On Fri, Apr 08, 2011 at 11:09:00PM -0700, email builder wrote:
> I'm wondering about the usefulness of smtp(d)_tls_CAfile(path) when using
> opportunistic encryption in both incoming and outgoing connections. The
> TLS_README suggests that certificate and key files be left empty for
> opportunistic smtp processes, but it doesn't talk specifically about
> smtp_tls_CAfile(path).
For the SMTP server, you should NOT leave the cert file empty, as many
clients won't support aNULL ciphers. Rather, you need to set a self-signed
cert, if one of the usual CAs is not suitable.
For the SMTP server, since you probably won't ask for client certs, you
never need a CAfile or CApath.
For the SMTP client, indeed, generally, your key and cert should be set
empty. On the other hand, it is a good idea in most cases to have a CAfile
and/or CApath with a few trusted roots.
> Am I correct to infer that both smtp(d)_tls_CAfile settings only serve
> a purpose when you want to verify client/server certificates?
> If that's the case, why does the example at the bottom of TLS_README
> use both the CAfile settings with only opportunistic encryption?
This reduces log noise, and improves the audit trail.
> Our system seems to work without any CAfile/CApath settings under
> opportunistic
> encryption both incoming and outgoing. Is there a performance or security
> difference between using them or not?
You should probably throw in a few trusted root CAs.
--
Viktor.
