Re: [PATCH] dns: realloc address buffer after each packet

[Josef Bacik <jbacik@fb.com>]

On 02/26/2016 05:22 AM, Andrei Borzenkov wrote:
> On Wed, Feb 24, 2016 at 10:11 PM, Josef Bacik <jbacik@fb.com> wrote:
>> Sometimes DNS responses come in slower than we poll for them which can lead us
>> to process multiple DNS packets which overflows the addresses array.  So instead
>> realloc the array each time to make sure we are accounting for any answers we
>> currently have in the address array.  We also move the caching of the addresses
>> outside of the recv hook so we can be sure to cache all the responses at once
>> instead of one packet at a time.  Thanks,
>>
>
> This still does not address the problem that we stop waiting for
> further packets as soon as we get any response, so we still depend on
> delivery order to get correct record.
>
> What about following
>
> - send both A and AAAA query concurrently if requested
> - keep track of both requests (i.e. have data.id[2] and data.addresses[2])
> - reset request ID (or otherwise mark it as "received") as soon as we
> got reply. Note that reply may contain no addresses - NXDOMAIN is
> prerfectly valid - so condition should not be "got any record of type
> A or AAAA" as it is now but rather simply "got reply to request with
> id XX". This also allows us to implement negative caching at some
> point :)

So we check the rcode so a NXDOMAIN response will just be discarded, we 
don't have to worry about this case.

> - return both A and AAAA results separately to grub_net_dns_lookup()
> - combine them in grub_net_dns_lookup() depending on preference - i.e.
> put either A or AAAA first in final result.
>
> This seems to cover all issues so far - we do not wait too long, we
> are guaranteed to get both A and AAAA if we request them and we return
> them in proper order for further processing.
>
> Do I miss something?
>

So my patch previous to this one changes it so DNS servers we get from 
dhcp are bound to either ipv4 or ipv6, so the only way we get 
PREFER_IPV* is if an admin sets it.  So now the only time we'll get both 
an A _and_ an AAAA record is if we have both types of interfaces on the 
system, so it won't matter which answer is first as we'll be able to 
send traffic to either.  I just side-stepped the issue by making it so 
that (at least automatically) we aren't asking for both records unless 
we are sure we have an interface to go along with both types of records. 
  Thanks,

Josef