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Description of problem:
Support for stale-answer-enable and others is missing. Allows responding from cache when authoritative servers are under attack or not responding.

Upstream change:

4700. [func] Serving of stale answers is now supported. This

                        allows named to provide stale cached answers when
                        the authoritative server is under attack.
                        See max-stale-ttl, stale-answer-enable,
                        stale-answer-ttl. [RT #44790]


First commit:
https://gitlab.isc.org/isc-projects/bind9/commit/df50751585b64f72d93ad665abf0f485c8941a3b

Check bug #1653111 comment #7 for more details.

Version-Release number of selected component (if applicable):
bind-9.11.4-14.P2.el8

Additional info:
Upstream has no support for stale cache in 9.11 ESV version, the feature was introduced in 9.12 version.

Note: Serve-stale feature introduced also some defects fixed in 9.12.2-P1 release. It has to be included right away.

https://ftp.isc.org/isc/bind9/9.12.2-P1/RELEASE-NOTES-bind-9.12.2-P1.html

Previous refractoring of resolver [1] makes backport hard. Not simple to check it is possible or not, have to decipher which changes would land where in the old code.

1. https://gitlab.isc.org/isc-projects/bind9/commit/96912e44b0b180de56f47d438f91c9e70925bd16

Some working prototype was successful. More changes is required, but it seems possible into 9.11.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2020:1845