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Explanation of this feature:
without this feature, libnl does not expose the IFLA_LINK_NETNSID attribute. That means, application who need this attribute, would have a hard time to work around it getting this information.
If an application (e.g. NetworkManager) is not aware of the IFLA_LINK_NETNSID, then it might wrongly think that the IFLA_LINK attribute advertised for a certain interface lies in it's own netns -- while in fact the ifindex in IFLA_LINK is only valid inside an other netns.
A common example are veth pairs, where one peer is in another netns of the application.
In this case, the IFLA_LINK of the visible peer is meaningless inside the netns of the application. Then the application either thinks that the linked-link is surprisingly missing, or it might wrongly think that it refers to another interface (that accidentally has an overlapping ifindex inside the netns of the application).
this can for example lead to wrong behavior inside NetworkManager.
Reproducer:
Create a veth pair and have their peers in two different netns. Make use of libnl3 and inspect the interface as returned from the library.
Check that the IFLA_LINK/rtnl_link_get_link() of that interface really shows the ifindex of the link in the other netns.
Check, that also IFLA_LINK_NETNSID/rtnl_link_get_link_netnsid() indeed indicates that the link is in the other netnsid.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHBA-2015-2105.html