Enable KSM Centos7/Debian

If you run a lot of Virtual Machines using qemu, you probably can save a lot of memory, at the expense of CPU cycles, by enabling Kernel Samepage Merging (KSM).
If you just want to test it and run only on the current boot run the following:

echo 1 > /sys/kernel/mm/ksm/run

After testing if you want to enable it permanently you can use systemd temporary files.
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Enabling Mattermost on the Gitlab Instalation

A Self-hosted installation of GitLab comes with an extremely interesting and useful tool that is disabled by default, the Mattermost chat server, think of it as a self-hosted and better Slack.
If you installed GitLab using the Omnibus package, the recommended way, you can enable it and set up a letsencrypt certificate in a few steps as described bellow.

Open the file ‘/etc/gitlab/gitlab.rb’ and find the line:
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Install python2-certbot-dns-route53 centos 7

If you’ve been bitten by the following bug during the installation of the certbot DNS auth plugin for route53.

---> Package python2-boto3.noarch 0:1.4.6-1.el7 will be installed
--> Processing Dependency: python2-s3transfer >= 0.1.10 for package: python2-boto3-1.4.6-1.el7.noarch
Package python2-s3transfer is obsoleted by python-s3transfer, but obsoleting package does not provide for requirements
--> Finished Dependency Resolution
Error: Package: python2-boto3-1.4.6-1.el7.noarch (epel)
           Requires: python2-s3transfer >= 0.1.10
           Available: python2-s3transfer-0.1.10-1.el7.noarch (epel)
               python2-s3transfer = 0.1.10-1.el7

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Enable(fix) write permissions on a NFS share mounted on Windows 10

One annoying “feature” of the windows 10 NFS client is that by default the anonymous user uid and gid is set to -2, and so you can create new files and directories on your NFS server, but you can’t edit or remove existing files.
To fix it, you can paste the code bellow in a text file and save it with a ‘.reg’ extension, run it and reboot the machine.
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Apache Reverse Proxy Example

The Simplest reverse proxy configuration possible is the code below, with this configuration your server will proxy all requests from example.com
to subdomain.example.com,

<VirtualHost *:80>
    ServerAdmin [email protected]
    ProxyRequests off
    ServerName example.com
    ProxyPreserveHost On
    <Location />
        ProxyPass http://subdomain.example.com/
        ProxyPassReverse http://subdomain.example.com/
        Order allow,deny
        Allow from all

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Disable IPv6 on Linux Via Kernel Boot Parameters

If for any reason you want to disable ipv6 and don’t want to do it with the sysctl directive, another easy route is to append ‘ipv6.disable=1’ to the kernel boot parameters.

The grub Way

Open /etc/default/grub and edit the line that starts with GRUB_CMDLINE_LINUX_DEFAULT.
Add the entry ‘ipv6.disable=1’ to the arguments that are already there.
Save and run

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Apache auto-create Virtual-Hosts*

In this guide we’ll create a generic apache config file to ‘automatically’ create virtual hosts for a specific domain in your server

Sometimes you have a development server with one primary domain, and some sub-domains with your projects and/or stuff that still need to be validated

Firstly you will need to disable SELinux or configure the folders appropriately ( not covered in this guide ).
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