This cabal package exposes module Network.FQDN, which provides one function, fullyQualifiedDomainName :: IO (Maybe String).

The FQDN is retrieved using getaddrinfo/getnameinfo POSIX functions. It should be reasonably portable, but has only been tested on Linux.

WARNING: Do not statically link your application when using glibc (most Linux distributions). See this bug: http://sourceware.org/bugzilla/show_bug.cgi?id=10652.