Comment # 13 on bug 1203420 from Jiri Bohac

I'm not a NetworkManager expert, but just quickly grepping through its code I
see it sets the value of disable_ipv6 in many places. So I suppose systemd sets
the value right (once) and then NM does what it wants with it. So it probably
works as expected or if this is not expected, it is a NM bug.

What does seem like a systemd bug is the logic where systemd checks the value
and does not write it if it is equal to the one being read. At least with this
sysctl, this is not technically correct. See
Documentation/networking/ip-sysctl.rst in the kernel sources:

conf/all/disable_ipv6 - BOOLEAN
        Changing this value is same as changing ``conf/default/disable_ipv6``
        setting and also all per-interface ``disable_ipv6`` settings to the
same
        value.

        Reading this value does not have any particular meaning. It does not
say
        whether IPv6 support is enabled or disabled. Returned value can be 1
        also in the case when some interface has ``disable_ipv6`` set to 0 and
        has configured IPv6 addresses.

Franck Bui: If nothing else comes out of this bug, this should be fixed in
systemd, what do you think?