What reads /etc/iproute2/rt_tables To do policy routeing, one needs to add a routing table to `/etc/iproute2/rt_tables`. I was wondering how this gets read. Is it read by the kernel, or is it read by the iproute2 userspace tools?

That's just the flat file used by the `iproute2` utilities which translates user friendly names given on the command line into integer values used by the kernel. It's just to maintain a consistent mapping of "this name is this integer"