Reverse Path Forwarding (RPF) as defined in RFC 3704 specifies 3 modes: 'strict' (drop traffic if the ingress interface does not have the best egress route to the source), 'feasible' (drop traffic if the ingress interface has no egress route to the source), and 'loose' (drop traffic if the entire system has no egress route to the source).

VyOS currently only implements the 'strict' and 'loose' modes:

set firewall source-validation 'strict'

and

set firewall source-validation 'loose'

I would like to see 'feasible' mode added as a third option, as it is a very pragmatic compromise between the existing 'strict' and 'loose' modes.