Comment 3 for bug 58071

I found a work-a-round for those who do not wish to use ipv6.

First disable net-pf-10 in /etc/modprobe.d/aliases by changing:

"alias net-pf-10"
to
"alias ipv6 off"
"alias net-pf-10 off"
"alias net-pf-10 ipv6 off"

and then add "blacklist" and "ipv6" to /etc/modprobe.d/blacklist

You will also need to remove or comment out all ipv6 addresses in /etc/hosts.

After that you can reboot(or possibly even restart your network, though I did not test that).

When you computer is back up type the following command in console:
ip a | grep inet6
If nothing shows up then you should be good to go.

After doing everything above, XDMCP started working on IPv4 again and as a positive side effect, I could use SCP again(didn't work before either, because SSH was listening on tcp6).

In conclusion, this doesn't look like a KDM specific problem.

Regards,
Jesse