- 27 July 2023
Systemd timers are a great way to replace cron jobs. They are more flexible and can have dependencies that can defined in easy statements instead of creating a script to check for the dependency. One of the use-cases is to use systemd timers to create PostgreSQL backups and remove old backups. In this example, we will create a backup of two databases, db1 and db2, and remove backups older than 10 days.
First step is to install PostgreSQL on the system. This example is for Debian or Ubuntu and uses the root user to connect to the database. It is recommended to create a user with limited privileges to connect to the database.
- 23 July 2023
Systemd timers are a great way to replace cron jobs. They are more flexible and can have dependencies that can defined in easy statements instead of creating a script to check for the dependency. One of the use-cases is to use systemd timers to create MariaDB backups and remove old backups. In this example, we will create a backup of two databases, db1 and db2, and remove backups older than 10 days.
First step is to install MariaDB on the system. This example is for Debian or Ubuntu and uses the root user to connect to the database. It is recommended to create a user with limited privileges to connect to the database.
- 21 July 2023
Running a backup MX host is a good idea if you want to ensure that e-mails are not lost if your primary MX host is down. With Postfix, this is easy to set up on a Debian or Ubuntu system as both are shipped with Postfix.
Mail servers that are misconfigured can be used to send spam. Therefore, you should closely monitor your mail server and make sure that it is not misused by spammers. In case of any doubt, you should shut down your mail server immediately.
- 17 July 2023
With systemd you can manage services like in post Environment variables set by systemd, but managing multiple instances of the same service is not so easy. For example, you have a service that runs a web server, and you want to run multiple instances of that service, each with different configuration. You can do that by creating multiple service files, but that is not very elegant. You can also use systemd templating, but that is not very flexible. The best way to do this is to use systemd instances.
For the example, we will create a service that runs a Python script. The script will print the value of an environment variable. The name of the environment variable will be the name of the instance. The script will be called
env_variables.py and will be located in
/usr/local/bin/. The service will be called
env_variables@.service and will be located in
/etc/systemd/system/. So let’s copy the script from the previous post as it can print the value of an environment variable to the standard output.
- 30 April 2023
Installing Linux on a physical or virtual machine can be fun for a couple of time, but configuring a system can be combursome after a couple of times. Red Hat developed Kickstart to install and configure machines via the network, but also as part of an ISO file that can used in a virtual CD-ROM drive or on a bootable USB-drive.
The following steps are required to create a bootable ISO file with a kickstart file and when the ISO file is booted, the kickstart file is used to install and configure the system. Creating a bootable ISO file is not a difficult task, but it is a manual procedure. The following steps are required to create a bootable ISO file with a kickstart file.
- 25 February 2023
Ansible allows for structured configurations on how to deploy applications and servers, but also for repetitive tasks. All these actions are defined in YAML files that rely on indentation and this is where Ansible can act in a way that isn’t expected. With yamllint most formatting and layout issues can be found, but with ansible-lint also common mistakes for Ansible can be detected.
As ansible-lint is a step in the CI-workflow a dependency upgrade started a CI-workflow to scan all Ansible Playbooks to be scanned with the latest version of Ansible-lint and found a violation that must be resolved before the pull request can be successfully merged. Running Ansible-lint after checking out the code the error shown below was found and indicates that the order of the keys in YAML isn’t in the correct order.
- 21 August 2022
Vagrant is a tool for building and managing virtual machines locally with just a few commands and a single file. It is a good way to get started with infrastructure-as-code on your local machine. Vagrant is also shipped with Fedora to make it easier to install and use as it uses libvirt to manage the virtual machines running on Linux with kernel virtualization to provide virtual hardware. This works well until you also want to install Terraform from the HashiCorp repository and later you upgrade Vagrant to a newer version with a regular package update.
In the example above Vagrant fails to find libvirt to connect to KVM and manage the virtual machine. Multiple providers are available for Vagrant to use, but the one that is used is not the one that is installed on the machine or can be used. This is a known issue with Vagrant on Linux and does not use the system Ruby environment with the lirbary for libvirt. Downgrading to the version of Vagrant that is shipped with Fedora will fix this issue.
- 30 April 2022
getent is not known to many users but it is very useful when you need to get information about users, groups, or other entities. The command can be used to get information about users, groups, or other entities for example without having to know where the information is stored.
