Awk & sed Snippets for SysAdmins

Remove Duplicate IPTables Rules

Duplicate entries may appear in your iptables configuration due to parallel efforts of various network security-related applications that may be running on your server. In most cases, having duplicate iptables rules is no big deal, …

Block WordPress Attackers With IPTables

After installing LogStash, I noticed server load went from around 1.2 to 50+. This was cause by a brute-force attack against WordPress wp-login.php from some IP in France (of all places). While LogStash should not …

Patching Binary Files in Bash

Just the other day I ran into a particularly annoying issue: after an application upgrade, all printed documents had “<<bbj>>~D” on the first line. The application sends EPS files to the CUPS print server. For whatever reason, …

CUPS Printing to a File

Using CUPS to print to a file instead of an actual printer is a great idea for troubleshooting and archiving purposes. This saves trees but, most importantly, saves you the walk to the printer. So, how …

Quick Setup of Salt on CentOS/RHEL

Just some quick notes on installing and configuring Salt master server and minions to run remote commands for various sysamdin needs.

Searching Files for Text Strings

In the past, I’ve posted a couple of versions of this script in Bash and Korn shell. Here’s a much-expanded and improved Bash version. The script will take your input and look for files containing …

Recovering Deleted /bin on CentOS

So, as the saying goes, shit happens and the other day I accidentally deleted /boot on a CentOS 6 box. I have NetBackup running on the system, so this would not have been a problem …

Disable Formatting When Pasting into Outlook

By default, Outlook will preserve source text formatting during copy/paste operations. At times this can be very annoying, especially when composing emails of technical nature. There’s a good chance your email will end up looking …

What To Do When Windows Ignores SD Card

Here’s a common scenario: you insert or swap and SD card in your laptop and… nothing happens. Sometimes Windows decides to ignore the SD card for reasons unknown (well, the true reason is it’s Windows). …

Permanently Ban IP with Fail2Ban

If you’re not familiar with Fail2Ban: it’s a log analysis tool that detects failed login attempts for your SSH, FTP, etc services and uses IPTools firewall to temporarily drop connection requests from the offending IP addresses. …

Looking for Recently Changed Files

Locating most recently-modified files in the current directory is easy: ls -alt | head. However, searching a directory structure for recent changes may prove a little more challenging. The most common approach is to use …

Installing CentOS 6.x From USB

If at all possible, don’t do it. Find a blank DVD-RW and burn the ISO file. Most current Linux distros can be easily put on a USB stick using freely-available utilities and installed without a hassle. CentOS is not one of them. It would appear, CentOS was designed specifically to make installation from a USB stick a professional challenge for a seasoned sysadmin.

Matching Installed RPMs on Two Systems

The basic problem is this: you have server “A” with a bunch of packages -let’s say PHP-related stuff – installed you need for you application. You need to get the same application working on server “B”, which is not an exact copy of server “A” and is missing some of the packages.

Searching Large Database Tables

Recently I ran into a little problem: I needed to make a simple Web UI allowing users to search a database table containing a copy of the Postfix maillog for quarantined mail addressed to them. …

VMware vCenter Converter Standalone Notes

The vCenter Standalone Converter is a handy app you can run on your Windows or Linux PC to to P2V a remote server. As convenient as the Converter is, there are a few gotchas that …

Script to Verify Passwordless SSH Access

If you have passwordless SSH configured on multiple servers, it’s a good idea to verify your access from time to time. This task may get rather tedious with a large number of remote systems. The simple script below will cycle through a list of servers and make sure you can access them without being prompted for a password. Any failures will be saved in the CSV file for later analysis.

Celerra NFS/CIFS Mixed Permissions Mode

This is a quick reminder on how to configure a Celerra filesystem for dual NFS/CIFS access where Windows-side permissions would translate correctly to the NFS side. By default, Celerra datamover will mount filesystems in “Native” mode. In this case, a file or folder created by a Windows user via the CIFS share will take on the default Unix permissions on the NFS side: 755 for folders and 644 for files.

Dealing with Full Filesystems

Filled up filesystems is a recurring condition eating up sysadmin time on a regular basis. Some studies show that filesystems running out of space are responsible for most day-to-day issues handled by IT departments. Disk …

Quickly Grow EMC Celerra Filesystem

As we all know, Celerra and its kin can get a bit annoying with their “filesystem over 90%” warning emails. Enabling the “autoextend” feature for the filesystems is one option, but if you are a control …

Watch the Log

In the past few days my Postfix server has been having occasional problems talking to the mail gateway. They problem would come and go. The Postfix server would timeout trying to connect to the gateway and keep …

Bash scripting: lists and random things

Imagine you have an HPC cluster with a hundred compute nodes named node001-node100. The two commands below will help you generate a list of node names – either all name on one line or one name per …

iPhone Stopped Syncing with Google Calendar

A small but potentially annoying problem: on rare occasions, your iPhone/iPad may decide your calendar looks full enough and will stop syncing with your Google calendar. This is a situation where:

Calendar sync between your iDevice and …