Time to validate the TLS configuration on your SMTP servers (was: Re: AS5 ipv6 hijack?)

Mon Apr 12 12:58:41 UTC 2021

OK, so that email bounced.  Or will eventually because this does not go
away with someone doing something:

  <dmitry at interhost.net>... Deferred: 403 4.7.0 TLS handshake failed.

I am posting this in public because it unfortunately is a very common
problem.

Debian buster was released on July 6th, 2019. It includes openssl 1.1.1
with this configuration update among number of other improvements:

openssl (1.1.1~~pre6-1) experimental; urgency=medium

  * New upstream version
  * Increase default security level from 1 to 2. This moves from the 80 bit
    security level to the 112 bit securit level and will require 2048 bit RSA
    and DHE keys.

 -- Kurt Roeckx <kurt at roeckx.be>  Tue, 01 May 2018 16:00:55 +0200

I assume similar policies have been applied to all modern and maintained
operating systems by now.

Everyone should verify their own SMTP servers to avoid losing email due
to TLS failures.  Doing so is simple from e.g Debian:

bjorn at canardo:/usr/local/src/openwrt$ cd                                                                                                                                                                                               
bjorn at canardo:~$ host interhost.net                                                                                                                                                                                                    
interhost.net has address 185.18.204.66
interhost.net mail is handled by 10 pineapp.interhost.co.il.

bjorn at canardo:~$ openssl s_client -quiet -connect pineapp.interhost.co.il:25 -starttls smtp
depth=2 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert Global Root CA
verify return:1
depth=1 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = RapidSSL RSA CA 2018
verify return:1
depth=0 CN = *.interhost.co.il
verify return:1
139901908640896:error:141A318A:SSL routines:tls_process_ske_dhe:dh key too small:../ssl/statem/statem_clnt.c:2150:

The fix obviously depends on the server, but is usually as simple as
regnerating the DH parameters.  See for example
https://forums.freebsd.org/threads/sendmail-dh-key-too-small.51985/

Bjørn