Quando o Postfix usa o IPv6 e quando o IPv4?

Question

Configurei meu servidor de e-mail para IPv6 e IPv4, mas na maioria dos casos, ele usa o IPv4 enquanto o IPv6 está disponível no host remoto. Eu uso o Postfix na versão 2.9. Então, meu pensamento foi que o postfix usa o IPv6, se disponível, e tem um fallback para o IPv4 ou estou errado?

    

Answer 1

Você configurou a opção smtp_address_preference para expliclity preferir o IPv6?

De postconf (5):

smtp_address_preference (default: any)
       The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client will try first, when a destination has IPv6 and IPv4 addresses with equal MX preference. This feature has no effect unless  the
       inet_protocols setting enables both IPv4 and IPv6.  With Postfix 2.8 the default is "ipv6".

       Notes for mail delivery between sites that have both IPv4 and IPv6 connectivity:

       ·      The setting "smtp_address_preference = ipv6" is unsafe.  It can fail to deliver mail when there is an outage that affects IPv6, while the destination is still reachable over IPv4.

       ·      The setting "smtp_address_preference = any" is safe. With this, mail will eventually be delivered even if there is an outage that affects IPv6 or IPv4, as long as it does not affect both.

       This feature is available in Postfix 2.8 and later.

    

Answer 2

Se o Postfix estiver configurado corretamente com o IPv6, ele será instruído a usar os dois protocolos e o host MX ao qual ele se conecta tiver registro A e AAAA, o Postfix fará uma escolha aleatória sempre, como Wietse Wenema mesmo explicou :

Recent Postfix SMTP clients randomly select between IPv4 and IPv6 so that mail won't get stuck when one of the two is down.