Home Cloud & Networking Ubuntu Server Cookbook

Ubuntu Server Cookbook

By Uday Sawant
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  1. Free Chapter
    Managing Users and Groups
About this book
Ubuntu is one of the most secure operating systems and defines the highest level of security as compared other operating system. Ubuntu server is a popular Linux distribution and the first choice when deploying a Linux server. It can be used with a $35 Raspberry Pi to top-notch, thousand-dollar-per-month cloud hardware. Built with lists that there are 4 million + websites built using Ubuntu. With its easy-to-use package management tools and availability of well-known packages, we can quickly set up our own services such as web servers and database servers using Ubuntu. This book will help you develop the skills required to set up high performance and secure services with open source tools. Starting from user management and an in-depth look at networking, we then move on to cover the installation and management of web servers and database servers, as well as load balancing various services. You will quickly learn to set up your own cloud and minimize costs and efforts with application containers. Next, you will get to grips with setting up a secure real-time communication system. Finally, we’ll explore source code hosting and various collaboration tools. By the end of this book, you will be able to make the most of Ubuntu’s advanced functionalities.
Publication date:
June 2016
Publisher
Packt
Pages
456
ISBN
9781785883064

 

Chapter 1. Managing Users and Groups

In this chapter, we will cover the following recipes:

  • Creating a user account

  • Creating user accounts in batch mode

  • Creating a group

  • Adding group members

  • Deleting a user account

  • Managing file permissions

  • Getting root privileges with sudo

  • Setting resource limits with limits.conf

  • Setting up public key authentication

  • Securing user accounts

 

Introduction


In this chapter, you will see how to add new users to the Ubuntu server, update existing users, and set permissions for users. You will get to know the default setting for new users and how to change them. Also, you will take a look at secure shell (SSH) access and securing user profiles.

 

Creating a user account


While installing Ubuntu, we add a primary user account on the server; if you are using the cloud image, it comes preinstalled with the default user. This single user is enough to get all tasks done in Ubuntu. There are times when you need to create more restrictive user accounts. This recipe shows how to add a new user to the Ubuntu server.

Getting ready

You will need super user or root privileges to add a new user to the Ubuntu server.

How to do it…

Follow these steps to create the new user account:

  1. To add a new user in Ubuntu, enter following command in your shell:

    $ sudo adduser bob
    
  2. Enter your password to complete the command with sudo privileges:

  3. Now enter a password for the new user:

  4. Confirm the password for the new user:

  5. Enter the full name and other information about the new user; you can skip this part by pressing the Enter key.

  6. Enter Y to confirm that information is correct:

  7. This should have added new user to the system. You can confirm this by viewing the file /etc/passwd:

How it works…

In Linux systems, the adduser command is higher level command to quickly add a new user to the system. Since adduser requires root privileges, we need to use sudo along with the command, adduser completes following operations:

  1. Adds a new user.

  2. Adds a new default group with the same name as the user.

  3. Chooses UID (user ID) and GID (group ID) conforming to the Debian policy.

  4. Creates a home directory with skeletal configuration (template) from /etc/skel.

  5. Creates a password for the new user.

  6. Runs the user script, if any.

If you want to skip the password prompt and finger information while adding the new user, use the following command:

$ sudo adduser --disabled-password --gecos "" username

Alternatively, you can use the useradd command as follows:

$ sudo useradd -s <SHELL> -m -d <HomeDir> -g <Group> UserName

Where:

  • -s specifies default login shell for the user

  • -d sets the home directory for the user

  • -m creates a home directory if one does not already exist

  • -g specifies the default group name for the user

Creating a user with the command useradd does not set password for the user account. You can set or change the user password with the following command:

$sudo passwd bob

This will change the password for the user account bob.

Note

Note that if you skip the username part from the above command you will end up changing the password of the root account.

