Systems utilizing gdm (or gdm-binary) through an XDMCP daemon may see a message which states that the session was declined because the maximum number of open sessions from your host was reached.
gdm limits the number of XDMCP sessions which can connect from a single IP address. Some Linux distributions limit the number to 2 connections. In this case, you will not be able to open more than 2 XDMCP sessions to the same Linux machine from the same PC.
If you attempt more than 2 sessions, the XDCMP daemon will block the connection and you will see an error message stating the maximum number of open sessions from your host was reached.
To change the limits of the number of connections allowed, please use the following instructions:
1. Edit the /etc/gdm/custom.conf or gdm.conf file.
DisplaysPerHost=10 (<=enter the desired limit)
2. Restart the top-level process within the gdm(or gdm-binary) processes.
#ps -aef |grep dm |grep -v grep
root 20483 1 0 16:01 ? 00:00:00 /usr/sbin/gdm-binary -nodaemon
root 24855 20483 0 23:04 ? 00:00:00 /usr/sbin/gdm-binary -nodaemon
# kill -1 20483