IP changeover

[pl12987]

The weirdest thing is going on with Verio. They sent me a notice that the root IP of my VPS server would "change" on April 1. That's all. So, I routinely asked 1.) if I could assume that change would be reflected on their own nameservers, and 2.) if there would be an overlap period where both IPs were live, to allow for propagation time.

I assumed both answers would be yes, but when I didn't get that response, I gently pointed out it would not be cool to leave customer web sites hanging for six hours or more...

Next thing I know, I get a new notice: Oops, we forgot a few things, forget the deadline and we'll get back to you.

Isn't this a little unusual for an IP change?

 

[ChrisAC]

In this case it would be normal to reduce the time-to-live periods for these DNS records a good 24 hours before the DNS change to make sure that other DNS servers refreshed their records say every 15 minutes or so! Once propagation has taken place and all records are updated then the TTL's can be put back to normal.

Chris.
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Chris Andrew, CCNA
chrisac@gmx.co.uk
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[GunJack]

I originally thought this was the best way to do an IP cutover but I later read in the Bind book from O'Reilly that the TTL's are for the slave servers so this would only affect the Verio dns system. All other DNS servers at ISPs would still potentially cache the old value for several days.

If you've seen where this isn't true, let me know as I thought this trick would allow for quick propogation but according to the docs, it won't work.

GJ

 

[ChrisAC]

The TTL's are for any servers that cache the DNS record! So, if the normal TTL value is say six hours (just an example) then any DNS server that has cached that record will hold it for six hours before flushing it and doing a lookup again when that domain is requested! If the new DNS record has a TTL value of say fifteen minutes and an ISP's DNS caches the domain again, it will only hold that record for the value of the new TTL before looking it up again.

We do this on a regular basis when changing customers IP ranges. We normally change the TTL values a couple of days before the planned changeover to ensure that all cached records are upto date. We then do the IP change, alter the DNS record and re-set the TTL's to whatever they previously were. Doing it this way you can usually see pretty fast propagation. We always do lookups on different DNS servers to check that the new details have been picked up.

Your ISP should make proper provision for your IP migration and they should be responsible enough to realise that they need to ensure fast propagation. The only way to do this is to plan well in advance and change TTL's well before the migration date, ensuring that all DNS servers have cached the new records and will therefore flush them in a short time and pick up the new IP's.

I hope that it goes well!

Chris.
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Chris Andrew, CCNA
chrisac@gmx.co.uk
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