> On Jan 8, 2019, at 5:17 PM, Bastian Schmidt <bastian.schm...@web.de> wrote: > > 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.
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 = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination 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. -- Viktor.