How to Make IP Forwarding Permanent

routing

I know that I can enable IP forward by echoing 1 to /proc/sys/net/ipv4/ip_forward, but how can I make this permanent?

By the way I want another method rather than start up scripts, is there any?

Edit /etc/sysctl.conf and search for the following lines:

# Uncomment the next line to enable packet forwarding for IPv4
#net.ipv4.ip_forward=1

Uncomment net.ipv4.ip_forward=1:

# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1

Or in one line command :

sudo sysctl -w net.ipv4.ip_forward=1

You can use nmcli to do it, the key is ipv4.never-default:

nmcli connection modify enp3s0 ipv4.never-default true

Di you try this:

To see which is your default gateway, run:ip route.
To delete the current default gateway, run: sudo route delete default gw <IP Address> <Adapter>.
To add a new default gateway, run: sudo route add default gw <IP Address> <Adapter>.

If route is not installed, run: sudo apt install net-tools to install it.
Credits:
How to Add or Change the Default Gateway in Linux