We want to only allow a machine to request a certificate for itself, not for
other machines. I've added a new taksgroup which will allow this.
The requesting IP is resolved and compared to the subject of the CSR to
determine if they are the same host. The same is done with the service
principal. Subject alt names are not queried yet.
This does not yet grant machines actual permission to request certificates
yet, that is still limited to the taskgroup request_certs.
There are some operations, like those for the certificate system, that
don't need to write to the directory server. So instead we have an entry
that we test against to determine whether the operation is allowed or not.
This is done by attempting a write on the entry. If it would succeed then
permission is granted. If not then denied. The write we attempt is actually
invalid so the write itself will fail but the attempt will fail first if
access is not permitted, so we can distinguish between the two without
polluting the entry.
