Routing iproute2, 2 gateways and 2 network interfaces

https://johnyuan2000.hatenadiary.org/entry/20120420/1334914291

https://www.thomas-krenn.com/en/wiki/Two_Default_Gateways_on_One_System

https://www.adampalmer.me/iodigitalsec/2014/10/05/linux-iproute2-multiple-default-gateways/

https://serverfault.com/questions/872818/routing-issue-with-iproute2-two-gateways-and-two-network-interfaces

auto eno0
iface eno0 inet static
    address aaa.aaa.127.18
    netmask 255.255.255.252
    gateway aaa.aaa.127.17

auto eno1
iface eno1 inet static
    address 172.20.4.1
    netmask 255.255.255.252
    post-up ip route add bbb.bbb.137.0/24 via 172.20.4.2
    post-up ip route add 10.0.0.0/8 via 172.20.4.2
    post-up ip route add 192.168.0.0/16 via 172.20.4.2
    post-up ip route add 172.16.0.0/12 via 172.20.4.2