How to selectively route traffic over openVPN on Openwrt

Here is the scenario: you have an openwrt router and an openvpn client configuration. You want to selectively route your traffic over openvpn, or expose your openvpn connection as a socks proxy everyone can use over the local network.

Install luci-app-openvpn and openvpn-openssl packages and add your openvpn configuration. For ProtonVPN openvpn configuration follow this guide: https://protonvpn.com/support/how-to-set-up-protonvpn-on-openwrt-routers

Do not modify firewall rules. Skip that step.

Add these options to your openvpn configuration:

route-nopull
pull-filter ignore "redirect-gateway"

Add an unmanaged network interface from luci. For name use anything you like, for example "openvpn". For device, choose tun0. Save and apply.

Connect to openvpn from luci.

Install Passwall2 and singbox on Openwrt.

In Passwall2 add a singbox node and select custom interface and then tun0.

This node works as any normal proxy node in Passwall and you can expose it as SOCKS proxy or route your entire traffic over it.