What is /dev/shm and its practical usage ?

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What is /dev/shm and its practical usage ?

/dev/shm is nothing but implementation of traditional shared memory concept. It is an efficient means of passing data between programs. One program will create a memory portion, which other processes (if permitted) can access. This will result into speeding up things on Linux.

If you type mount command you will see /dev/shm as a tempfs file system. Therefore, it is a file system, which keeps all files in virtual memory. Everything in tmpfs is temporary in the sense that no files will be created on your hard drive. If you unmount a tmpfs instance, everything stored therein is lost. By default almost all distro configured to use /dev/shm.

Nevertheless, where can I use /dev/shm?
You can use /dev/shm to improve the performance of application software or overall Linux system performance. On heavily loaded system, it can make tons of difference. For example VMware workstation/server can be optimized to improve your Linux host’s performance (i.e. improve the performance of your virtual machines).

For example, if you have 8GB RAM then remount /dev/shm as follows:

# mount -o remount,size=8G /dev/shm

To be frank if you have more than 2GB RAM and if you running multiple Virtual machines this hack always improves performance.

SSH Authorized Keys Automatic Login

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SSH can automatically authenticate connections when the client presents an authorized key. A client gives it’s public key to a server and then when it connects the server knows it’s allowed in and automatically allows the connection. The Keys are specific to users, so a key for user_a will not let user_b in.

Few Simple Steps

  1. Create Keys

    If the keys don’t exists already you must create them. Look in your ~/.ssh for files called id_rsa and id_rsa.pub. If those files don’t exist say ssh-keygen -b 2048 -t rsa to create them.

  2. Place Keys on Server

    Copy id_rsa.pub to the server then append that to ~/.ssh/authorized_keys

Optionally you can use a DSA key, simply replace `rsa` with `dsa` above

SSH automatic login assistance script

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The script automatically pushes your SSH private key to the server specified so you can have automatic SSH,scp and sftp authentication. If you don’t have a key the script will generate one for you, also this script assumes that the user running the script has a home directory on the remote host. Should note that this script will only push the RSA encrypted key.

The Script

 

 

#!/bin/bash

# say: ./ssh_script.sh hostname

# Uploads your id_rsa.pub to the specified host, wrapped for readability

if [ ! -r ${HOME}/.ssh/id_rsa.pub ];then

ssh-keygen -b 2048 -t rsa

fi

# Append to the copy on the remote server

cat ~/.ssh/id_rsa.pub | ssh ${USER}@$1 “cat – >> .ssh/authorized_keys”

if [ $? -eq 0 ]; then

echo “Success”

fi

save the script say as  ssh_script.sh and execute it ./ssh_script.sh hostname

Virtuozzo glibc-dummy-centos-4 Error (1)

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If you come across this error when trying to do a “yum update” on your Virtuozzo Virtual Dedicated Server

–> Processing Dependency: glibc-common = 2.3.4-2.25 for package: glibc-dummy-centos-4–> Finished Dependency ResolutionError: Missing Dependency: glibc-common = 2.3.4-2.25 is needed by package glibc-dummy-centos-4

You can make use of the following steps to fix the problem. First you need to remove the package that is causing the problem.

yum remove glibc-dummy-centos-4

After you have removed that package you need to reinstall gcc with all the extras. You can do that simply by running the following command.

yum install gcc & yum install gcc*

If everything installed correctly you should now be able to run a yum update and it should not error out at all.

Enable SLM on Virtuozzo

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 What is SLM, “Virtuozzo Service Level Management (SLM) is a system that maintains and allows administrators to configure and control the levels of service (i.e., the set of ‘promises’) provided to VPS owners.”I came across some different ways of enabling SLM on the Virtuozzo Node, by far the best was the information about how to enable it with out a Node reboot. run the following commands. Edit the following files and make said changes

/etc/sysconfig/vz and change the SLM entry to slm=yes/etc/sysconfig/vzagent/vzagent.conf Locate the SLM entry, change it to 1. 

