It would depend on how popular the sites hosted on the servers were. If you set a the times to be too low, say 1 minute, then every time someone looks up the DNS records, then BLAM... your dns servers are hit because things aren't cached anywhere.
So I would use something like an hour (we use this). An hour is reasonable unless you need total 100% uptime. If you needed 100% uptime, you wouldn't just rely on DNS for this anyway. You'd need something more reliable like IP takeover, dedicated hardware solutions, etc. Depends greatly on what your budget is. The dns servers are queried randomly, so say you have 4 DNS servers listed, then each 4, in theory, should get approximately the same amount of traffic. If one of them goes down, then the client SHOULD try the next available dns server. You'd also want to colocate somewhere WAY out of the same network neighbourhood. Interestingly a few of our clients from the USA do this. Since we are located in Hong Kong, our networks are totally seperate from anything you use in the USA. So when these california blackouts (is that the right term?) hit them, they were fine. If you really want to keep everything in the USA, try and find totally seperate networks... and i mean totally (if you want to be real safe). UUnet and the big boys in the USA tend to have a few core NOCs (even if they tell you everything is distributed and safe, blah blah blah), and if any one of them is hit with a blackout, earthquake, etc. then the whole network is affected. This happened to UUnet in one of the countries in Asia (won't mention which country it just in case UUnet is watching this) once... something happened to one of their core international-link routers, and many countries were affected, including the one our client was in. UUnet may deny it but we... the people who actually use them... know the true story ;-) Anyway, if you're really into reliability, you might want to colocate in hong kong. Can't get much more diversified network-wise than that. Email me back if you're interested in working something out. Otherwise, consider the above carefully about the US networks. Sincerely, Jason ----- Original Message ----- From: ":yegon" <[EMAIL PROTECTED]> To: <debian-isp@lists.debian.org> Sent: Sunday, June 17, 2001 8:50 PM Subject: redundancy via DNS > we have several servers colocated with several ISP's > i am trying to sort out some configuration that would ensure good uptime for > customers > > i want to place the html documents of every customer on two separate servers > connected to separate ISP's > the dns servers will point to one server and the second one will be just a > backup, in case the main server goes down we just change the DNS and point > the affected domains to the backup server. when the main server is back up > the dns changes back to normal > > and now my questions: > 1. what should the times in zone files be set to to enable the dns change to > be propagated very quickly, say 5 minutes max. > is it possible/wise to use TTL=0 > > 2. if a domain has 2 name servers set during registration, are both of these > servers used for lookups? Or is it so that just the primary is querried if > it works, and the secondary is querried only if the primary is not > responding? > > 3. is this whole idea worth consideration anyway or should I forget it? > > > thanks for answers > > Martin Dragun > > > -- > To UNSUBSCRIBE, email to [EMAIL PROTECTED] > with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED] > >