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From Tom Evans <>
Subject Re: [users@httpd] rewrite rule tutorial on ask apache I could not understand syntax
Date Tue, 15 Jun 2010 15:47:43 GMT
On Tue, Jun 15, 2010 at 3:56 PM, Tapas Mishra <> wrote:
> On Tue, Jun 15, 2010 at 8:16 PM, Tom Evans <> wrote:
>> I dont understand what you are asking? - the reply comes from apache.
> I could not understand I am newcomer to apache.I have used Reverse
> Proxy and other settings but do not completely understand it.
> So trying to understand how apache behaves when it gets a request to
> serve a webpage what is happening in background.
>> HTTP_HOST is a variable that holds the value of the http header
>> 'Host'. If the browser doesn't send one, it isn't going to have a
>> value.
> What is Host in this reply which you are referring to.

Host is a request header sent with HTTP/1.1 requests indicating the
host that the user requested.
Apache reads that header in and stores it on the request.
mod_rewrite makes that variable available to read with the name HTTP_HOST.

If the request is a HTTP/1.0 request, no host header is sent, so
HTTP_HOST will be empty if examined.

I can't explain the HTTP protocol + Apache on a mailing list. Do what
I did and read RFC 2616 to understand HTTP, and read "Apache modules
with Perl and C" to understand Apache (although it is a little dated
now, it has an excellent section on the request life cycle). This may
also help:


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