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The SMAPIv3 plugin architecture requires that storage plugins are able to work in the absence of xapi. Amongst other benefits, this allows them to be tested in isolation, are able to be shared more widely than just within the XenServer community and will cause less load on xapi’s database.
However, many of the currently existing SMAPIv1 backends require inter-host operations to be performed. This is achieved via the use of the Xen-API call ‘host.call_plugin’, which allows an API user to execute a pre-installed plugin on any pool member. This is important for operations such as coalesce / snapshot where the active data path for a VM somewhere in the pool needs to be refreshed in order to complete the operation. In order to use this, the RPM in which the SM backend lives is used to deliver a plugin script into /etc/xapi.d/plugins, and this executes the required function when the API call is made.
In order to support these use-cases without xapi running, a new mechanism needs to be provided to allow the execution of required functionality on remote hosts. The canonical method for remotely executing scripts is ssh - the secure shell. This design proposal is setting out how xapi might manage the public and private keys to enable passwordless authentication of ssh sessions between all hosts in a pool.
On firstboot (and after being ejected), the host should generate a host key (already done I believe), and an authentication key for the user (root/xapi?).
Three new fields will be added to the host object:
host.ssh_public_host_key : string: This is the host key that identifies the host
during the initial ssh key exchange protocol. This should be added to the
‘known_hosts’ field of any other host wishing to ssh to this host.
host.ssh_public_authentication_key : string: This field is the public
key used for authentication when sshing from the root account on that host -
host A. This can be added to host B’s
authorized_keys file in order to
allow passwordless logins from host A to host B.
host.ssh_ready : bool: A boolean flag indicating that the configuration
files in use by the ssh server/client on the host are up to date.
One new field will be added to the pool record:
pool.revoked_authentication_keys : string list: This field records all authentication keys that have been used by hosts in the past. It is updated when a host is ejected from the pool.
On pool join, the master creates the record for the new host and populates the
two public key fields with values supplied by the joining host. It then sets
ssh_ready field on all other hosts to
On each host in the pool, a thread is watching for updates to the
ssh_ready value for the local host. When this is set to false, the host
then adds the keys from xapi’s database to the appropriate places in the ssh
configuration files and restarts sshd. Once this is done, the host sets the
ssh_ready field to ‘true’
On pool eject, the host’s ssh_public_host_key is lost, but the authetication key is added to a list of revoked keys on the pool object. This allows all other hosts to remove the key from the authorized_keys list when they next sync, which in the usual case is immediately the database is modified due to the event watch thread. If the host is offline though, the authorized_keys file will be updated the next time the host comes online.