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

[Josef Bacik]

On 02/27/2016 12:39 PM, Andrei Borzenkov wrote:
> 26.02.2016 16:52, Josef Bacik пишет:
> > On 02/26/2016 05:22 AM, Andrei Borzenkov wrote:
> > > On Wed, Feb 24, 2016 at 10:11 PM, Josef Bacik <address@hidden> 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.
>
> In this case I do not follow why you want to collect multiple answers in
> the first place. The only reason for me was to make sure we have both A
> and AAAA replies; otherwise every reply packet is supposed to contain
> exactly the same content. Following this logic the right thing to do is
> to just drop all subsequent duplicated replies.

I don't want to collect multiple answers.  We bail as soon as we get an 
answer we care about.  If a user sets --prefer-ipv6 then they must have 
an environment that also gives them an ipv4 address so they can handle 
an A record response.  I want to make sure that we only get both an A 
and AAAA record when we have both kinds of ipvX interfaces, and if not 
we bail as soon as we get one we want.

> >   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.
>
> User can set preference using net_add_dns --prefer-ipvX | --only-ipvX.
> We should respect it.

And we still will, prefer means we try but we don't make guarantees and 
the --only will work as it has always worked.  Thanks,

Josef