Category Archives: Tips & Tricks

Display SSH Warning Message to Users

One of the easiest way to alert unauthorized people that they are trespassing when they try to access the SSH of your server (maybe by mistake) is to add a warning message before and/or after login. Of course, this does not mean that your server is secure when you add it, but at least they… Read More »

How to compute the MD5 hash of a file

MD5 is one of the most common types of hash used to verify the integrity of a file after a transfer (and not only). You may even find some files with .md5sum extension which contain the MD5 checksum of some existing files. The MD5 hash is commonly expressed as a 32 digit hexadecimal number, e.g.: 1bc29b36f623ba82aaf6724fd3b16718… Read More »

How to view all the open ports

One of the first thing you learn as a system administrator is that programs (applications, scripts) need TCP connections to communicate with other devices. This can sometimes also be a problem. Some exploits open a new port and can cause some serious problems. However, there is a very simple way to find out which programs… Read More »

Useful yum repositories

Yum is the easiest tool to find and install packages on Red Hat/CentOS. However, yum needs to know where to look for packages. Here is where repositories come into play. Of course, you always need to make sure the repositories you use are trusted! Installing packages from untrusted source can be pretty nasty since you… Read More »