root@:~# ip rule
0: from all lookup local
32766: from all lookup main
32767: from all lookup default
root@:~# ip rule add prio 100 fwmark 1 lookup 1
root@:~# ip rule add prio 100 fwmark 2 lookup 1
root@:~# ip rule add prio 100 fwmark 2361 lookup 1
root@:~# ip rule
0: from all lookup local
100: from all fwmark 0x439894 lookup 1
100: from all fwmark 0x439894 lookup 1
100: from all fwmark 0x439894 lookup 1
32766: from all lookup main
32767: from all lookup default
root@:~# /bin/busybox ip rule add prio 100 fwmark 1 lookup 1
root@:~# /bin/busybox ip rule
0: from all lookup local
100: from all fwmark 0x1 lookup 1
32766: from all lookup main
32767: from all lookup default
It is possibly something similar incompatibility between iproute and kernel version, that cwbsw noticed in the other thread
Maybe the recent upgrade of iproute to 4.19 was too early. "ip rule add sport" can not work as expected