What does DNS flushing do?

Question

I came across a website that instructed me to flush my DNS in order to view what was online. What does this do and why would it help display the website?

Answer 1

DNS is the Domain Name System. DNS servers convert a domain name (such as example.com) into an IP address (in this case 192.0.32.10). The mapping of names to numbers can change from time to time.

Your computer holds a record of DNS entries to save looking them up every time. This is your DNS cache. You can delete those records (flush the cache) any time you like.

If a website has recently moved servers, you might see the old website for a while. Flushing your DNS cache might help.

Answer 2

Your system holds the IP address of named sites... When you do a ipconfig /flushdns, your system clears the cache of name to ip entries and reloads them from the connected DNS server.

Answer 3

Unless you're on old versions of Windows, it means that the website operators messed up. They moved the service to a new IP address; before doing that, they should have lowered some timeouts on the name data in DNS, so that they said something like "this mapping is valid for 5 minutes" instead of "this mapping is valid for the next day". This needs to be done some time in advance.

The exception to this is that old Windows releases (through XP? I forget) would ignore timeouts shorter than a day and just remember the old mapping anyway. While it's common for caches to have a "lower bound" on how long the timeout can be, it's usually around "5 minutes", not "24 hours". This means that a website moving IP address has to actually be available on two different IP addresses for that length of time.

The solution for the website operators is to have a forwarding "proxy" run on the old IP address, passing traffic to the new IP address, for a couple of days, to give time to deal with the sad reality of stupidly excessive caching.

Either way, the problem has been punted onto you. But there's a reason that big professional sites don't ask you to do this -- they work around the problem themselves.