Automatic STARTTLS upgrades (was: gnutls status)

[Lars Magne Ingebrigtsen <larsi@gnus.org>]

Lars Magne Ingebrigtsen <larsi@gnus.org> writes:

> What the hey.  I've now altered NNTP to use proto-stream, and it'll
> switch on STARTTLS now if the server supports it.

But I'm wondering about the defaults a bit.  This could be brittle and
slow if things don't actually work.

So let's take the worst-case scenarios:

* First we open a normal network connection, and we say "CAPABILITY" in
  one way or another.  This should be safe, but it's one extra round
  trip.

* If the server says that it supports STARTTLS, we enable either:

** If Emacs has gnutls support, we just enable STARTTLS.  One extra
   round trip.

** If Emacs doesn't, we sever the connection, and start it again with
   starttls.el, which defaults to gnutls-cli.  This may fail because of
   a faulty gnutls-cli installation, in which case we error out.  But it
   it's successful, then it's a close, an open, and an STARTTLS
   enablement.  So that's at least a ... big handful of extra round
   trips. 

* We then do the TLS negotiation.  This is at least one round trip, but
  it's probably more.  It may fail for any number of reasons.  For
  instance, inn may be compiled with TLS support, but not have the
  certificates.  Or they may be grossly invalid.  In that case, we error
  out.

* On success, we do CAPABILITY again, since they have a tendency to
  change after STARTTLS in some protocols.

* We're done!

So, as you can see, this introduces many new, interesting failure modes,
which have to be mitigated somehow.

First of all, I've now switched off opportunistic STARTTLS upgrades if
we don't have an Emacs with built-in gnutls support.

Secondly, I think if the STARTTLS stuff fails for any reason, the
connection should go back to being a plain network connection.  Is that
possible with the built-in gnutls stuff, Ted?  Otherwise, proto-stream
has to reopen the connection...

-- 
(domestic pets only, the antidote for overdose, milk.)
  larsi@gnus.org * Lars Magne Ingebrigtsen