No IPv6 outbound from Sendmail starting with 20.04

[Brian Smith]

With Bionic, Sendmail would send email over IPv4 and IPv6, which I could see in logs and DMARC aggregate reports. Since upgrading to Focal, it's only attempting to send over IPv4, but I can receive email just fine over IPv4 or IPv6. I'm seeing that both with an upgrade and a fresh install.

On further testing, it looks like Sendmail in 20.04 isn't even trying to get IPv6 addresses for servers. If only an IPv6 address is available, I get a "host unknown" bounce.

What changed, then? Not my configuration, in the case of the upgrade, and I don't see anything relevant in the package changelogs between the 2 versions.

[Ubuntu Foundations Team Bug Bot (crichton)]

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[Brian Smith (avalon73)]

After more testing, this seems to be a resolver issue in sendmail, since I can specify an IPv6 address directly as a smart host and it works. If it's a config problem, I don't know what that might be, and it doesn't explain the existing install no longer trying to use IPv6 after upgrading to Focal.

[Brian Smith (avalon73)]

Not sure what you guys did or didn't do, but it's apparently working again with Jammy.

It took a while, but... thanks.

[Dan Cooper (pcfan1234)]

Can you reproduce that it affects the version in Focal, but not Jammy?
Can you use Wireshark to check if sendmail queries the DNS for an AAAA record for the specific domain name?
What happens if you remove all IPv4 addresses on your system?
Does it then use IPv6?
Remember there is "happy eyeballs" fallback to IPv4.

[Brian Smith (avalon73)]

Unfortunately I don't have any machines running Focal anymore to test with, and I didn't think to check if a AAAA DNS request was actually going over the wire when I had one.

It's easy enough to replicate, though, by using the smart host feature. On the machine running Focal:

- IPv6 address literal as a smart host works
- Named smart host with A and AAAA records uses the IPv4 address
- Named smart host with only a AAAA record results in a "host not found" bounce

There is no IPv4 fallback when no IPv4 addresses are in the DNS results.