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Creating a Chroot Jail for SSH Access

April 17, 2019 – 4:51 pm

Just a quick collection of notes on – rather than a definitive guide to – setting up an SSH chroot jail on RHEL 6. The same should work on RHEL 7 and unrelated flavors. For …

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Networking

Unix and Linux network configuration. Multiple network interfaces. Bridged NICs. High-availability network configurations.

Applications

Reviews of latest Unix and Linux software. Helpful tips for application support admins. Automating application support.

Data

Disk partitioning, filesystems, directories, and files. Volume management, logical volumes, HA filesystems. Backups and disaster recovery.

Monitoring

Distributed server monitoring. Server performance and capacity planning. Monitoring applications, network status and user activity.

Commands & Shells

Cool Unix shell commands and options. Command-line tools and application. Things every Unix sysadmin needs to know.

Commands & Shells, Networking »

Adding and Removing sshd instances on CentOS 6

April 20, 2019 – 8:24 pm
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This is a process and a couple of script to allow you to setup an additional sshd service on an alternate port.

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The Unix Oriental

April 14, 2019 – 4:16 pm
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Placed quite appropriately in the “Security” category – my favorite Oriental cocktail recipe. Distinguishing it from the classic preparation, are absence of sweet vermouth and lime juice, as well as addition of just a couple …

Notes on ownCloud configuration

April 12, 2019 – 4:13 pm
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The ownCloud file sharing application has been around for some time now, but somehow evaded my attention. My limited imagination makes me say “think of it us Dropbox hosted on the server in your basement”.

Multi-Dimensional Arrays in Bash

March 30, 2019 – 4:06 pm
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Bash does not support multi-dimensional arrays, but there is a way to imitate this functionality, if you absolutely have to.

Asciinema Notes

March 27, 2019 – 4:04 pm
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Asciinema is an awesome tool for recording your console sessions. It’s great for documenting processes and, in general, showing people how shit works: one look is worth a thousand words, as they say. Unfortunately, I …

Late Night Rant: Facebook

March 25, 2019 – 4:15 pm
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According to media reports, since 2012, millions of Facebook and Instagram logins and plaintext passwords have been sitting on some internal Facebook system, accessible by  thousands of the company’s employees.

Measure DNS Server Performance

March 25, 2019 – 4:04 pm
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This is not an entirely proper way to benchmark a DNS server, but, in a pinch, it should give you some idea of its responsiveness and stability.

The Facebook Fracas

January 9, 2019 – 4:48 pm
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The Facebook privacy saga is getting ridiculous. You’re using a free service that you signed up for – nobody was holding a gun to your giant head. And it’s not Gulag either: just delete your profile, uninstall the damned app, and forget Facebook ever existed.

Generating and Running Multiple Scripts

January 9, 2019 – 4:45 pm
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Admittedly, this has a limited range of practical applications and is more of a scripting exercise. The command shown here generates a bunch of temporary scripts each containing the sleep command for up to one minute.

Removing Chef Server Installation

January 9, 2019 – 4:44 pm
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Just a quick note on how to completely remove (more or less) Chef server installation from a CentOS/RHEL box.

Auto-Update /etc/hosts

January 9, 2019 – 4:39 pm
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This was a silly solution to a silly problem, but may come in handy in the future for other things. A user process was establishing frequent connections to a handful of FQDNs. Not having any IP caching capability, every connection attempt was preceded by DNS lookup.

Find Large Folders

January 9, 2019 – 4:37 pm
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When you get a filesystem space alert (and they tend to arrive at 3:37 on Saturday morning), deleting various supersized log files is one option I already covered in adequate detail. That is if you get so lucky as to find such a file to delete…

Deleting Hidden Files

August 7, 2018 – 4:14 pm
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This simplest of problems just never fails to find a new victim. For whatever reason you want to delete all hidden files in the current directory and subfolders. Without much thinking you just type rm …

Quickly Grow tmpfs filesystem

August 6, 2018 – 4:13 pm
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Just a quick command to double the size of the /tmp filesystem that uses tmpfs. This is  for those situations when you can’t be bothered to think.

Duck Duck Go Search from Shell

August 5, 2018 – 4:10 pm
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The ddgr python script is a command line interface for Duck Duck Go  – a privacy-oriented search engine that, unlike Google, will not give away your data to the likes of Cambridge Analytica. In the …

Obfuscating Shell Scripts

August 1, 2018 – 4:04 pm
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Unix shell scripting language is run by the command-line interpreter and, as such, can be read and understood by anyone with sufficient access and experience. Sometimes this is not a good thing. Sometimes you want …

The Mystery of a Hemisphere

July 31, 2018 – 4:00 pm
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To make this long (and embarrassing) story short: a friend’s kid had a homework assignment in geometry to calculate the surface area of a conjoined cone, a cylinder, and a hemisphere.

Raspbian ‘partitions on different devices’ Error

July 30, 2018 – 3:59 pm
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After downloading a Raspbian image and writing it to an SD card, you get the following error when trying to boot the Pi: “Could not expand filesystem, please try raspi-config or rc_gui. Boot and root …

Tmux Bootstrap Configuration

February 17, 2018 – 3:22 pm
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Just a list of commands to quickly configure tmux with some of the best (i.e. my favorite) plugins and options. Feel free to disagree – you may be right – but I don’t think so.

Find Large Files

February 17, 2018 – 3:21 pm
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Every time a filesystem is running out of space and I need to find something to delete, I end up googling the syntax for the find command to make sure it stays off NFS and …

Show Logged In Users During a Time Window

February 17, 2018 – 2:59 pm
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Let’s say you want to see who was logged in on the server during last weekend. This includes users who, say, logged in on Thursday and haven’t logged out until Saturday afternoon.

Extracting Email Addresses from TCP Streams

December 5, 2017 – 7:06 pm
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Here’s a quick example of using tshark to extract email addresses from TCP streams. Let’s say some application on your server is sending emails and you want to find out who is receiving those emails.

Export WP Posts and Sync to Sharepoint

November 14, 2017 – 12:24 pm
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Long story short: old wiki is overloaded and broken; new wiki is not a wiki but SharePoint – a glorified file server; people are forced to improvise and put documentation on a WP wiki; bosses …