Monday, April 13, 2009

Why Load-Balancers are Cool !

I suppose the term "load-balancer" is out of date and has been replaced by the term "Application Delivery Controller", but regardless of what you call them, they are pretty powerful and can do a lot of cool things ! Sysadmin types have known this for years, but as a network guy who just recently started digging into these, I'm a bit geeked about what you can do with these.

The background here is that we use load-balancers from both Zeus and F5 depending on the application. In preparing for the move to our new data center, we're testing some new F5 hardware and software and reconsidering how these things get connected into the network.

One goal we have is to enable failover between our data centers in Indianapolis and Bloomington (see my previous post on this). We had been looking at DNS based solutions (Global Server Load-Balancers), but for a number of reasons Route Health Injection (RHI) is a much better option for us. A couple of weeks ago we got together with our Messaging team to setup and test RHI. Without too much manual reading and just a little bit of poking around, we were able to get RHI working within about 15 minutes and boy was it slick. We injected a /32 BGP route for a DNS Virtual IP from our F5's at Indy and Bloomington and weighted the routing preferences so the Bloomington path was preferred. DNS queries resolved on the Bloomington server until we shutdown 'named' on the Bloomington server. Within a few second, queries to the same IP address were resolved by the server in Indy. Turned 'named' back up in Bloomington, and queries went back to Bloomington. One problem solved !

Operationally this points out how load-balancers are both network and server ! Server-wise they do things like SSL-offload so your SSL certs actually live on the load-balancer --- so your server admins probably want to manage these. Network-wise, they're now running BGP routing with your core routers and the routing configuration on the F5 (based on Zebra) looks a lot like Cisco's IOS --- so your network admins probably want to have some control of these functions.

Now, what if I want to add IPv6 support to those DNS servers ? Well, I could go and enable IPv6 on all my DNS servers, but with a load-balancer, I could just enable IPv6 on the load-balancers and have them translate between v6 and v4 . After all, the load-balancer is essentially acting like an application-layer proxy server. In under 2 minutes I added a new Virtual IP (IPv6 in this case) and associated it with the pool of DNS servers we already configured in our test F5s and, without touching the servers, I was resolving DNS queries over IPv6 transport ! According to their documentation Zeus supports this IPv6 functionality as well. So, instead of hampering IPv6 deployment, as is the case with many network applications such as firewalls and IDPs, these load-balancers are actually making it easy to support IPv6 !

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