[root@mvp gdm]# cat /etc/X11/xdm/Xaccess 
# $XdotOrg: Xaccess,v 1.3 2000/08/17 19:54:17 cpqbld Exp $
# $Xorg: Xaccess,v 1.3 2000/08/17 19:54:17 cpqbld Exp $
# $XFree86: xc/programs/xdm/config/Xaccess,v 1.4 2003/07/09 15:27:40 tsi Exp $
#
# Access control file for XDMCP connections   #xdmcp訪問控制
#
# To control Direct and Broadcast access:
#
#    pattern
#
# To control Indirect queries:
#
#     pattern        list of hostnames and/or macros ...
#
# To use the chooser:
#
#    pattern        CHOOSER BROADCAST
#
# or
#
#    pattern        CHOOSER list of hostnames and/or macros ...
#
# To define macros:
#
#       %name        list of hosts ...
#
# To control which addresses xdm listens for requests on:
#
#    LISTEN        address [list of multicast groups ... ]
#
# The first form tells xdm which displays to respond to itself.
# The second form tells xdm to forward indirect queries from hosts matching
# the specified pattern to the indicated list of hosts.
# The third form tells xdm to handle indirect queries using the chooser;
# the chooser is directed to send its own queries out via the broadcast
# address and display the results on the terminal.
# The fourth form is similar to the third, except instead of using the
# broadcast address, it sends DirectQuerys to each of the hosts in the list
# The fifth form tells xdm which addresses to listen for incoming connections
# on.  If present, xdm will only listen for connections on the specified 
# interfaces and/or multicast groups.
#
# In all cases, xdm uses the first entry which matches the terminal;
# for IndirectQuery messages only entries with right hand sides can
# match, for Direct and Broadcast Query messages, only entries without
# right hand sides can match.
#

*                    #any host can get a login window  任何主機都可以登錄

#
# To hardwire a specific terminal to a specific host, you can
# leave the terminal sending indirect queries to this host, and
# use an entry of the form:
#

#terminal-a    host-a


#
# The nicest way to run the chooser is to just ask it to broadcast
# requests to the network - that way new hosts show up automatically.
# Sometimes, however, the chooser can't figure out how to broadcast,
# so this may not work in all environments.
#

#*        CHOOSER BROADCAST    #any indirect host can get a chooser

#
# If you'd prefer to configure the set of hosts each terminal sees,
# then just uncomment these lines (and comment the CHOOSER line above)
# and edit the %hostlist line as appropriate
#

#%hostlist    host-a host-b

#*        CHOOSER %hostlist    #

#
# If you have a machine with multiple network interfaces or IP addresses
# you can control which interfaces accept XDMCP packets by listing a LISTEN
# line for each interface you want to listen on.  You can additionally list
# one or more multicast groups after each address to listen on those groups
# on that address.
#
# If no LISTEN is specified, the default is the same as "LISTEN *" - listen on
# all unicast interfaces, but not for multicast packets.  If any LISTEN lines
# are specified, then only the listed interfaces will be listened on.
#
# IANA has assigned FF0X:0:0:0:0:0:0:12B as the permanently assigned 
# multicast addresses for XDMCP, where X in the prefix may be replaced
# by any valid scope identifier, such as 1 for Node-Local, 2 for Link-Local,
# 5 for Site-Local, and so on.  The default is equivalent to the example shown
# here using the Link-Local version to most closely match the old IPv4 subnet
# broadcast behavior.
#
# LISTEN        * ff02:0:0:0:0:0:0:12b

# This example shows listening for multicast on all scopes up to site-local
#
# LISTEN    * ff01:0:0:0:0:0:0:12b ff02:0:0:0:0:0:0:12b ff03:0:0:0:0:0:0:12b ff04:0:0:0:0
:0:0:12b ff05:0:0:0:0:0:0:12b

[root@mvp gdm]# cat /etc/gdm/custom.conf 
# GDM configuration storage

[daemon]

[security]
AllowRemoteRoot=true   #許可root遠程登錄
[xdmcp]
Enable=true 
Port=177
[greeter]

[chooser]

[debug]

[root@mvp xdm]# lsof -i:177
COMMAND    PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
gdm-binar 3780 root   11u  IPv4  26854      0t0  UDP *:xdmcp