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Originally Posted by cliveb
Or maybe I've completely misunderstood how NTL's proxies work. I am only concluding that they do a reverse-DNS lookup based on the symptoms. It's possible that it was coincidence that everything started working at the same time I switched to the non-NTL proxy. What I'd really like to find out is exactly how NTL's proxies are set up.
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Nothing is a simple as it first looks;
The Proxy servers don't do reverse lookups, very little would work if they relied on that system as a large number of website IP's don't have a reverse lookup available (non of mine do). Many websites share an IP as well.
When you request a page, your pc does a DNS lookup and sends the request to the identified IP. The proxy intercepts this request [on port 80] and checks the http headers for the host & page you requested. If it can supply this from its cache then it will, otherwise it will request the page itself to then pass it onto you. This will involve it doing a DNS lookup using the DNS servers it is set to use (which would presumably be NTL's).
If NTL's DNS is broken and you are using your own, you will be able to request & browse pages that the proxy has cached. If the proxy does not have them cached then it would not be able to request them for you because of its lack of DNS. You would not be able to access those pages.
However, if NTL's DNS were ok, but links to the US were down, you could still have a problem, even for web sites hosted in the UK (even with .co.uk domain names) if the domains master name servers are based in the US (like someone using zoneedit or register.com).
This is becuase if the entry for
www.yourdomain.co.uk has expired from the local DNS servers cache then it has to be looked up again from the master name server [in the US]. If this cannot be done due to a link failure then you would not be able to access the site, you would get a DNS lookup failure even though the site is fine, uk based and NTL's DNS servers are fine.
A domains DNS record could also have expired on NTL's DNS servers but not the DNS servers you are using which would make the situation look even more strange (http to the site could fail, but https/ftp etc would work).
This will have been some of the problems last night, making DNS look broken, and some sites apparently working [for some people] and others not - the failed link was still the root of the problem.