TLS client certificates and auth external


I have an email client (K-9 on Android), which, when using TLS client
certificates insists on sending an auth external. However, postfix/SASL
does not advertise external auth, which causes the client to not being
able to use client certificates with postfix.

As I see it, postfix is missing the external mechanism as specified in
RFC 2222 (SASL) completely. Thus, I have implemented this feature (for
TLS CA client certs) and I am currently successfully running this on a
local installation using cyrus sasl.

I would be willing to provide a patch and would really like to see this
integrated in future versions of postfix.

I hope this is the right postfix mailing list for this request.



Re: TLS client certificates and auth external

By Viktor Dukhovni at 01/08/2019 - 17:35

Well perhaps postfix-devel is equally or more appropriate.

There is a key design issue here:

* In typical Postfix configurations we see relay restrictions of
the form:

smtpd_relay_restrictions =

which is fine, when the user has enrolled for a login account
on the receiving system. But with client certs, anyone can get
a client certificate from some CA, or even mint their own.

So what does "SASL authenticated" mean with client certs? Is
there a particular issuing CA that's the only one trusted to
issue client certs? Or does the client certificate fingerprint
need to match a lookup table for it to be considered authenticated?

My advice is that a trusted CA, and likely often accidentally every
CA on the planet from one of the usual CA bundles, is much too risky
in this context, and would drag in revocation lists, OCSP, and that
whole dumpster-fire of PKI issues.

Therefore, the meaning of SASL authenticated for EXTERNAL should be
that the client certificate fingerprint matches a lookup table that
maps the client certificate to something resembling a SASL user name.

You would then either "permit_sasl_authenticated" without distinguishing
between one user and another, or else use "check_sasl_access" based on
username obtained from the fingerprint->username map. You could also
then use the "sender login mismatch" features by matching the username
with valid sender addresses, ...

Otherwise, "EXTERNAL" should be fairly straight-forward. Feel free to
move the discussion to postfix-devel, or continue here to the extent
the discussion stays high level, rather than dives into the implementation.