APACHE - failed to start

[On]

Red Hat Enterprise Linux Server release 7.4 (Maipo)

Apache was not running and attempt to start the httpd service failed. The natural thing as a sysadmin you would check the error_log. 

# systemctl restart httpd

# tail -f /var/log/httpd/error_log (location could be different on your installation)

--- error snippet---

[Tue Feb 20 17:57:32.667940 2018] [auth_digest:error] [pid 6256] (28)No space left on device: AH01762: Failed to create shared memory segment on file /run/httpd/authdigest_shm.6256

[Tue Feb 20 17:57:32.667948 2018] [auth_digest:error] [pid 6256] (28)No space left on device: AH01760: failed to initialize shm - all nonce-count checking, one-time nonces, and MD5-sess algorithm disabled

--- error snippet---

2 things to observe here. 

1. no space left of device
2. failed to create shared memory

As it turns out /run partition was used up 100% - which explains the subsequent error -  no. 2. 

Basically - the issue here is Apache has been leaving some stuff behind in shared memory. So the next command list those stuffs 

# ipcs -s (s is semaphores)

--- output ---

------ Semaphore Arrays --------

key        semid      owner      perms      nsems

0x00000000 3375104    apache     600        1

0x00000000 3407873    apache     600        1

0x00000000 2359298    apache     600        1

0x00000000 3440643    apache     600        1

0x00000000 3473412    apache     600        1

0x00000000 3506181    apache     600        1

--- output ---

You may proceed to remove the semaphores (apache only) by executing this command

# ipcrm -s  {semid} (if possible one after another)

Then proceed with restarting the httpd service

# systemctl restart httpd

# systemctl status httpd (verify Apache is running)

# ipcs -s (verify the semaphores are cleared)

Also, it is a good practice to reboot your machine once in 2 or 3 months that will actually clear the unused semaphores. 

PostgreSQL - Step by step guide installation from source

[Tested On]
CentOS Linux release 7.4.1708 (Core)

# wget https://ftp.postgresql.org/pub/source/v9.6.6/postgresql-9.6.6.tar.gz
# tar xvzf postgresql-9.6.6.tar.gz
# cd postgresql-9.6.6
# mkdir /opt/postgresql-9.6.6
# ./configure --prefix=/opt/postgresql-9.6.6
--- possible error output ---
configure: error: readline library not found
--- possible error output ---
# yum install readline-devel (install the missing library as reported above)
# make
# make install
# adduser postgres
# passwd postgres (set password)
# mkdir /opt/postgresql-9.6.6/data
# chown postgres:postgres /opt/postgresql-9.6.6/data
# su - postgres
$ /opt/postgresql-9.6.6/bin/initdb -D /opt/postgresql-9.6.6/data/
$ /opt/postgresql-9.6.6/bin/postmaster -D /opt/postgresql-9.6.6/data >/opt/postgresql-9.6.6/data/log 2>&1 &
$ ps -eaf | grep postgres (verify postgres is running)
$ vi ~/.bashrc
export PATH=$PATH:/opt/postgresql-9.6.6/bin
-- Save the file
-- Exit to root

--- Start and Stop postgresql using systemctl (systemd) ---
# cd /usr/lib/systemd/system
# vi postgres.service

--- Insert the following script  (change the postgresql location paths accordingly) ---
# START SCRIPT
[Unit]
Description=PostgreSQL database server
After=network.target

[Service]
Type=forking

User=postgres
Group=postgres

# Where to send early-startup messages from the server (before the logging
# options of postgresql.conf take effect)
# This is normally controlled by the global default set by systemd
# StandardOutput=syslog

# Disable OOM kill on the postmaster
OOMScoreAdjust=-1000
# ... but allow it still to be effective for child processes
# (note that these settings are ignored by Postgres releases before 9.5)
Environment=PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj
Environment=PG_OOM_ADJUST_VALUE=0

# Maximum number of seconds pg_ctl will wait for postgres to start.  Note that
# PGSTARTTIMEOUT should be less than TimeoutSec value.
Environment=PGSTARTTIMEOUT=270

