What is OOM Killer? The OOM killer, a feature enabled by default, is a self-protection mechanism employed the Linux kernel when under severe memory pressure. If the kernel cannot find memory to allocate when it's needed, it puts in-use user data pages on the swap-out queue, to be swapped out. If the Virtual Memory (VM) cannot allocate memory and can't swap out in-use memory, the Out-of-memory killer may begin killing current userspace processes. it will sacrifice one or more processes in … [Read more...] about How to adjust a process score calculations for Out-of-memory situations in CentOS/RHEL
Linux
How To Setup mutt in CentOS/RHEL
Mutt is a full-featured text-based Mail User Agent that supports the typical mail formats and protocols while providing comprehensive support for MIME, GPG, and PGP. By its very nature Mutt is a very useful package, and because it is a tool that all server administrators should become familiar with, it is the purpose of this recipe to show you how Mutt can be used to deliver customized reports and messages to a destination of your choice. Installing and Configuring mutt 1. Install the mutt … [Read more...] about How To Setup mutt in CentOS/RHEL
Which network ports are reserved by the Linux Operating System?
For the Linux Operating system, there are 3 different kinds of network service ports: The Well Known Ports (0-1023) - which are reserved for the operating system and core services. The Registered Ports (1024-49151) - which can be used by applications, specific services, and users. The Dynamic and/or Private Ports (49152-65535) There is a standard list of these network service port assignments for Linux documented … [Read more...] about Which network ports are reserved by the Linux Operating System?
How to Reserve a Port Range for a Third Party Application in CentOS/RHEL
This post provides guidance on how to reserve a specific port range on CentOS/RHEL systems when these ports are needed for a third party application. For example, one need a port range between "10000 to 15000" to be used by Application A and one need a port range between "20000 to 25000" to be used by Application B. Those ports in between "10000 to 15000 " and " 20000 to 25000" are going to be exclusively reserved for Application A and B. In CentOS/RHEL operating system side one have to … [Read more...] about How to Reserve a Port Range for a Third Party Application in CentOS/RHEL
How to reserve port for an application using the portreserve service
The Ask The user needs to reserve a port(s) for the exclusive use of an application. User wish to make sure that no other application can bind to those ports before our application has been started. The Solution The portreserve service can be used to bind to a set of ports when the system is first booted. Until the ports are released via the portrelease command any attempt to bind to those ports will result in a EADDRINUSE error. Lets add few ports in the configuration file … [Read more...] about How to reserve port for an application using the portreserve service
Linux OS Service ‘portreserve’
The portreserve program aims to help services with well-known ports that lie in the portmap range. It prevents portmap from a real service's port by occupying it itself, until the real service tells it to release the port (generally in the init script). For example the cups package provides /etc/portreserve/cups. # cat /etc/portreserve/cups ipp In /etc/services, it is defined "service-name ipp" is equal to 631 ports. When portreserve service starts, portreserve reserves 631 ports. # … [Read more...] about Linux OS Service ‘portreserve’
“shutdown” Command Examples in Linux
Introduction In this article, we are going to learn Linux shutdown command with examples. The shutdown command is used to halt/shutdown, reboot the Linux Operating System. When shutdown is entered correctly—as root, and with a valid time argument — it does the following. Notifies all users the system is going down Blocks additional logins Notifies all running programs so they can close/save files and exit gracefully Notifies init to change the runlevel Psssst! Tell init to take us to … [Read more...] about “shutdown” Command Examples in Linux
How to Remove a file with space in Linux
The rm command is used to remove files or to be more technical, unlink a file. The rm command is provided by the coreutils RPM. See the remove man page, man rm, for usage, additional options, and command details. Removing a File Without a Space In order to remove a file, you can use the ls command to list the contents of the directory you are currently is and find the filename you want to remove. For example, I have a file in the /tmp/ directory called "file_without_a_space" that I will … [Read more...] about How to Remove a file with space in Linux
How to Force umount an NFS share on an NFS Client When NFS Server is Down
The Problem The user is unable to umount the NFS share on the NFS client while the NFS server is down. How can we forcefully umount the mount point? The Solution The following two umount options are the most relevant and are described in the umount(8) man page: # man umount -f, --force Force an unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.) ... -l, --lazy Lazy unmount. Detach the filesystem … [Read more...] about How to Force umount an NFS share on an NFS Client When NFS Server is Down
How to configure NIS services to use static port in CentOS/RHEL
The Network Information Service (NIS) facilitates the sharing of critical data stored in flat files among systems on a network. In managing a network consisting of many systems/hosts, it is often ideal to have a common repository (such as NIS) for storing user and group information traditionally stored in flat files such as /etc/passwd and /etc/group. Making such files available via NIS would allow any remote and properly configured NIS client machine to access or query the data contained in … [Read more...] about How to configure NIS services to use static port in CentOS/RHEL