Making the equivalent of iproute2's "bridge mdb|fdb" commands using libnl

[Thomas Haller]

On Wed, 2014-10-01 at 20:41 -0400, Laine Stump wrote:
> In libvirt, I need to be able to get and set the unicast and multicast
> tables of an interface from within libvirt. Since libvirt uses libnl, it
> would be most consistent to use libnl to do this; iproute2's "bridge
> mdb|fdb" command does this, so it could be used as an example, but that
> is all written to directly use netlink (with help from libnetlink.c in
> the iproute2 source). I see from that source that the  RTM_GETNEIGH and
> RTM_GETMDB messages are used to request these tables, but I've been
> unable to find any examples of using RTM_GETNEIGH or RTM_GETMDB with
> libnl. Looking at neigh.c in libnl, it seems the information of
> RTM_GETNEIGH might be stored in a cache by libnl, but I'm not even sure
> that's the same information that bridge fdb is retrieving, and looking
> for parallel use of RTM_GETMDB, I see nothing in libnl.
> 
> So can anyone unconfuse me? Is there a reasonable example of retrieving
> these tables using libnl anywhere?


Hi Laine,


I am not entirely sure, but...



libnl3 doesn't even mention the RTM_GETMDB message. So you have to do it
yourself -- unless you send us a patch for libnl to support it (patches
welcome).


But still, using libnl probably helps you significantly with that.

Have a look at this random patch from NetworkManager, that parses
netlink messages itself (using libnl3):
https://git.gnome.org/browse/network-manager-openswan/commit/?h=dcbw/cleanups&id=b8fab11967ff4cbb05bfc170c8590c3d1233c67a




Regarding RTM_GETNEIGH, neigh.h should give you all you need.
Looking at neigh.c which reads:


 * @par 1) Retrieving information about configured neighbours
 * @code
 * // The first step is to retrieve a list of all available neighbour within
 * // the kernel and put them into a cache.
 * struct nl_cache *cache = rtnl_neigh_alloc_cache(sk);
 *
 * // Neighbours can then be looked up by the interface and destination
 * // address:
 * struct rtnl_neigh *neigh = rtnl_neigh_get(cache, ifindex, dst_addr);
 *·
 * // After successful usage, the object must be given back to the cache
 * rtnl_neigh_put(neigh);
 * @endcode


Putting tgraf in CC. @Thomas, please correct me if I am wrong.

best,
Thomas
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