[Dnsmasq-discuss] TTL override for clients?

[Simon Kelley]

Fredrik Ringertz wrote:
> Hi All,
> 
> I am currently using dnsmasq with a few Perl scripts which
> automatically reloads dnsmasq with different options on the fly. I
> use this to allow the user switch different firewall profiles which
> add/removes different "--address" options to redirect certain
> websites (I redirect windows update etc to another IP address if the
> user wants to block it temporarily etc).
> 
> The only problem with this method is that the clients OS caches the
> response for the period of time specified in the TTL. I therefore
> want to get around this issue by overriding the TTL given to them.
> However I don't want to change the actual TTL added to the dnsmasq
> cache (I don't want to add additional load to the upstream DNS
> servers, I just want my local clients to query dnsmasq more often).
> So I am interested in somehow changing the TTL given to the client,
> rather then the TTL added to the cache list.
> 
> Where is a good place for me to start? Could someone familiar with
> the source code point me in the right direction where I could hard
> code this option (and then I will later add a configurable
> parameter)? 


The function crec_ttl() in src/rfc1035.c should be all you need: just
implement your  ceiling  as part of the final return statement.


> Also what are people's general opinions on this? Does
> this break any RFC standards? I can imagine this might be a useful
> feature for a lot of other people so if I manage to do this in a tidy
> way then I could release this as a patch for other people to use?

As long as you don't allow TTL _longer_ than would otherwise be the
case, I can't see any objection on standards grounds. Don't forget that
making the TTL zero is valid and means "never cache this data locally"
so that's useful.

I don't know how generally useful this would be, but it's not a big
addition, so the bar is pretty low.


Cheers,

Simon.