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error recovery

When you cannot monitor your server for service availability, it is better to take help of automated monitor and restart utility. Last 4 days I was away from my server as I was enjoying my vacation. During this time due to load my lighttpd webserver died but it was restarted automatically within 2 minutes. I had utility configured for monitoring services on a Linux system called monit. It offers all features you ever needed for system monitoring and perform error recovery for UNIX like system.

Before monit I had my own shell and perl script for monitoring service. If service failed script will try to restart service and send an automated email to me. However monit is a superior solution.

monit is a utility for managing and monitoring processes, files, directories and devices on a Unix system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations. For example, monit can start a process if it does not run, restart a process if it does not respond and stop a process if it uses to much resources. You may use monit to monitor files, directories and devices for changes, such as timestamps changes, checksum changes or size changes.

Monit logo

You may also use monit to monitor files, directories and devices on localhost. Monit can monitor these items for changes, such as timestamps changes, checksum changes or size changes. This is also useful for security reasons you can monitor the md5 checksum of files that should not change.

Personally, I always install and configure monit on all boxes which are under my control.

Install monit under Debian or Ubuntu Linux

Use apt-get command to install monit
# apt-get install monitOR$ sudo apt-get install monit

Install monit under Red Hat enterprise Linux / CentOS Linux (source code installation)

Many distributions include monit. However monit is not included in official Red hat enterprise Linux. Just download monit source code from official web site using wget command:
# cd /opt
# wget http://www.tildeslash.com/monit/dist/monit-4.8.2.tar.gz
Untar monit
# tar -zxvf monit-4.8.2.tar.gz
# cd monit-4.8.2

Configure and compile monit:

# ./configure
# make

Install monit

# make install

Copy monit configuration file:

# cp monitrc /etc/monitrc

By default monit is located at /usr/local/bin/monit

How do I Configure monit?

monitrc is name of monit configuration file and it is by default located at /etc/monitrc location. However each distribution places file in different location: .
=> Source code installation : /etc/monitrc
=> Debian / Unentu Linux installation : /etc/monit/monitrc

Open monit configuration file and setup values as follows:
# vi /etc/monitrc

a) Run it as daemon and check the services (such as web, mysql, sshd) at 2-minute
intervals.
set daemon 120

b) Set syslog logging with the 'daemon' facility:
set logfile syslog facility log_daemon

c) Set mail server name to send email alert
set mailserver mail.cyberciti.biz
Set email format such as from email
set mail-format { from: alert@nixcraft.in
subject: $SERVICE $EVENT at $DATE
message: Monit $ACTION $SERVICE at $DATE on $HOST: $DESCRIPTION.
}

d) Now most important part, restart lighttpd or apache web server if failed or killed by Linux kernel due to any causes:
check process lighttpd with pidfile /var/run/lighttpd.pid
group lighttpd
start program = "/etc/init.d/lighttpd start"
stop program = "/etc/init.d/lighttpd stop"
if failed host 75.126.43.232 port 80
protocol http then restart
if 5 restarts within 5 cycles then timeout

Where,

  • check process lighttpd with pidfile /var/run/lighttpd.pid : You are specifying lighttpd pid file and daemon name
  • group lighttpd: Specify group name, which is allowed or used to start/restart lighttpd
  • start program = "/etc/init.d/lighttpd start" : Command to start lighttpd server
  • stop program = "/etc/init.d/lighttpd stop" : Command to stop lighttpd server
  • if failed host 127.0.0.1 port 80 : Server IP address and port number (80)
  • protocol http then restart : If above IP and port failed restart the webserver
  • if 5 restarts within 5 cycles then timeout : Try to restart 5 times; if monit cannot restart webserver 5 times; just time out to avoid race condition.

Here is my mysql server restart configuration directives:
check process mysqld with pidfile /var/run/mysqld/mysqld.pid
group database
start program = "/etc/init.d/mysqld start"
stop program = "/etc/init.d/mysqld stop"
if failed host 127.0.0.1 port 3306 then restart
if 5 restarts within 5 cycles then timeout

Here is my sshd server configuration directives:
check process sshd with pidfile /var/run/sshd.pid
start program "/etc/init.d/sshd start"
stop program "/etc/init.d/sshd stop"
if failed host 127.0.0.1 port 22 protocol ssh then restart
if 5 restarts within 5 cycles then timeout

Here is my Apache serverrestart configuration directives:
check process httpd with pidfile /var/run/httpd.pid
group apache
start program = "/etc/init.d/httpd start"
stop program = "/etc/init.d/httpd stop"
if failed host 127.0.0.1 port 80
protocol http then restart
if 5 restarts within 5 cycles then timeout

Replace IP address 127.0.0.1 with your actual IP address. If you are using Debian just start monit:
# /etc/init.d/monit start

If you are using Red Hat Enterprise Linux, start monit from /etc/inittab file:
Open /etc/inittab file:
# vi /etc/inittab
Append following line:
mo:2345:respawn:/usr/local/bin/monit -Ic /etc/monitrc

Now start monit:
# init -qOR
# telinit -q

You can verify that monit is started from /var/log/messages log file:
# tail -f /var/log/messagesOutput:

Nov 21 04:39:21 server monit[8759]: Starting monit daemon
Nov 21 04:39:21 server monit[8759]: Monit started

If lighttpd died, you will see something as follows in log file:

Nov 21 04:45:13 server monit[8759]: 'lighttpd' process is not running
Nov 21 04:45:13 server monit[8759]: 'lighttpd' trying to restart
Nov 21 04:45:13 server monit[8759]: 'lighttpd' start: /etc/init.d/lighttpd

You may use monit to monitor daemon processes or similar programs running on localhost or started from /etc/init.d/ location such as
=> Apache Web Server
=> SSH Server
=> Postfix/Sendmail MTA
=> MySQL etc

Further readings