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[th/addr-gen-mode-default] all: make "ipv6.addr-gen-mode" configurable by global default

Thomas Haller requested to merge th/addr-gen-mode-default into main

It can be useful to choose a different "ipv6.addr-gen-mode". And it can be useful to override the default for a set of profiles.

For example, in cloud or in a data center, stable-privacy might not be the best choice. Add a mechanism to override the default via global defaults in NetworkManager.conf:

# /etc/NetworkManager/conf.d/90-ipv6-addr-gen-mode-override.conf
[connection-90-ipv6-addr-gen-mode-override]
match-device=type:ethernet
ipv6.addr-gen-mode=0

"ipv6.addr-gen-mode" is a special, because its default depends the component that configures the profile.

  • when read from disk (keyfile and ifcfg-rh), a missing addr-gen-mode means to default to "eui64".

  • when configured via D-Bus, a missing addr-gen-mode means to default to "stable-privacy".

  • libnm's ip6-config::addr-gen-mode property defaults to "stable-privacy"

  • when some tool creates a profile, they either can explicitly set the mode, or they get the default of the underlying mechanisms above.

    • nm-initrd-generator explicitly sets "eui64" for profiles it creates.
    • nmcli doesn' explicitly set it, but inherits the default form libnm's ip6-config::addr-gen-mode.
    • when NM creates a auto-default-connection for ethernet ("Wired connection 1"), it inherits the default from libnm's ip6-config::addr-gen-mode.

Global connection defaults only take effect when the per-profile value is set to a special default/unset value. To account for the different cases above, we add two such special values: "default" and "default-to-eui64". That's something we didn't do before, but it seams useful and easy to understand.

Also, this neatly expresses the current behaviors we already have. E.g. if you don't specify the addr-gen-mode in a keyfile, "default-or-eui64" is a pretty clear thing.

Note that usually we cannot change default values, in particular not for libnm's properties. That is because we don't serialize the default values to D-Bus/keyfile, so if we change the default, we change behavior. Here we change from "stable-privacy" to "default" and "eui64" to "default-or-eui64". That means, the user only expiriences a change in behavior, if they have a .conf file that overrides the default.

This MR is not finished (probably there are bugs, and the tests need adjustment), because Lubomir already indicated that he might not like this. It's only to show how it could be. Opinions?

Edited by Thomas Haller

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