[libvirt] "migration_address must not be the address of the local machine: 127.0.0.1"

[Laszlo Ersek]

I recently upgraded my laptop from RHEL-7.1 to RHEL-7.2.

I always pay attention to *.rpmnew config files, and I manually diff and
merge them with the ones I have in place.

I did the same with "/etc/libvirt/qemu.conf" this time.

Now libvirtd doesn't start for me. Systemd doesn't actually notice the
startup failure (insert bitter joke about systemd being so much better
than startup scripts); it only reports the service inactive/dead (=
unstarted), rather than failed.

But, the libvirtd log file gives the reason:

    migration_address must not be the address of the local machine:
    127.0.0.1

The error is easy to fix up in the config file, but my question is:

Why must migration_address not be the address of the local machine?

This comes from:

commit 5e0561e115de10da342296bb7c7361e91e368d9c
Author: Chen Fan <chen.fan.fnst at cn.fujitsu.com>
Date:   Tue Oct 7 12:07:32 2014 +0800

    conf: Check whether migration_address is localhost

    When enabling the migration_address option, by default it is
    set to "127.0.0.1", but it's not a valid address for migration.
    so we should add verification and set the default migration_address
    to "0.0.0.0".

    Signed-off-by: Chen Fan <chen.fan.fnst at cn.fujitsu.com>
    Signed-off-by: Ján Tomko <jtomko at redhat.com>

I checked the mailing list archive. AFAICS, the earliest appearance of
this check is:

https://www.redhat.com/archives/libvir-list/2014-August/msg01503.html

The validity of the proposed check was never questioned, nor explained.

What gives?

BTW, my purpose is not in-host migration (perhaps that's indeed
unsupported, I don't know); I just want to lock down the incoming
migration port (and not just with firewall rules).

If there's a way to disable incoming migration in libvirtd, I'd be
interested in that.

Thanks!
Laszlo