Upgrade to 9.6-ESV-R4-P1, which address the following issues:

1. Very large RRSIG RRsets included in a negative cache can trigger

an assertion failure that will crash named (BIND 9 DNS) due to an
off-by-one error in a buffer size check.

This bug affects all resolving name servers, whether DNSSEC validation
is enabled or not, on all BIND versions prior to today. There is a
possibility of malicious exploitation of this bug by remote users.

2. Named could fail to validate zones listed in a DLV that validated

insecure without using DLV and had DS records in the parent zone.

Add a patch provided by ru@ and confirmed by ISC to fix a crash at
shutdown time when a SIG(0) key is being used.