Configuring nscd cache for mDNS

Configuring nscd cache for mDNS

[Trevor Vallender]

Hello


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'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.

Thanks,

Trevor Vallender.

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’.