Activate 177 port for Windows X-manager s/w .

For Linux Environment :
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XDMCP For Redhat 4 or 5:

In Red Hat® Enterprise Linux® 4 or 5 , Edit the file '/etc/gdm/custom.conf' could be modified to enable xdmcp. where xdmcp enable the port 177 .

There are two ways that xdmcp can be enabled:
Modify the file /etc/gdm/custom.conf and under the [xdmcp] section, add this line: 'Enable=true'

Alternately, the tool "gdmsetup" can be used. This is a gui-based tool to modify gdm.
Once one of these two steps are done - simply run "gdm-restart" command from gui terminal
and then login as desired user.

cat /etc/gdm/custom.conf
[xdmcp]
Enable=true

For AIX 5.3/6.1 Environment :
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In AIX xdmcp(177) port is not working , Then need to run the Following script :
sh /etc/rc.dt