interfaces/network: workaround Go's need for NETLINK_ROUTE with 'net'. LP: #1689536

[jdstrand]

https://bugs.launchpad.net/snapd/+bug/1689536

This should be in 2.26 point release.

It looks like with 4.4 kernels, these go calls require NETLINK_ROUTE:

• net.Interfaces()
• net.InterfaceAddrs()

Curiously, NETLINK_ROUTE is not needed for these on 4.10 x86 kernel. Technically, these two calls are in the domain of 'network-observe', 'network-bind' and 'network-control' and not 'network'.

For series 16 I think we should add this to 'network' to not break existing applications on ARM that only plugs 'network', in part because both network and network-bind are autoconnected and there is therefore no appreciable difference security-wise wrt install time interface connections.

For series 18 (or whenever we start having different policy), we can consider removing NETLINK_ROUTE from the 'network' policy since that is more correct.

[zyga]

LGTM

[morphis]

Thanks for this! LGTM