The *ssh-host-cert-sign@.service* unit does what it says on the tin:
requests a signed host certificate from an SSHCA server. It is a
template unit, whose instances correspond to SSH key types (RSA, ECDSA,
and Ed25519). The *ssh-host-certs.target* unit depends on the three
instances of the template unit, so they can all be activated together.
This target is only activated on the first boot of the system, to
initially request the certificates.
The *ssh-host-certs-renew.timer* unit periodically renews the SSH hosts
certificates. Its corresponding target unit depends on the three
instances of *ssh-host-cert-sign@.service*, so each certificate will be
renewed independently.
Dustin