musl - Re: DNS resolution happenning only after timeout

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Date: Wed, 04 Oct 2017 19:28:35 +0000
From: Srinivasa Raghavan <raghav135@...il.com>
To: musl@...ts.openwall.com
Subject: Re: DNS resolution happenning only after timeout

Hi Markus,

Thanks for the reply.

The problem is not only in nslookup, it is there in ping, tracert, curl,
node.js, wget etc. :(

I will debug and find the exact c api that is used for each of the
scenarios.

I am just wondering if there is any workaround ?

Lot of folks are facing this issue (slow dns name resolution in alpine
linux, with some dns servers) , and this may be the root cause?

Kind Regards,
Rsr


On Wed, 4 Oct 2017 at 10:16 PM, Markus Wichmann <nullplan@....net> wrote:

> On Wed, Oct 04, 2017 at 07:18:10PM +0530, Srinivasa Raghavan wrote:
> > Hi Rich,
> >
> > Thanks for the reply.
> >
> > Some updates:
> > 1. Our DNS server is "Infoblox appliance".
> > 2. When we had a delay, we found that there was a "AAAA" query along with
> > "A" query.
> >
> > I did further debugging with "tcpdump" and able to narrow down on the
> > difference in behavior between "debian" and "alpine" images.
> >
> > In debian:
> > If ipv6 is disabled (net.ipv6.conf.default.disable_ipv6 = 1)
> > Then the "nslookup" (or name resolution) does *not* do a "AAAA" query
> >
>
> That's probably because glibc's DNS resolver only generates AAAA queries
> if it can create an IPv6 socket.
>
> > In alpine:
> > If ipv6 is disabled (net.ipv6.conf.default.disable_ipv6 = 1)
> > Then the "nslookup" (or name resolution) does an "AAAA" query along with
> > "A" query
> >
> > Is this intentional?
> >
> > Also, I was wondering if there was any way to disable AAAA query in name
> > resolution?
> >
>
> There does not appear to be a way without changing code. In musl, the
> function name_from_dns() will always generate both the AAAA and the A
> query unless "family" is explicitly set to one of the address families.
> No input from resolv.conf or similar is used for this. And "family"
> comes directly from the caller, i.e. nslookup. You'd have to change the
> nslookup code to only ask for IPv4 addresses.
>
> > Kind Regards,
> > Srinivasa Raghavan.
>
> Ciao,
> Markus
>

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