Linux User & Group File Permissions

In Linux, every file and directory have some permission’s set by default. But now the very first thing comes in mind is that what is need of any file permission? When we work within files and directories,we do not want other users to do any changes in our files. If other users have the access to our file, it is possible that we can lose our data with a small mistake of other user. So to escape from this situation, we can configure file permission’s on our files and directories.

Basically,a file permission is a set of permission for controlling access to a file. In Linux,different files and directories have the permission that specify which user or group can read, write or modify and access the files and directories. Now question is; who is user, who is group and who is other user? Don’t be confused, i am explaining it here:

User: user is owner of file or directory,who creates a file.
Group: this is a group,who owns the file and directory.
Other: All users,which are not the owner of file or not the part of the group who owns the file.
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Linux User & Group Administration

In Linux, User and Group administration is manged by some special files. User related information is always stored in ‘/etc/passwd’ and ‘/etc/shadow’ and Group related information is stored in ‘/etc/group’.

Now lets see what command can be used for the administration of User and Group.

To add a new user

[root@localhost ~]# useradd

To set password for the user

[root@localhost ~]# passwd

Note: By default every user account is locked, when it is created.

To set blank password for user

[root@localhost ~]# passwd -d

To delete user

[root@localhost ~]# userdel -r

Note: If you dont use ‘-r’ option with this command, user entries from ‘/etc/passwd’ and ‘/etc/shadow’ will be removed, but their home directories will be remain exist.
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How Email Server Works

E-Mails have become an integral part of our daily routine. We can communicate with anyone in world who has an Internet connection and an e-mail account. E-mailing is so easy, thats why people think that they don’t need to understand about the email’s working. But i think you should know about this process to better understand how your mails are going from your system to others system and from others system to your system. So here i am going to show you how your mail server works.

Mail server is a computer which is used to interchange the emails between the systems. There are some parts of emails that are necessary for emails to work. Before knowing how all these parts work,lets have a look what all they work.

1] MUA: It stands for Mail User Agent. It is used to read and write emails. It is also known as email client. Novell Evolution, Mozilla Thunderbird,Microsoft Outlook are example of MUA.

2] MTA: It stands for Mail Transfer Agent. It is used to transfer email between server and clients.It is also called as outgoing mail server. Sendmail, Postfix, Qmail are example of MTA.

3] MDA: It stand for Mail Delivery Agent. It retrieve message from MTA and drop the message into users mailbox. It is also called as incoming mail server. Procmail is the example of MDA, which is installed and running by default in most of systems.
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How DNS Works

Today we spend a huge amount of time on Internet. We browse many different websites and get valuable information from it. But have you ever wondered how all this happens? I mean to say that how these websites open instantly when you type their name on address bar of your browser and they open instantly. May be you never thought of this. But i think it is very interesting to know that how all this happens. Keep reading this article if you want to know about this interesting process.

When you enter a website name like “www.linuxonlinesolutions.com” at the address bar of your browser, the website ‘www.linuxonlinesolutions.com’ opens instantly. But you will be amazed to know that this website doesn’t open directly, as this request doesn’t go to the webserver, but to the DNS first. DNS is the reference for Domain Name Server. It converts Domain name like “www.linuxonlinesolutions.com” to an IP address like “216.18.203.210”. It uses port number 53 to response the DNS queries. You will be thinking that what is the need to convert Domain name into an IP address? Can’t a website open directly? Well let me clear it to you with an example. Suppose i tell you my name is ‘vinod’ and my phone number is “9898658547”. Now what is easier for you to remember? My name or My phone number? I think you all will agree that my name is easier for you to remember. It is our human tendancy that we can remember names easily as compare to the numbers. But computers uses a unique strategy. They can only remember the number. Computers knows binary language and they uses this language to communicate with other computer and programs. This is the reason why we use DNS.
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