A practical, hands-on, real world approach to learning and implementing SDN and OpenFlow. Python for Network Engineers: Netmiko, NAPALM, pyntc, Telnet, SSH and more! Learn Network Programmability and Network Automation using GNS3 and Python.
4. Python (version 3) code to telnet to R1
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Next we create Python code to telnet to our R1 router, login with our provided username and password. It will also enter privileged mode (Router#) with “cisco” as the secret password. Then create a loopback interface 1 with IP address of 18.104.22.168/32 on our router:
The source code can also be downloaded here.
Note: The letter “b” in front of the string (for example in line: tn.read_until(b”Username:”)) indicates that the literal should become a bytes literal in Python 3. Bytes literals are always prefixed with ‘b’ or ‘B’; they produce an instance of the bytes type instead of the str type.
Save this code to the “telnet.py” file to any location you want (we saved it in “C:/Users/PC/PycharmProjects/Python_Telnet/telnet.py”). Then open command line and run this file as follows:
Of course you have to change the correct path to your Python file. Notice that we are using Python version 3 to run this file. If we are lucky, we will get a nice result like this:
Note: If you use some Python IDEs (like PyCharm) to run this file then the “Password: ” prompt will not be shown so please make sure to run it under “cmd”.
Now on R1 issue the “show ip interface brief” command to see our new Loopback 1 interface has been created:
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5. Some errors you may see during the configuration
The hardest error you may see in this lab is the telnet connection to R1 does not work with the following error:
Password: (type “12345” here)
Successfully passed getpass
Traceback (most recent call last):
File “C:/Users/PC/PycharmProjects/Python_Telnet/telnet.py”, line 8, in <module>
tn = telnetlib.Telnet(host)
File “C:Program Files (x86)Python37-32libtelnetlib.py”, line 218, in __init__
self.open(host, port, timeout)
File “C:Program Files (x86)Python37-32libtelnetlib.py”, line 234, in open
self.sock = socket.create_connection((host, port), timeout)
File “C:Program Files (x86)Python37-32libsocket.py”, line 727, in create_connection
File “C:Program Files (x86)Python37-32libsocket.py”, line 716, in create_connection
TimeoutError: [WinError 10060] A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
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To solve this problem we recommend you to:
+ Check step 3 – Verify the connection between our laptop and router in GNS3 carefully to make sure two devices can communicate normally
+ Try to run command line in Administrator mode (Search > Cmd > Right-click and choose “Run as administrator”)
+ Disable any firewall, antivirus software during the lab
+ Check the IP address of “host” in “telnet.py” file carefully (it should be “host = “10.1.1.1” in our lab)
Another problem we may encounter is after typing “Username: R1” and waiting for the “Password” prompt. The program is suspended:
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As we said above, if we run this Python file through some Python IDEs (like PyCharm), the “Password” function of “getpass.getpass()” will not interpreted correctly and interrupted.