Using a Cisco AUX port as a console port with reverse Telnet


Aux-Port

Problem:

Often times when you need console access to a Cisco device and you maybe too far away or due to various reasons you cannot console to/from a server with a serial port.

Solution:

Fear not! Providing you have a Cisco router which is reachable, you can connect a Cisco device’s console port to the routers AUX port for remote access by using “reverse telnet”

The method is simple:

Step 1.
Connect the routers AUX port to console port of the device you wish to connect to with a “Straight Through” cable.

Step 2.
Setup the AUX port

router#config t
router(config)#line aux 0
router(config-line)#modem InOut
router(config-line)#transport input all
router(config-line)#speed 19200
router(config-line)#exit

Step 3.
Create a loopback address for this port

router#config t
router(config)#int loopback 0
router(config-if)#ip address 10.101.0.1 255.255.255.0
router(config-if)#no shut
router(config-if)#exit

Step 4.
You must find out what LINE the router used for the AUX port

router# sh line
 Tty Typ     Tx/Rx    A Modem  Roty AccO AccI   Uses   Noise  Overruns   Int
      0 CTY              -    -      -    -    -      0       0     0/0       -
      5 AUX  19200/19200 - inout     -    -    -      7       0     0/0       -
*     6 VTY              -    -      -    -    - 101226       0     0/0       -
      7 VTY              -    -      -    -    -  33754       0     0/0       -
      8 VTY              -    -      -    -    -   5395       0     0/0       -
      9 VTY              -    -      -    -    -   1180       0     0/0       -
     10 VTY              -    -      -    -    -    280       0     0/0       -

Mine uses line 5.

Step 5.
Telnet to the console port with the IP address set on the Loopback interface to the port number 2000 + line number.

#Note if you setup the routing correctly and point a route for the loopback address to the router, you can telnet remotely without needing to login to the router to telnet.

In my case I will

telnet 10.101.0.1 2005

Step 6.
Once you are complete you must clear the line assigned to the AUX port to drop the connection.

In my case:

router# clear line 5

That’s all you need to know to reverse telnet!

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About the Author

Alfred Tong
Author and owner of this blog. A Networking enthusiast, full time networking and systems Engineer. Generally curious about all things IT.Certifications: GIAC GSEC, CCNP-S, CCNP, CCSP, CCDP, CCNA, RHCE, JNCIA - FWV