The If-Modified-Since HTTP Header

The “If-Modified-Since” HTTP header is sent from an agent (browser/bot) to the web server in order to know if the requested page has been changed since its last visit. The server responds with a 200 code (Ok) if the page has been modified or with a 304 code (Not modified) if the page has not been modified.

Here is an easy way to check if a web server supports the if-modified-since HTTP header:

Telnet the web site on port 80:

# telnet www.example.com 80

Then enter the following (modify it according to your requested web site) :

GET /index.html HTTP/1.0
User-Agent: Mozilla/5.0
From: something.somewhere.net
Accept: text/html,text/plain,application/*
Host: www.example.com
If-Modified-Since: Wed, 19 Oct 2005 10:50:00 GMT

I assume that you know what is the last modification time of the web page, so substitute the date and time to something later than that. Just remember to use GMT time.

If the returned code is 304 (Not modified), then the server supports this header.

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