How to redirect to an available IP?

Question

Computer networks

Good afternoon. Faced such a problem - on the server side there is software through which you can drive in an IP or a domain where it will knock, on the other side a router with 1 wired Internet with a static IP and 2 Usb 3g modem also with a static IP

The essence of the problem - you need to from the server it was knocked on an available Ip, that is, if the Internet suddenly disappears over the wire, then it automatically switched to a 3G modem.
I couldn't find any solutions on google, maybe I'm looking in the wrong place!

Answer 1

If I understand you correctly, the server needs to be able to access a resource that is accessible via two globally routable IP addresses. The proposed solution is shown in the diagram:
The bottom line is that we need one more server/vps with a globally routable ("white") IP address. In the diagram, the address is specified as CCCC On this server, you need to install a load balancer that will monitor the availability of the target computer through different addresses (AAAA, BBBB) and accordingly proxy connections/datagrams (if TCP/UDP protocol is used). I implemented a similar scheme using an "iron" load balancer (F5 LTM), but, as far as I know, free haproxy will help in your case. In addition, on the routers closest to the target computer, port-forwarding must be configured accordingly.
Another solution could be a script on the server that monitors the availability of AAAA/BBBB addresses and changes the static DNS records on the server accordingly. The program will need to specify the target host by specifying a name (hostname). Then, in different situations (in terms of the health of the channels), this name will be translated differently into an IPv4 address.