Re: QNAME minimisation question

On 6/3/25 11:29, Nick Tait wrote:
> On 02/06/2025 23:30, Petr Špaček wrote:
>> In short, with an empty cache, BIND will exceed pre-configured limit 
>> on number of queries it can do. This is protection from various 
>> attacks which misuse DNS to attack itself.
> 
> Thanks for the explanation!
> 
> This particular recursive query doesn't seem especially out-of-the- 
> ordinary to me, in terms of the number of name servers returned for each 
> authoritative zone, so it was a little surprising to me that it would 
> hit the default limit setting. However when I took a closer look at the 
> combined impact that QNAME minimisation and DNSSEC and IPv4+IPv6 has on 
> the number of queries it is actually not so surprising after all...
> 
>> Here's how I found out.
>>
>> To test cold-cache scenario, the easiest is to run:
>>
>> delv +ns +qmin -d99 195.5.90.45.in-addr.arpa. PTR &> log
> 
> Using this as a starting point, I created a mawk expression to count the 
> queries being performed by delv to answer this recursive query. In this 
> test delv made 47 queries to get an answer.
> 
> But I also noticed that delv only makes A queries (not AAAA), and even 
> if I specify "-6" on the command-line it makes no difference? So if I 
> add one extra AAAA query for every A query, to match what I can see in 
> my original packet capture (from BIND), the total number of queries that 
> BIND would make to get an answer would be 70!
> 
>     $ delv -x 45.90.5.195 +ns +qmin +maxqueries=100 | mawk -- 'BEGIN { count = 0; acount = 0; } /^;; sending packet to / { while ($0 != "") { getline; if ($0 == ";; QUESTION SECTION:") { getline; sub(/^;/, ""); print; count++; if (/\tIN\tA$/) acount++; break; } } } END { print "NUMBER OF QUERIES = " count; print "DOUBLE-COUNTING A QUERIES TO COMPENSATE FOR MISSING AAAA = " count + acount; }'
>     .				IN	NS
>     arpa.				IN	NS
>     .				IN	DNSKEY
>     arpa.				IN	DNSKEY
>     arpa.				IN	DS
>     a.ns.arpa.			IN	A
>     b.ns.arpa.			IN	A
>     c.ns.arpa.			IN	A
>     d.ns.arpa.			IN	A
>     e.ns.arpa.			IN	A
>     .				IN	NS
>     in-addr.arpa.			IN	NS
>     f.ns.arpa.			IN	A
>     g.ns.arpa.			IN	A
>     h.ns.arpa.			IN	A
>     i.ns.arpa.			IN	A
>     45.in-addr.arpa.		IN	NS
>     net.				IN	NS
>     arin.net.			IN	NS
>     r.arin.net.			IN	A
>     u.arin.net.			IN	A
>     x.arin.net.			IN	A
>     y.arin.net.			IN	A
>     z.arin.net.			IN	A
>     90.45.in-addr.arpa.		IN	NS
>     ripe.net.			IN	NS
>     45.in-addr.arpa.		IN	DNSKEY
>     authdns.ripe.net.		IN	NS
>     arin.authdns.ripe.net.		IN	A
>     45.in-addr.arpa.		IN	DNSKEY
>     in-addr.arpa.			IN	DNSKEY
>     in-addr.arpa.			IN	DNSKEY
>     5.90.45.in-addr.arpa.		IN	NS
>     netcup.net.			IN	NS
>     systems.			IN	NS
>     xaas.systems.			IN	NS
>     acns01.xaas.systems.		IN	A
>     acns02.xaas.systems.		IN	A
>     acns03.xaas.systems.		IN	A
>     acns04.xaas.systems.		IN	A
>     acns05.xaas.systems.		IN	A
>     root-dns.netcup.net.		IN	A
>     third-dns.netcup.net.		IN	A
>     second-dns.netcup.net.		IN	A
>     195.5.90.45.in-addr.arpa.	IN	PTR
>     90.45.in-addr.arpa.		IN	DS
>     5.90.45.in-addr.arpa.		IN	DS
>     NUMBER OF QUERIES = 47
>     DOUBLE-COUNTING A QUERIES TO COMPENSATE FOR MISSING AAAA = 70
> 
> On that basis I've added this option to my BIND configuration:
> 
>     max-recursion-queries 100;
> 
> And when I repeated my original test, I get the expected answer. FYI My 
> packet capture shows that the total number of queries transmitted on the 
> wire was 46, which sounds about right considering that the root zone 
> queries are answered from the mirror zone (and therefore don't appear in 
> the packet capture).
> 
> Thanks again for helping to solve my problem.

Happy to help!

I've created
https://gitlab.isc.org/isc-projects/bind9/-/issues/5351
so we can improve logging. Your input on what sort of information is 
useful would be much appreciated.

-- 
Petr Špaček
-- 
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