Thunderbird can't connect to Dovecot (bad certificate: SSL alert number 42)

[Jaroslaw Rafa]

Dnia 18.09.2022 o godz. 10:09:34 Stuart Henderson pisze:
> 
> The CA/Browser Forum baseline requirements say that certificates must
> include subjectAlternativeName. This doesn't strictly apply to non-browser
> applications but it does mean that all CA-issued certs can be relied upon
> to have SAN.
> 
> RFC 6125 6.4.4 says that clients must not check CN if the identifiers
> used in subjectAlternativeName are present. So for certs following the
> baseline requirements, checking CN is redundant. It also says that
> clients *may* check CN but it's not required.
> 
> There are differences in handling of name constraints between certs
> using just CN and those using SAN. Name constraints don't really work
> for certs using CN (by adding dc= components to the Subject, you can
> comply with the directoryNameconstraints that apply to Subject
> while providing a CN that is not in the expected domain). The dNSName
> constraint applicable to SAN doesn't have this problem.
> 
> So there's a good reason to avoid using CN when checking the name: it
> gives defence against a CA or sub-CA with a trusted but constrained root
> certificate that goes rogue.
> 
> Practically this means you need to make sure that if you use self-
> signed or internal CA certificates you include subjectAlternativeName
> otherwise they won't work with some client software. If you use public
> CA-signed certs you typically don't need to do this yourself because
> the CA adds SAN if missing from the CSR (their only other option is
> to reject issuance).

I have a question regarding this:
I always understood (maybe wrong) SAN as literally *alternate* DNS names for
the server in addition to its basic, "canonical" DNS name, which was
specified in the CN.

For example if the server is example.com, but it also can be accessed as
www.example.com (and both names have A records resolving to the same IP
adddress), I put example.com into CN and www.example.com into SAN.

>From what you have written above, I cannot figure out if this is correct, or
maybe should I put both names example.com and www.example.com into SAN (in
addition to example.com being in CN)?
-- 
Regards,
   Jaroslaw Rafa
   raj at rafa.eu.org
--
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