The syntax of the
getent command is simple to use as it takes the name of the data source and possibly the name of the entry to retrieve.
- 18 July 2020
A lot has changed since 2018 when exFAT was kept out of Fedora due to patent issues and a third-party FUSE-driver needed to be used. Until recently the GPLv2 based driver from Microsoft wasn’t enabled in the kernel as it was based on an older specification and wasn’t fully functional for everyday use.
Fedora 32 recently received an upgrade to kernel 5.7 and with that, the native exFAT driver was enabled during compile time. The driver got a lot of updates from Samsung to work correctly to the latest specifications.
- 18 May 2020
For Developing inside a Container with Visual Studio Code, one of the requirements is to use Docker Community Editon as the version of Docker that ships with Fedora is too old and misses certain features. Also the new Docker alternative Podman from Red Hat isn’t supported by Visual Studio Code.
After installing Docker CE on Fedora 31, cgroups version 1 needs to be enabled as Linux switched over to cgroups version 2, but Docker still depends on version 1. With the commands below cgroups version 1 can be enabled again and requires a rebooting of the system.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 15 September 2018
exFAT has been chosen by the SD Card Association as the standard file system for SDXC cards with 32 GiB or more of storage. Sadly the Fedora Project has chosen not to bundle support for exFAT due to patent issues. A free implementation of exFAT has been made and is available via RPMFusion Free for RPM-based systems.
If you now try to mount your SD-card in Nautilus for example it should mount your drive. The performance should also be better than with NTFS as there is less overhead.
- 27 December 2017
Solid-state drives sound ideal as they have no spinning parts and are very quiet, but they have a limited lifespan as you can’t write a memory cell only an X amount of times. But how to check your SSD on Linux to see if it is still in good shape? S.M.A.R.T. has become the standard for disk health years ago and can be queried by
smartctl. So if we query for the health status and show all available attributes we get a good overview.
The most interesting attributes are 202 on how much lifetime is left, but also 5, 180, and 9 that show you the number of replaced storage cells and how many hours the disk has been running. If attributes 5 and 180 are changing it is most definitely time to replace this solid-state drive as memory cells have been worn out.
- 13 December 2017
With the creation of RFC 4408 also new a record type 99 for DNS was created to identify SPF Resource Records. It was advised to have both TXT and SPF records in DNS with the same content. RFC 4408 was obsoleted by RFC 7208 in 2014 with paragraph 3.1 stating the following:
SPF records MUST be published as a DNS TXT (type 16) Resource Record (RR) [RFC1035] only. The character content of the record is encoded as [US-ASCII]. Use of alternative DNS RR types was supported in SPF’s experimental phase but has been discontinued.
- 15 October 2017
Last month CentOS 7.4 was announced and it was time to rebuild some servers from scratch to make sure all playbooks were still correct as it is always good to know you can quickly (re)build servers when needed. For some other servers, the impact would be big due to huge amounts of data that needed to be moved around and an in-place upgrade would be sufficient.
Upgrading is very straightforward as it the same as the update option with “-obsoletes” flag set which removes obsolete packages. So let start with CentOS 7.3.
- 08 September 2017
Disk space seems endless, until you run out and/or have to pay the bill. The question is how to find tables with a high disk storage usage and with the query below it shows the table and index size, but also the size of TOAST data for PostgreSQL.
After running this query on the development schema and exporting the results to CSV, we can see that a ManyToMany table consumes a total of 39 MB. With over 330.000 entries this seems numbers seem to be fine as the table size is in line with the amount of data stored in it.
- 29 July 2017
For now the following commands increase the inotify limit to 512k of files.
- 08 May 2017
For many Secure Shell (SSH) is a magic sauce to get access to a server and transfer files between servers. But when things go wrong this magic sauce becomes a problem. Let’s start with one example of when things go wrong and how to debug them. First, we start add to option -v to our command to connect to another server to get some basic debug information about the SSH handshake and get to the point the user has to authenticate.
Just before the SSH client prompts for the user’s password two interesting debug lines are shown. The first line is about the authentication methods we can use and the next line shows our client selected method password as we don’t have any methods configured in our SSH client like public key. So we manually disable publickey authentication and set the preferred authentication methods to keyboard-interactive.
