Linux Server

Ubuntu Release Download checksum

A quick note on the links related where to find the ubuntu releases and how to do a checksum to ensure that the downloaded copy

If you are downloading Ubuntu from the official site, MD5/SHA1 checksum (ubuntu hashes) for latest releases may not be listed there.

If it is not there, you can visit the site http://releases.ubuntu.com/. This site lists a comprehensive list of ubuntu releases and their checksums. Checksums ensure that the download is complete without corruption and is 100% accurate. So, it is always advisable to do a checksum after each download.
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Linux: How many Run Levels are there and what are they?

Linux systems have 7 Run Levels and they are as follows.

0 – system halt
1 – single user mode (system maintenance mode)
2 – multiuser mode without network access
3 – full multiuser mode with network access
4 – not used
5 – multiuser mode with X windows
6 – reboot or restart

Each run level in linux has a very specific purpose.

Run level 1 is also known as system maintenance mode and at this level the system prompt appears without root level password. It should be used only when it is absolutely necessary for system maintenance.

Run level 4 is not used and it can be used by the user to customize his own level.

Note: Solaris Run Level is different from Linux run levels.

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Linux: What is Run Level?

Run Level is a term to indicate your computer’s state of operation. When your computer starts it initially starts the the first process called the init process. Init process in turn starts associated sub-processes for that state of operation.

Run Level is the state of the machine – indicating the processes and sub-processes that needs to be started in order for the Operating System to be in that state.

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What is Kerberos Authentication System?

Kerberos Authentication is a means by which a communicating entity on a non-secure network can prove itself to another entity about its identity in a secured way.

In a system developed for a client-server model, it is inconceivable to identify the threat posed by the network users and the intruders.

Primary goal of Kerberos Authentication System is to prevent free text based transmission of passwords over the network.

Kerberos was created by MIT with a license similar to BSD and the current version is Version 5. It is implemented across various OS – Linux, Unix, Windows, MacOS.

Kerberos system is built on a symmetric key algorithm.
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Apache: How to save configuration settings without disconnecting users from web server?

There are times when you would like to do some changes in your web server configuration file which might necessitate the server to be restarted in order for the changes to take effect.

Issuing a apache restart command will forcefully close all the connections made to the web server which will end any user related activity on the server.

If you would want to avoid such scenarios then issue the following command

$ sudo /etc/init.d/apache2 graceful (in ubuntu)

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How to display the history of commands executed by the user?

Linux: History command

Each bash sessions stores the history of commands executed in .bash_history file which in turn in stored under each user’s home directory

usr100@dev01:/tmp/user$ ls
07-14.txt a.txt b.txt d.txt

usr100@dev01:/tmp/user$ cat ~/.bash_history
clear
su root
rm -i d.txt
ls -ltr
touch c.txt

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Nagios: Temporary failure in name resolution

Troubleshooting the above warning message in Nagios:

i) If you can specify the IP address of the host with its port number try specifying it that way instead of the host name

ii) Check the the time that is set for the http check parameter and based on it try setting the -w (warning message) parameter (sample setting: command_line /usr/lib/nagios/plugins/check_http -H mywebsite.com -t 60 -w 55)

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How to find the environment settings in Linux?

The following command will give you the environment settings in Linux

$ env

This is equivalent of PATH in windows.

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How to set Tomcat to run on port 80?

Setting Tomcat to work on port 80 instead of port 8080

There may be instances when you want Tomcat to run on port 80 and not on port 8080 which is the default port allocated to it on installation.

If Apache is installed in the machine, then you need to stop the apache server (if this were to be for a temporary cause) and if you need it permanently then you should disable apache on start up.
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How to configure Tomcat in Ubuntu?

Configuring Tomcat in Ubuntu with Apache server running on it

Login with the root account. Check if java is already installed on the machine. If not, first install java on ubuntu.

Web get the apache-tomcat tar file from the below location
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