 

After you modified configuration file you need to restart VZAgent, use the following command on a hardware node:

vzagent_ctl restart 

 

How to install Cpanel and Fantastico in a VPS

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Problem:
cPanel installs a little bit differently in a VPS that it does on your traditional server.

Solution:
Install cPanel in CentOS / Fedora VPS:

You will need yum installed on the VPS

cat /etc/redhat-release

rpm -q yum centos-yumconf

 

CentOS 5 :

rpm –import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/python-elementtree-1.2.6-5.i386.rpm
rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/m2crypto-0.16-6.el5.1.i386.rpm

rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/python-urlgrabber-3.1.0-2.noarch.rpm
rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/sqlite-3.3.6-2.i386.rpm
rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/python-sqlite-1.1.7-1.2.1.i386.rpm
rpm -ivh http://mirror.centos.org/centos/5.0/os/i386/CentOS/rpm-python-4.4.2-37.el5.i386.rpm
rpm -ivh http://mirror.centos.org/centos/5.0/os/i386/CentOS/libxml2-2.6.26-2.1.2.i386.rpm
rpm -ivh http://mirror.centos.org/centos/5.0/os/i386/CentOS/libxml2-python-2.6.26-2.1.2.i386.rpm
rpm -Uvh http://mirror.centos.org/centos/5.0/os/i386/CentOS/yum-3.0.5-1.el5.centos.2.noarch.rpm

Fedora 6:

rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/python-elementtree-1.2.6-5.i386.rpm
rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/python-sqlite-1.1.7-1.2.1.i386.rpm
rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/rpm-python-4.4.2-32.i386.rpm
rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/python-urlgrabber-2.9.9-2.noarch.rpm
rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/yum-metadata-parser-1.0-8.fc6.i386.rpm
rpm -Uvh http://mirrors.kernel.org/fedora/core/6/i386/os/Fedora/RPMS/yum-3.0-6.noarch.rpm

Install cPanel :

mkdir /home/cpins
cd /home/cpins
wget http://layer1.cpanel.net/latest
sh latest

cd /usr/local/cpanel/whostmgr/docroot
rm -Rf fantastico*
wget -N http://netenberg.com/files/free/fantasticoinstaller.tgz
tar xzpf fantasticoinstaller.tgz
rm -f fantasticoinstaller.tgz

In /etc/xinetd.d/pop3ds

Change ‘disable = no’ to ‘disable = yes’

restart xinetd
restart cpanel

Fatal error: Allowed memory size of 123456 bytes exhausted (tried to allocate 234567 bytes) in /path/file.php

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Php is setup is to limit memory usage per process. If you require more, this limit can be increased.
Edit

/usr/local/lib/php.ini

If you are unsure about the php.ini path, You can find your server php.ini using command :

php -i | grep php.ini

Configuration File (php.ini) Path => /usr/local/lib

Loaded Configuration File => /usr/local/lib/php.ini

and set:

memory_limit = 8M ; Maximum amount of memory a script may consume (8MB)

to a higher value, like 20M. Save, exit, then restart apache.

DirectAdmin Apache won’t restart: [crit] (98)Address already in use: make_sock: could not bind to port 8090

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Apache won’t restart: [crit] (98)Address already in use: make_sock: could not bind to port 8090

[crit] (98)Address already in use: make_sock: could not bind to port 8090
or
[crit] (98)Address already in use: make_sock: could not bind to port 443
or
[crit] (98)Address already in use: make_sock: could not bind to port 80

If you see this error in your /var/log/httpd/error_log, it would mean that apache isn’t shutting down completely before trying to restart. This means that the new process won’t be able to bind to the given ports because they’re still being used by the old copy of apache that wasn’t completely shut down.

To solve this, use a different boot script that will wait for all apache processes to stop before starting the new one:

cd /usr/local/directadmin/customapache
rm -f httpd*
./build update

Once you have the new scripts downloaded, you need to install them:

RedHat:

cp httpd /etc/init.d/httpd
chmod 755 /etc/init.d/httpd
chkconfig httpd reset

FreeBSD:

cp httpd_freebsd /usr/local/etc/rc.d/httpd
chmod 755 /usr/local/etc/rc.d/httpd