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Synology NAS appliances are proven, reliable storage appliances that are full-featured, powerful, and reasonably affordable. They use Disk Station Manager (DSM), which employs a Linux-based operation system and offers root-level access via a secure shell or telnet. They also support a plethora of first- and third-party packages, and advanced users can seek to compile their own software for the devices using guides available from Synology.
More information can be found on Synology's website.