Documentation should explicitly mention that xdmcp is likely not enabled by distribution
By default GDM only enables XDMCP if tcp_wrappers is explicitly enabled. It's not enough anymore to just have the right dependencies. A user filed a bugreport with Mageia regarding the lack of XDMCP support. The documentation explains that perhaps XDMCP/tcp_wrappers might not be enabled. However, that comment is pretty far into the document. Further, it seems to be about the tcp_wrappers. It would be nicer if the entire section (5.4.6) started with a warning box saying something like:
XDMCP support needs to be explicitly be enabled by your distribution. For XDMCP to work it needs to be enabled in the package as well as in the GDM configuration file.
Secondly, the following is incorrect:
If GDM is compiled to support it, access from remote displays can be controlled using the TCP Wrappers library. The service name is gdm
Per the meson.build file, unless tcp_wrappers is enabled you wouldn't even have XDMCP. Meaning: it should say something like
If GDM is compiled to support XDMCP, access from remote displays can be controlled using the TCP Wrappers library. The service name is gdm
alternatively, maybe just get rid of the remark/sentence?
See https://help.gnome.org/admin/gdm/3.6/configuration.html.en#xdmcpsection and https://bugs.mageia.org/show_bug.cgi?id=27210 for the Mageia report.
Alternatively, maybe find a replacement for tcp_wrappers (some systemd thing)?