- 05 May 2017
Recently I generated kickstart files for a virtual environment where people could experiment and virtual machines could be rebuilt quickly. Sadly enough a typo slipped into the generated files that would make the anaconda installer stops. Every kickstart file could be corrected by the hand of course, but one sed command could also correct the typo in all files in one go.
The Unix toolkit is full of handy tools and options like this and it pays to get to know your environment. Especially when it is your work environment and you’re familiar with the Unix philosophy.
- 01 May 2017
Generating documents in PDF form is becoming the standard nowadays, but how to generate them easily when they’re mostly free format? One of the goals of the Offensive Security Certified Professional (OSCP) Certification is writing a report based on the evidence you find. This is where LaTeX comes into the picture as you can easily have multiple files with data and one or more TeX-files combining this into a proper document. The question then also comes “How to optimize this pipeline?”
The first step is to see every report as a git repository where you can store and version all data. And running rubber locally solves the problem to quickly create a PDF from your sources, but wouldn’t it be nice if this part also could be automated? Who didn’t make the last moment change and forgot to run rubber if the document would still compile into a PDF? GitLab CI can luckily also compile LaTeX into a PDF and the notification if your update broke the build process comes for free.
- 04 September 2016
Last week Kali Linux 2016.2 was released so it was time to make a new VirtualBox instance for it to see the difference from the release in January. But let’s automate a little bit to quickly rebuild virtual machines for Kali Linux.
Let’s create the virtual machine and boot it. In this example it is bound to the wireless network card and allocates an 16 GB disk image as the default 8 GB size for Debian is too small and 10 GB is the minimum advised.
- 29 April 2015
Yesterday I wrote a post about disabling SSLv3 in Postfix and today we take a close look at Apache. While taking a closer look at the current installation of Apache and the version shipped with Debian 8 that was released a few days back it showed that the Apache project or Debian has taken the responsibility to completely disable SSLv2. Hopefully, SSLv3 will get the same treatment soon, as broken security is worse than no security due to a false sense of security.
After a clean install on Debian Wheezy
/etc/apache2/mods-available/ssl.conf contains the following entries:
- 28 April 2015
The POODLE attack was made public in late 2014 and most vendors have taken action to solve possible issues related to POODLE. The time definitely has come to close SSLv3 in all parts of the public-facing infrastructure. By default Postfix still only disallows SSLv2 and hopefully, this will change in the form of stricter default behavior in Postfix or distributions/vendors that stop shipping SSLv3 libraries.
For now, you can set with the
postconf command restrictions which protocols shouldn’t be used by Postfix.
- 28 January 2015
Installing and configuring SSL certificates is always an issue as to how to create them and where to store them. Most of the time people can find the procedure on how to create them, but they forget all the places where they have placed them. Some initiatives exist to have centralized key stores on systems, but getting applications to use them is still a problem.
Also on Debian is this an issue and key material is all over the system if youre not careful. Some Debian developers tried to fix it, but it ended in a “stalemate” and for now, an additional package called
ssl-cert exists to create self-signed certificates. This package also provides a structure for storing commercial certificates and accessing them in a safer way. We install the package
ssl-cert for this.
- 04 April 2013
Sometimes you have a system with legacy naming standards, but you really want to switch over to the new standard to keep all the scripting clean without some exceptions no one is going to remember in 12 months. Oracle had the command ALTER DATABASE, but since Oracle 10 you need to take the database offline and do some magic. MySQL got the RENAME DATABASE option with release 5.1.7 and lost the option again with release 5.1.23 as it was eating data.
Luckily PostgreSQL still has the command ALTER DATABASE so let’s rename a database and the owner. Before we start we need the password and then we need to log in as the PostgreSQL superuser
postgres or another account with similar privileges. So first we check the database name and owner.
- 12 February 2013
SpamAssassin has a feature to blacklist and unblacklist certain e-mail addresses. But recently I noticed something interesting that may need some more investigation. I have all addresses for domain example.org blacklisted, but also unblacklisted certain functional addresses as is shown in the example below.
Now I expected that firstname.lastname@example.org was going to be unblacklisted, meaning the mail would have a spam score of both +100 and -100 making it effective 0 again. This modification resulted in a spam score of +100 and makes me worry that unblacklisting will demand that the domain part needs to be specified instead of having a wildcard. This will require some more testing in the near future, but for now, it may affect other installations.
