from making direct links to the pictures or downloadable files).

To sum up, you usually should know very well what you are doing and be able to recognize the problems if they arise if you want to modify your requests significantly. The two variables provided in the Defaults are unimportant but at the same time it is usually they that provide the most sensitive information about your system (browser and operating system versions and your language). Others, even Referer, are not that safe to modify.

Your operating system information is usually a part of the User-Agent field.The information about plugins may be present in the "Accept" field, looking like this:

Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/msword, application/vnd.ms-excel, */*

Note the elements referring to winword, excel etc. If you decide to modify it, make sure that you keep the ordinary elements referring to graphics formats (gif, bitmap, jpeg etc.), and the "*/*" element. If you have a problem with a particular site, first try disabling the modifications.

Excellent, very detailed article on the Referer field, its modification and possible issues:

http://www.deny.de/txt/misc/referring.txt
(if the link above doesn't work, a copy of the article is available here)

Some further reading:

http://www.webtools.com/story/browsers/TLS19991213S0001 http://www.oreilly.com/catalog/httppr/chapter/http_pkt.html
http://www.garshol.priv.no/download/text/http-tut.html
http://www.cs.tut.fi/~jkorpela/http.html

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