Negative Caching of DNS Responses for Different RCODES

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Negative Caching of DNS Responses for Different RCODES

Harshith Mulky
Hello experts,

If a DNS server looks up a record and it's missing, it will often "negatively cache" the fact that this record is missing, and not try to look it up again for a while.
From RFC 2308, Negative Caching of DNS Queries, I understood, the TTL for NXDOMAIN RCODE responses is taken from SOA Record at the top of the Zone.

My Questions
1. How is Negative Caching Applied for other RCODES : FORMERR, SERVFAIL, REFUSED and NOTIMPL? What is the minimum TTL Value for these responses?
2. Are the clients free to re-query the same DNS server if no caching is applied for the above RCODES?

Thanks
Harshith

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Re: Negative Caching of DNS Responses for Different RCODES

Tony Finch
Harshith Mulky <[hidden email]> wrote:
>
> 1. How is Negative Caching Applied for other RCODES : FORMERR, SERVFAIL,
> REFUSED and NOTIMPL? What is the minimum TTL Value for these responses?

Good question: this isn't well specified. BIND has servfail-ttl (1s by
default) and lame-ttl (600s by default). The lame-ttl can take effect in
as a result of REFUSED responses amongst other things. NOTIMPL should not
normally occur. FORMERR can trigger EDNS downgrade.

> 2. Are the clients free to re-query the same DNS server if no caching is
> applied for the above RCODES?

In general the same question will yield the same answer so a good
implementation should avoid and preferably suppress repeat queries.

Tony.
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f.anthony.n.finch  <[hidden email]>  http://dotat.at/
Biscay, Fitzroy: Variable 3 or 4, becoming northerly or northwesterly 5 at
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Good.
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