Rsync remote backup fails after target Linux server upgraded to OpenSSH 8.8

Backup of a Linux server to ReadyNAS (6.10.3) using Rsync remote failed after updating the Linux server to OpenSSH 8.8. This is because RSA signatures using the SHA-1 hash algorithm are now disabled by default for security reasons - see https://www.openssh.com/txt/release-8.8 

Rsync remote uses a generated RSA public key to enable Rsync over SSL and hence OpenSSH. The fix is to generate DSA and / or ECDSA public private keys using SSH-KeyGen, save them down to /root/.ssh on the ReadyNAS and add the public keys to the to the authorized_users file for SSH on the Linux server.

"Test Connection" on the backup setup dialogue now succeeds and the backup executes correctly.

This fix requires root SSL access to the ReadyNAS so all the usual warnings apply.

Andy