Re: Better control over what mDNS announces

[is%netbsd.org@localhost]

On Fri, Jul 14, 2017 at 06:38:53PM +0200, Edgar Fuß wrote:

> So I'm thinking about implementing some mdnsd/mDNSResponder options to 
> re-gain some control over what it announces. I haven't looked into how 
> feasable each option is; but I would like some comments, especially from 
> mDNS experts, if possible.
> 
> -- If the only IPv6 address on an interface is link local, regard that 
>    interface as not set up vor IPv6 (and don't announce the LL address).
> 
> -- Don't announce link local addresses at all.

Both of these (but especially the first) violate (one of) the
point(s) of mdns - finding services without explicit network
configuration.

Unless you (ipf,pf,npf)-filter them unoperational - those should
work, as they're (well, should be) only announced on the local link
where the connection will work.

However, if you want that as an _option_, ok with me.

> -- Don't announce any alias address, only the main one.

My personal opinion: having a seperate notion of alias address is
a remnant from the old century when the idea of more than one address
for a machine was new and exotic. They're not distinguished as well
as you might think even on IPv4, and I think not at all on IPv6.
OTOH, IPv4 addresses can change their role from alias to "main" - I'm
not sure if back - implicitly.

I think more useful would be to have options to only announce  or
ignore a specific set of addresses. Will also help for the former
case.

> -- Ignore a given list of interfaces (or only serve a given list)

Sure.

(To a certain degree your perceived problem seems to come from cups
not exercising control over what addresses it uses and which it 
wants announced? That is, less than optimal integration of mdns into
cups?)

	-is