Re: Configuring nscd cache for mDNS

[Ludovic Courtès]

Hi Trevor,

Trevor Vallender <trevor@tsvallender.co.uk> skribis:

> I have a Docker container which advertises its name using mDNS,
> e.g. hostname.local. However, when I restart the container,
> Docker assigns the container a new IP (not sure why, doesn't
> happen on other distros but that's probably not the point
> here). nscd has the old IP cached and doesn't refresh even though
> the container is advertising the new IP. I'm not sure why this
> behaviour is occurring, or if it is the expected behaviour?

I think so.

> I'm don't know the correct fix here but am open to suggestions. I
> presume one of the following:
>
> - Configure nscd not to cache mDNS addresses
> - Shorten the length of time these addresses are cached for
>
> If someone could suggest why this is occurring, or provide an
> example of how to do one of the above, that would be brilliant.

You can clear nscd’s host name cache with:

  sudo herd invalidate nscd hosts

Better, you can configure nscd to disable caching along these lines:

  (operating-system
    ;; …
    (services
      (modify-services %base-services  ;or ‘%desktop-services’, etc.
        (nscd-service-type
         config => (nscd-configuration
                     (inherit config)
                     (caches '()))))))   ;zero caches

More details here:

  
https://guix.gnu.org/manual/devel/en/html_node/Base-Services.html#index-nscd_002dconfiguration

HTH!

Ludo’.