I have a dialup Internet connection, thru an interface (say eth0) which provides a ppp0 interface when set up using pppoeconf. I have another Ethernet interface (say eth1) which is connected to another ISP. In addition to these two interfaces, I have a third interface that connects to other machines on the same network. These other machines have their default gateway set to this server. Both the ISPs give a set of static IPs.
I have iptables rules for NAT and a Nginx reverse proxy but that may be irrelevant for the question.
Initially the websites we host have DNS A address set to static IP offered by the non-ppp ISP. My goal is to check if this interface goes down for any reason, and if so, use the ppp-ISP to dynamically change the DNS A record of the websites to point to the static IP provided by the ppp-ISP.
Initially both ISPs are functioning and all interfaces are up.
My initial question is: What should the default route be set to when both ISPs are functioning? Or should the ppp-ISP/interface itself be brought down until the non-ppp interfaces goes down? Any other issue that I need to address?