There's more…

With adduser, you can do five different tasks:

  • Add a normal user

  • Add a system user with system option

  • Add user group with the--group option and without the--system option

  • Add a system group when called with the --system option

  • Add an existing user to existing group when called with two non-option arguments

Check out the manual page man adduser to get more details.

You can also configure various default settings for the adduser command. A configuration file /etc/adduser.conf can be used to set the default values to be used by the adduser, addgroup, and deluser commands. A key value pair of configuration can set various default values, including the home directory location, directory structure skel to be used, default groups for new users, and so on. Check the manual page for more details on adduser.conf with following command:

$ man adduser.conf

See also

 

Creating user accounts in batch mode


In this recipe, you will see how to create multiple user accounts in batch mode without using any external tool.

Getting ready

You will need a user account with root or root privileges.

How to do it...

Follow these steps to create a user account in batch mode:

  1. Create a new text file users.txt with the following command:

    $ touch users.txt
    
  2. Change file permissions with the following command:

    $ chmod 600 users.txt
    
  3. Open users.txt with GNU nano and add user account details:

    $ nano users.txt
    
  4. Press Ctrl + O to save the changes.

  5. Press Ctrl + X to exit GNU nano.

  6. Enter $ sudo newusers users.txt to import all users listed in users.txt file.

  7. Check /etc/passwd to confirm that users are created:

How it works…

We created a database of user details listed in same format as the passwd file. The default format for each row is as follows:

username:passwd:uid:gid:full name:home_dir:shell

Where:

  • username: This is the login name of the user. If a user exists, information for user will be changed; otherwise, a new user will be created.

  • password: This is the password of the user.

  • uid: This is the uid of the user. If empty, a new uid will be assigned to this user.

  • gid: This is the gid for the default group of user. If empty, a new group will be created with the same name as the username.

  • full name: This information will be copied to the gecos field.

  • home_dir: This defines the home directory of the user. If empty, a new home directory will be created with ownership set to new or existing user.

  • shell: This is the default login shell for the user.

The new user command reads each row and updates the user information if the user already exists, or it creates a new user.

We made the users.txt file accessible to owner only. This is to protect this file, as it contains the user's login name and password in unencrypted format.

 

Creating a group


Group is a way to organize and administer user accounts in Linux. Groups are used to collectively assign rights and permissions to multiple user accounts.

Getting ready

You will need super user or root privileges to add a group to the Ubuntu server.

How to do it...

Follow these steps to create a group:

  1. Enter the following command to add a new group:

    $ sudo addgroup guest
    
  2. Enter your password to complete addgroup with root privileges.

How it works…

Here, we are simply adding a new group guest to the server. As addgroup needs root privileges, we need to use sudo along with the command. After creating a new group, addgroup displays the GID of the new group.

There's more…

Similar to adduser, you can use addgroup in different modes:

  • Add a normal group when used without any options

  • Add a system group with the--system option

  • Add an existing user to an existing group when called with two non-option arguments

Check out the manual page for the addgroup(man addgroup) to get more details.

See also

  • Check out groupadd, a low level utility to add new group to the server

 

Adding group members


Once you have groups in place, you can add existing users as well as new users to that group. All access rights and permissions assigned to the group will be automatically available to all the members of the group.

Getting ready

You will need super user or root privileges to add a group member to the Ubuntu server.

How to do it…

Follow these steps to add group members:

  1. Here, you can use adduser command with two non-option arguments:

    $ sudo adduser john guest
    
  2. Enter your password to complete addgroup with root privileges.

How it works…

As mentioned previously, you can use the adduser command to add an existing user to an existing group. Here, we have passed two non-option arguments:

  • john: This is the name of the user to be added to the group

  • guest: This is the name of the group

There's more…

Alternatively, you can use the command usermod to modify the group assigned to the user:

$ sudo usermod -g <group> <username>

To add a user to multiple groups, use the following command:

$ sudo usermod -a -G <group1>,<group2>,<group3> <username>

This will add <username> to <group1>, <group2>, and <group3>. Without flag –a, any previously assigned groups will be replaced with new groups.

