DNS - recursion/forwarding

VantageUK

Simple question.

What is the difference between recursion and forwarding in DNS?

As far as I can see (MS Press) they both send DNS queries from a client that the local server can't resolve on to upstream DNS servers and then pass on the reply to the requesting client.

Can anyone clarify the difference, if there is one?

Cheers

Charlie.

dynamik

Basically, recursion occurs if the 2003 DNS server cannot resolve the name itself and doesn't have it in it's cache. It'll start at the root zone and work it's way down. For example, if you're trying to get to support.microsoft.com, it'll query the root zone (.), which will refer it to a dns server responsible for com, which will finally refer it to the dns server responsible for microsoft.com.

Forwarding basically just sends a DNS to another DNS server, such as your ISPs. Then that server will either resolve queries, refer to it's cache, or perform recursion itself. Forwarding can have security and/or performance benefits.

There's a lot more information (with diagrams) here:
http://technet2.microsoft.com/windowsserver/en/library/0bcd97e6-b75d-48ce-83ca-bf470573ebdc1033.mspx?mfr=true
http://technet2.microsoft.com/windowsserver/en/library/0104be3c-0405-4455-b011-6950875c04461033.mspx?mfr=true

royal

Also, you can set a forwarder, and if the forwarder can't resolve it, you can have your DNS server still provide recursion via root hints (default).

VantageUK

And lo, it all became clear!

Thanks both.

Charlie.