Linux – Creating a multi-wan/isp failover router where wan IPs are dynamic on Debian

debianlinuxnetworkingrouterrouting

I'm having an incredibly hard time creating a basic router on Debian which has connections to two ISPs where the ip addresses are dynamic. I don't need bonding, I really just need some way to detect when WAN1 goes down and automatically failover to WAN2 (or reverse)

I have seen a lot of documentation on how to do this when you have static IPs/Gateways but not if IPs/Gateways are set via DHCP.

I don't really have any code to show, but in general it would start with something like this, I'm assuming:

/etc/network/interface

#loopback,etc 
#LAN
auto eth0 
iface eth0 inet static
address 192.168.1.1
netmast 255.255.255.0
network 192.168.1.255

#WAN1 / ISP1
auto eth3
iface eth3 inet dhcp
up #add iptables/nat/routing?
down #??

#WAN2 / ISP2
auto eth4
iface eth4 inet dhcp
up #add iptables/nat/routing?
down #???

Thank you for any help, or pointers!

Best Answer

One solution would be to make a script that does the following:

  1. Fetch network details of the WAN interfaces.
  2. Test their connectivity with ICMP probes by binding the probes to their appropriate interface (-I flag on linux ping command)
  3. Depending on connectivity status act accordingly (if primary link is down, switch-over to secondary)

The above can be accomplished with a simple shell script.

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