Jay Swan

Author Archives: Jay Swan

Installing netmiko on Windows

Netmiko is a Python module by Kirk Byers that provides a wrapper around the Paramiko SSH module for doing screen scraping and CLI automation on network devices.

Paramiko has some dependencies that make installation on Windows a tad tricky. Here's a quick way to get it done:

  1. Install Anaconda.
  2. From the Anaconda shell, run "conda install paramiko".
  3. From the Anaconda shell, run "pip install scp".
  4. Install git for Windows.
  5. Clone netmiko with "git clone https://github.com/ktbyers/netmiko"
  6. cd into the netmiko directory and run "python setup.py install".
Done! Screen scrape away, and don't forget to hound your vendors for real APIs... :-)

Extracting Traffic from Rolling Capture Files

Every so often I need to extract a subset of traffic from a set of rolling timestamped pcap files. One common place I do this is with Security Onion; one of the great features of SO is its full-packet-capture feature: you can easily pivot from Snort, Suricata, or Bro logs to a full packet capture view, or download the associated pcap file.

But what if you don't have an associated alert or Bro log entry? Or if you're doing pcap on some system that's not as user-friendly as Security Onion, but nonetheless supports rolling captures?

The way I usually do this is with find and xargs. Here's an example of my most common workflow, using timestamps as the filtering criteria for find:

> find . -newerct "16:07" ! -newerct "16:10" | xargs -I {} tcpdump -r {} -w /tmp/{} host 8.8.8.8
> cd /tmp
> mergecap -w merged.pcap *.pcap

Translated:
  1. Find all files in the current directory created after 16:07 but not created after 16:10. This requires GNU find 4.3.3 or later. It supports many different time and date formats.
  2. Using xargs, filter each file with the "host 8.8.8. Continue reading

More ADN (Awk Defined Networking)

Want to know how many IPv4 nodes are in each of your VLANs? Use ADN:

ssh myswitch 'sh arp | i Vlan' | awk '{print $NF}' | sort | uniq -c | sort -rn

     79 Vlan38
     65 Vlan42
     58 Vlan34
     22 Vlan36
     21 Vlan32
     20 Vlan40
      9 Vlan3
      7 Vlan8
      5 Vlan6
      5 Vlan204
      5 Vlan203
      5 Vlan2
      4 Vlan74
      3 Vlan82
      3 Vlan4

ADN – Awk Defined Networking

Because I have yet to transition to a completely software-defined network in which everything configures itself (wink wink), I still have to do tasks like bulk VLAN changes.

Thanks to a recent innovation called ADN, or "AWK Defined Networking", I can do this in a shorter time window that the average bathroom break. For example, I just had a request to change all ports on a large access switch stack that  are currently in VLAN 76 to VLAN 64:

# ssh switch_name.foo.com 'show int status | i _76_' | grep Gi | awk '{print "int ",$1,"n","description PC/Phone","n","switchport access vlan 64"}'
Password: ***

int  Gi1/0/25
 description PC/Phone
 switchport access vlan 64
int  Gi1/0/26
 description PC/Phone
 switchport access vlan 64
[many more deleted]

Then I copied and pasted the results into config mode. Back to lounging on the beach.

Not even any Python skills required!

Filtering .raw fields with Python Elasticsearch DSL High-Level Client

It took me a while to figure out how to search the not_analyzed ".raw" fields created by Logstash in Elasticsearch indices, using the high-level Python Elasticsearch client. Because keyword arguments can't have attributes, Python throws an error if you try it the intuitive way (this assumes you've already set up a client as es and an index as i, as shown in the docs):

Instead, you create a dictionary with your parameters and unpack it using the ** operator:

This produces the Elasticsearch query we want:

Experimenting with the CSR1000v REST API

This all started because we occasionally want to block traffic from an IP address or two for a short time. Our firewall is a pain to configure for this sort of thing: adding a drop for a single IP address literally takes 10 minutes. You have to open a fat client, create an object, add the object to a group, save the config, verify the config, push the config, etc.

I thought that SRTBH (Source-based Real-Time Black Hole) implemented by BGP would be the ticket: fast, easy, and theoretically easy to automate with the REST API in the Cisco Cloud Services Router 1000v. SRTBH is a simple and elegant way of dropping selected traffic on BGP speaking routers. In a nutshell:
  1. You configure a "trigger router" that speaks iBGP with the rest of your BGP-speaking routers (usually your Internet edge or transit routers), but doesn't participate in traffic forwarding.
  2. On each edge/transit router you configure a static route to null0 for an unused /32, usually 192.0.2.1: ip route 192.0.2.1 255.255.255.255 null0
  3. On each edge/transit router you configure loose-mode unicast RPF filtering on your outside interfaces: ip verify source reachable-via any. Continue reading

Problems with kvm-ok in VIRL with VMWare Player

I'm installing Cisco VIRL, and despite following the instructions regarding nested virtualization settings, the kvm-ok command was still complaining. I needed to edit the .vmx file for the VIRL VM and add/edit the following:

monitor.virtual_mmu = "hardware"
monitor.virtual_exec = "hardware"
vhv.enable = "TRUE"
monitor_control.restrict_backdoor = "true"
 