 

Deleting a user account


If you no longer need a user account, it is good idea to delete that account.

Getting ready

You will need super user or root privileges to delete a group from the Ubuntu server.

How to do it...

Follow these steps to delete the user account:

  1. Enter the following command to delete a user account:

    $ sudo deluser --remove-home john
    
  2. Enter your password to complete addgroup with root privileges:

How it works…

Here, we used the deluser command with the option --remove-home. This will delete the user account named john and also remove the home and mail spool directories associated with john. By default, the deluser command will delete the user without deleting the home directory.

It is a good idea to keep a backup of user files before removing the home directory and any other files. This can be done with an additional flag along with the deluser command:

$ deluser --backup --remove-home john

This will create a backup file with the name john.tar.gz in the current working directory, and then the user account and the home directory will removed.

There's more…

When called with the --group option, the deluser command will remove the group. Similarly, when called with two non-option arguments, the deluser command will try to remove a user from a specific group:

$ deluser john guest # this will remove user john from group guest
$ deluser --group guest # this will remove a group

If you want to disable the user account rather than delete it, you can do it with the following commands:

$ sudo usermod --expiredate 1 john # disable the user account john
$ sudo usermod --expiredate "" john # re-enable user account john
$ sudo usermod -e YYYY-MM-DD john # specify expiry date

See also

  • Refer to the manual page for deluser with man deluser

 

Managing file permissions


We have created users and groups. In this recipe, you will work with default file permissions for users and groups, as well as see how to modify those permissions.

Getting ready

Create two users, user1 and user2. Create new group editor and add user1 and user2 as members.

How to do it…

Follow these steps to manage file permissions, follow these steps:

  1. To change groups for files and directories:

    1. Log in with user1.

    2. Create a new directory documents under home:

      	user1@ubuntu:~$ mkdir documents
      
    3. Create a text file under documents:

      	user1@ubuntu:~$ echo "hello world"> documents/file.txt
      
    4. Now log in with user2:

      	user1@ubuntu:~$ su user2
      
    5. Try to edit the same text file. It should say Permission denied:

      user2@ubuntu:/home/user1$ echo "hello again">documents/file.txt
      
    6. log in as user1 and change the group of documents to editor:

      	user1@ubuntu:~$ chgrp -R editor documents
      
    7. Switch to user2 and try editing the same file. Now it should work:

  2. To set permissions with chmod, follow these steps:

    1. Create simple shell script with the following command:

      	$ echo 'echo "Hello World!!"'> hello.sh
      
    2. Execute a shell script with the following command:

      	$ ./hello.sh
      
    3. Set executable permission to hello.sh with the following command:

      	$ chmod u+x hello.sh
      
    4. Check new permission with the following command:

      	$ ls -l
      
    5. Execute hello.sh again:

  3. To protect shared files with sticky bit, follow these steps:

    1. Log in as user1 and set sticky bit for directory documents:

      	user1@ubuntu:~$ chmod +t documents
      
    2. Log in as user2 and create a new file.

    3. Try to delete any file under documents. It should fail:

How it works…

When you create a new file or directory in Ubuntu, the default permissions for files are read and write access to owner and owner's private group, along with read, write, and execute access for directories. You can check the default setting with umask -S.

In our example, we have user1 and user2. Both of them are members of the editor group. When user1 creates a file, the default permissions are limited to user1 and its private group (user1) named after the user account. This is the reason user2 sees Permission denied on editing file. By changing the group of documents to editor we allow all members of editor to read and write to files in documents.

With the chmod command, we can set permissions at a more granular level. In our example of hello.sh, we have set the executable permission for hello.sh. Similarly, we can set read permission as follows:

$chmod +r filename

To set write permission, use the following command:

$chmod +w filename

You can set more selective permissions with additional parameters before mode expression as follows:

$chmod ugo+x filename

Here, u sets the permission for user, g for group, and o for all others.

