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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Using SSH for Secure Access

Problem

You want to use SSH for secure encrypted remote access to your router.

Solution

You can configure your router to run an SSH Version 1 server for VTY access:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#hostname Router1
Router1(config)#ip domain-name oreilly.com
Router1(config)#crypto key generate rsa
The name for the keys will be: Router1.oreilly.com
Choose the size of the key modulus in the range of 360 to 2048 for your
  General Purpose Keys. Choosing a key modulus greater than 512 may take
  a few minutes.

How many bits in the modulus [512]: 1024
Generating RSA keys ...
[OK]

Router1(config)#
Jun 27 15:04:15: %SSH-5-ENABLED: SSH 1.5 has been enabled
Router1(config)#ip ssh time-out 120 
Router1(config)#ip ssh authentication-retries 4
Router1(config)#end
Router1#

SSH became available in Cisco’s IOS, starting with release 12.1(1)T. However, only versions of IOS that support IPSec (DES or 3DES) encryption include SSH support. Note that there are severe restrictions on exporting any software that includes 3DES outside of United States and Canada.

Beginning with IOS Version 12.3(4)T, Cisco introduced support for SSH Version 2. The configuration is identical. However, only IOS versions that support 3DES encryption will support SSH Version 2. Also, the RSA key-pair size must be greater than or equal to 768.

Discussion

SSH provides a secure method of communication between network entities by the means of ...

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