Sometimes, there is a need, from within a web application written in Java and running on Linux, to authenticate a user as a system user. There are packages that provide Java bindings to the password file, but since everyone uses PAM, that does not cut it.
JAAS is Java’s PAM analogue. But only conceptually: there is no PAM JAAS “module”! I do not need such a module, but if one was there, I could use it to authenticate the users.
I never understood why such a module is not part of the JAAS release, but it isn’t, and so the problem of authenticating as a system user is not solved by JAAS :(
While researching the area, I found two projects:
SysyAuth by Matthew M. Lavy is posited as a simple alternative to JAAS. In addition to PAM, it supports Windows authentication. It uses hard-coded PAM service, which needs to be configured for it to work. It is not clear what license does it have.
jpam by Greg Luck, Barrow Kwan, Jon Eaves supports assorted Linux/Unix flavors. It provides JAAS module, but does not support operations other than authentication.
Over the years I attempted - more than once - to write the code to do the authentication. Native library (libpam) has to be interfaced to, so the usual unpleasantness associated with JNI and native code layer apply :) I prefer to minimize amount of native code and do as much as possible in Java. PAM “conversation” involves callbacks, which I prefer to handle in Java, not in the native wrapper itself. Native code has to work both ways: Java to native and back.
Somewhere around 2008-9 I (re)discovered JNA (I heard about the library that became JNA in 1998). It allows to interface with native libraries without writing any native code! And it can handle callbacks! It seemed clear that this is the way to go!
Kohsuke’s code is readable and well-researched. Some things I would have done differently from Kohsuke. Actually, since the library is very small (thanks to JNA), every time I need something like that, I just adopt it :)
And here we see, that if you wait long enough, someone will do what you were thinking of doing ;)