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I can not understand following error from my VPS (LAMP)

175.101.XX.XX - - [27/Aug/2013:17:41:48 +0530] "GET /undefined HTTP/1.1" 404 694 "" "Mozilla/5.0 (Windows NT 5.1; rv:23.0) Gecko/20100101 Firefox/23.0"

I am getting these kind of error, anyone can help me to know source and nature of this error.

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Some web client with the IP 175.101.XX.XX wants to retrieve some file named "undefined" from your server. Since that file doesn't exist, your server answers with a 404 error.

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cpanel error log shows [Tue Aug 27 19:46:54 2013] [error] [client 175.101.xx.xx] File does not exist: /home/sitename/public_html/foldername/undefined, referer: – user1844933 Aug 28 '13 at 7:35
So it looks as if on the site you mentioned ( ) there's a link towards that 'undefined' filename. That's what's called Referer (see original log line) – vautee Aug 28 '13 at 8:48
I will receive email from my site at the time of user registration, login and activities performed in member page, but there is no mail from 175.101.XX.XX and there is no entry in my database for this IP, then how it possible to enter to member page? is it client system infected by virus? – user1844933 Aug 28 '13 at 9:06

undefined is the keyword of choice for Javascript when you try to do something that is, well, undefined. Maybe something on your website is trying to do an AJAX request using a file name that's pulled from a form element, but the form element doesn't exist.

This is not something to be fixed server side - I would be using your website with something like FireBug enabled and keeping an eye on the requests that you're sending to see what's causing the page undefined to be accessed.

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