Ok, taking this over to postfix-devel. I understand the concern of default security.As the default smtpd_relay_restrictions are permit_my_networks, permit_sasl_authenticated, defer_unauth_destination. Silently adding external auth is not an option, as then verified client certs are permitted by default due to permit_sasl_authenticated succeeding. This means, in order to not have problems with existing configurations allowing more access suddenly, we would need an option smptd_tls_ccert_verified_is_sasl_authenticated or something.
I took cyrus imap server as an example for what SASL authenticated means with a client cert. It simply uses the CN of the subject of the cert as a username. And one can just specify one or several CAs, which are trusted. Cyrus also lacks support for ocsp and crls.
Exactly what cyrus imap does is what I already have implemented.I personally don't use the fingerprint solution from postfix due to adminstrative effort. Thus, I have not yet implemented something like the suggested "fingerprint to user map" but I see this fits very well to postfix's current security design and would be willing to implement this if it helps auth external to be integrated.
You are right the implementation is rather straight forward as SASL libraries already suppport it.
Bastian On 08.01.19 23:35, Viktor Dukhovni wrote:
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.
smime.p7s
Description: S/MIME Cryptographic Signature
