E-mail IPv6 Fallback to IPv4

[Xavier Bonfond]

Hello,

imagine we have 2 mail relay that need to communicate and both are dual stack. What happens if the ipv6 communication is down between both end ? does the sender will try the ipv4 after a delay ?

[marce1000]

 - That question is 'not connected' the mail-app does not care about  the transport layer which is lower in  the network stack, it only tries to send the mail to the destination-host which can be either reachable or not. It will also seem difficult to use ipv4 if the destination-sender resolved to an ipv6-address only.

 M.

[Xavier Bonfond]

Thanks for your reply,

My question was in a dual stack context. But lets be more concrete.

Imagine an enterprise connected to 2 tiers1 providers and lets say one of them is for example "Cogent" who don't provide a complete IPv6 routing because of conflict with some service provider like "Google".

The enterprise and Google have A/AAAA records on each MX records and mail relay.

If the enterprise have an outage with the provider with the complete ipv6 table, the routing to Google is no more possible over ipv6.

Does the mail gateways on each side will try to use the A record when the ipv6 one don't reply ?

[marce1000]

 >Does the mail gateways on each side will try to use the A record when the ipv6 one don't reply

                 It will try all addresses returned for the MX record -  normally.

 M.

[Georg Pauwen]

Hello,

I have read the IETF draft linked below, and in theory, the fallback should be automatic.

That said, are you in a position to actually test if it works as suggested ?

https://tools.ietf.org/id/draft-martin-smtp-ipv6-to-ipv4-fallback-00.xml