There is a problem with nat in at least 2.2.16 and 2.2.18. If you
specify routing first and nat second,
ip route add nat x.x.x.x via y.y.y.y
ip rule add from y.y.y.y nat x.x.x.x
the rule doesnt have an effect, ping to x.x.x.x says it got response
from y.y.y.y. With turned order of commands,
ip rule ...
ip route ...
it works.