Monthly Archives: November 2011

Utilize Cisco AUX Port for Reverse Telnet

Since I’m too cheap/poor to buy an asynchronous serial module (NM-16A/NM-32A) for a Cisco terminal server, or an OpenGear console server, I’ve figured out how to configure the AUX port to perform the same function.  Typically the AUX port is used to connect a dial-up modem to the device for dial-in access.  You can identify the line number by running “show line” from the command line:

#show line
   Tty Typ     Tx/Rx    A Modem  Roty AccO AccI   Uses   Noise  Overruns   Int
      0 CTY              -    -      -    -    -      0       0     0/0       -
*     5 AUX   9600/9600  -    -      -    -    -      2       0     0/0       -
*     6 VTY              -    -      -    -    -      5       0     0/0       -
      7 VTY              -    -      -    -    -      0       0     0/0       -
      8 VTY              -    -      -    -    -      0       0     0/0       -
      9 VTY              -    -      -    -    -      0       0     0/0       -
     10 VTY              -    -      -    -    -      0       0     0/0       -

Line(s) not in async mode -or- with no hardware support:
1-4

As you can see, the AUX port is showing up as line 5.  This is important.

Next, put some configuration on the AUX port:

line aux 0
 transport input all
 flowcontrol hardware

Now you can either access the line from your device’s console, or from a remote host.  Assuming the device’s IP address is 172.30.1.2, from a remote host you can telnet to 172.30.1.2 on port 2005 (05 is the two digit line number), enter your credentials to the device, and gain access to the AUX port.  Once attached to the AUX port you have access to the console’s connection.  In this case, it is connected to a Juniper SRX which requires authentication for console access.

$ telnet 172.30.1.2 2005
Trying 172.30.1.2...
Connected to cme.
Escape character is '^]'.


User Access Verification

Username: eric
Password: 

ROUTER (ttyu0)

login: eric
Password:

--- JUNOS 11.1R2.3 built 2011-05-06 05:59:38 UTC
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