Basic VRF support
Add basic VRF (virtual routing and forwarding) support. Protocols can be associated with VRFs, such protocols will be restricted to interfaces assigned to the VRF (as reported by Linux kernel) and will use sockets bound to the VRF. E.g., different multihop BGP instances can use diffent kernel routing tables to handle BGP TCP connections. The VRF support is preliminary, currently there are several limitations: - Recent Linux kernels (4.11) do not handle correctly sockets bound to interaces that are part of VRF, so most protocols other than multihop BGP do not work. This will be fixed by future kernel versions. - Neighbor cache ignores VRFs. Breaks config with the same prefix on local interfaces in different VRFs. Not much problem as single hop protocols do not work anyways. - Olock code ignores VRFs. Breaks config with multiple BGP peers with the same IP address in different VRFs. - Incoming BGP connections are not dispatched according to VRFs. Breaks config with multiple BGP peers with the same IP address in different VRFs. Perhaps we would need some kernel API to read VRF of incoming connection? Or probably use multiple listening sockets in int-new branch. - We should handle master VRF interface up/down events and perhaps disable associated protocols when VRF goes down. Or at least disable associated interfaces. - Also we should check if the master iface is really VRF iface and not some other kind of master iface. - BFD session request dispatch should be aware of VRFs. - Perhaps kernel protocol should read default kernel table ID from VRF iface so it is not necessary to configure it. - Perhaps we should have per-VRF default table.
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