[Dnsmasq-discuss] Feature Proposal: DNS Recursor

[Loune Lam]

Hi All,

I've been using dnsmasq for a long time and it has been working 
brilliantly. One feature I find that it's missing though, is a DNS 
recursor. Having the ability to recurse would make it the most 
lightweight self-containing DNS server/Cache. I've done some preliminary 
investigations on how this could be done, which I've outlined below. If 
Simon and others here are interested I'd be happy to create a patch for 
inclusion in the mainline.


*** Possible Recursor Implementation for DNSMasq

-frec will have 4 new fields - nameservers, recurse_depth, lookup 
callback & closure

-There will be 3 new functions recurse() resolve() recursor_resolved() 
inside forward.c (or recurse.c?)

-In process_reply() if lookup results in 'refused to do a recursive 
query', then recurse() is called with HEADER and frec.

-recurse() take the NSes obtained from HEADER and if available the 
additional RR (glue) IPs are populated into frec.nameservers and 
forward_query() run with same frec and rebuilt question HEADER (ie. 
retry with next namewserver). If there are no glue records, then 
additional resolution of nameserver IP is neccessary. resolve() is 
called providing recursor_resolved() as a callback and frec as closure 
(new branch of resolution).

-everytime recurse() is called frec.recurse_depth is increased. if 
recurse_depth > 10, then automatic failure

-resolve() will take recurse_depth, callback, closure and question 
HEADER, lookup the cache (answer_request()) and if found call the 
callback before returning. If not, forward_query() will be called with 
generated header and new frec (setting frec.recurse_depth, frec.callback 
and frec.closure).

-Any place which triggers send_from will need to be amended to: if 
callback is not NULL to run the callback with HEADER & closure, 
otherwise send_from as usual

-callback recursor_resolved() will be called when resolution finishes. 
It will accept HEADER and the closure frec as parameters. It will 
populate the NS answer IPs from HEADER into closure frec and trigger 
forward_query() run with closure frec and rebuilt question HEADER.

-forward_query() will, if available, look up using nameservers stored in 
frec


*** Questions

-How to best store nameservers in frec?

-How are stale frecs handled? ie. If nameservers don't respond?

-Should Additional RRs be cached as well? (poisoning implications)

-Seems like that DNSMasq doesn't resolve CNAMEs - mechanism to 
recursively resolve CNAMEs will need to be built. The answer HEADER will 
need to be modified to include the CNAME and the A record in the answer.

Looking forward to comments,

cheers,

-l