[Impact]
Only affects IPv6 connections, but we established that the default should be to maintain our users' privacy on IPv6 connections; we want all IPv6 external connections to be established using temporary, dynamic addresses.
[Development Fix]
Very minor patch to threat an unknown value (-1) as (2), aka, enabling and preferring privacy extensions. Patch is attached.
[Stable Fix]
See "Development fix".
[Test case]
Connect a wire to your system. With NetworkManager running, the value returned by 'sysctl net.ipv6.conf.eth0.use_tempaddr' should be 2 at all times unless configured otherwise (though the default is set to 2 by default). Without the patch, the value is reset to 0 when NetworkManager is running, and correctly set to 2 when NetworkManager is stopped (since it reverts the value to the value when NM started).
[Regression Potential]
Low, impact may be failing connections in case of issues with the privacy extensions system in the Linux kernel.
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The use_tempaddr sysctl defaults to 0 in NetworkManager if it's not set elsewhere, such as it is the case for the default NM-created, automatic wired connection. That connection is never "completed" (because it's internal magic), and nm_backend_ipv6_use_tempaddr() doesn't have a chance of returning a valid system-wide default value since the function it calls, nm_generic_ipv6_use_tempaddr() only checks in /etc/sysctl.conf and /lib/sysctl.d/sysctl.conf for a system default value.
(Regardless, this wouldn't work since we generate a setting based on a number of files in /etc/sysctl.d)
Change the behavior slightly to default to a string of -1 as a default value in nm-system.c; then make sure that if that value is never changed by the result of nm_backend_ipv6_use_tempaddr(), we just don't touch that sysctl at all.
Change looks good and matches what we said we'd do at UDS, what NM does for all new connections and what's currently happening for all non-NM connections.
Please upload to precise-proposed :)