Rd is a remote desktop client for Plan 9 operating system implementing RDP, the Microsoft's Remote Desktop Protocol. Initially started as a feature-cut fork of the X11 rdesktop client, it eventually grew into something different. It does not support pre-RDP5 protocol versions and requires the server to support an equivalent of STARTTLS (Windows XP SP2/2003 and up). The X.509 certs presented by servers are checked with okThumbprint against the list kept in /sys/lib/tls/rdp file.