Re: What are the intended semantics of IFLA_LINK_NETNSID?

Le 25/02/2015 20:03, Eric W. Biederman a écrit :

Reading through the code of rtnl_newlink I am perplexed with what your
intended semantics of IFLA_LINK_NETNSID are supposed to be.

My expectation was something with the same semantics IFLA_NET_NS_PID and
IFLA_NET_NS_FD just a different data representation and something that
could be used in more netlink messages, so you could report the network
namespace where the other end of a tunnel or the the network namespace
of an underlying device is.

Being very slow I would expect that ILFA_LINK_NETNSID would replace
dest_net or possibly net in rtnl_newlink but it does not replace either
of those completely.  Which causes me to think that the implementation
of IFLA_LINK_NETNSID in rtnl_newlink is broken.

I suspect the correct fix for rtnl_newlink is to just use
IFLA_LINK_NETNSID in rtnl_link_get_net and have it be an alternative way
of setting dest_net.  But you may intend some different semantics that
I don't understand.

ILFA_LINK_NETNSID is used to point to the i/o netns of the interface, ie the
opposite netns of dest_net.
The interface is first created in link_net and moved at the end in dest_net.

IP tunnels interfaces (ipip, sit, ip6_tunnels, gre[v6]) does not use src_net,
thus when you create an ipip interface by specifying an attribute
IFLA_NET_NS_PID it will result to an interface which is not across two netns but
only in the netns pointed by IFLA_NET_NS_PID. If you use IFLA_LINK_NETNSID, it
allows you to create this kind of x-netns interfaces.
In fact, the goal of this attribute is to replace the two following command by
only one:
ip link add foo ...
ip link set foo netns bar
=> ip link add foo link-netnsid barID


Regards,
Nicolas