IDL

IDL is available from ​rsinc, for a licensing fee (apparently $480 for a "floating node license fee" for educational institutions). The current version appears to be 6.2.

They do support linux, and are moving to a kernel/glibc support indicator strategy (see ​here). This strategy apparently won't be ready until 6.3. However, that doesn't mean that the earlier systems won't have comparable support.

They also have a ​linux FAQ which seems somewhat out of date.

licensing

CAL has a "floating" 50 user license (meaning that a properly configured IDL installation requests a license while running and then returns the license when finished). The current license server runs on njord.phys.columbia.edu (a sun) and will serve licenses to any version of IDL running on any operating system. Installation of IDL on a client machine means both installing the software and a "license.dat" file that contains the information required by the license server.

license server

The license server used by IDL is FLEXlm. Instructions for installing this are available ​here.