Getting NAT working for Internet Local Apps under LTSP

[John Hupp]

As I understand it, the Local Apps settings are enabled by default in 
Lubuntu 12.04.  Once LTSP is also installed, that means that one can run 
on the LTSP client hardware (instead of the LTSP server hardware) any 
app that is installed in the LTSP chroot image.  For instance, xterm can 
be run on the client via "ltsp-localapps xterm" (ltsp-localapps being 
the script command by which an app must be launched locally).

To run an additional app locally, just install it in the chroot and 
update the image:
     sudo chroot /opt/ltsp/i386
     sudo apt-get install <app-of-your-choice>
     sudo ltsp-update-image

HOWEVER, Internet apps will launch but not work at this point due to 
Name Resolution failure.  So the LTSP server has to be set up as a NAT 
gateway.

As far as I know, the most relevant instruction for doing that is here: 
https://help.ubuntu.com/community/UbuntuLTSP/ThinClientHowtoNAT/

But I have not gotten that working, so I want to summarize the steps of 
that instruction here and insert comments about my own implementation.  
I trust someone will spot the problem.

** Edit /etc/ltsp/dhcpd.conf (sudo gedit /etc/ltsp/dhcpd.conf) setting 
the routers option to the ip address of the ltsp server on the ltsp 
network. eg
    option routers 192.168.0.254;
and restart the dhcp server
    sudo /etc/init.d/dhcp3-server restart
ON 11.10 restart the dhcp server
    sudo /etc/init.d/ish-dhcp-server restart
Test: Reboot the test PC on the thin client network and check if it sees 
the new default gateway (on linux type ip route and look at the line 
beginning "default"; on windows type "route print" into a command prompt 
and look for default gateway).

[COMMENT: I understand that LTSP servers have variously been set up with 
default 192.168.0.254 or 192.168.0.1, but that now people are pressing 
for a uniform default of 192.168.0.1, which is the way mine is set up.  
I found dhcpd.conf already had "option routers 192.168.0.1" enabled.  
And my test PC did indeed show a default gateway of 192.168.0.1.]

** On the LTSP server edit /etc/sysctl.conf (sudo gedit 
/etc/sysctl.conf) and either add this line or uncomment if it's already 
there:
    net.ipv4.ip_forward=1
which will take effect at next server reboot. To make the setting 
effective immediately, now run the command
    sudo sysctl -w net.ipv4.ip_forward=1

[COMMENT: I only had to un-comment the line.]

** Enable network address translation. By default the ltsp network 
address/mask is 192.168.0.0/24. Remember to change the example to fit 
your network setup. Use a command like:
    sudo iptables --table nat --append POSTROUTING --jump MASQUERADE  
--source 192.168.0.0/24
to enable NAT. To make this setting permanent run
    sudo sh -c 'iptables-save > /etc/ltsp/nat'
and add the extra line up iptables-restore < /etc/ltsp/nat to the bottom 
of the eth0 (or whichever is your LTSP interface) stanza of 
/etc/network/interfaces so it looks something like
     auto eth0
     iface eth0 inet static
             address 192.168.0.254
             netmask 255.255.255.0
             broadcast 192.168.0.255
             network 192.168.0.0
             up iptables-restore < /etc/ltsp/nat
This creates a script called /etc/ltsp/nat which restores the NAT next 
time the eth0 network interface comes up. Test: ping the dns server on 
the main network from the test PC.

[COMMENT: I used the first two commands as written.  My network is the 
default 192.168.0.0/24.  And since my LTSP interface it eth1, I added 
the following to interfaces:
     auto eth1
     iface eth1 inet static
             address 192.168.0.1
             netmask 255.255.255.0
             broadcast 192.168.0.255
             network 192.168.0.0
             up iptables-restore < /etc/ltsp/nat
But I thought it of interest that neither eth0 nor eth1 was initially 
configured via /etc/network/interfaces.  Lubuntu seems to prefer 
Preferences: Network Connections, a nice GUI, and that is what I used to 
set up eth1 for LTSP.  But here, I didn't know how to use Network 
Connections to add "up iptables-restore < /etc/ltsp/nat."  So I hoped 
editing interfaces as instructed would do the job.]

** Again, edit /etc/ltsp/dhcpd.conf, as above, setting the dns option to 
the ip address of the main network DNS server (listed in 
/etc/resolv.conf on the LTSP server beside the word "nameserver"), e.g.
    option domain-name-servers 192.168.1.1;
and again restart the dhcp server
    sudo /etc/init.d/dhcp3-server restart
Test: Reboot the test PC again, to update its DNS server. Try pinging a 
real world server from the test PC by name, eg
    ping www.ubuntu.com

[COMMENT: /etc/resolv.conf showed 127.0.0.1 for the nameserver.  Given 
the instruction's example of 192.168.1.1, this address seems unexpected, 
but I tried it and it did not work.  I also tried 192.168.0.1.  Also 
192.168.1.1, the address of the router, which I would have taken to be a 
good bet.  I also set the router to 10.0.0.1/255.255.255.0 and tried 
that for the dhcpd.conf domain-name-server.  (See more on that below.)  
Finally I tried the openDNS servers that are set up in the router: 
208.67.222.222; 208.67.220.220.  Most of my failures simply resulted in 
Name Resolution failures when local app chromium-browser was trying to 
find a site.  But at this point, something in my configuration seems to 
have disabled LTSP DHCP altogether -- the LTSP client does not boot.]

-----------------------------------

MORE GENERAL INFO: My DSL modem is an AT&T that uses a non-configurable 
LAN-side address of 192.168.0.1/255.255.0.0.

Initially I had the router set to 10.0.0.1/255.255.255.0, and computers 
on the main network can ping and open the configuration page for the 
modem.  I was concerned that this would conflict with the LTSP server at 
192.168.0.1, but LTSP clients worked OK -- perhaps because of the 
differing netmasks of 255.255.255.0 vs. 255.255.0.0??

I also tested setting the router at 192.168.1.1/255.255.0.0.  In this 
configuration computers on the main network cannot see the modem at 
192.168.0.1.
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