Does AirPort Extreme support XDMCP on wireless?

It is not something I have seen requested before or people having issues with.

But it doesn't surprise me. what model AE do you have with what firmware??

Can you test wireless to wireless? Often the issue is when you go wireless to ethernet or visa versa.. since they are different connections and packets need to be passed correctly by the firmware.. whereas ethernet - ethernet is local switch without involvement from the firmware.

Opening ports is not relevant.. that is WAN to LAN. What your problem is, is more protocol is not recognised correctly. This is much harder to correct because the firmware is written by Apple for a box designed by Apple for Apple with only Apple in mind. It is NetBSD v4 if that is of any interest. And you can get a serial console onto the Apple (with some tricky soldering).. if you can make heads or tails of the results.. if you are deeply into linux you might be.

Can you put two computers on the wireless and see if the XDCMP is passed correct with two computers on wireless.. if so I think you can say pretty positively the issue is the Apple firmware. And you can send Apple a request to fix it.. I would expect the priority to fix the fault to be about minus 6 on the scale of 1-10..

BUT I always recommend earlier firmware. If you have a Gen5 or earlier extreme.. go back to 7.6.1 or even 7.5.2 firmware.. they seem to handle ports that Apple doesn't recognise better than the later two.

it's model A1521 running 7.7.1.

The AE is new AC version hardware and is pretty much a whole new field of pain. After 5 revisions of the old hardware Apple almost had it right.

Apple of course could never use OEM linux firmware so they have to rewrite it from scratch in NetBSD so they don't have to release the source code.

Your two firmwares back and running insecure version.. so you should at least upgrade to 7.7.3 which is one way street.. you cannot go back.. but since it doesn't work on 7.7.1 it will not work, equally well, on 7.7.3. So you see how egalitarian it all is.

I think about another 2 hardware revisions and 3 or 4 firmware revisions and it will be really good.

In the meantime replace the AE as your main router.. grab something cheap like Netgear WNDR3800 which is a really excellent router.. load it with OpenWRT or Gargoyle firmware and try again with your Linux XDCMP and I think you will find it has a better hope of working on a straight linux firmware. You can keep using the AE bridged for wireless. It is much better than the Netgear as a Wireless AP. Meanwhile Gargoyle in particular will give you excellent QoS even throttling and quotas, plus things like built in vpn server, decent firewall controls.. decent parental controls. Excellent network analysis. Load of stuff really. I cannot give you URL as my post will then be deleted.. but a short google search will get you what you need.