Many test scenarios need to configure resolvers on test machines. Most
notable patterns are:
* using IPA master as DNS resolver on clients and replicas
* intentionally breaking name resolution
Now it is done by directly editing /etc/resolv.conf file. While being
simple this approach has following issues:
* NetworkManager restores this file periodically and on specific events
* This is not how users are expected to manage resolvers on modern
systems with NetworkManager and systemd-resolved.
This patch introduces three classes for main types of resolvers management:
* plain file
* NetworkManager
* systemd-resolved
For each resolver manager the native way of configuring of nameserves is
used: direct editing for /etc/resolv.conf or drop-in config files for
NM and resolved.
The type of resolver is automatically detected for each host and an
appropriate instance is added to Host object.
The Resolver class (and it's subclasses) provide convenience functions
for changing nameservers and restoring the original config.
During all operations (backup, modify, restore) it checks that resolver
configuration has not been altered unexpectedly and raises exception if it
was. This helps to detect unexpected changes in resolvers.
Related to https://pagure.io/freeipa/issue/8703
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
