Reminiscing another problem I encounter during my internship at IBM, I encountered an issue with some internal tools during a server migration.
One particular issue out of many I would like to discuss is how a
misconfiguration to /etc/hosts can cause your script to fail. After
identifying all the dependencies that needed to be installed and migrating
the database and webserver, we decided to
do a simple test to see if the website does what it is supposed to do.
To our surprise, a specific feature on the webpage failed to work. This came
as a shock because we made no code modifications, downloaded all the
dependencies, and all external applications such as the database were working
fine. After much debugging, I narrow down the bug to a particular Perl script
that was not behaving the way it should be.
Side Track - Talking about the old Internet
To many modern and young web developers, it may seem very odd that a webpage feature failed due to a Perl program not running correctly. Before it was popular to make websites using some JS framework, dynamic pages could be rendered using a CGI (Common Gateway Interface) script or using PHP. This is a side tangent from the post, but I would like to inform newer developers that the web was very messy in the old times. There is a mash of languages and scripts that were used to have dynamic websites. Bash and Perl or PHP were quite popular when I was young as CGI languages. PHP can still be seen such as on WordPress. To make interactive websites such as games, Java applets and Flash were very common (to be run on the client-side) but those are now “dead” (In the sense that it reached EOL). The Web was and is still weird. I’ve seen and talked with people who worked on web development using many different languages such as Python (Flask), C#, Ruby on Rails, Coldfusion, and Java. This shocked me in Highschool because the books I read and worked with at school and at my Highschool internship were all using the LAMP stack. Nowadays, various Javascript frameworks are being used such as React.js or Vue.js as the frontend with Node.js in the backend. The rapid progress of technologies makes me feel very old even though it’s only been a year since I graduated from University.
Anyhow, back to the main subject. Upon debugging the feature, I came across a Perl script where the object it created was returning null. While I cannot delve into what the purpose of the script was, some key values in the script relied on knowing which SITE the script was run on. SITE in this case refers to the campus the script was run on. For instance, if I ran the script in a machine in Aloha, then the script will know the SITE of the machine based on the machine’s domain name which would contain the SITE’s name.
How the script got the site name specifically was calling a Perl function
gethostbyname() which in the Perl Doc has the following information
(perldoc -f gethostbyname):
These routines are the same as their counterparts in the system
C library. In list context, the return values from the various
get routines are as follows:
...
my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
The script grabbed the official name of the host and performed some regex to
get the SITE. However, the script was unable to grab the SITE from $name
because the full domain name (FQDN) was not being displayed. From the perldoc,
we find that the Perl function works the same way as the one in the C library,
so if we look at the manpages, we find that it looks at the file
/etc/hosts for the hostname’s information. Upon inspecting the file, I saw
that the file did not have the FQDN which would explain the bug. The file
looked like this
192.168.2.1 yukikaze yukikaze
So to resolve the problem, all I needed to do was to place the full FQDN on
the file where aloha would be considered the SITE of the lab.
192.168.2.1 yukikaze.aloha.net yukikaze
Conclusion
Aside from all the random commentary in today’s post, a misconfiguration to
/etc/hosts may mess with your program or script if you are relying on
gethostbyname function to get the FQDN.