Rd is a remote desktop client for Plan 9 operating system implementing RDP, the Microsoft's Remote Desktop Protocol.
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.