fib_frontend: Add network specific broadcasts, when it takes a sense

[Brandon Philips <brandon.philips@coreos.com>]

Hello-

A number of us are working on an OSS overlay network system called flannel.
It is used in a variety of Linux container systems and one of the backends
is VXLAN.

The issue we have: when creating the VXLAN interface and assigning it an
address we see a broadcast route being added by the Kernel. For example if
we have 10.4.0.0/16 a broadcast route to 10.4.0.0 is created. This route is
unwanted because we assign 10.4.0.0 to one of our VXLAN interfaces.

However, the Kernel interface bring-up comment reads: Add network specific
broadcasts, when it takes a sense. The code is here:
https://github.com/torvalds/linux/blob/master/net/ipv4/fib_frontend.c#L859-L872

Can someone explain why creation of the broadcast route is non-optional?
Would a patch to make it optional be acceptable? Is it safe for us to
simply delete the route? We have a patch that simply deletes the broadcast
route after interface creation but don't know why the Kernel code "makes
sense".

You can read more information about the issue here:
https://github.com/coreos/flannel/pull/569

Thank You,

Brandon