DNS Round-Robin on SSL

Question

We're adding a second web server for redundancy and load sharing purposes. All connections are mandated to be SSL, and adding a dedicated appliance is not possible at this moment.

I'd like to use round robin DNS, where both servers answer to the same domain using different IPs (we have a wildcard SSL certificate, so that's OK). I can get the DNS to return in random/round robin order no problem.

Is this a bad setup when using SSL?

Our user pattern is consistent -- users will consistently be utilizing the web app for 8-10 hours. We want each page view to be as fast as possible, and my concern is users could constantly flip between the servers, potentially negating any SSL handshake caching/keep alive.

Thanks!

SimonJ SimonJ · Accepted Answer · 2009-12-12T20:17:06

Firstly, SSL has the ability to resume an earlier session, so flipping between servers will cost you a few hundred ms per request (longer if several clients are accessing the site simultaneously, since this is CPU time we're talking about).

Whether the clients will actually flip depends, though - DNS "load balancing" is fiddly business:

if many of your users are using the same recursive nameservers, they'll get the same "first IP" hence no load balancing
if the DNS record has a high TTL (several hours), caching nameservers will store a particular permutation of IP addresses until they expire (good so long as your users aren't all using the same recursive nameservers)
if your users have multiple recursive nameservers configured, they may flip if each nameserver has a different "first IP" (bad)
if you have no mechanism for removing "bad" records, and a low TTL, then if one server goes down 50% of your clients will get the "bad" server and have to wait for a timeout before they can see your site

As you can see there are various tradeoffs depending on whether you're more concerned about redundancy/failover or load balancing; DNS isn't really the best tool here - you really need the servers to share an IP using either a reverse proxy, or something like Heartbeat (assuming you're Linux-based).

An aside: if both servers are answering to the same domain then you don't need a wildcard cert, although CAs often charge more if you intend to use a cert on more than one server.

DNS Round-Robin on SSL

4 Answers