Guide on how to migrate chatmail to a new host

[missytake]

This guide doesn't require knowing about firewalls, but utilizes the cmdeploy run --disable-mail command from #428 and the cmdeploy run --ssh-host command from #439. Should be merged after those.

supercedes #417

Tested by migrating c2 back and forth, especially the second time worked like a charm :)

[missytake]

I wonder whether we should also preserve the DKIM key, as it might lead to rejected messages if multiple keys are around for the same dkim selector.

[hpk42]

To be honest, it all looks workable but also rather error-prone/tedious, especially the constant overwriting of host identity keys.
I think we need a "cmdeploy --ssh-host=sshname" option because then one can just add "old" and "new" to /etc/hosts and do "cmdeploy --ssh-host=old run" etc, and skip all the hostkey-overwriting. It should also help with the NAT-issue and should not be hard to do.

[link2xt]

I wonder whether we should also preserve the DKIM key, as it might lead to rejected messages if multiple keys are around for the same dkim selector.

I think we should generate a new DKIM selector for each deployment. Using current date like 20241017 for the selector name. Ideally DKIM selector to key mapping should be possible to cache indefinitely, so reusing DKIM selectors is not a good idea.

[missytake]

I wonder whether we should also preserve the DKIM key, as it might lead to rejected messages if multiple keys are around for the same dkim selector.

I think we should generate a new DKIM selector for each deployment. Using current date like 20241017 for the selector name. Ideally DKIM selector to key mapping should be possible to cache indefinitely, so reusing DKIM selectors is not a good idea.

Do you consider #195 a blocker for merging this?

[missytake]

To be honest, it all looks workable but also rather error-prone/tedious, especially the constant overwriting of host identity keys. I think we need a "cmdeploy --ssh-host=sshname" option because then one can just add "old" and "new" to /etc/hosts and do "cmdeploy --ssh-host=old run" etc, and skip all the hostkey-overwriting. It should also help with the NAT-issue and should not be hard to do.

With c9b574b we're down from 16 to 9 steps :)))

[hpk42]

overall looks great, some minor comments only.
Haven't run it myself though.
Changelog entry is missing.