Just installed a new MC 1.11.2 server on Ubuntu. Flying is turned on in server.properties, and the mod works normally when I play in my SP worlds. When playing on the MP server, though, I take damage whenever I land. The damage seems to track with my highest altitude minus landing altitude.

I assume you are talking about Simply Hax? Well, all it does is tell a client "yes this is creative mode you can totally fly". If the server imposes fall damage on it's lonesome i dont think a client-only mod can change that.

Yes, Simply Hax. I am unfamiliar with Minecraft modding, so I'm not sure where the line is between server and client. Your point sounds reasonable...do you know how I can configure the server to not inflict damage on landing, in the case of flying?

This thread has some suggestions for how to cancel damage. Is that applicable to this problem?

It's not that i don't know how to fix it, i do. The solution would be to run Simply Hax as a server and clientside mod both. But that's against the idea of Simply Hax. I could make an optional serverside similar to how Finder Compass works but i am too busy currently to do any new development. Or do much of anything really.

Do you have any suggestions on how to address this while keep the client-side approach? It seems to work just fine in single player..

Not using Simply Hax, unfortunately. You need some mod that has a serverside component to avoid the issue.