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I have a question about DNSSEC and how many times a DNS server must sign or in other words, how many signatures are there in a day. I know that DNSSEC builds on a chain of trust starting at the root level.

Now lets assume that I run a DNS server that covers a zone with 1 million domains and the TTL of each record is 1 hour. Furthermore, the zone changes once a week. How many new signatures are there in one day?

I can think of the two possibilities below:

  1. there are new signatures only when the zone changes, i.e. around 1M / 7 = 143k signatures per day

  2. Every time the TTL expires, the record must be re-signed. This would lead to 1M * 24 = 24M signatures per day

Which one of them is right (if any)? And if none of them, how and way is it?

Thanks a lot!

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The DNS server is free to do it as often as it wants/needs: signatures liftetime is a policy issue, covered by a DPS (DNSSEC Policy Statement). You can find examples and good practices, but if a server wants to create signatures even on the fly, it can (see https://doc.powerdns.com/authoritative/dnssec/modes-of-operation.html "In PowerDNS live signing mode, signatures, as served through RRSIG records, are calculated on the fly, and heavily cached." or https://www.cloudflare.com/dns/dnssec/dnssec-complexities-and-considerations/ "Cloudflare’s DNSSEC implementation leverages ECDSA’s efficient signature generation to sign DNSSEC records on-the-fly.")

Signatures in RRSIG records have two dates: a beginning of validity and an end.

This has nothing to do with the TTL value, nor the zone changes (signatures will change even if the zone does not change).

See for example in A Framework for DNSSEC Policies and DNSSEC Practice Statements section 4.6.5. "Signature Lifetime and Re-Signing Frequency" that mandates a good policy document to have:

This subcomponent describes the life cycle of the Resource Record Signature (RRSIG) record.

Now let us look at some published DPS:

17.6 Signature Life-time and Resigning Frequency: The KSK signatures of the TLD zone DNSKEY RRset will have a validity period of 40 days. The ZSK signatures of the TLD zone authoritative data will have a validity period of 10 days

(default TTL: 86400s aka 1 day)

We re-sign our zones daily with a signature lifetime of 30 days.

(default TTL: 3600s aka 1 hour)

RR sets are signed with ZSKs with a validity period of fourteen days and are resigned every two days. Zone file generation and the signing of new records takes place every hour.

(default TTL: 3600s)

The signing practice of the COM Zone is divided into quarterly continuous time cycles of approximately 90 days. Time cycles begin at the following dates each year: January 15th April 15th July 15th October 15th

The time cycle will never be less than 90 days, except in emergency situations (in which a key has been compromised) or if Verisign decides to begin using different key lengths.

(default TTL: 172800s aka 2 days)

etc.

Also, take into account this part from RFC 4035 (section 5.3.3):

If the resolver accepts the RRset as authentic, the validator MUST
   set the TTL of the RRSIG RR and each RR in the authenticated RRset to
   a value no greater than the minimum of:

   o  the RRset's TTL as received in the response;

   o  the RRSIG RR's TTL as received in the response;

   o  the value in the RRSIG RR's Original TTL field; and

   o  the difference of the RRSIG RR's Signature Expiration time and the
      current time.

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