- 31 July 2012
Ext3 is a successor of Ext2 with support for journaling which means it can have a log of all the recent changes it made or is going to make to the file system. This allows
fsck to get the file system back in a good state when a power failure happens for example. But what is the size of the journal? Reading the manpage for
tune2fs it says it needs to be between 1024 and 102400 blocks which means it can start with 1MB on a file system with a 1KB block size and 4M on a file system with a 4KB block size.
So let’s start to see which inode contains the journal and normally this should be inode 8 unless you have a file system that was upgraded from Ext2 to Ext3 or Ext4.
- 08 July 2012
With the transition towards
/run some temporary files will move towards
/run/user/, but enough files remain in
/tmp. Files that may leak information or be a point of code injection as shown with CVE-2012-3355. A first step is to create a temporary directory for every user when he or she logs in to restrict the exposure of temporary files.
After installing the right module for PAM and enabling it, every user that logs in will get its own directory for temporary files. In this case, based on the user’s ID number, but is still only accessible to the user themself.
- 07 July 2012
Creating home directories for new users can be a difficult task and especially in an LDAP-based environment, but most PAM installations have the option to create a new home directory before the user login is completed. Debian also ships the module mpam_mkhomedir, but without a manifest to set it up correctly. Bug 640918 covers this issue, but for now, creating the file
/usr/share/pam-configs/mkhomedir with the content below resolves the problem.
After creating the file, the command below updates the PAM-config to create the home directory when a user’s home directory doesn’t exist. In the example configuration above the default umask is 0027 so only the user and group will have access to the home directory.
- 23 June 2012
Over the years PAM (Pluggable Authentication Module) has become the standard on Solaris and Linux, and others like AIX and the known BSDs are following. But by default, all services that use PAM are allowing all users to use it unless the service itself takes action. So why not bring the authorization part to PAM and make the decision to allow access directly in PAM?
In this example, we want to allow only access to Dovecot for users who are members of POSIX-group
ac_mail. For this, we use a module called
pam_succeed_if which can verify if a user is in a certain group or not. Based on the standard PAM file for a service, we create a new file for Dovecot and added the required line to do the authentication.
- 21 January 2012
In a previous posting, I started with BtrFS, and as mentioned BtrFS supports snapshotting. With this, you can create a point-in-time copy of a subvolume and even create a clone that can be used as a new working subvolume. To start we first need the BtrFS volume which can and must always be identified as subvolid 0. This as the default volume to be mounted can be altered to a subvolume instead of the real root of a BtrFS volume. We start with updating /etc/fstab so we can mount the BtrFS volume.
As /media is a temporary file system, meaning it is being recreated with every reboot, we need to create a mount point for the BtrFS volume before mounting. After that, we create two read-only snapshots with a small delay in between. As there is currently no naming guide for how to call snapshots, I adopted the ZFS naming schema with the @-sign as a separator between the subvolume name and timestamp.
- 18 January 2012
After using ZFS on Sun Solaris, I missed the ZFS features on Linux and with no chance of ZFS coming to Linux, I had to do with MD and LVM. Or at least until BtrFS became mature enough and since the Linux 3.0 that time slowly has come. With Linux 3.0 BtrFS supports auto defragmentation and scrubbing of volumes. The second is maybe the most important feature of both ZFS and BtrFS as it can be used to actively scan data on a disk for errors.
The first tests with BtrFS were in a virtual machine already a long time ago, but the userland tools were still in development. Now the command btrfs follow the path set by Sun Microsystems and basically combines the commands zfs and zpool for ZFS. But nothing compares to a test in the real world and so I broke a mirror and created a BtrFS volume with the name datavol:
- 09 August 2011
After upgrade Debian to kernel 3.0.0, I saw a hidden file called
/etc which I didn’t noticed before. Checking other machines gave the same result as shown below, but both without a matching Debian-package or manpage.
As time match at least the installation date of the machine and exists on other machines it appears to be a valid file, but with what purpose? After reading the Linux Programmer’s Manual two functions called lckpwdf and ulckpwdf where candidates for using this file. Checking the source code at Sourceware confirmed that both lckpwdf and ulckpwdf are using the file. And reading the manpage about these functions also confirms its purpose, a lock file the commands like passwd.