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Originally Posted by pem
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).
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Let me make sure I've got this right....
When my browser is set up without an explicit proxy and I request, say, "www.bbc.co.uk", it is my understanding that my browser does a DNS lookup on that name and receives an IP address in reply. Then my browser attempts to connect to that IP address on port 80, at which point NTL's transparent proxy intercepts it. Now, at this stage, all the connections are being done at the IP address level, so why does the proxy have to do a DNS lookup?
You say that the proxy checks the http headers: am I to understand that in those headers will be the host name "www.bbc.co.uk", and it is this which causes the proxy to do a DNS lookup (if the page is not in its cache)? Seems a bit strange: why not just use the IP address in the original request that was intercepted, thus saving an unnecessary DNS lookup? (It would be a bit ironic if the answer is because NTL don't trust others' DNS servers :-)