Environment=PGDATA=/opt/postgresql-9.6.6/data


ExecStart=/opt/postgresql-9.6.6//bin/pg_ctl start -D ${PGDATA} -s -w -t ${PGSTARTTIMEOUT}
ExecStop=/opt/postgresql-9.6.6/bin/pg_ctl stop -D ${PGDATA} -s -m fast
ExecReload=/opt/postgresql-9.6.6/bin/pg_ctl reload -D ${PGDATA} -s

# Give a reasonable amount of time for the server to start up/shut down.
# Ideally, the timeout for starting PostgreSQL server should be handled more
# nicely by pg_ctl in ExecStart, so keep its timeout smaller than this value.
TimeoutSec=300

[Install]
WantedBy=multi-user.target
# END SCRIPT

--- Save the file

# systemctl daemon-reload
# systemctl enable postgres
# systemctl start postgres

--- Verify postgres is running
# systemctl status postgres

LINUX - Installing a package after a fresh installation

[Tested On]

CentOS Linux release 7.4.1708 (Core) x64

I installed CentOS on my VM with "minimal selection" which later I came to realize that I required cc, gcc and python compilers to install a couple of third party software. 

I performed these steps :

# yum group list

The required compilers are available in the "Development Tools" package which you will find under the Available Groups:

To proceed with the installation, execute the following command:

# yum --setopt=group_package_types=mandatory,default,optional groupinstall "Development Tools"

This will basically install "all" packages under the Development Tools which I prefer to avoid any dependencies issues later on. 

POSTGRESQL - Comparison between PostgreSQL vs EDB Postgres Advanced Server

Your new IT project requires you to setup postgresql database and most of the time we get to the postgresql website and download the version (either the source or rpm files)  and proceed with installation accordingly.

Before you do that, please understand the specific requirement for the database or consult with the project manager before you make a decision to choose either postgresql or EDB Postgresql Advanced Server for your project. 

Please go through the comparison by clicking the link below. 
https://www.enterprisedb.com/products/compare-postgres-databases

LINUX - Delete user account and its home directory

[Tested On]

CentOS Linux release 7.4.1708 (Core)

First please verify user home directory 

# cat /etc/passwd | grep {user}

testuser:x:1001:1002::/home/testuser:/bin/bash

--- /home/testuser - is the user home directory

To delete the user and everything in its home directory, issue the command below

# userdel --remove test

LINUX - Test ping ipv6 address

[Tested On]
Red Hat Enterprise Linux Server release 7.4 (Maipo)

--- Obtain your ipv6 address and network device information

# ifconfig

--- Test ping using ping6

# ping6 {ipv6-address}%{network-device}

SMTP - Testing SMTP communication using telnet client

[Tested On]
Red Hat Enterprise Linux Server release 7.4 (Maipo)

Note: For Windows Server, please make sure telnet client is installed.

# telnet
open {mail-server} 25
EHLO {mail-server}
MAIL FROM:{sender}
RCPT TO:{recipient}
DATA
Subject: Test from SMTP Mail Relay
This is a test message
. (this is a dot)
QUIT

--- expected output ---
open {mail-server} 25
EHLO {mail-server}
-- output --

250-mail-server Hello mail-server [IP], pleased to meet you

250-ETRN

250-AUTH LOGIN CRAM-MD5 PLAIN

250-8BITMIME

250-ENHANCEDSTATUSCODES

250 SIZE
-- output --
MAIL FROM:{sender}
-- output --
250 2.1.0 Sender OK
-- output --
RCPT TO:{recipient}
-- output --
250 2.1.5 Recipient OK
-- output --
DATA
-- output --

Start mail input; end with <CLRF>.<CLRF>

-- output --

Subject: Test from SMTP Mail Relay

This is a test message
.

250 2.6.0 Ok, message saved

QUIT

221 2.0.0 See ya in cyberspace

Connection closed by foreign host.

--- expected output ---