Curl only uses the specified OpenVPN interface if it is run as root

I am using Ubuntu Server 18.04.1 LTS (Bionic Beaver). I run OpenVPN with the redirection gateway disabled because I do not want traffic to use it by default, only a specific application. This is the command that I use:

sudo openvpn --config / etc / openvpn / CA  Montreal.ovpn --pull-filter ignore redirect-gateway --daemon

And then if I use the following command:

curl --interface tun0

I get my real external IP address, not my VPN address. But if I execute the command:

sudo curl --interface tun0

Then I get my VPN IP address. I am also experiencing this problem in other programs, such as traceroute. How do I fix it so that a program does not need root to go through the VPN?