[Semibug] Self-resolved: answer to my technical question this month
jeff at nucleus.mi.org
Wed Nov 21 21:49:14 EST 2018
Nicely done, Josh! Happy Thanksgiving!
On Wed, Nov 21, 2018, 20:45 Josh Grosse <josh at jggimi.net wrote:
> The TL;DR - I figured out my error. As expected, it was
> in my test configuration files.
> For anyone actually interested:
> I'd asked if anyone had an inkling why my perfectly valid
> self-signed x509 certificates were failing in a lab that I
> had prepared for a migration to the new OpenSMTPd release.
> It's an isolated test environment to prevent any test
> results from getting away from me and annoying the Internet.
> I set up the test environment because much of the mail server
> logical flow and the provisioning syntax has changed, and
> I have two mail servers with complex flows between them,
> using PKI and authentication to eliminate several forms
> of attack on the main internet-facing MTA, and vxlan(4)
> to tunnel SMTP traffic between these servers.
> But my test environment was not functioning properly,
> because of certificate failures. My production
> environment runs fine, using a public CA and their
> provided certificates.
> For this test environment, I'd followed the starttls(8)
> man page, studied and re-studied the smtpd.conf(5) man
> page, and was at a loss. So I took the opportunity this
> week to ask at the meeting if anyone had advice.
> My own thought was to reach out to the lead developer
> Gilles Chehade, who has been kind enough to offer
> support to the user community on provisioning problems.
> But I was hoping to avoid bothering him. Again. :)
> This evening, one of the *many* tests I ran was without
> authentication. And suddenly my certificates were
> acceptable between my two test MTAs. .
> Authentication is a form of password protection, and can
> be used -- when the session is encrypted -- of ensuring
> that the two ends of the session are from known and
> trusted mail systems. The authentication in SMTP happens
> in plain text, so OpenSMTPd rightly only permits it when
> the SMTP session occurs in encrypted form.
> With OpenSMTPd, this encryption requires TLS or SMTPS.
> You can't configure it without encryption. And encryption
> requires an X.509 certificate. And in my enclosed little
> lab (virtual machines on a laptop), self-signed seemed
> like it should have been a perfect solution.
> What I discovered was that if one is using TLS or SMTPS
> without authentication ... then a self-signed certificate
> is perfectly OK. But, if you enable authentication, then
> you must have a cert signed by a valid CA.
> This restriction is hidden in plain sight, in smtpd.conf(5),
> but I missed it, as it just happened to be the last line
> in a paragraph on authentication tables:
> The label corresponds to an entry in a credentials table,
> as documented in table(5). It is used with the
> "smtp+tls" and "smtps" protocols for authentication.
> Server certificates for those protocols are verified by
> My next tests will be to override the default with "tls no-verify"
> and if succesful, I can complete my testing without having to
> deal with the hassle of obtaining new certs just for my lab
> machines, and then openg up my lab's network to the Internet
> for verifying the certs. I want to make sure my mail servers
> are operating properly and not spewing traffic they shouldn't
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