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Category: How To’s

How To: Enable SSH On A Cisco 2950

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First you have to make sure you are running a version of code that has encryption. See my previous article for instructions on how to upgrade the code. Once your code is upgraded, here are the steps to enable SSH on a Cisco 2950.

Generate An SSH Key

Verify your key like so

Wow 1993, feels good to be a time machine 🙂

Configure the allowed number of retries

Enabling SSH on the VTYs

Configuring A Username

When you’re running telnet you don’t need a username. But when you are using SSH, you do.

Now were golden, let’s test.

Awesome ! That concludes this short tutorial.

How To: Upgrade IOS On A Cisco 2950

Published / by tuxninja / Leave a Comment

My cisco 2950 came with an older IOS version 12.1, but more importantly, one that does not support encryption, and thus I cannot use SSH. I need to upgrade the code aka IOS Image on this switch to enable SSH. So here we go, I’ll be referencing the following guide : http://kb.promise.com/KnowledgebaseArticle10139.aspx throughout this how to article. Note: You should be in enable/privilege 15 mode for the duration of this article.

Existing version info

Cisco Image & TFTP Server

I had to register for the Cisco site to download the latest image, with support for encryption. This is the image I will be installing : c2950-i6k2l2q4-mz.121-22.EA14.bin

After the image is downloaded, we need to configure the TFTP server. Mac OS X comes with tftpd automatically. You are going to want to place the imagine in /private/tftpboot.  After you have copied the image there, make absolutely sure you update the permissions. Otherwise, your tftp request will timeout from your device.

After you update the permissions you are ready to start TFTP.


Great TFTP is running. Now we are ready to request it from the Cisco switch, aka the client in this scenario.

Copy TFTP Flash

Uh oh shaggy ! I am out of space. After doing a ‘dir flash’ I saw that really I had no choice, but to delete my existing flash image to make room for the new one. Feels dangerous and scary, but luckily this is my lab environment 🙂

Deleting From Flash

Copy TFTP Flash Again (This time with our fingers crossed)

Whew…close one 🙂 To be on the safe side we can verify our image like this.

Next we make our new flash image bootable.

Great, that looks good, now we are ready to reload our switch !

Once the switch comes back to life, validate the version info.

We went from 2004, to 2010…but 2010 is the latest image available for my ancient switch ! Awesome. Now we are ready to enable SSH in the next article !

Thanks for reading,
Jason Riedel


Object-Oriented Programming With Python : Inheritance (2/3)

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Continuing on our Object-Oriented Programming with Python series, this is article 2 of 3 on the 2nd pillar, Inheritance. You can find the first article about Encapsulation in this series here.

Inheritance – The Second Pillar

Inheritance in OOP is the ability to have one class inherit the attributes of another class.

Inheritance is simple to implement, but arguably the most powerful pillar in OOP, providing the most value to a programmer. It is the pillar that allows for effective re-use of code organized in classes reducing the amount of lines a programmer has to write to do something useful in their program.

In case you don’t remember from the previous article in this series on encapsulation, a class is a blueprint or template that contains methods, objects and attributes. Once defined a class can be inherited by another class. At which point the child class (aka derived class or subclass) contains all the goodness that the parent class ( aka super class or base class) provides.

Here is an example of inheritance :

The code above downloads the webpage ‘tuxlabs.com’ and outputs it’s contents to STDOUT using the ‘requests’ module.

<rant>If you are a Python developer who is not already familiar with the requests module you should be ashamed of yourself 🙂 It is the most fantastically simple implementation of Python HTTP client functions available.</end rant>

In this example we have defined two classes, one called ‘Tuxlabs’ and one called ‘Crawl’. Tuxlabs inherits a base class ‘object’ and the ‘Crawl’ class inherits ‘Tuxlabs’. If you’re wondering why Tuxlabs inherits from object see here.  Our ‘Tuxlabs’ class has two methods a setter() & a getter() you will remember from the previous article on encapsulation. Now let’s focus on the real point in this example, the ‘Crawl’ class.

The ‘Crawl’ class as previously mention inherits the Tuxlabs class when it is defined or implemented ‘class Crawl(Tuxlabs):’. This means when an instance of Crawl initialized it will inherit all the attributes from the Tuxlabs class in addition to it’s own methods defined in the ‘Crawl’ class. The ‘Crawl’ class has only one method defined ‘download’ that takes a single argument, website.

Inside the download method in the crawl class we see ‘self.setter(website)’, but this is interesting, because we are using ‘self’ to call ‘setter()’, but self is actually referring to the ‘Crawl’ class and the ‘Crawl’ class  only has download defined. So shouldn’t an error be thrown when we try to access ‘self.setter(website)’ inside the ‘Crawl’ class, when setter() lives in ‘Tuxlabs’ ?

No, and the answer is because self, which refers to our ‘Crawl’ class, which is actually inheriting from our ‘Tuxlabs’ class. So this actually works ! Now from within ‘Crawl’ we can access all methods and attributes in Tuxlabs or Crawl, nifty, handy and powerful are the words that come to mind. Before moving on to some even cooler features, let’s show the abbreviated output of this script (I can’t show the entire webpage output, yuck!).

Nothing magical there, but I did want to show you that this code works !

Now then, we should talk about __init__ & super().

__init__() if you don’t already know is the initial method called in a class. It is what other languages might refer to as a constructor. When you create an instance object of a class the __init__() method within that class is called and whatever code is in it is executed. Init can be good for setting up default/static values used within the scope of the class and other methods. A typical example of an init class might look like this.

