I've recently seen something like this myself, using DNS for DR, so if one server goes down the A record is modified right away so there is minimal downtime, to me this is a bad way of setting up DR. The problem I've seen is that some DNS servers don't read the 0 value and stick a default TTL of about an hr onto the zone, is this common? Regards Barry
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.
The scary thing is that both sites probably put a lot of time into making the actual pages load as fast as possible.
-- Simon Lyall | Very Busy | Web: http://www.darkmere.gen.nz/ "To stay awake all night adds a day to your life" - Stilgar | eMT.
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