How to make failover from two providers on linux?

Question

linux

There are two lines from two different providers connected to one PC with three network cards.
It is necessary to make Internet fault tolerance through two network cards, if the first provider falls off - the second one turns on and when the first one rises - it is again in priority. The same PC then has to distribute the Internet to the local network through the third network card. That is, it will have dhcp, dns, nat...
All this must be done on linux ubuntu/debian.
Now such a scheme works on mikrotik. But now you need to do the same on the PC.
pfsense - not suitable, maybe there are analogues for / under linux? Or a ready OS or some solutions.

Answer 1

Assign a smaller metric to the first network interface, and a larger one to the second. Both interfaces must have gateways for 0.0.0.0.
For NAT use -j MASQUERADE.
This is from automatic routing.
Or write a script that will check the availability of the default gateway of interface 1 and, depending on this, manage routes.

Answer 2

it's called BONDING
https://www.cyberciti.biz/faq/howto-configure-dual...