 

My Network Toolkit

A while back, Chris Marget of Fragmentation Needed posted a run-down of his comprehensive and extremely clever network toolkit. Because I'm something of a weight weenie, mine is a lot more slimmed down. I thought I'd post it here:




The contents:

  1. Two random USB drives (in case I need to leave one with somebody).
  2. Single-mode and multi-mode LC fiber loopback plugs.
  3. Rack PDU plug adapter.
  4. Awesome PicQuic compact screwdriver (thanks to Chris's post).
  5. T1 loopback plug (red) (because we still have T1s out here in the boonies).
  6. Cat-6 pass-through plug (white).
  7. Crossover adapter (orange).
  8. Sharpie.
  9. Console setup:
  1. USB-to-DB9 adapter.
  2. DB9-to-RJ45 adapter.
  3. Flat Cat-6 cable.
  4. Rollover adapter.
  5. Velcro tie
  6. Flat Cat-6 cable with velcro tie.
The console setup could probably be improved by adding a DB9 null-modem adapter. The coolest thing (IMO) that I'm missing from Chris's setup is the Bluetooth console adapter -- maybe one day.

The Fenix AA light and Leatherman Skeletool CX almost always live in a pocket rather than the kit and go with me everywhere. The kit all fits into a small zippered case that used to hold a Dell laptop power supply.

My main goal here was to have all the hard-to-find professional stuff in Continue reading

Imposing Artificial Limitations to Develop Skills

I'm a big fan of imposing artificial limitations on yourself in order to aid skill development. Here are some quick ideas:

  • When troubleshooting network devices from the CLI, try not to look at the configuration. Use only "show" or "debug" commands instead. I found this enormously beneficial when practicing for CCIE.
  • When troubleshooting larger operational issues or learning a new environment, try not to log into individual devices at all. Force yourself to use only your network management system, NetFlow, packet captures, or host-based tools like ping, traceroute, or nmap.
  • When learning automation or orchestration skills, force yourself to write scripts, run API calls, or use your favorite orchestration tool to do simple things, even if it doesn't seem like they merit the extra effort.

Simple Python Syslog Counter

Recently I did a Packet Pushers episode about log management. In it, I mentioned some of the custom Python scripts that I run to do basic syslog analysis, and someone asked about them in the comments.

The script I'm presenting here isn't one of the actual ones that I run in production, but it's close. The real one sends emails, does DNS lookups, keeps a "rare messages" database using sqlite3, and a few other things, but I wanted to keep this simple.

One of the problems I see with getting started with log analysis is that people tend to approach it like a typical vendor RFP project: list some requirements, survey the market, evaluate and buy a product to fit your requirements. Sounds good, right? The problem with log analysis is that often you don't know what your requirements really are until you start looking at data.

A simple message counting script like this lets you look at your data, and provides a simple platform on which you can start to iterate to find your specific needs. It also lets us look at some cool Python features.

I don't recommend pushing this too far: once you have a decent idea Continue reading

Python Sets: Handy for Network Data

My Python-related posts seem to get the most reads, so here's another one!

A problem that comes up fairly often in networking is finding the number of occurrences of unique items in a large collection of data: let's say you want to find all of the unique IP addresses that accessed a website, traversed a firewall, got denied by an ACL, or whatever. Maybe you've extracted the following list from a log file:

1.1.1.1
2.2.2.2
3.3.3.3
1.1.1.1
5.5.5.5
5.5.5.5
1.1.1.1
2.2.2.2
...

and you need to reduce this to:

1.1.1.1
2.2.2.2
3.3.3.3
5.5.5.5

In other words, we're removing the duplicates. In low-level programming languages, removing duplicates is a bit of a pain: generally you need to implement an efficient way to sort an array of items, then traverse the sorted array to check for adjacent duplicates and remove them. In a language that has dictionaries (also known as hash tables or associative arrays), you can do it by adding each item as a key in Continue reading

Fun with Router IP Traffic Export and NSM

The Basics
I finally got around to setting up Security Onion (the best network security monitoring package available) to monitor my home network, only to discover that my Cisco 891 router doesn't support support the right form of SPAN. Here's how I worked around it. The topology looks like this:


The 891 router has an integrated 8-port switch module, so the simple case would have been a traditional SPAN setup; something like this:

! vlan 10 is the user VLAN
monitor session 1 source interface vlan 10
monitor session 1 destination interface FastEthernet0

with the server's monitoring NIC connected to FastEthernet0.

The problem is that the 891 doesn't support using a VLAN as a source interface, and because of the way the embedded WAP works, a physical source interface won't work either. Hence, I turned to an obscure feature that's helped me occasionally in the past: Router IP Traffic Export. This is a feature for IOS software platforms that enables you to enable SPAN-like functions for almost any source interface.

