Reverse Telnet means that instead of initiating a telnet session from the PortServer into the network, the session will be initiated from the network into one of serial ports.

One popular benefit of this feature is the ability to eliminate multiple terminals that connect to various console ports by simply cabling each of the console ports to the Digi PortServer.

Before starting the reverse telnet set-up, first verify that the correct ip address, submask setting, and firmware of the PortServer or PortServer II is current. To verify, type set config from the PortServer prompt. The information will display on your screen.

Basic PortServer Configuration:

Instructions for basic setup follow. Afterwards, instructions for more advanced setups will be covered.

The setup below assumes that you are going to set up reverse telnet on port 16 (but any port can be used).

Once you have logged in as root, issue the following commands:

#> set config ip=[the_ip_you_want_to_assign_to_the_portserver]

#> boot action=reset (reboots the PortServer)

PortServer Port Configuration:

1. Configure the ports to provide a reverse telnet session. This is done by logging into the PortServer, and typing:

   #> set port range=16 dev=prn auto=off

   The above example will set port 16 to a "Printer or incoming only" setting. The Printer setting is the optimal setting for a reverse telnet session; it puts the port into a "passive" mode, which will allow a session of data to go in "reverse."

2. You may possibly need to set up a baud rate and protocol settings for the reverse telnet port. By default, the ports will be set to 9600. If you need to change the rate, run the following command:

   #> set line range=16 baud=(baud_rate)

3. Depending on the attached device it may be necessary to change the flow control method for the port. This will also involve the necessary cable. By default, the ports will all be set to use xon/xoff flow control. In nearly every case, xon/xoff is fine for reverse telnet situations. Type the following to check flow control settings:

   #> set flow range=16

Testing the Port(s):

Digi usually ships a loopback plug with its products. This plug will look like a telephone jack, without any cable coming out of it. Insert the loopback plug into the port you want to test, in this case, port 16.

Connect to the ports using the following standard:

2001 - 2099 Telnet Connect socket numbers.

2101 - 2199 Raw Connect socket numbers.

In most cases for reverse telnet, you will use the first standard. Think of this as the number 2000 + the port number you want to go to. For example, if you want to connect to Port 16, you would type the following from your server prompt:

telnet [ip_of_the_PortServer] 2016

When you get connected, you probably won’t see anything, although it's possible you would see a "Digi International PortServer" header along with the word "Connected."

With the loopback plug in the port, when you type some characters on your keyboard they should echo back at your screen. This is an indication that everything is working correctly.

The last thing to verify is the cabling from the PortServer port to the console port. Make sure that the cable you are using is a crossover cable. Refer to the link below for appropriate cabling pin-outs.

http://supportold.digi.com/support/techsupport/hardware/portserver/common/cables/

Connect your cable, and make sure your devices "console" port is enabled for login. When you connect to the port again and press Enter, you should get a login prompt from the attached device, whether it be a Sun Sparc server, a Cisco router, or any other serial device.

Advanced Features and Typical Problems:

There are a few common things that users ask how to do, beyond the basic reverse telnet setup.

1. "I don’t want to remember what port has what system. Can I make a menu, or assign each port an ip?"

   Since Digi does not support a "menu" for reverse telnet sessions, the next best thing is to assign an ip for each port. To set up this scenario use the "altip" feature of the PortServer. In this example you will assign an ip address of 10.0.0.16 to port 16 of the PortServer. Type the following 3 commands from the PortServer prompt:

   #> set altip ip=10.0.0.16 group=16

   #> set port range=16 group=16

   #>boot action=reset

   Now the port can be accessed by telneting to 10.0.0.16.

2. "How can I set up authentication so that the PortServer will check for login and password (local authentication)?"

   For the PortServer use the following:
   
   > set auth ra=1 login=16
   
   NOTE: If you have the PortServer II, go to the PortServer
   Console Management Guide.

Removing the PortServer Banner:

1. "When I reverse telnet into a port, I get an annoying PortServer banner, and it's causing my scripts trouble!"

   For the PortServer, depending upon the firmware version, it will provide a banner that is displayed when you reverse telnet into the port. To turn it off, type:

   #>set logins mess=1 range=0

   #>set logins mess=2 range=0

2. "When I reverse telnet into one of my ports, I see an "Access Denied," or a "Refused connection" message, even though I know the ports are set up right!"

   In this case, most likely the port has a process/user on it. This can be checked by telneting to the PortServer, loging in as root, and typing the following commands:

   #>who

   Look for the tty port. For example, if you are getting the error from a reverse telnet into port 16, look under the TTY column for 16. If you see it, type:

   #>kill tty=16

   Retype the "who" command. It should not show an entry for the TTY. Then try to reverse telnet into that port again.

3. "Is it possible to set up the PortServer to allow dialup access to all our console ports in the event of a total network failure?"

   Yes! This is one of the most elegant and useful features that we have for the reverse telnet capability of our PortServers.

   1. Decide which port you want to set a modem up on. Then follow the setup guide below for installing a modem:

      http://supportold.digi.com/support/techsupport/hardware/portserver/NPM.html

   2. Simply set up the rest of the ports to provide the reverse telnet capability to each of your console ports as described above.

      In the event of a network failure, simply call the phone number of the modem you have set up on one of our ports. When connected, you can issue the usual "telnet ip_of_portserver 20##." Even though the network is down, the PortServer will realize you want to come back to the same ip, and it will loopback into itself and out to your console ports.