8. Zone Considerations

There are several differences between signed and unsigned zones. A signed zone will contain additional security-related records (RRSIG, DNSKEY, DS, and NSEC records). RRSIG and NSEC records may be generated by a signing process prior to serving the zone. The RRSIG records that accompany zone data have defined inception and expiration times that establish a validity period for the signatures and the zone data the signatures cover.

8.1. TTL Values vs. RRSIG Validity Period

It is important to note the distinction between a RRset's TTL value and the signature validity period specified by the RRSIG RR covering that RRset. DNSSEC does not change the definition or function of the TTL value, which is intended to maintain database coherency in caches. A caching resolver purges RRsets from its cache no later than the end of the time period specified by the TTL fields of those RRsets, regardless of whether the resolver is security-aware.

The inception and expiration fields in the RRSIG RR ([RFC4034]), on the other hand, specify the time period during which the signature can be used to validate the covered RRset. The signatures associated with signed zone data are only valid for the time period specified by these fields in the RRSIG RRs in question. TTL values cannot extend the validity period of signed RRsets in a resolver's cache, but the resolver may use the time remaining before expiration of the signature validity period of a signed RRset as an upper bound for the TTL of the signed RRset and its associated RRSIG RR in the resolver's cache.

8.2. New Temporal Dependency Issues for Zones

Information in a signed zone has a temporal dependency that did not exist in the original DNS protocol. A signed zone requires regular maintenance to ensure that each RRset in the zone has a current valid RRSIG RR. The signature validity period of an RRSIG RR is an interval during which the signature for one particular signed RRset can be considered valid, and the signatures of different RRsets in a zone may expire at different times. Re-signing one or more RRsets in a zone will change one or more RRSIG RRs, which will in turn require incrementing the zone's SOA serial number to indicate that a zone change has occurred and re-signing the SOA RRset itself. Thus, re-signing any RRset in a zone may also trigger DNS NOTIFY messages and zone transfer operations.