The configuration looks like this:

ip traffic-export profile RITE_MIRROR
  interface FastEthernet0
  bidirectional
  mac-address 6805.ca21.2ddd

interface Vlan10
 ip traffic-export apply RITE_MIRROR

This takes all traffic routed across Continue reading

Quick Thoughts on the Micro Data Center

Here's something that's been on my radar lately: while all the talk in the networking world seems to be about the so-called "massively scalable" data center, almost all of the people I talk to in my world are dealing with the fact that data centers are rapidly getting smaller due to virtualization efficiencies. This seems to be the rule rather than the exception for small-to-medium sized enterprises.

In the micro data center that sits down the hall from me, for example, we've gone from 26 physical servers to 18 in the last few months, and we're scheduled to lose several more as older hypervisor hosts get replaced with newer, denser models. I suspect we'll eventually stabilize at around a dozen physical servers hosting in the low hundreds of VMs. We could get much denser, but things like political boundaries inevitably step in to keep the count higher than it might be otherwise. The case is similar in our other main facility.

From a networking perspective, this is interesting: I've heard vendor and VAR account managers remark lately that virtualization is cutting into their hardware sales. I'm most familiar with Cisco's offerings, and at least right now they don't seem to Continue reading

Packet Capture in Diverse / Tunneled Networks?

(With the usual caveats that I am just a hick from Colorado, I don't know what I'm talking about, etc.)

I just read Pete Welcher's superb series on NSX, DFA, ACI, and other SDN stuff on the Chesapeake Netcraftsmen blog, and it helped me think more clearly about a problem that's been bothering me for a long time: how do we do realistically scalable packet capture in networks that make extensive use of ECMP and/or tunnels? Here's a sample network that Pete used:






Conventionally, we place packet capture devices at choke points in the network. But in medium-to-large data center designs, one of the main goals is to eliminate choke points: if we assume this is a relatively small standard ECMP leaf-spine design, each of the leaf switches has four equal-cost routed paths through the spine switches, and each spine switch has at least as many downlinks as there are leaf switches. The hypervisors each have two physical paths to the leaf switches, and in a high-density virtualization design we probably don't have a very good idea of what VM resides on what hypervisor at any point in time.

Now, add to that the tunneling features present in hypervisor-centric Continue reading

Programmatically Configuring Interface Descriptions from Phone Descriptions

I wrote some Python code that allows you to do the following:
  1. Query a Catalyst switch CDP neighbor table from its HTTPS interface,
  2. Extract the device names of the attached IP phones,
  3. Query Communications Manager for the IP phone device description, 
  4. Apply the device description as the switch interface description.
Obviously, this makes it much easier to see whose phone is attached to a switch port.

I hope that this example saves someone the head-banging that I incurred while trying to figure out the AXL XML/SOAP API for Communications Manager.

I haven't tested this extensively; all my testing has been on Catalyst 3560 and 3750 switches and CUCM version 8.6. Using the --auto switch to automatically configure the switch is quite slow; this is a limitation of the HTTPS interface rather than the script code. It may be faster to leave that option off and manually copy/paste the printed configuration if you're in a hurry.

Note that your switch must be configured to allow configuration via the HTTPS interface; you may need to modify your TACACS/etc. configurations accordingly.

All the relevant info is in the Github repo.

Why Network Engineers Should Learn Programming

Because Microsoft Excel is not a text editor. Seriously.

This is a followup to the previous post, inspired by Ethan Banks of Packet Pushers. If you do operational networking at all, you deal with text files all the time: logs, debug output, configuration files, command line diagnostics, and more. I'm constantly amazed when I see people open Word or Excel to do their text editing, often one keystroke at a time.

The number one reason to learn basic programming is to automate that stuff. Personally, I use a combination of traditional Unix shell tools and Python to get the job done, but you could probably do it all with one or the other.

There are lots of other reasons to learn programming too, many of which will be discussed on an upcoming Packet Pushers episode. But if you don't believe any of those, this one alone makes it worth the effort.

Step away from the spreadsheet. Do it now.

Quick Thoughts on Learning Python

I was scheduled to be a guest on an upcoming episode of the Packet Pushers podcast, on the topic of Python for network engineers. Unfortunately due to bad luck I'm not going to be able to make the recording. Here are some quick thoughts on learning Python. If you're already an expert programmer you already know how to learn languages, so this post isn't for you.

Scenario 1: You've coded in another language, but you're not an expert.
I would start with the basic Python class at Google Code. It's targeted specifically at people who know basic programming skills in some other language. It was perfect for me; I went through the exercises and was able to quickly start writing simple, useful Python scripts.

Scenario 2: You don't know how to write code at all.
Start with the Udacity CS101 class if you like guided learning, or Learn Python the Hard Way if you prefer books. Be prepared to spend a lot of time on either. It's not easy the first time around.

After you've gotten through one of those two scenarios, do the following:


  1. Spend time browsing the documentation for the Python Standard Library. Python is a large language, Continue reading
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