theckman

Earlier in 2011 I wrote an article providing instructions for setting up your own IPv6 dual-stack setup at home using a Hurricane Electric Tunnel Broker account.  The process works great if you are running the tunnel on a network where your IPv4 address will not change.  However, for most people who would want this configuration (including myself), your external IP address isn’t always static.  This left the door open for your IPv6 networking to not work properly if your external address changed.  Shortly thereafter I wrote a shell script to help keep the tunnel alive using the Tunnel Broker API (link).

The shell script had a few limitations I had to work around so I planned, awhile back, to rewrite the script.  It took longer than I had wanted but I finished it just in time for the New Year.  On an off-topic note I hope you all had a great Holiday and a prosperous New Year.

I’ll be providing a quick history of the script as well as an overview of the functionality after the break.  If you just want to skip that and get the script head on over to the GitHub page.  All the information you need is provided in the README.

Hurricane Electric IPv4 BASH Script

The first tool I used to keep my address up to date was a shell script (GitHub) that only updated Hurricane Electric if the external address changed.  I also implemented some URL rotation so I wasn’t hitting someone’s IP script too often.  There were some limitations I had to overcome so the script is fairly hacked together to do what I want.  You can also see my explanation of the script in this blog post.

The Reimplementation

Even while writing the above script I knew that at some point I should rewrite the script using a language that was more versatile.  For awhile I’ve been toiling with the decision of what language I wanted to work on next.  Perl or Python?  I was a bit more familiar with Perl syntax and would need to spend extra time learning the semantic of Python (which is still on my todo list).

The Goal

The goal of the script is to be ran on a regular schedule by cron with root permissions.  It was also to require no interaction from the sysadmin as well as to provide plenty of information if the sysadmin wishes to fiddle with it.  The final goal was to only update the external IPv4 address when needed and avoid any repetitive unnecessary calls to the Hurricane Electric Tunnel Broker API as well as the different sites that are used to pull the IP address.

How It Works

The script uses a few Perl modules to log to Syslog, save the external IP and last URL for external IP used to a YAML file to ensure smooth operation, and to pull the IP and update the IP via HTTP/HTTPS.  The script is designed to be resilient if one, two, or even three of the external IP address sites are down.  I’ll provide some detailed information about each process.

Logging

I wanted to implement a logging system with varying verbosity to let you know what’s going on.  In the configurations section there is a commented out $debug variable that allows you to set the verbosity.  The script has a subroutine for logging called slog.  The slog subroutine then uses Logger::Syslog to spit the information to syslog.  It may also print depending on how verbose the debugging is.

YAML!

I needed a lightweight and easy solution for saving/retrieving the previous external IP address as well as which URL was used last.  Immediately Yet Another Markup Language (YAML) came to mind.  By default this information is saved in “`/var/cache/he-ipv4.yml“` using YAML::Tiny.  This is configurable by changing the $configFile variable to an absolute location.  The data is also validated to make sure it looks right.  Is the IP and IP, and is the last used URL index an integer?

Obtaining External IP

The process by which the external IP address is obtained leaves room for some unexpected results/data.  Garbage in, garbage out.  Interactions with websites for obtaining the IP address, as well as interacting with Hurricane Electric Tunnel Broker API, use LWP::Protocol::https and WWW::Mechanize.  The subroutine that obtains the external IP address is the most complex in the script as it has to make sure the data it received is valid.

The code that pulls the external IP address is looped until either a valid IP is obtained, or all URLs are unable to produce a valid IPv4 address.  The data received from the server is mathed via Regex as well as verifying that the HTTP status code is 200.  If the script is unable to obtain valid data it exits with an error.

Finalizing The Update

Once the script obtains the external IP address it does a quick check to see if it has changed.  If not the script writes the last external IP address and URL used to the YAML file and exits.

If the IP address has changed it submits a call to the subroutine that updates the external IP address.  It uses the UserID, UserPass, and TunnelID configuration options to set the new IPv4 address.  Once the API has been updated the tunnel interface is brought down and back up.  Also, RAdvD is restarted to make sure the native IPv6 on the LAN is fully woking.

Closing Notes

This gives you an overview of how the process actually works.  The project with full code, commit history, comments, and README are available on GitHub.  I do encourage you to read, fork, and submit patches for my code as I still have plenty to learn with Perl and do welcome any enhancements to the script.  I look forward to seeing what you come up with.

As usual leave any comments or feedback you have about the article!