could there be any disadvantages to this kind of URL validation

Question

In some parts of my site, I'll allow users to enter an URL. I'm working on appengine, so what I was thinking, is to validate it like this.

try
    response = urlfetch.fetch(url,follow_redirects=True, deadline=1)
    if not response.status_code==200:
       raise SomeCustomException
except: Exception as e:
    my_exception_handling_code(e)

advantages

• checks for url pattern automatically
• also makes sure that the url isn't broken and the loading time is no longer than 1s, helping to ensure a good user experience when clicking a link on my site.

possible disadvantages I can see:

• the request has to wait for the fetch to complete. This will probably not be a very big deal, as there are very few occasions where users can enter URL's (currently only on signup).

Answer 1

This is a bad idea. As S. Lott pointed, url might not be available now. Or it might be available only from some specific subnetwork. Or it gets more than 1 second to load it. Just don't do this. Use a traditional approach, e.g. urlparse.

Answer 2

If a user of your site figured out that you poll a URL every time they post one they might be able to exploit it for other purposes, like voting in a poll, or racking up pageviews.