How to prevent, or change priority, of dovecot's FAILed relay-submission to relay's IPv6 address, and submit ONLY/first to IPv4?

[PGNet Dev]

How do you turn OFF, or reduce priority of, IPv6 connect attempts by submission relay?

On 4/3/21 8:03 PM, PGNet Dev wrote:
> my server is a linux, dual-stack IPv4/IPv6 host
> 
> it runs multiple services, including, but not limited to, postfix & dovecot
> 
> the hostname is
> 
>      internal.mx.example.com
> 
> its DNS config,
> 
>      host internal.mx.example.com
>          internal.mx.example.com has address 10.1.1.15
>          internal.mx.example.com has IPv6 address fd80:10:1::15
>          internal.mx.example.com mail is handled by 5 internal.mx.example.com.
> 
> here, for dovecot
> 
>      dovecot --version
>          2.3.13 (89f716dc2)
> 
> submission is configured to relay to the same-host postfix instance, listening @ port 465
> 
>      ./conf.d/10-master.conf
>          ...
>          protocols = imap submission lmtp sieve
>          ...
>          submission_relay_host       = internal.mx.example.com
>          submission_relay_port       = 465
>          submission_relay_ssl        = smtps
>          submission_relay_ssl_verify = yes
>          submission_relay_trusted    = yes
> 
> whereas other services listen at both IPv4 & IPv6 addresses, with IPv6 preferred over IPv4, postfix listens ONLY on IPv4,
> 
>      postconf inet_protocols
>          inet_protocols = ipv4
> 
> and, as intended, simply refuses ipv6 connections
> 
>      telnet 10.1.1.15 465
>          Trying 10.1.1.15...
>          Connected to 10.1.1.15.
>          Escape character is '^]'.
>          ^]
>          telnet> quit
>          Connection closed.
> 
>      telnet -6 fd80:10:1::15 465
>          Trying fd80:10:1::15...
>          telnet: connect to address fd80:10:1::15: Connection refused
> 
> on each/every mail submit -- via dovecot -- dovecot makes the connection
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-server: conn 10.1.2.163:35388 [1]: Server accepted connection (fd=7)
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-server: conn 10.1.2.163:35388 [1]: Connection created
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: Connection created
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: Disconnected
> 
> looks up IP address
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: Looking up IP address
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: Performing asynchronous DNS lookup
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-server: conn 10.1.2.163:35388 [1]: Sent: 235 2.7.0 Logged in.
> 
> finds BOTH IPs -- IPv4 & IPv6
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: DNS lookup successful; got 2 IPs
> 
> then first tries to connect via the host's IPv6 address,
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 [1]: Connecting to fd80:10:1::15:465
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 ([fd80:10:1::15]:465) [1]: Connecting
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 ([fd80:10:1::15]:465) [1]: Waiting for connect (fd=8) to finish for max 0 msecs
> 
> ############
>   FAILS
> ############
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 ([fd80:10:1::15]:465) [1]: Client connection failed (fd=8)
> 
> then proceeds to connect to the host's IPv4 address
> 
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 ([fd80:10:1::15]:465) [1]: Connecting to 10.1.1.15:465
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: Connecting
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: Waiting for connect (fd=11) to finish for max 0 msecs
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: Client connected (fd=11)
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: Connected to server (from 10.1.1.15:52880)
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: Starting SSL handshake
>      2021-04-03 19:35:35 submission(mailer at mx.example.com)</tnO6xm/PIqsHgsH>: Debug: smtp-client: conn internal.mx.example.com:465 (10.1.1.15:465) [1]: SSL handshake successful
>      ...
> 
> and submission continues/completes
> 
> 
> I need to get Dovecot to stop trying/failing @ those IPv6 address submission connections.
> 
> Either by
> 
>      (1) trying IPv4 *first*, before IPv6, to avoid the FAIL on submission
> 
> or
> 
>      (2) turning off submission relay by IPv6 altogether, as I'll never use it
> 
> What's the config required to do either/both?
>