Linux - how to set a default route with 0.0.0.0 as gateway

Question

I have a problem where I need to manually set up a route on Ubuntu 18.04 during the init (ram) process. I cannot use any of the new networking tools, only Busybox is available.

The routing table should looks like this:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.31.1.1      0.0.0.0         UG    100    0        0 ens3
172.31.1.1      0.0.0.0         255.255.255.255 UH    100    0        0 ens3

The host is a VPS with an IP of 95.xx.xx.xx, so the gateway IP is not on the same network. I can't figure out what command to build the above table with. I get various SIOCADDDRT errors with everything I try.

Here's what I've tried so far:

> route add default gw 172.31.1.1
SIOCADDDRT: Network uncreachable
> route add default gw 172.31.1.1 via 0.0.0.0 
..invalid format
> route add 172.31.1.1 gw 0.0.0.0
SIOCADDDRT: Invalid argument
> route add 0.0.0.0 gw 172.31.1.1
SIOCADDDRT: Network uncreachable
> route add default gw 0.0.0.0
SIOCADDDRT: Invalid argument

Is there any way to make this work with Busybox? Or any other way to make it happen inside initram?

Solution

On your desired output in flags, 'G' indicates a gateway, and 'H' represents a host. Hence for the 0.0.0.0 output you are trying to create for the route, its not really used.

To add a host IP to the interface that will generate the same output:

ip addr add 172.31.1.1/32 dev ens3