Replace the custom DNS servers used in the "forward" system test with
new code based on the isctest.asyncserver module.
For ans6, instead of configuring the responses to send at runtime, set
them up when the server is started. Make sure the server supports
toggling response sending at runtime to enable simulating forwarder
timeouts as required by one of the checks.
For ans11, put most of the responses to be provided by that server into
a zone file, only retaining code modifying zone-based answers in the
form of a response handler, to improve code readability. Use explicit
domain names instead of variables as that server only handles a single
domain and fixed strings improve readability in this case. Make sure
the server supports toggling response sending at runtime to enable
simulating forwarder timeouts as required by one of the checks.
Migrate sendcmd() and its uses to the new way of sending control queries
to custom servers used in system tests.
Implement TCP support in the `ans11` Python-based DNS server.
Implement a control command channel in `ans11` to support an optional
silent mode of operation, which, when enabled, will ignore incoming
queries.
In the added check, make the `ans11` the NS server of
"a.root-servers.nil." for `ns3`, so it uses `ans11` (in silent mode)
for the regular (non-forwarded) name resolutions.
This will trigger the "hung fetch" scenario, which was causing `named`
to crash.
- Check that an NS in an authority section returned from a forwarder
which is above the name in a configured "forward first" or "forward
only" zone (i.e., net/NS in a response from a forwarder configured for
local.net) is not cached.
- Test that a DNAME for a parent domain will not be cached when sent
in a response from a forwarder configured to answer for a child.
- Check that glue is rejected if its name falls below that of zone
configured locally.
- Check that an extra out-of-bailiwick data in the answer section is
not cached (this was already working correctly, but was not explicitly
tested before).