To remove permissions, replace + with -. For example, $chmod o-w filename. Alternatively, you can use the Octal format to specify permissions:

$chmod 777 filename

This gives read, write, and execute permission to user group and others, whereas the command $chmod 600 filename gives set, read, and write permissions for owner and no permission to groups and others. In Octal format [777], the first bit is used for the user or owner of the file, the second bit is for group, and the third bit is for everyone else. Check out the following table for more information:

Notation

Octal value

Permissions

-|---|---|---

0|000|000|000

Regular files, no permissions

d|r--|r--|r--

d|400|400|400

Directory, read permission to owner, group, and others

-|rw-|r--|r--

-|644|644|644

Regular file, read and write permission to owner and read permission to group or others

-|rwx|rwx|rwx

-|777|777|777

Regular file, all permissions to everyone

Finally, when you share files within a group of users, there are chances that someone deletes the file that is required by other users. Sticky bit can protect these file from deletion. When sticky bit is set, only the owner or a user with root privileges can delete a file.

You can set sticky bit with the command chmod as $chmod +t directoryName. Sticky bit is shown in long listing (ls -l) with symbol t or T. Additionally, sticky bit works only with directories and is ignored on ordinary files.

There's more…

Many times when working as a root user, all files and directories created are owned by root. A non-root user can't write to these directories or files. You can use the command chown to change the ownership of such files and assign them to respective users.

To change ownership of a file, use the following command:

$chown newuser filename

To change the owner as well as the group of file, use the following command:

$chown newuser:newgroup filename

You can skip changing owner and change only the group with the following command:

$chown :newgroup filename

Note that the chown command can only be used by users with root privileges.

 

Getting root privileges with sudo


When you create a new Ubuntu server in the cloud, by default you get the root account. This account has full system access with no restrictions at all and should only be used for administrative tasks. You can always create a new user account with fewer privileges. But there are times when you need extra root privileges to add a new user or change some system setting. You can use the sudo command to temporarily get extra privileges for a single command. In this recipe, you will see how to grant sudo privileges to a newly created user.

Getting ready

You will need a root account or an account with root privileges.

How to do it...

Follow these steps to get the root privileges with sudo:

  1. Add new user if required:

    $sudo adduser john
    
  2. Make john a member of sudo group with the following command:

    $sudo adduser username sudo
    

How it works…

All sudo access rules are configured in a file located at /etc/sudoers. This file contains a list of users and groups that are allowed to use the sudo command:

alan ALL=(ALL:ALL)ALL // allow sudo access to user alan
%sudo  ALL=(ALL)  ALL // allow sudo access to members of sudo

The line alan ALL=(ALL:ALL) ALL specifies that the user alan can run any command as any user and optionally set any group (taken from man pages for sudoers: man sudoers).

The entry %sudo ALL=(ALL) ALL specifies that any member of system group sudo can run any command as any user.

All we have to do is add a new user to the group sudo and that user will automatically get sudo privileges. After getting the membership of the sudo group, user needs to log out and log back in for the changes to take effect. Basically, the user shell needs to be restarted with new privileges. Optionally, you can always go and change the sudoers file for a specific condition.

Note

Make sure that you use the visudo tool to make any changes to sudoers file.

There's more…

Here, we will discuss how to set a password-less sudo and some additional benefits of sudo.

Setting password less sudo

sudo is a useful and handy tool for temporary root privileges, but you need to enter your password every time. This creates problems especially for users with no password set. This problem can be solved by setting the NOPASSWD flag in the sudoers file. Make sure you use the visudo tool to edit the sudoers file:

  1. Open the sudoers file with the visudo command:

    $sudo visudo
    
  2. Select the line for user or group you want to allow password-less sudo access.

  3. Add NOPASSWD after closing the bracket:

    %sudo   ALL=(ALL:ALL) NOPASSWD: ALL
    
  4. Press Ctrl + O and then confirm with the Enter key to save the changes.

  5. Press Ctrl + X to exit visudo.

Now, the users of the group sudo should be able to use the sudo command without providing a password. Alternatively, you can add a separate entry to limit password-less access to a specific user.

