DNS Servers

Which DNS servers are people on Zen using?

I had been using the default DNS servers allocated when I connected (212.23.3.100, 212.23.6.100 and 2a02:8010:1::212:23:3:100). The fritz box was using 212.23.6.100 as the preferred server.

Around midday my hosting provider moved my mail and websites to a new server (needed to get me on the latest php). Within half an hour the 212.23.3.100 and the IPv6 server were returning the new IP addresses for <mydomain> and mail.<mydomain>.

Around 5 hours later and 212.23.6.100 still isn't giving the new IP addresses, although once or twice it has (I suspect there must be multiple actual servers behind each DNS servers IP address.

I've used an online DNS propagation checker and the new address has gone all round the world, just not all Zen's servers. I've never known a DNS change to take so long to propagate to an ISP's DNS caching server!

Edit: It gets worse - 212.23.3.100 has just returned the wrong answer!

Edited by jelv (Thu 23-Dec-21 17:25:32)

It depends on the TTL of the record as to whether the Zen resolver has done anything wrong. Remember, the servers around the world probably won't have cached the records as they've never looked them up. The zen one will have as you have been doing lookups for those DNS records with it before the changes.

We'd need to know the TTL of your DNS records to speculate further.

Edited by Pipexer (Thu 23-Dec-21 19:00:41)

OK - just checked it's 12 hours.

Can you explain why each of the three servers I've listed is giving variable responses? In the last hour or so I've seen all three respond with both the old and the new IP!

In reply to a post by jelv:

Can you explain why each of the three servers I've listed is giving variable responses?

I'm only speculating here but it could be a cluster of servers that each have the same IP address (a single public IP address need not be assigned to a single physical server and if they're doing any sort of load-balancing or have redundant servers then this is probably the case) so sometimes you will get the new record if you hit a server that doesn't have it in its cache and other times you will get the older stale record if you hit a server that has queried it in the past and still has it in its cache since the TTL hasn't yet expired.

I use Cloudflare's DNS servers - 1.1.1.1 and 1.0.0.1.

In reply to a post by jelv:

OK - just checked it's 12 hours.

Can you explain why each of the three servers I've listed is giving variable responses? In the last hour or so I've seen all three respond with both the old and the new IP!

Clustered. One end point with multiple servers behind that. Each will store their own TTL.

Your host should have lowered the zones TTL before changing the IP to say 60 seconds, you'd have noticed it for a few minutes at most.

Matt

But wouldn't the TTL need to have been changed 12 hours before changing the IPs?

Yes.

If this was an expected/planned IP change, you'd change the TTL to a lower value at earlier than the TTL was currently set, i.e 13 hours.

This means that any DNS servers that had old records with a TTL of 12 hours, would have all timed out and gotten the new, lower TTL.

Once the IP had changed, they could then put the TTL back.

This isn't an issue with Zen's DNS servers, or anything they have control of. Your host just did the migration poorly, especially if they knew the new IP address before the change was made.

Matt