Re: [exim] DANE(TA) doesn't work with self signed certificat…

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Author: Viktor Dukhovni
Date:  
To: exim-users
Subject: Re: [exim] DANE(TA) doesn't work with self signed certificates


> On Sep 4, 2018, at 8:52 AM, Jeremy Harris via Exim-users <exim-users@???> wrote:
>
> As the docs say:
>
> "DANE-TA usage is effectively declaring a specific CA to be used; this
> might be a private CA or a public, well-known one."
>
> That CA needs to be known by the Exim configuration.


Sorry, that's simply wrong. Exim MUST support validation via
DANE-TA(2) trust-anchors that ARE NOT configured locally. Indeed
Exim SHOULD ignore the local trust-anchors when validating usage
DANE-TA(2) TLSA records. All that's required is that the remote
server include the trust-anchor certificate in its TLS certificate
message.

If Exim is to claim DANE support it MUST either correctly handle
non-public trust-anchors, or else MUST ignore "unusable" TLSA
RRsets that contain DANE-TA(2) TLSA records. Indeed even "mixed"
TLSA RRsets with some DANE-EE(3) records and some DANE-TA(2)
records should probably be ignored until this issue (if not user
error), because quite often only the DANE-TA(2) records are valid.

My advice to the user would be to use a version of Exim that
is linked with OpenSSL and NOT GnuTLS. The Exim DANE support
in combination with GnuTLS is not nearly as well tested or
supported.

-- 
    Viktor.