Why NX 3.5.0 needs OpenSSH to be installed on the system
The ssh-keygen tool is needed during the installation of NX Server
During the installation of NX Server, the ssh-keygen tool, provided by the OpenSSH suite, is needed to generate a pair of private/public ssh keys to get NX Node to authenticate to the server.
This implies that the OpenSSH subsystem is already available on the server host machine. More information about OpenSSH may be found at:
Why OpenSSH is not listed among the NX Server package dependencies
Even if the OpenSSH set of tools is needed to complete the NX Server installation, NoMachine doesn't retain listing OpenSSH among the NX Server package dependencies necessary for numerous reasons.
Firstly, NX doesn't link to the system OpenSSH. Secondly, adding OpenSSH to the package dependencies can generate possible conflicts with different versions of OpenSSH already present on the system. Furthermore, OpenSSH could be present on the system as a non-default installation. Finally, adding a package dependency on OpenSSH means forcing the system administrator to uninstall NX Server everytime he/she wants to uninstall SSH.
A running SSHD is needed on each NX Node host
A further requirement is that the SSH server daemon (SSHD) needs to be up and running on each of the NX Node machines (and consequently on each of the NX Server machines, since NX Node is a prerequisite of the server), due to the fact that NX relies on the mechanism provided by the SSH subsystem for handling users authentication.
NX 3.5.0 support for multiple versions of the SSH daemon
NX 3.5.0 doesn't support password-less SSH login. Additionally NX server supports only OpenSSH, even though NoMachine is evaluating the possibility to add support for multiple versions of SSH daemon, included commercial ones such as F-Secure in future versions of the product.
System login with password or private SSH key is supported in NoMachine 4.