Redhat Firewall configuration: from iptables to firewalld

Tools to manage firewall Packet filter rules in Linux Kernel is managed by an user-space application named iptables in CentOS and RedHat. Since CentOS 7, firewalld is introduced as an alternative to iptables. Firewalld can be installed and executed as a systemd service, and it is supposed to replace iptables. This article describes how to … Read moreRedhat Firewall configuration: from iptables to firewalld

Log shipping through ELK

A common devops task is build logging pipeline with ELK stack (Elasticsearch, Logstash, Kibana). Suppose the application is written in Java and currently use log4j’s RollingFileAppender to generate log files locally. We can use log4j’s socket appender to write to Logstash, which further pushes the log stream to Elasticsearch cluster. In this model, failure to … Read moreLog shipping through ELK

Coordination between resources in AWS CloudFormation

Update 2023: the practice outlined in this post has been outdated. This post is for archive ony. One of the reasons I prefer CloudFormation over Terraform is access to Helper scripts. Many legacy applications are not built with statelessness and the installation depends on host information of other layers in the stack. This requires communication … Read moreCoordination between resources in AWS CloudFormation

The IT journey across industries

Back in the days, in the absence of handy automation tool, I used to install 40+ servers all manually, following exact same sequence of steps 40 times. I did not neglect the existence of automation tools. At the get-go, I estimated the efforts between sorting out automation and just plowing through the install manually. The … Read moreThe IT journey across industries

Linux Admin Basics 3 of 3 – text processing, regex, sed & awk

Most of the text processing can be processed by awk and sed. Sed is non-interactive stream editor that allows you to specify all editing instructions in one place and execute them on a single pass through the file. Awk is a pattern-matching programming language. Using sed and awk requires some understanding of regular expressions. Here’s … Read moreLinux Admin Basics 3 of 3 – text processing, regex, sed & awk

cron and anacron in RedHat Linux (How logrotate works)

Cron and anacron We all know cron is a job scheduler. Many admin uses crontab to manage scheduled task. It is also important to know that crontab works at different levels as well, as well as the distinction between cron and anacron. They are similar, but different, managed by different sets of files. Below is … Read morecron and anacron in RedHat Linux (How logrotate works)