Posted in 2019
Environment variables set by systemd
- 02 July 2019
Applications sometimes need environment variables to be set for triggering certain behavior like giving debug output or routing traffic via a HTTP-proxy for example. A common way is to modify the start-stop script, but with systemd on most Linux systems, like Debian and Red Hat based distributions, this can also be directly set within the unit file and you don’t have to export the variables anymore.
Let’s start with a Python script to read and print the environment variables set by the environment to see how this works. The Python script below that we run via systemd checks if environment variable VAR1 has been set and will generate different output based on that.
Connecting to legacy servers with OpenSSH
- 25 June 2019
Phasing out legacy cryptographic algorithms can always be an interesting endeavor as terminating too early breaks stuff and too late can lead to a compromise. OpenSSH disabled Digital Signature Algorithm (DSA) with version 7.0 in March 2015 as 5 years earlier it was discovered that DSA was compromised and labeled as insecure. Normally this shouldn’t be a problem with a normal software life cycle, but sometimes you will encounter a legacy box that will not be upgraded as it will break things. Now it will stop new connections from being set up from being upgraded to machines as with SSH.
For an incidental connection from the command line, the algorithm can be enabled again to connect with a legacy machine.
Setting a different libvirt uri for Vagrant
- 18 June 2019
HashiCorp Vagrant normally selects the right hypervisor, but the version shipped with Fedora 30 prefers to run within the QEMU user session of the hypervisor. A .Vagrantfile it would match the default behavior which doesn’t require any system privileges is shown below.
In some cases a virtual machine needs to run on QEMU system level and that can be done by changing the domain.uri from “qemu:///session” to “qemu:///system”. Vagrant now creates the virtual machine at the system level of the hypervisor and isn’t depending on any user environment to run.
Using bare variables in Ansible 2.8
- 02 June 2019
Ansible 2.8 was released in May 2019 and later in May came to Fedora 30 in package form. So the first tests could be done to see what needed to be done to switch from 2.7 to 2.8 and don’t generate a lot of stopped GitLab CI jobs due to new warnings and errors. Let’s start with one warning that needs to be resolved before the 2.12 release and also is given on many third-party roles.
The example code above is simple enough to get the warning about CONDITIONAL_BARE_VARS. We could opt for disabling the warning in ansible.cfg and move forward, but as this is the technical debt we don’t want to get more and resolve the current debt as quickly as possible.
Redirecting to mobile Wikipedia
- 05 April 2019
Wikipedia both has a traditional and progressive website that is shown on mobile devices. After years the progressive website is still not shown on desktops sadly enough, but with a browser plugin a redirect can be triggered to the mobile site in most browsers. This as the Redirector plugin works at least in Mozilla Firefox, Google Chrome and Chromium, and will most likely also work in future when Microsoft Edge switches to the Chromium engine.
The configuration in JSON-format below can be imported to setup the Redirector plugin. After enabling the redirect rules, the browser should redirect the Wikipedia to the mobile Wikipedia website.
Using YUM history to see package changes
- 03 February 2019
When you install or update packages on your system, then changes may occur that were not expected. Recent security updates on a server and left Nagios in a failed state, but what exactly happened, and can it be traced back as yum-cron installs all required security updates? Luckily YUM keeps a history database of all actions and with yum history can you list all transactions.
As transaction 15 was the latest and only transaction before the defect occurred it is the one to look into. With yum history info the details of the transaction can be shown. It shows when and who triggered the transaction, but also with which version of RPM, YUM, and which plugins for YUM were used. Most importantly it also shows which package was updated with versions used and from which repository. This narrows the search down to the packages shown as updated and sees what they changed on the system.
Check HTTP with telnet
- 26 January 2019
HTTPS may become the standard quickly, but Hypertext Transfer Protocol (HTTP) is still the base, and understanding how to verify an HTTP server without a web browser can be very useful. A lot of situations simply don’t allow you to install a web browser or gives only a blank page.
As HTTP is a plain-text protocol you can simulate a connection with telnet on the command line. So let connect to a fresh Linux machine with Apache running and see what happens. After connecting you type in GET /index.html HTTP/1.1 to tell web server which files you want to get and in this case the file in /index.html. The second line tells the web server for which website you make the request which is 192.168.121.7.xip.io in the example. And finally, you give an additional entry to tell your request is complete and can be processed after which you get the response.
PHP 7.3 and forbidden functions
- 03 January 2019
Last month PHP 7.3.0 was released and with that a lot of functions or aliases were deprecated that may lead to issues down the road. While Xdebug still needs to be released for PHP 7.3 an automated test with GitLab isn’t possible yet as the build phase of Xdebug fails. Luckily I’m using PHP CodeSniffer and extending phpcs.xml.dist with the lines below make the build already fail if any of the forbidden functions are being used in the code.
Hopefully PHP Code Sniffer will be extended to check on deprecated constants as well, but for now all code running on PHP 7.2 can be checked to run smoothly on PHP 7.3 and later.