How to use XDMCP to create a login prompt for VNC sessions

The basic process for giving yourself a login prompt to a Linux box via vncviewer is:

Install the vncserver package for your distribution
Find out which display manager you are running and enable XDMCP remote logins
Test to make sure your XDMCP is working
Add connection details to /etc/services
Using either inetd or xinetd configure an Xvnc session on the ports specified in step 3.
Make it secure

1. Make sure you have the vncserver package for you Linux distribution

This may be any of the following

vncserver

vnc4server

tighvncserver

or other.

Sometimes you have to try different VNC server packages until one works

2. Find out if you are using a display manager or not

ps -ef | grep dm /usr/bin/gdm-binary -nodaemon

Depending on what desktop environment you are runing this could be gdm | xdm | kdm enabling XDMCP on each display is covered in the next few sections

Enabling XDMCP on KDE/KDM (Mandrake 9.1)

vi /etc/sysconfig/desktop

# add DISPLAYMANAGER

DESKTOP=KDE
DISPLAYMANAGER=KDM

vi "/etc/X11/xdm/Xaccess"

# add a line with 'localhost' as the only entry
# delete the `*' this will stop it accepting Xdmcp logins from any host

localhost

vi /usr/share/config/kdm/kdmrc

# Enable Xdmcp
[Xdmcp]
Enable=true

Enabling XDMCP on GDM (on RedHat 9.0 or any gdm based system)

gdmsetup

# enter root password
# make sure the Enable XDMCP option is enabled

Change the gdm.conf file to read as follows
[xdmcp]
# Enable=false change it to true
Enable=true

On CentOS 5.2 gdmsetup will look as follows

Enable plain Style login

Uncheck Honor indirect request leave the rest as the defaults.

XDM Setup

/etc/X11/xdm/Xaccess

* #any host can get a login window

/etc/X11/xdm/xdm-config

! SECURITY: do not listen for XDMCP or Chooser requests
! Comment out this line if you want to manage X terminals with xdm
! DisplayManager.requestPort: 0

3. Testing to see if your XDMCP is working

you may have to either reboot or just run telinit 3 then telinit 5 to bring the display manager down then up to get XDMCP working.

# make sure you have the XFree86-Xnest package installed then run
Xnest :3 -query localhost # should produce a login window as follows

4. Editing the /etc/services File

# add this to /etc/services

vnc-800x600 5901/tcp # vnc 800x600 session
vnc-1024x768 5902/tcp # vnc 1024x768 session

5. Editing xinetd.d or inetd.conf configuration

xinetd.d config

# create two files for xinet.d to handle launching the Xvnc server in /etc/xinetd.d/

# note that the name in /etc/services does not dictate what size of screen but is simply a description. You need to edit the -geometry parameter to get the screen size you want.

service vnc-800x600

{
        disable = no
        socket_type     = stream
        protocol        = tcp
        wait            = no
        user            = nobody
        server          = /usr/X11R6/bin/Xvnc
        server_args     = :1 -inetd -query localhost -geometry 800x600 -depth 16 -once
        # note on some brands of linux the server value will be /usr/bin/Xvnc (check yours for the correct value)
        # also the server_args value may need -securitytypes=none to stop the remote vncviewer being prompted for a 
        # vnc password instead of the GDM/XDM login prompt
}
service vnc-1024x768
{
        disable = no
        socket_type     = stream
        protocol        = tcp
        wait            = no
        user            = nobody
        server          = /usr/X11R6/bin/Xvnc
        server_args     = :2 -inetd -query localhost -geometry 1024x768 -depth 16 -once
        # note on some brands of linux the server value will be /usr/bin/Xvnc
        # also the server_args value may need -securitytypes=none to stop the remote vncviewer being prompted for a 
        # vnc password instead of the GDM/XDM login prompt
}

I have recently tried this on CentOS 5.2 with this xinet.d config

service vnc-800x600 { disable = no socket_type = stream protocol = tcp wait = no user = nobody server = /usr/bin/Xvnc server_args = -inetd -query localhost -geometry 800x600 -depth 16 -once -securitytypes=none }

service vnc-1024x768 { disable = no socket_type = stream protocol = tcp wait = no user = nobody server = /usr/bin/Xvnc server_args = -inetd -query localhost -geometry 1024x768 -depth 16 -once -securitytypes=none }

Ubuntu inetd.conf configuration (tested on Dapper 6.06)

/etc/inetd.conf

vnc-800x600 stream tcp nowait nobody /usr/sbin/tcpd \
/usr/bin/Xvnc :21 -rfbport 5901 -inetd -query localhost -geometry 800x600 -depth 16 -once
vnc-1024x768 stream tcp nowait nobody /usr/sbin/tcpd \
/usr/bin/Xvnc :22 -rfbport 5902 -inetd -query localhost -geometry 1024x768 -depth 16 -once

Note: If you specify a Xvnc server port eg :21 then your vnc port listen port will be 5921 and when launching via inetd then the vnc server will be listening on the wrong port. To make the inetd setup work you need to force Xvnc to use the same port as the one you specify in /etc/services passing the “-rfbport” <portnumber> parameter fixes this i.e. -rfbport 5902

Restart xinetd

# run this to get xinet.d to reread it's config
/etc/rc.d/init.d/xinet.d reload
or
service xinetd reload

# restart X or telinit 3 and then telinit 5 or kill -SIGHUP pid of gdm

6. Adding Firewall Rules

# add some firewall rules to stop connections from anywhere except localhost

iptables -A INPUT -s 127.0.0.1 --dport 177 -j ACCEPT
iptables -A INPUT -s 0/0 --dport 177 -j DENY

Troubleshooting

I recently installed on Debian and had trouble until I did an
apt-get install xfs