mgr/ssh invocation of ceph-daemon

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Right now the way ceph-daemon is used by the ssh orchestrator is designed 
to minimize the dependencies/setup complexity.  The only requirements for 
a host to be added to the cluster are

 - python (2 or 3)
 - systemd
 - either podman or docker installed
 - the ceph cluster's pub key in /root/.ssh/authorized_keys

No other software (including Ceph) needs to be installed.  The mgr/ssh 
module invokes ceph-daemon on the remote host by running /usr/bin/python 
over ssh and piping the cluster's version of ceph-daemon to stdin.

The downside to this approach is that some users might not like the idea 
of ceph having an ssh key with root access.  For large clusters I'm not 
sure how much this really matters--if you pwn ceph you can delete TB to PB 
of data so do you really care if someone has root?--but for hyperconverged 
cases this might be a problem.

One alternative might be to

 - create a ceph user on the node, and put the cluster's key in 
   that user's authorized_keys
 - install a package that includes ceph-daemon (/usr/bin/ceph-damaen)
 - install an /etc/sudoers.d/ceph file that lets the ceph user 
   'sudo ceph-daemon ...'

Cons:
 - This makes the bootstrap process slightly more complicated: (1) install 
   package, (2) create user, (3) install ssh key (vs just #3).
 - The remote version of ceph-daemon can get out of sync with the 
   cluster.. either stale and missing some feature, or even too new and 
   not behaving the way the cluster expects.

Pros:
 - This limits the attack surface area (if someone manages to get the 
   cluster's ssh key) to the functions that ceph-daemon implements, vs 
   full root.

We could mitigate the 'keep ceph-daemon up to date' problem somewhat by 
implementing a 'ceph-daemon update' function that will apt/dnf/yum install 
ceph-daemon on the local host, so that the cluster could self-update the 
remote host.

Or... we could skip the package entirely and install the ceph-daemon 
script in /home/ceph/ceph-daemon, and include an update function that 
updates the script in place.  That is less complicated than knowing how to 
apt/dnf/yum install a package for all the random distros and repo location 
combinations (one of the biggest benefits of containers IMO).  But it 
still requires some sort of process to keep ceph-daemon up to date.  
Maybe if we always pass an md5sum to ceph-daemon whenever we invoke it to 
assert that we are running the version we want, and if there is a 
mismatch, ceph-daemon bails out with a special exit code that triggers and 
update and retry?

Anyway, what are people's thoughts here?  How much more complicated are we 
interested or willing to make this to make people more comfortable with 
the idea that ceph owns an ssh key?

sage
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