rdesktop is superseeded by the freerdp project. rdesktop doesnt support the newer NLA/TLS connections provided by default on Windows 2008/2012 servers. freerdp continues where rdesktop left, implementing most new features, including NLA support and a host of others like video passthrough.
Freerdp can connect to any Windows version to date, it has support for tls/nla, and whatnot. I administer a Windows 2012 server and 3 2008 servers and it works with both versions using the default NLA security (without enabling the old version tickbox in the remote connections control panel).
The easiest way to use it is through Remmina - the best Linux remote admin client RDP/VNC/NX client that uses the freerdp libs for RDP support. It has support for saving connections, tray icon, full/scaled screen support etc.
PS. To connect to a Windows 2012 server only for administering (ie does not have terminal services installed and lets only administrators to connect remotely) you might need to use the admin switch - Remmina has it in the "advanced" tab (Attach to console...).
rdesktop/freerdp can use this by passing -0 (rdesktop/freerdp) or /admin (new freerdp) to the command line.