What CAN cause 'RTNETLINK answers : No such process' when adding a route

I have come up against this error a number of times, but haven't yet found an explanation of what the error means, or what can cause the problem.

I'm not posting my configs or the specific route I'm trying to add as I don't want someone to tell me what I've done wrong THIS time, I'd like an understanding of what the error means (what is RTNETLINK for a start), and what are the common/known causes of this error, so that I can solve it for myself this and next time.

# ip route add .....
RTNETLINK answers: No such process

Solution 1:

Rtnetlink allows the kernel's routing tables to be read and altered. It is used within the kernel to communicate between various subsystems, and for communication with user-space programs. Source

The most common cause for the message I've seen is that you're trying to route via a path that is not known to the host. For example, LAN is 192.168.56.100 and WAN is 10.0.3.15. ip route add 8.8.8.8/32 via 192.168.6.1 results in RTNETLINK answers: No such process because 192.168.6.1 is not known to the host.