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2024-03-28

system library:BIO_connect:Connection refused:../crypto/bio/bio_sock2.c

I got the strange error message 
system library:BIO_connect:Connection refused:../crypto/bio/bio_sock2.c
on executing 
openssl s_client -starttls smtp -connect mail.area23.at:587
when testing sendmail smtp tls with an Let's Encrypt SSL certificate.

If you got the same error message, ensure that
  1. sendmail is running (on mail server / relay host)
    test it with init rc script 
      /etc/init.d/sendmail status
    or simple by process listing tools, like 
      pstree | grep sendmaail
      pidof sendmail-mta
      ps fauxwww | grep sendmail
    and network server socket is bound 
      netstat -avn 2>&1 |less
    or fuser and your sendmail port
      fuser -v -n tcp 25
    or various variants of lsof, like
      lsof -i TCP:25

  2. sendmail is well configured (on mail server / relay host)
    this will take a little bit too long for that article
    in my case it was a wrong IPv6 address in sendmail.mc

    DAEMON_OPTIONS(`Family=inet6, Name=MTA-v6, Port=smtp, Addr=2600:1f14:1d6d:f202:8d53:fc57:c45d:6590')dnl
     
  3. mail server is reachable from outside 
    test it with 
     telnet [mailserver] [port]
    or brutal way with a network scanner, like nmap
      nmap -p 25,465,587 [mailserver]

    if mailserver is not reachable from outside, 
    then you must lookup on server and on client firewall rules, e.g
    .
      iptables -t filter --list
      iptables -6 -t filter --list
You could also simply run strace to see, what happens:
  strace openssl s_client -starttls smtp -connect mail.area23.at:587

strace openssl s_client -starttls smtp -connect [mailserver] [port]

openssl s_client -starttls smtp -connect [mailserver] [port]

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