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On physical layer AUX port works on asynchronous serial RS-232 protocol. However, one can specify various application layer protocols like telnet or SSH as allowed input protocols under AUX line configuration. Am I correct that those protocols are specified only to enable or disable reverse-telnet or reverse-SSH?

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On physical layer AUX port works on asynchronous serial RS-232 protocol. However, one can specify various application layer protocols like telnet or SSH as allowed input protocols under AUX line configuration. Am I correct that those protocols are specified only to enable or disable reverse-telnet or reverse-SSH?

Short Answer:

On an aux line, transport input tells the router what protocols are permitted to reverse-telnet on the aux line.

Example:

I have the following lab setup...

  lab-rt1                                       sw1
+--------+ aux                           con +-------+
|  c1841 |-------{rolled serial cable}-------| c3560 |
+--------+                                   +-------+
           speed 115200                  speed 115200

A "rolled cable" is just a normal Cisco console cable, which has

  • pin 8 connected to 1
  • pin 7 connected to 2
  • etc...

If you have some older Cisco cables lying around, it usually looks like this...

console cable

These days, Cisco doesn't ship rolled cables with two RJ45 connectors; now they have DB9 on one end. I made my rolled cable with some crimpers.

Lab Configuration:

I have the following configured on a Cisco 1841 lab router (transport input none is default on aux in my IOS)...

lab-rt1#sh runn | b line aux
line aux 0
 no exec
 exec-timeout 15 0
 speed 115200

The aux reverse-telnet port is tcp/2001 on the c1841 (you have to add 2000 to whatever line is shown below)...

lab-rt1#sh line | i CTY|AUX
      0    0 CTY              -    -      -    -    -     0      0    0/0      -
      1    1 AUX 115200/115200-    -      -    -    -     5      5    3/0      -
lab-rt1#
lab-rt1#

Example 1 (without transport input ...):

If I try to telnet to that router without transport input telnet, then it fails...

[mpenning@tsunami]$ telnet rt1 2001
Trying 172.16.1.210...
telnet: Unable to connect to remote host: Connection refused
[mpenning@tsunami]$

Example 2 (using transport input ...):

However, if I configure this on the c1841's aux port, reverse-telnet and reverse-ssh work...

lab-rt1(config)#line aux 0
lab-rt1(config-line)#transport input telnet ssh
lab-rt1(config-line)#end
lab-rt1#

Now I telnet again to port 2001...

[mpenning@tsunami]$ telnet rt1 2001
Trying 172.16.1.210...
Connected to rt1-lo0.lab.pennington.net.
Escape character is '^]'.


User Access Verification

Username: mpenning
Password:


sw1 con0 is now available


Press RETURN to get started.

SSH reverse telnet is a little strange...

  • You might be tempted to ssh to port 2001 on the terminal router (i.e. ssh -p 2001 rt1), but that fails; use port 22 and see the next bullet.
  • You also have to mangle your username as <username>:<line_number>... in my case, I have to use line 1 to get to the aux (from show line, above)

Example... I have username mpenning configured on rt1, and I'm making a reverse-ssh to aux 0 (which is line number 1)...

[mpenning@tsunami]$ ssh -l mpenning:1 rt1
Password:

sw1 con0 is now available


Press RETURN to get started.
1

You can make it do more fun stuff like having it talk IP: https://glennmatthys.wordpress.com/2012/11/02/using-a-cisco-routers-aux-port-to-talk-ip/

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