A sysctl variable represents file locations under the /proc/sys directory. The dot(".") notation is used when setting in a configuration file. Setting a sysctl parameter value immediately (not persistent) on any version of CentOS/RHEL Use the 'sysctl -w' command. Parameter which take multiple values should have the values enclosed in quotes. For example, to set net.ipv4.ip_forward to 1 and net.ipv4.ip_local_port_range to 1025-65535, use the below commands: # sysctl -w … [Read more...] about How to set sysctl variables on CentOS/RHEL
CentOS/RHEL 8
How to build a Systemtap kernel module in CentOS/RHEL 6,7,8
Question: It is not possible to install kernel-debuginfo and kernel-devel packages on a production server. Is there any alternate way to run SystemStap script on production machine without installing kernel-debuginfo and kernel-devel packages? The following are the requirements to build and run a systemtap kernel module: Development machine having the same architecture as the target (production) system. The kernel-devel, kernel-debuginfo and, kernel-debuginfo-common-x86_64 package … [Read more...] about How to build a Systemtap kernel module in CentOS/RHEL 6,7,8
How to disable firewalld and nftables and use iptables instead in CentOS/RHEL 8
CentOS/RHEL 8 now uses iptables as the default packet filtering framework, and is also used by firewalld. But if you try starting iptables, it would fail by default. Follow the steps outlined below to disable firewalld and nftables and enable iptables instead. 1. Install the iptables-services package: # dnf install iptables-services 2. Disable nftables and firewalld: # systemctl disable nftables firewalld Removed /etc/systemd/system/multi-user.target.wants/nftables.service. Removed … [Read more...] about How to disable firewalld and nftables and use iptables instead in CentOS/RHEL 8
How to add an Ethernet connection using nmcli in CentOS/RHEL 7
Question: How to use nmcli to add an Ethernet connection with manual or via DHCP configuration? The NetworkManager command line tool, nmcli, provides those who prefer to use the command line way to configure networking by controlling NetworkManager. It is installed along with NetworkManager. Installing NetworkManager Use nmcli with the object connection and command add. (This can be abbreviated to con add). The first step should be to have the latest version of NetworkManager package, … [Read more...] about How to add an Ethernet connection using nmcli in CentOS/RHEL 7
How to set the order to load certain modules in CentOS/RHEL 7 and 8
If you want to load modules that have dependencies then you can use the modprobe command. It will automatically resolve the dependencies and load the modules. Here are three ways to accomplish loading modules at boot automatically: 1. Using systemd-modules-load.service (Recommended) Use systemd-modules-load.service to load the modules during boot. 1. Add a file in /etc/modules-load.d/[name].conf containing module names. It will read the lines in order and load the modules … [Read more...] about How to set the order to load certain modules in CentOS/RHEL 7 and 8
How to configure initrd / initramfs to including kernel modules in CentOS/RHEL
The method for including kernel modules in the initrd / initramfs varies based on RHEL version. In all examples replace [kernel module] with the module name, not including .ko. For CentOS/RHEL 4 and 5 Three methods can be used to include a kernel module in the initial RAM disk: 1. Use the --withM option when manually building the ram disk with mkinitrd: # mkinitrd --with=[kernel module] /boot/initrd-$(uname -r).img $(uname -r) 2. For storage-related modules put an alias in … [Read more...] about How to configure initrd / initramfs to including kernel modules in CentOS/RHEL
How to configure systemd.path to trigger an event when any changes made to a directory
What is systemd.path systemd.path works similar to auditd in earlier versions with extra features as triggering an event or a run a script when a change is made to file or directory. From the man page of systemd.path: A unit configuration file whose name ends in ".path" encodes information about a path monitored by systemd, for path-based activation. Path files must include a [Path] section, which carries information about the path(s) it monitors. How to configure systemd.path … [Read more...] about How to configure systemd.path to trigger an event when any changes made to a directory
How to Configure Persistent logging for the systemd journal in CentOS/RHEL 7,8
By default in Red Hat Enterprise Linux 7 and 8, systemd is not configured to maintain logs persistently. The systemd journal is configured by default to store logs only in a small ring-buffer in /run/log/journal, which is not persistent. Journal database logs do not survive a system reboot. Follow the steps outlined below to configure a persistent systemd journal logging in CentOS/RHEL 7 and 8. 1. Configure systemd to persistently store journal logs in /var/log/journal by running the … [Read more...] about How to Configure Persistent logging for the systemd journal in CentOS/RHEL 7,8
How to disallow or disable HTTP TRACE and TRACK requests in httpd (Apache)
My security team communicated that we are vulnerable to "Apache HTTP TRACE / TRACK Methods Allowed" issue, How to fix it? What is Apache TRACE and TRACK methods TRACE and TRACK are two HTTP methods used to debug web applications. These methods could be leveraged by malicious users to perform Cross-site Tracing attacks which are used to bypass authentication token protections. Disabling the TRACE and TRACK methods Add the line "TraceEnable off" if not already present to the httpd … [Read more...] about How to disallow or disable HTTP TRACE and TRACK requests in httpd (Apache)
How to audit all Commands run on a Linux system using auditd
The audit package ties into the Linux kernel audit subsystem. The audit system audits system calls and other kernel level events, not user-space events, so we need to audit the execve() system call which is what starts executing new programs. For the example in this post, we have taken the CentOS/RHEL system, but the steps remain more or less the same for other *NIX distributions as well. For CentOS/RHEL 7 Apply audit rules into the system 1. To keep the rules persistent after reboot or … [Read more...] about How to audit all Commands run on a Linux system using auditd