Note that the sudoers program performs cache authentication for a small time (default is 15 minutes). When repeated within timeout, you may notice password-less sudo without setting the NOPASSWD flag.

Other uses of sudo

In addition to running a single command with sudo, you might want to execute a list of commands with the sudo privileges. Then, you can open a shell with root access (# prompt) with the command $sudo -s. The shell environment remains same as original user, but now you can execute commands as a root user.

Alternatively, you can switch user to root with the command $sudo su -. This command will open a new shell as a root user.

See also

  • Check manual pages for sudo with $man sudo

  • For more details on adduser, check the Creating user account recipe

 

Setting resource limits with limits.conf


Ubuntu is a multiuser and multi-process operating system. If a single user or process is consuming too many resources, other processes might not be able to use the system. In this recipe, you will see how to set resource limits to avoid such problems.

Getting ready

User account with root privileges is required.

How to do it...

Following are the steps to set the resource limits:

  1. Check the CPU use limit with $ulimit –t.

  2. To set new limit, open limits.conf with the following command:

    $sudo nano /etc/security/limits.conf
    
  3. Scroll to the end of the file and add following lines:

    username  soft  cpu  0  # max cpu time in minutes
    username  hard  cpu  1000 # max cpu time in minutes
    
  4. Enter Ctrl + O to save the changes.

  5. Enter Ctrl + X to exit GNU nano editor.

How it works…

PAM stands for pluggable authentication module. The PAM module pam_limits.so provides functionality to set a cap on resource utilization. The command ulimit can be used to view current limits as well as set new limits for a session. The default values used by pam_limits.so can be set in /etc/security/limits.conf.

In this recipe, we are updating limits.conf to set a limit on CPU uses by user username. Limits set by the ulimit command are limited to that session. To set the limits permanently, we need to set them in the limits.conf file.

The syntax of the limits.conf file is as follows:

<domain> <type> <item> <value>

Here, <domain> can be a username, a group name, or a wildcard entry.

<type> denotes the type of the limit and it can have the following values:

  • soft: This is a soft limit which can be changed by user

  • hard: This is a cap on soft limit set by super user and enforced by kernel

<item> is the resource to set the limit for. You can get a list of all items with $ulimit –a:

In our example, we have set soft limit on CPU uses to 0 minutes and hard limit to 1000 minutes. You can changes soft limit values with the ulimit command. To view existing limits on open files, use the command $ulimit -n. To change limits on open files, pass the new limit as follows:

$ulimit -n 4096

An unprivileged process can only set its soft limit value between 0 and hard limit, and it can irreversibly lower hard limit. A privileged process can change either limit values.

There's more…

The command ulimit can be used to set limits on per process basis. You can't use the ulimit command to limit resources at the user level. You can use cgroups to set a cap on resource use.

 

Setting up public key authentication


In this recipe, you will see how to set up secure public key authentication.

Getting ready

You might need root privileges for certain tasks.

How to do it...

Follow these steps to set up public key authentication:

  1. Add a new user. You can skip this step if you have already created a user:

    $sudo adduser john
    
  2. Log in as john and change to the home directory with cd ~/:

  3. Create a .ssh directory if it doesn't already exist:

    $ mkdir .ssh
    
  4. Create a file named authorized_keys under the .ssh directory:

    $ touch .ssh/authorized_keys
    
  5. Set permissions on the .ssh directory to 700:

    $chmod 700 .ssh
    
  6. Set permissions for authorized_keys to 600:

    $ chmod 600 .ssh/authorized_keys
    
  7. Generate public key pair on your local system with the following command:

    $ ssh-keygen
    
  8. Copy the generated public key from the .ssh/id_rsa.pub file to the authorized_keys file on the server.

  9. Now, open an ssh connection from local to server with the following command:

    $ ssh john@server
    
  10. If asked for confirmation, type yes and press the Enter key to continue:

How it works…

Logging in with SSH supports different authentication methods. Public key authentication and password-based authentication are two common methods. To log in with public key authentication, we need a public private key pair. We generate this key pair with the ssh-keygen command. This command creates two files under the .ssh directory in the user's home:

  • id_rsa: This is the private key file

  • id_rsa.pub: This is the public key file

You can view the contents of the files with $cat id_rsa.pub. It should start with something like ssh-rsa AAAA...(except for the trailing dots).

