[msmtp-users] tls_hostname: TLS verification without fingerprint or FQDN

[ilf <ilf@...273...>]

I do a lot of TLS verification via $tls_fingerprint. A usual lifetime of 
a certificate used to be a year, with Let's Encrypt it's currently 90 
days. And that's a good thing. However, that makes the task of verifying 
fingerprints out-of-band and unautomated more work-intensive.
Most people just use a $tls_trust_file, either a system-wide one with 
many certs, or a specific one for the CA of the server. However, since 
this compares the domain name of $host with that in the certificate, 
this requires using a correct FQDN in msmtp config. Sometimes it's 
desirable to use different forms of $host, like an IP-address (IPs of 
mailservers rarely change, no point to check for a new one every few 
minutes via an unencrypted and unsigned protocol like DNS) - or a .onion 
address as Tor Hidden Service.
It would be awesome to combine the "best of two worlds." In order to use 
an IP or onion as $host combined with $tls_trust_file, I would propose 
to add something like a $tls_hostname setting which we will be verified 
against the hostname in the certificate.
This isn't completely new, f.e. unbound does something like this for 
DNS-over-TLS:
> forward-addr: 1.1.1.1#cloudflare-dns.com
https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=658#c10

What do you think about this?

Thanks, and keep up the good work!

--
ilf

If you upload your address book to "the cloud", I don't want to be in it.

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