Monthly Archives: September 2015

How To Troubleshoot DNS Client Issues in Linux

DNS resolution is an important service, without it functioning properly domain names will not be correctly resolved to IP addresses preventing other network services from working correctly. Therefore it is equally important to know how to troubleshoot DNS issues on a Linux client and fix any problems to reduce disruption.

There are multiple potential points of failure during the DNS lookup process such as at the system performing the lookup, at the DNS cache, or on an external DNS server. Here we will cover how to check these and perform various tests to identify where exactly the problem lies.

Edit the XenServer Storage Heartbeat

Posted by Jarrod Farncomb on September 24, 2015 No comments

A Citrix XenServer host that is managed by Apache CloudStack will reboot itself along with all running virtual machines running on the host if there is a problem with the primary storage after a period of time. In general this is a good protection method, if the primary storage is no longer available it can cause problems for the virtual machines, Windows VMs may blue screen of death (BSOD) while Linux VMs may enter a read only file system state to protect against data loss. However you may have multiple NFS mount points to act as primary storage and may not want every VM on the XenServer host to power off with the host should this happen. It is possible to work around this by modifying the heartbeat script created by CloudStack on the XenServer host.

How To Configure a Caching-Only DNS Name Server

Posted by Jarrod Farncomb on September 22, 2015 7 comments

DNS resolution is used to lookup a domain name and resolve it to an IP address. Instead of having various systems in our network querying directly out to the Internet, we can configure a DNS caching server within our network and have other systems point to this for DNS resolution which will improve efficiency. The more systems sending queries through the caching server, the larger the cache will grow, further improving the hit rate.

The local cache of DNS records is stored closer to the systems initiating the requests, resulting in faster resolution time. These records will be cached until their TTL expires, or the cache is flushed. If any other system performs a DNS lookup for a query that resides in the cache it can be returned much faster as the caching server will not have to forward the query to some other DNS server out on the Internet.

There are many packages capable of acting as a DNS cache such as Bind, Dnsmasq and Unbound. In our example we will be working with Unbound.

Gzip vs Bzip2 vs XZ Performance Comparison

Posted by Jarrod Farncomb on September 17, 2015 30 comments

Gzip, Bzip2 and XZ are all popular compression tools used in UNIX based operating systems, but which should you use? Here we are going to benchmark and compare them against each other to get an idea of the trade off between the level of compression and time taken to achieve it.

For further information on how to use gzip, bzip2 or xz see our guides below:

Backup and Restore a MariaDB/MySQL Database

Posted by Jarrod Farncomb on September 15, 2015 12 comments

It is critical to regularly take a backup of important data that is stored within a database so that the data can be restored in the event of any problem. Examples of such problems include database server failure, data tampering, corruption and any other number of file system specific problems that are looking to do your database harm.

Here we will cover how to backup and restore an SQL database from MariaDB/MySQL. Read more »

13 Simple XZ Examples

Posted by Jarrod Farncomb on September 10, 2015 6 comments

xz is another general purpose data compression tool with syntax similar to the older and more popular gzip and bzip2 options. With xz we will typically get a better compression ratio.

We are going to cover 13 examples of xz here, showing you common tasks that can be completed and just how easy it is to use.

How To Configure Key-Based Authentication for SSH

Posted by Jarrod Farncomb on September 8, 2015 No comments

SSH keys can be used to increase the level of security for a user remotely authenticating to a Linux server through SSH. SSH keys are typically preferable in terms of security when compared to passwords as they are far less vulnerable to brute force attack, there is simply a lot more entropy in a key than password.

Here we are going to cover how to configure and use key-based authentication for SSH in Linux. Read more »

How To Install and Configure MariaDB

Posted by Jarrod Farncomb on September 3, 2015 1 comment

MariaDB is a drop in replacement for MySQL and is an open source fork of MySQL created by the original MySQL developers. As of Red Hat Enterprise Linux (RHEL7) and CentOS 7 MariaDB is now the default SQL database, rather than MySQL which was the default in previous releases of the operating system.

Here we’re going to cover how to install and configure MariaDB.

23 Hardening Tips to Secure your Linux Server

Posted by Jarrod Farncomb on September 1, 2015 5 comments

It is important to secure a Linux system as much as possible in order to reduce the likelihood of compromise.

Here are 23 security tips to guide you through hardening your Linux operating system.
Read more »

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