# Dynamic Trust Anchor Management

BIND is able to maintain DNSSEC trust anchors using RFC 5011 key management. This feature allows `named` to keep track of changes to critical DNSSEC keys without any need for the operator to make changes to configuration files.

## Validating Resolver

To configure a validating resolver to use RFC 5011 to maintain a trust anchor, configure the trust anchor using a `managed-keys` statement. Information about this can be found in .

## Authoritative Server

To set up an authoritative zone for RFC 5011 trust anchor maintenance, generate two (or more) key signing keys (KSKs) for the zone. Sign the zone with one of them; this is the "active" KSK. All KSKs which do not sign the zone are "stand-by" keys.

Any validating resolver which is configured to use the active KSK as an RFC 5011-managed trust anchor will take note of the stand-by KSKs in the zone's DNSKEY RRset, and store them for future reference. The resolver will recheck the zone periodically, and after 30 days, if the new key is still there, then the key will be accepted by the resolver as a valid trust anchor for the zone. Any time after this 30-day acceptance timer has completed, the active KSK can be revoked, and the zone can be "rolled over" to the newly accepted key.

The easiest way to place a stand-by key in a zone is to use the "smart signing" features of `dnssec-keygen` and `dnssec-signzone`. If a key with a publication date in the past, but an activation date which is unset or in the future, " `dnssec-signzone -S`" will include the DNSKEY record in the zone, but will not sign with it:

    $ dnssec-keygen -K keys -f KSK -P now -A now+2y example.net
    $ dnssec-signzone -S -K keys example.net

To revoke a key, the new command `dnssec-revoke` has been added. This adds the REVOKED bit to the key flags and re-generates the `K*.key` and `K*.private` files.

After revoking the active key, the zone must be signed with both the revoked KSK and the new active KSK. (Smart signing takes care of this automatically.)

Once a key has been revoked and used to sign the DNSKEY RRset in which it appears, that key will never again be accepted as a valid trust anchor by the resolver. However, validation can proceed using the new active key (which had been accepted by the resolver when it was a stand-by key).

See RFC 5011 for more details on key rollover scenarios.

When a key has been revoked, its key ID changes, increasing by 128, and wrapping around at 65535. So, for example, the key "`Kexample.com.+005+10000`" becomes "`Kexample.com.+005+10128`".

If two keys have IDs exactly 128 apart, and one is revoked, then the two key IDs will collide, causing several problems. To prevent this, `dnssec-keygen` will not generate a new key if another key is present which may collide. This checking will only occur if the new keys are written to the same directory which holds all other keys in use for that zone.

Older versions of BIND 9 did not have this precaution. Exercise caution if using key revocation on keys that were generated by previous releases, or if using keys stored in multiple directories or on multiple machines.

It is expected that a future release of BIND 9 will address this problem in a different way, by storing revoked keys with their original unrevoked key IDs.