Supermicro iKVM not working when using HAProxy

I have a X10SLH-F board which supports HTML5 based iKVM. When I use the DHCP reserved IP address to access the IPMI, the iKVM works as expected.

I also have an Opnsense router and I have set up SSL offloading on HAProxy and am using HTTPS for all my internal sites via a Lets Encrypt wildcard cert. When I use x10slh.mydomain.com, I can access the IPMI page, and also do remote starts, bios updates etc. However when I try to launch iKVM, it always fails with the error

Server disconnected (code 1006)

Searching for the error only indicated that the iKVM needs to be reset or re-installed which I have already done but none of it has helped. I feel that this is more of a firewall issue. Would anyone be able to point out what firewall rules or HAProxy conditions/rules need to be setup in order to get this working?

Thanks

Unless someone can confirm otherwise, I believe the HTML5 iKVM uses the same ports/methods for actually connecting to the KVM functionality as the Java client - TCP/5900 aka the common port for VNC (not really a surprise because VNC is an well known open protocol, they just extended it a bit with some extra functions). You can see this on https://www.supermicro.com/manuals/other/IPMI_Users_Guide.pdf Page 3-1 (155 of 195 in the PDF).

Therefore my next step would be to search whether HAProxy can work with VNC. I’ll leave that step to you.