What to do when you have a server behind a natted internet connection whitout public IP and you want to access it from your home or somewhere?
SSH is your best friend. So you can enable a reverse ssh tunnel that allow a connection from above the intranet.
server “S0″ in the intranet —-> Router (without access) —-> (OoOo Internet oOoO) <---- router (my CISCO 803) <---- "C1" remote computer (my.example.cxm)
In the server S0 I use ssh to create a tunnel that allow my C1 to connect it.

On S0
ssh -R 5000:localhost:22 my.example.cxm

On Router CISCO

ip nat inside source static tcp 10.1.1.1 5000 interface Dialer 1 5000

On C1

simply launch: ssh -p 5000 localhost

and you are connected to your server in the office intranet.

What appen if the connection goes down?

we can assure that the tunnel back up again. So we must consider the idea to insert a cron job which check periodically the tunnel status.

next: example shell script

The “problem” is caused by dhcp server that overwrite our resolv.conf script and we lose this personalization during the IP release session. Not even is a good idea to replace the dns configuration proposed by the dhcp server, if it push that to you, probably there is a reason. But some time you want to use your own configuration such as when you use your broadband router and you want your /etc/resolv.conf has been preserved fron updates.

To prevent this nasty situation we can operate in two ways, determined by which dhcp client you are using: dhcpcd ord dhclient.

In the first case, dhcpcd (as in my Slackware laptop), prevent overriding can be obtained simply add the -R option to the dhcpcd command:

# dhcpcd -R eth0

In the second case, we must read the man page about dhclient-script which is invoked any time you use dhclient.
In:

man 8 dhclient-script

at the HOOKS section we can read:

HOOKS

When it starts, the client script first defines a shell function, make_resolv_conf , which is later used to create the /etc/resolv.conf file. To override the default behaviour, redefine this function in the enter hook script.

This means that we must to create /etc/dhclient-enter-hooks and redefine the make_resolv_conf function to satisfy our needs.
If we simply wants to prevent resolv.conf updates only, the fastest way is to redefine the function to do nothing:

# cat /etc/dhclient-enter-hooks
make_resolv_conf() {
exit 0
}

Then save the file and ensure it is executable:

# chmod a+x /etc/dhclient-enter-hooks

Note that, as explained in the man page, the dhclient-script is not standard so if this configuration doesn’t work, please read the man page.

Next: How to merge dns addresses pushed by the dhcp server and my own dns.