What is MaxDisplayNumber sesman option?

[Mogaba]

Hello,

I upgraded xrdp from v0.9.24 to v0.10.1 and found out there's a new sesman option MaxDisplayNumber. It seems it affects the maximum number of sessions like MaxSessions. The default value 63 doesn't allow more than 54 sessions. After 54 is reached, any attempt to log in results in an error: X server -- no display in range (10 to 63) is available.
I couldn't find any info on that except the description in sesman.ini:

IANA only allocates TCP ports up to 6063 for X servers. If you are not
allowing TCP connections to your X servers you may safely increase this
number.

I don't fully understand how it works and if increasing the value is OK. I set it to 110 because I have about 90 users that need to connect to the server. Could someone please explain this option (especially the part about not allowing TCP connections to X server)?

[matt335672]

I'll have a go at explaining it!

On Unix/Linux, there are two ways to connect to an X server:-

1. Over a local socket (Unix Domain Socket)
2. Over a TCP socket.

Here's the relevant section from Xorg(1):-

NETWORK CONNECTIONS
       Xorg  supports  connections  made  using  the  following reliable byte-
       streams:

       Local
           On most platforms, the "Local" connection  type  is  a  UNIX-domain
           socket.   On  some System V platforms, the "local" connection types
           also include STREAMS pipes, named pipes, and some other mechanisms.
           See the "LOCAL CONNECTIONS" section of X(7) for details.

       TCP/IP
           Xorg listens on port 6000+n, where n is the display  number.   This
           connection  type is usually disabled by default, but may be enabled
           with the -listen option (see the Xserver(1) man page for details).

The X server can listen on both network connection types at once, if that's what you want.

If you're not using the -listen tcp option on your Xorg instances, you're not using TCP to connect to your X servers, and you can safely increase MaxDisplayNumber

If you are using the -listen tcp option, the MaxDisplayNumber value is important to keep the TCP ports used by the X server in the allowable range of 6000-6063.

Is that clear? I'm happy to provide more information if I've not explained it well enough.

[Mogaba]

Thanks for the explanation, I checked my Xorg processes:

Xorg :73 -auth .Xauthority -config xrdp/xorg.conf -noreset -nolisten tcp -logfile .xorgxrdp.%s.log

-nolisten tcp - this means everything's fine, right?

[matt335672]

Definitely.