Weird DNS behaviour resolution issues when more labels are present in a zone
marka at isc.org
Mon Dec 14 01:20:52 UTC 2020
> On 12 Dec 2020, at 20:36, Prasanna Mathivanan (pmathiva) via bind-users <bind-users at lists.isc.org> wrote:
> Hi everyone,
> I have an issue in resolving a domain, from logs I see its timing out.
> And from dig output we are getting SERV fail response.
> The bind version we are using 9.14.10, same domain resolves in bind version 9.11 and lower.
> Example domain:- a.b.c.eample.com
> When we took tcpdump and saw what’s happening when we do a dig, we see its querying the wrong domain“_.b.c.example.com” , and it’s not able to query the NS for this domain and we get timeout in logs.
> Adding to that we get SERVFAIL response when doing dig.
If you are getting a timeout then the server for _.b.c.example.com is broken or it is unreachable.
“_” is a legal label in the DNS. If the server for b.c.example.com responds to b.c.example.com but
does not respond to _.b.c.example.com then the server is broken or there is a broken firewall in
front of it.
> We don’t see this behaviour for bind version 9.11 or lower and works with +trace as well.
> If anyone can explain why this behaviour is happening, it will be very helpful in understanding the issue.
> After looking into the problem, it appears that bind 9.14 ships with Query Name Minimisation feature as defined by RFC 7816 enabled by default.
> few have experienced this behaviour and solution was to disable QNAME minimization.
> How does QNAME Minimisation alter this behaviour? To quote from RFC 7816:
> Instead of sending the full QNAME and the original QTYPE upstream, a resolver that implements QNAME minimisation and does not already have the answer in its cache sends a request to the name server authoritative for the closest known ancestor of the original QNAME. The request is done with:
> • the QTYPE NS
> • the QNAME that is the original QNAME, stripped to just one label more than the zone for which the server is authoritative
> A resolver using QNAME Minimisation implicitly assumes that each label in the query name corresponds to a zone cut. The resolver queries a parent zone server, using an abbreviated query name that is truncated after the name of the immediate child label and uses a query type of NS.
> Am adding the following links to justify this behaviour, but just wanted a suggestion if we are good with doing this.
> Disabling QMIN does fix the issue, but I would like to understand why delegation breaks if there are more labels.
> And why the query goes to underscore domain even though it doesn’t exist.
Because people deploy non-RFC compliant nameservers. If you find one complain to the operator of it.
_.<label-sequence> is chosen so that the resulting NXDOMAINs are not being cached for <label-sequence>.
The initial QMIN implementation used NS queries but that caused problems when servers returned NXDOMAIN
to NS queries but there were really records there. Querying for A / AAAA at <label-sequence> also distorts
QTYPE statistics for <label-sequence>. Prepending “_” prevents that by using a QNAME that almost never
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Mark Andrews, ISC
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