We then copy the contents of public key to the server's authorized_keys file. Ensure that all contents are listed on single line in the authorized_keys file.

Also, ensure the permissions are properly set for the .ssh directory, and ensure that the authorized_keys file and directory are owned by the user. The permissions for the .ssh directory limits read, write, and execute permissions to the owner of the file. Similarly, for authorized_keys file, permissions are limited to read and write for owner only. This ensures that no other user can modify the data in the .ssh directory. If these permissions are not properly set, the SSH daemon will raise the warning Permission denied?.

Working of SSH authentication

When the SSH client initiates a connection with the server, the server sends public key identification of server to client. If a client is connecting to the server for the first time, it shows a warning and asks for user confirmation to store the server key in the known_hosts file under the .shh directory. After receiving the identity, the client authenticates server to ensure that it is really the intended server.

After server authentication, the server sends a list of possible authentication methods. The client selects the authentication method and selection to the server. After receiving the authentication method, the server sends a challenge string encrypted with client's private key. The client has to decrypt this string and send it back to server along with previously shared session key. If the response from the client matches the response generated by the server, then client authentication succeeds.

There's more…

You might be searching for a secure option to install key on server. Here's one way!

If your local system has the ssh-copy-id tool installed, you can directly add your public key to the server's authorized_keys file with a single command:

$ ssh-copy-id john@serverdomain

After providing the password, your local public key will be added to the authorized_keys file under the .ssh directory of the user john.

Troubleshooting SSH connections

Most of the connection issues are related with configuration problems. If you happen to face any such issue, read the error message in detail. It is descriptive enough to understand the mistake. You can also go through following checklist:

  • Check if the SSH daemon is running. Check the port in use and port conflicts, if any

  • Check whether the firewall configuration allows SSH ports

  • Check the list of configuration methods that are enabled

  • Check permissions for your private keys on your local system

  • Check authorized_keys file for your public key on the server

  • Check for any entry with the old address of the server in known_hosts on the local system

Additionally, you can use the verbose flag (-v or -vvv) with the ssh command to get details of every step taken by the SSH client. Also, check SSH daemon logs on server.

SSH tools for the Windows platform

If your local system runs Windows, then you can use tools provided by puTTYto generate new keys and connect to the server:

  • putty.exe: This is the SSH client on Windows

  • puttygen.exe: This tool generates public or private keys

  • pscp.exe: This is the SCP client for secure file transfer

When using public key generated by the puttygen.exe tool, make sure that you convert the key to OpenSSH key format. Remove all comments and prepend ssh-rsa. Additionally, the entire key should be listed on a single line.

Another easy option is to use puttygen.exe. Load your private key in PuTTYgen and then copy the public key from the Key section of the PuTTYgen window.

See also

 

Securing user accounts


In this recipe, we will look at ways to make user profiles more secure.

How to do it...

Follow these steps to secure the user account:

  1. Set a strong password policy with the following steps:

    • Open the /etc/pam.d/common-password file with GNU nano:

      	$ sudo nano /etc/pam.d/common-password
      
    • Find the line similar to this:

      password    [success=1 default=ignore]  pam_unix.so obscure sha512
      
    • Add minlen to the end of this line:

      password    [success=1 default=ignore]  pam_unix.so obscure sha512 minlen=8
    • Add this line to enforce alphanumeric passwords:

      password requisite pam_cracklib.so ucredit=-1 lcredit=-1 dcredit=-1  ocredit=-1
      
    • Save changes and exit GNU nano editor.

