* Add a simple username/password back-end in addition to PAM

* Fix SIGHUP handling on the main server.

* Think how the DTLS part can use better negotiation of algorithms and DTLS
  versions than the current openssl string approach (using PSK ciphersuites
  seem to be like a solution, but then we could not use the session ID to
  forward the UDP connection to the proper worker).

* Try adding salsa20-12 and UMAC as encryption algorithms for DTLS to reduce 
  CPU load.

* Handle users being in multiple groups.

* Keep the TLS key and certificates into the privileged process and use IPC
  for operations. This currently cannot be done, because the main process is
  the one that spawns the workers and there is no sane way to make the key
  (or any temporal value depending on it) disappear from the main process
  memory. Moreover, such a move will make the privileged process a bottleneck
  on a multi-core system. If that functionality is desirable, it can be
  obtained using a software security module (e.g. pkcs11 softoken), an HSM
  or TPM.

* Certificate authentication to the main process. Possibly that is just
  wishful thinking. To verify the TLS client certificate verify signature one
  needs instead of the signature, the contents of all the handshake messages, 
  and knowledge of the negotiated TLS version, in addition to being able to
  select the server hello random. That could be done sanely only if gnutls 
  provided facilities to set the server hello random, and override the client
  signature verification at an early stage before data are hashed (to verify
  that the set random value was present in the handshake).

* When a TUN device is in use and cannot be assigned mark it as such and
  continue.