Hello,
I have two DSL in family. One is main, another is backup.
When the main DSL get outage, can debian system choose backup DSL as gateway and keep internet not disconnected? If so, how to deploy it?
You need a Router with two WAN ports! On a router I would configure two Static Routes one default router for the main DSL and one for the backup DSL that is weighted to a lower priority. This is known as surprisingly Weighted Static Routes.
What are you currently using as a WAN router? You can use Debian as a router! You can configure two default Gateway's with the primary using a metric of 600 and the backup with a metric of 700. This will require two NICs.
I do not recommend connecting to the Internet without at-least a WAN State-full Packet Inspection SPI Firewall Router.
Tim
Thanks.
Yong