On 13/03/2007, at 11:01 AM, Simon Lyall wrote:
I'm doing a bit of DNS testing and I start noticing some weird results for a few domains. The two below appear to be specific problems since they are popular and I noticed them but I suspect there are others.
The problem is that www.anz.co.nz and www.anz.com have TTLs of zero seconds. This means that *every* DNS look up for them takes either around 40 or 140ms (one server is closer/quicker than the other) since it is never cached.
Similarly the TTL for www.trademe.co.nz (and www.oldfriends.co.nz) is just ten seconds so this must be constantly rechecked. Not as bad since their servers are in NZ but still there is a delay.
Some companies (like google or yahoo) have very sophisticated systems that constantly check user performance and switch them from datacenter to datacenter in seconds if things start going slow.
However unless your company has such a system (and very reliable and nearly DNS servers) then a TTL of a few minutes is good enough for manual updates to quickly propagate. Lower values than that will result in decreased performance for your customers. Even google and yahoo have TTLs of a minute or two.
That's not uncommon these days. Things like Foundry GSLB (Global Server Load Balancing) and Cisco DistributedDirector sit in front of DNS servers (or are DNS servers in some cases), and return records with low (or zero) TTLs. This is configurable with Foundry (default 10s), I'm not sure about Cisco and others. I believe TradeMe have some Foundry kit, so it's not inconceivable that they have GSLB turned on. I did read somewhere recently that zero-second TTLs have a bad effect on some clients resolvers, something like they instead use their 'default' which is 15 minutes in many cases. Google doesn't love me today though, so I can't give you references to it. -- Nathan Ward