Floating IPs usage on Digital Ocean

Question

I am looking for a basic thing yet I have not found not even a single good documentation on getting it done.

I want to allocate a floating IP, then associate it to a network interface of a droplet other than eth0. The reason is I want to have the ability to very easily switch from one IP to the other with a programming language.

In a few words, I want to be able to do these two commands and both should provide a different response.

curl --interface eth0 https://icanhazip.com
curl --interface eth1 https://icanhazip.com

Also, I want to know what to do once I release the Floating IP, how do I roll back to the starting point.

All documentation I read, rely heavily on "ip route" and "route", most did not even work, some worked but replaced completely the old IP by the floating and that's not what I want, and also they did not show how to rollback the introduced configuration changes.

Please help, I spent 1 whole day now trying to get this to work for a project, and no results so far.

I guess there is no need to know DigitalOcean, how to make this work on other Cloud Providers would apply here too I think.

Update

After asking this on DigitalOcean community forum (https://www.digitalocean.com/community/questions/clear-guide-on-outbound-network-through-floating-ip), they claim that is not supported, although there may be some solutions to this if somebody can provide such a "hacky" solution I would take it too. Thanks

Answer 1

In the cloud (AWS. GCP etc.) ARP is emulated by the virtual network layer, meaning that only IPs assigned to VMs by the cloud platform can be resolved. Most of the L2 failover protocols do break for that reason. Even if ARP worked,the IP allocation process for these IPs (often called “floating IPs”) would not integrate with the virtual network in a standard way, so your OS can't just "grab" the IP using ARP and route the packets to itself.

I have not personally done this on Digital Ocean, but I assume that you can call the cloud's proprietary API to do this functionality if you would like to go this route.

See this link on GCP about floating IPs and their implementation. Hope this is helpful.

Here's an idea that needs to be tested:

• Let's say you have Node1(10.1.1.1/24) and Node2(10.1.1.2/24)
• Create a loopback interface on both VMs and set the same IP address for both like (10.2.1.1/32)
• Start a heartbeat send/receive between them
• When NodeA starts it automatically makes an API call to create a route for 10.2.1.1/32 and points to itself with preference 2
• When NodeB starts it automatically makes an API call to create a route for 10.2.1.1/32 and points to itself with preference 1
• The nodes could monitor each other to withdraw the static routes if the other fails. Ideally you would need a 3rd node to reach quorum and prevent split brain scenarios, but you get the idea right?