[nsd-users] nsd process restarting

[Anand Buddhdev]

On 15/09/2022 15:28, Franky Van Liedekerke wrote:

Hi Franky,

> So the process restarts to serve freshly updated zones? Is there any

Yes, kind of. The master creates new child processes, which have the old 
zone data in them. These child processes apply the updates to their 
in-memory copy of the zones, and then take over serving zones from the 
previous processes.

> reason to that logic? Because it is indeed a server that serves more
> than 7000 zones with the real masters indeed updating their zones
> regularly, and restarting a dns process because a zone was updated is a
> very costly step ...

Well, there are different ways to apply updates to existing zones, and 
the NSD developers chose this one (fork new child processes to apply the 
update). Other name servers like BIND and Knot DNS do things differently.

I agree with you that it's not the most efficient way to do things. For 
starters, NSD temporarily causes memory usage to double when applying 
zone updates. So you either have to provision a server with double the 
amount of RAM, or play tricks with swap and/or tune the kernel's memory 
variables (eg. overcommit) to ensure that fork() doesn't fail.

However, this is how NSD works, so if you're using it, then it's good to 
understand it, and tune your server accordingly.

If you feel that the reloads are too frequent, you can slow them down by 
adjusting "xfrd-reload-timeout" from the default of 1s to 10s or even 
higher. This causes the server processes to be restarted less 
frequently. The consequence is that more updates are batched together, 
and zone updates will not be visible immediately.

Regards,
Anand