[Dnsmasq-discuss] non-recursive DNS ansewers patch

[Simon Kelley]

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The risk with this is if you forward a query to a non-recursive
nameserver that it _isn't_ authoritative for. In that case you'll get
a referal - ie a reply packet with an empty answer section, but one or
more DNS servers in the authority section.

Returning that answer to a stub resolver will cause the stub resolver
to conclude that the name has no values. Caching it in dnsmasq will do
that same thing. This is why dnsmasq logs an warning if any of its
upstream nameservers are not recursive.

If you insist on forwarding to an authoritative nameserver, it only
makes sense to do that with queries for domains it is authoritative
for. The patch doesn't make that any more unsafe than it already is,
you'll still get the wrong answer if any replies are CNAMES to domains
that the server doesn't cover.

Dnsmasq really wants recursive upstream servers.

Cheers,

Simon.




On 15/02/15 18:33, Nikolay P wrote:
> This question is for maintainers of Dnsmasq
> 
> I want to consult you if the attached patch is safe.
> 
> I am trying to develop a workaround for this:
> 
> /* Don't put stuff from a truncated packet into the cache. Don't
> cache replies from non-recursive nameservers, since we may get a 
> reply containing a CNAME but not its target, even though the
> target does exist. */
> 
> As currently implemented in src/rfc1035.c any answer from
> non-recursive DNS servers will not be cached.
> 
> if (!(header->hb3 & HB3_TC) && !(header->hb4 & HB4_CD) && 
> (header->hb4 & HB4_RA) && !no_cache_dnssec) cache_end_insert();
> 
> The attached patch enables caching of DNS answers from
> non-recursive servers IF the answer DOES NOT contain a CNAME
> record.
> 
> Could you check the patch and let me know if I got it right and it
> is safe to implement?
> 
> The patched code compiled successfully and worked OK so far.
> 
> Best, Nikolay
> 
> 
> 
> _______________________________________________ Dnsmasq-discuss
> mailing list Dnsmasq-discuss at lists.thekelleys.org.uk 
> http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> 
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