[Dnsmasq-discuss] backend for dynamic data

Simon Kelley simon at thekelleys.org.uk
Mon Jan 14 17:21:33 GMT 2008

Eric S. Johansson wrote:
> let me state up front that I'm not in a position to dive into this
> seriously yet but I have an distributed application were I am
> considering using an rbl like DNS lookup.  Obviously, I could write my
> own UDP client/server but I would prefer to recycle existing tools
> whenever possible.  I have come to count on dnsmasq as one of my "highly
> trusted" DNS tools and I'm interested to see if it's reasonably
> practical to stretch it just a little bit further to use either a
> program or a DBM file to translate requests into responses.

Think hard about concurrency issues. When dnsmasq gets a query, it
either answers it from the cache (so the transaction doesn't block) or
it forwards it upstream (which doesn't block either). Once the query is
forwarded, only minimal information is kept about it; just enough to
send the eventual reply. In particular, no copy of the query, either in
raw packet form, or parsed, is kept.

That makes lots of stuff very simple, a single, statically allocated
packet buffer and working space, and a  simple table of "forwarding

Once you start execing programs or using database lookups, then the
amount of "stuff" you will need to keep around gets much greater, and if
you're to avoid blocking whilst you handle one query all that stuff will
have to be dynamically allocated.

Take home message: there's lots of code you can re-use, but there's also
a lot of stuff not there which you'll need, and the end result will be a
very different animal from what you started with.



> Thanks for supplying a tool I don't have to worry about.
> ---eric
> PS, I'm currently using dnsmasq to handle DNS and DHCP for vmware
> servers.  It's really nice to be able to control IP addresses in one
> spot (on the host machine) and make everything a nicely self-contained
> unit.  when I have the cycles, I'm planning on experimenting with
> dnsmasq to also allocate IP addresses for the virtual network feature of
> VM Ware.  Their DHCP server is really primitive and not at all connected
> to the DNS tool.

Sounds good. There's a RedHat effort called libvirt(?) which might be
similar, I think.

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