Hobbyist developer

martin@rosenau-ka.de

I-D Internet-Draft E-Mail Mail Email SMTP IPv6 During the introduction phase of IPv6 (as of Jan 2013) many existing mail servers are not able to do IPv6 communication, yet. However new networks will be assigned IPv6 addresses only so new mail servers will not be able to do IPv4 communication. This document proposes the installation of special servers that are able to route mail between IPv4-only and IPv6-only mail servers to solve this problem.

As of the end 2012 IPv4 addresses are no longer available to companies. For this reason new mail servers (especially mail servers of companies that do not have IPv4 addresses, yet) are IPv6-only. The introcuction of IPv6 on existing mail servers, however, is often not done right now. RFC 3974 proposes that every mail server should have an IPv4 address even if it is able to do IPv6. However this is no longer possible due to the depletion of IPv4 addresses. For this reason it may be impossible to send an electronic mail from a domain that has an IPv4-only server to a domain using an IPv6-only server and vice versa.

This document proposes the installation of IPv4-to-IPv6 translation servers to manage the problem. Such a server would work the following way: Accept incoming SMTP connections on IPv4 AND IPv6 For each mail "received" simply forward the received mail to the SMTP server that is really responsible for the destination address In our example such servers have the addresses 192.0.1.30 and 192.0.1.31. The DNS configuration may look like this:

example.org. IN MX 1 mx1.example.org. example.com. IN MX 1 mx1.example.com. ip4only.example. IN MX 1 mx1.ip4only.example. mail-tunnel.example. IN AAAA 2001:db8:ffff::1 mail-tunnel.example. IN AAAA 2001:db8:ffff::2 mail-tunnel.example. IN A 192.0.1.30 mail-tunnel.example. IN A 192.0.1.31 mx1.example.org. IN AAAA 2001:db8:ffff::1:1 mx1.example.org. IN A 192.0.1.30 mx1.example.org. IN A 192.0.1.31 mx1.example.com. IN AAAA 2001:db8:ffff::2:1 mx1.example.com. IN A 192.0.1.30 mx1.example.com. IN A 192.0.1.31 mx1.ip4only.example. IN A 192.0.2.1

Note that mx1.example.com and mx1.example.org are different (!) servers. However the IPv4 address in the DNS configuration is the same. The DNS query for their IPv4 address will result in the IPv4 address of the server "mail-tunnel.example" because they do not support IPv4.

Scenario: An Email is sent from example@ip4only.example to example@example.com. The mail server of the domain ip4only.example will now query the IPv4 address of mx1.example.com. The DNS server will return the address 192.0.1.30 which is in fact the address of the tunnel server. The mail is now delivered to the tunnel server, that then forwards the mail to the actual mail server.

Scenario: An Email is sent from example@example.com to example@ipv4only.example. The mail server of the domain example.com will now query the IPv6 address of mx1.ipv4only.example. The DNS server will return no result. In this case the mail server's implementation must be done in a way to allow transmission to IPv4-only domains: It must detect this case and directly connect the tunnel server or use other tunneling mechanisms that allow connecting the IPv4 mail server using a TCP connection from an IPv6-only network directly (like NAT64). Note: If an IPv6 mail server has access to NAT64 and DNS64 it can send mail directly to an IPv4 mail server