Tuesday, June 11, 2019

Steps to fix or resolve Error 15 or Error 17 or No files under /boot.

We get to see “Error 15” or “Error 17” when the ‘/boot’ file system is corrupted, so system fails to boot up. This is mainly because of primary boot loader files missing or not found. It could also be possible that the ‘/boot’ file system has been overwritten or formatted by mistake. 

Friday, May 17, 2019

Common Debugging Methods In Linux

The top 6 common debugging methods in Linux.

( 1 ) Increase logging verbosity
As usual, in Linux systems we could look out for errors/warnings/info related messages in "/var/log/messages" file. By default only ‘info’ ( information) and above messages gets logged in to this file, so to increase this logging verbosity one could edit the configuration line of "/var/log/messages" in "/etc/rsyslog.conf" file.

Change this line from :
*.info;mail.none;authpriv.none;cron.none                /var/log/messages

*.debug;mail.none;authpriv.none;cron.none                /var/log/messages

After making this change, save and exit the file. Restart the “rsyslogd” service ( 'systemctl restart rsyslogd.service' or 'service rsyslogd restart' in case of RHEL6.x and older) to make the changes come into effect. NOTE: Keep in mind that this would generate a lot of logs and "/var" may get filled up fast, so after the diagnosis make sure to revert the changes.

Sunday, March 3, 2019

What Is Firewalld & How To Configure Rules?

Image result for firewallIn this post we’d talk about ‘firewalld’ and it is benefits and how to get it configured in RHEL7 and above systems.

A Note on netfilter

In Linux ‘netfilter’ is being used which is a powerful network filtering sub-system. This allows kernel modules to inspect each packet traversing the system, and it could be dropped or rejected or allowed in a programmatic way and this applies to all incoming, outgoing and forwarded packets. So, in earlier RHEL6.x version there is ‘iptables’ being used which is a well-known netfilter program. However, in RHEL7.x version and above we've ‘firewalld’ in-place of ‘iptables’.

Thursday, January 17, 2019

Let's Automate System Health Check In Linux

Most of the modern day applications uses virtual systems which are cloud based hosted by some popular cloud service providers such as Amazon, Google, Azure, Verizon etc,. Since these systems are provisioned and maintained by the cloud service providers, the task of a Linux Administrator has gone reduced. Gone are the days when a Linux Admin spends hours of time in analyzing crash dump to understand the symptoms of reboot/crash and finding out ways to tune the system. So, in this virtual world, application vendors doesn't really bothered or interested if a virtual machine crashes, because they get to host the services on another virtual system with absolutely zero or minimal downtime. All such tasks of maintaining virtual systems are done by cloud providers. Hence, from application perspective there are no worries as such. However, there are times when it is required to track/monitor a Linux system health which is hosting some applications.

Tuesday, December 4, 2018

RHEL 8 Beta: Bits & Pieces

Read this page to understand differences between RHEL 8 and 7:

Yes, Red Hat has recently released the Beta version of Red Hat Enterprise Linux (RHEL) 8.  This is still in a rigorous testing and development phase, so there could be some changes compared with ‘Stable’ release later. As everyone I was also keen on installing the ‘beta’ version and checking how it works, what are the differences, what’s new, what is not available etc., hence, I thought of writing up this post to give a glimpse of RHEL 8 beta.

Thursday, November 1, 2018

How-To Recover Accidentally Deleted Disk Partition In Linux

There are many a times that we’ve come across this situation where an “hard drive partition table has been erased accidentally” or “partition data is missing or corrupted” and hence, unable to mount respective block devices. Yes, “how to recover or repair corrupted or deleted partition table of a hard drive” or “how can I recreate hard drive partition table without losing data” are some of the keywords that we hit on Internet to find out a solution. There are a few third party tools (many from open-source) available which could get this job done, however, I wanted to show how this could be done using native tools/commands in Linux operating system. There are at times that we could not use those third party tools, hence, we’ve to go with native way of restoring partition table data. So, let’s see how could we get this achieved. NOTE: If a hard drive is extremely damaged (physically) or if it is re-formatted (making a file system) or if meta-data itself is gone corrupted or severely damaged then there are very less chances of recovering data, in such cases users may have to rely on some sophisticated third party tools which may recover some or part of data. The situation that I’m doing to cover here is wherein hard drive partition table is erased, however, no re-formatting has been done yet.