    • Press Ctrl + O to save changes.

    • Press Ctrl + X to exit GNU nano editor.

  2. Secure the home directory with the following steps:

    • Check home directory permissions with the following command:

      $ ls -ld /home/username
      
    • Restrict permissions to user and group with the following command:

      $ chmod 750 /home/username
      
    • Change adduser default permissions by editing /etc/adduser.conf. Find DIR_MODE=0755 and change it to DIR_MODE=0750.

  3. Disable SSH access to root user with the following step:

    • Open /etc/ssh/sshd_config and add or edit PermitRootLogin to PermitRootLogin no

  4. Disable password authentication with the following step:

    • Open /etc/ssh/sshd_config and add or edit PasswordAuthentication no

  5. Install fail2ban with sudo apt-get install fail2ban.

How it works…

This recipe discussed a few important steps to make user accounts more secure.

A password is the most important aspect in securing user accounts. A weak password can be easily broken with brute force attacks and dictionary attacks. It is always a good idea to avoid password-based authentication, but if you are still using it, then make sure you enforce a strong password policy.

Password authentication is controlled by the PAM module pam_unix, and all settings associated with login are listed at /etc/pam.d/login. An additional configuration file /etc/pam.d/common-password includes values that control password checks.

The following line in the primary block of common-password file defines the rules for password complexity:

password [success=1 default=ignore] pam_unix.so obscure sha512

The default setting already defines some basic rules on passwords. The parameter obscure defines some extra checks on password strength. It includes the following:

  • Palindrome check

  • Case change only

  • Similar check

  • Rotated check

The other parameter, sha512, states that the new password will be encrypted with the sha512 algorithm. We have set another option, minlen=8, on the same line, adding minimum length complexity to passwords.

Tip

For all settings of the pam_unix module, refer to the manual pages with the command man pam_unix.

Additionally, we have set alphanumeric checks for new passwords with the PAM module pam_cracklib:

password requisite pam_cracklib.so ucredit=-1 lcredit=-1 dcredit=-1  ocredit=-1

The preceding line adds requirement of one uppercase letter, one lowercase letter, one digit (dcredit), and one special character (ocredit)

There are other PAM modules available, and you can search them with the following command:

$ apt-cache search limpam-

You might also want to secure the home directory of users. The default permissions on Ubuntu allow read and execute access to everyone. You can limit the access on the home directory by changing permission on the home directory as required. In the preceding example, we changed permissions to 750. This allows full access to the user, and allows read and execute access to the user's primary group.

You can also change the default permissions on the user's home directory by changing settings for the adduser command. These values are located at /etc/adduser.conf. We have changed default permissions to 750, which limits access to the user and the group only.

Additionally, you can disable remote login for the root account as well as disable password-based authentication. Public key authentication is always more secure than passwords, unless you can secure your private keys. Before disabling password authentication, ensure that you have properly enabled public key authentication and you are able to log in with your keys. Otherwise, you will lock yourself out of the server.

You might want to install a tool like fail2ban to watch and block repeated failed actions. It scans through access logs and automatically blocks repeated failed login attempts. This can be a handy tool to provide a security against brute force attacks.

About the Author
  • Uday Sawant

    Uday Sawant has completed Master's in Computer Applications from Mumbai University. He is skilled with more than four years of experience in software development and operations field. Before working as a software developer, he worked extensively with server hardware as a system administrator. He is an expert with the LAMP stack, JavaScript, and cloud infrastructure. Before starting as a software developer, he worked extensively with server hardware and has 2+ years' experience as system administrator. Currently, he is working as Software Scientist in a Mumbai based start-up called Sweetcouch. His responsibilities include developing back-end services, setting up a real-time communication server, and automating various daily tasks. With an immense interest in Machine Learning, he likes to spend his spare time exploring this subject. His first book was Instant Building Multipage Forms with Yii published by Packt publishing

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