i have added a route (a default route, actually) to my route table with the "ip route add default ..." command. the route works for about a few minutes then disappears and is no longer in the table. how can i make a manually entered route stay in place until the system goes down?

It depends. Is it a systemd abomination? If yes, persistent routes should be added in the [route] section of systemd-networkd configuration. Read, despair and beware of many bugs which definitely and categorically are present in systemd-networkd handling of static-dynamic route mix. https://wiki.archlinux.org/title/systemd-networkd

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it is Ubuntu 20.04 LTS so it does run systemd.

what is going on.

i have added other routes that are not default routes via my script that starts up my VPN. those routes all work fine. when i reboot, they get added, again, when my script starts the VPN. the route that just disappears after a few minutes is manually added. really manually. i type it in. it works exactly as intended and expected until it disappears in a few minutes. it is manual because it is for testing. i want to test how a certain website behaves when traffic comes from a different IP address.

i want my manual route table entry to stay until i reboot or manually delete it or override it with a lower metric (have done that before and it works). i don't want to have to reconfigure something. it should lose what i manually do with just doing only a reboot.