Here we are defining a class ‘Foundation’ and an init constructor that accepts a dictionary as an argument containing SQL Credentials. Then within the init constructor we define several attributes who’s values are populated from that dictionary. We also made ‘port’ an optional argument with a default port of ‘3306’ the MySQL default port. This is a very useful start to a class, and it is my actual code from a project. From here we would build on top of this class many other methods like SQL queries we frequently used and other data processing techniques we frequently need like parsing JSON or XML.

Now let’s assume for a moment, this is a larger scale project. One that requires many classes. Classes that are inheriting from each other. Because the ‘Foundation’ class above is so generic, it can be used over and over again in other classes. However, those other classes will have slightly different purposes. In the case of databases, perhaps each class represents a different database. This makes sense, but we would still want to make use of the lovely constructor code inside of ‘Foundation’, thanks to it’s well written generic-ness. Here is an example of how we would do that, using something called super().

We have our original class ‘Foundation’, and a new child class ‘Cloud’ that inherits ‘Foundation’. In addition ‘Cloud’ is defining it’s own __init__ constructor and accepting a dictionary for SQL credentials ‘sqlCredsCloud’. Finally, we call super() and pass our ‘Cloud’ class to the inherited class ‘Foundation’  (in this case self) and pass our SQL Creds to the ‘Foundation’ class constructor.

Whew.. I think I did an ok job explaining that, but you may need to read it back a couple of times and re-visit after trying to use this concept. Let’s take a look at how we would use these classes.

First we modify our Python path to locate the foundation module and import it. Then we create a subroutine for setting up our connection to the database and creating our instance object of our Cloud class. Notice we are passing sqlCredsCloud to the Cloud class and remember internally this is passing sqlCreds to the Foundation class constructor.  Finally, in the main routine of the program, after we setup the credentials and after we setup our database connection, we call ‘TruncateTable’, which is a method within the Cloud class that I have not shown in our code above, but you get the idea. Well I think that is about all I can muster, you now know what I know about inheritance.

For more information on super() click here

See ya next time !
Jason Riedel

Upgrading to Python 2.7 on CentOS 6.5

Published / by tuxninja / 1 Comment on Upgrading to Python 2.7 on CentOS 6.5

Hey Folks,

The systems running Tuxlabs are currently running CentOS 6.5 to emulate a production RHEL like setup for an Openstack Cloud. Running an operating system this old has it’s drawbacks such as dependencies. I was recently installing a well know Python framework and ran into compatibility issues. The framework required Python 2.7 and CentOS 6.5 comes with 2.6. The below is a step by step procedure for how to upgrade to Python 2.7 on CentOS 6.5 if  you ever should need it. However, as a reminder run a newer OS when possible and for god sakes if you don’t need Redhat support, run Ubuntu.

Step one, we verify we are indeed running Python 2.6

Ok then, let’s upgrade Python to 2.7. First let’s update all of our system applications, just in case for version dependencies and it’s good for security etc.

Next, we have to install Develop Tools, it is a required dependency to install Python.

Additionally, we will need these…

Now, let’s install Python 2.7

It is important to use ‘altinstall’ otherwise you will end up with two different versions of Python on your filesystem, both named ‘python’.

You can verify the install like so

That’s it ! Enjoy.

SSH Tunneling

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In my last post about Runner I briefly explained needing to modify your ~/.ssh/config to use a ProxyCommand to allow for automatic tunneling with SSH.

What I didn’t explain is there is an alternative method that is arguably simpler. It requires creating three small shells scripts & placing them in your path or a common host path like /usr/local/bin/ with the chmod +x permission. Here is the script that sets up the ssh tunnel.

Script: starttunnel

Running starttunnel, will connect you to your bastion/jump box and then background this connection with keep alives on. It will listen / dynamically forward ssh requests to 8081 through or to tlbastion.tuxlabs.com. Additionally, if you wanted to tunnel a web port specifically on a machine that sits within your network back to the machine you are tunneling from, you can add it to the script. Such that the required host/port always gets tunneled and is available on your machine when you run starttunnel. Example config would look like.

Script: starttunnel + forwarding http


Now that you have authenticated to your bastion and have a working tunnel you need to get ssh requests to go through this tunnel. However, if your like me you still want the ability to ssh to other stuff without going through that tunnel. So I created a new script called ‘sshp’. When I want to ssh through the tunnel / proxy I use ‘sshp’, when I want to ssh to somewhere else on the internet or another network I use plain old ‘ssh’. Here is my sshp script used to connect to machines behind the bastion.

Script: sshp

Now, when you run sshp tuxlabs1@tuxlabs.com you will be connection through the tuxlabs bastion into tuxlabs1. Also notice in my previous post I used sconnect as the proxy command in this one we are using ‘nc’ aka netcat. I have found this method of tunneling to be the most simplistic and effective in my daily life. One more script you need is if you want to copy files you need to use scp. So you have to make a similar command ‘scpp’ for tunneling your copying of files. Here’s the script.

Script: scpp

One final note…if you need use ‘*’ aka splat for copying many files you cannot use the script above, because the shell or script converts that incorrectly. Instead just use the full command yourself from the command line.

scp’ing with *

This would copy all files named ‘copy.all.<whatever>’ to the  bastion. Hope this hopes the folks out there feeling limited by bastions. They provide great security and are an absolute requirement in secure environments so learning tricks that make sure you only need to authenticate once for an extended period of time can come in real handy.

Jason Riedel