How to disable IPv6-based SMTP delivery with exim on Debian

Disabling outgoing mail delivery over IPv6 with exim on Debian.md

IPv6 is great, but sometimes it's troublesome. SMTP over IPv6 is particularly troublesome if you're talking to Google (and probably others as well)

If you for some reason don't have a correct PTR record, you'll end up with a lots of refused mail and logs pointing you to https://support.google.com/mail/answer/81126?p=ipv6_authentication_error&rd=1#authentication

One way to fix this is to fall back to IPv4 only for outgoing SMTP. Given a Debian installation with split-config exim, the simplest way to do this seems to be as follows:

• copy /etc/exim4/conf.d/router/200_exim4-config_primary to 200_exim4-config_primary.rul
• The second "dnslookup:" router is used for remote smtp-delivery, adding 0::0/0 to the ignore_target_hosts will make sure that no AAAA-records are used for later transport

change :

ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8 : 192.168.0.0/16 :\
                  172.16.0.0/12 : 10.0.0.0/8 : 169.254.0.0/16 :\
                  255.255.255.255

to :

ignore_target_hosts = 0.0.0.0 ; 127.0.0.0/8 ; 192.168.0.0/16 ;\
                  172.16.0.0/12 ; 10.0.0.0/8 ; 169.254.0.0/16 ;\
                  255.255.255.255 ; 0::0/0

ignore_target_hosts = <

What's the "<" for?

That very much looks like a copy-pasta typo on my behalf.

I guessed so ... and thanks for this gist.

You need the <; at the beginning of the option so that Exim understands the change of separator from : to ;.

ignore_target_hosts = <; 0.0.0.0 ; 127.0.0.0/8 ; 192.168.0.0/16 ;\
                  172.16.0.0/12 ; 10.0.0.0/8 ; 169.254.0.0/16 ;\
                  255.255.255.255 ; 0::0/0

Ah, yes. That makes sense. Well, for some values of sense at least.