OPS235 Lab 4 - CentOS7

LAB PREPARATION

Purpose / Objectives of Lab 1

There are many other tasks that a Linux system administrator must perform other than installing Linux and installing software.

A few additional tasks are user management and managing services.

Main objectives:

• Administer (add, remove, modify) users on a Linux system.
• Save time while adding new users using a template of start-up files.
• Create and manage groups on a Linux system.
• Start and Stop services on a Linux system.
• Display the status of running services on a Linux system.

Minimum Required Materials

Removable Hard Disk Pack (SATA)

USB key (for backups)

Lab4 Log Book

My Toolkit (CLI Reference)

User Management: useradd userdel usermod groupadd groupdel

Managing Services chkconfig service systemctl

Miscellaneous /etc/passwd /etc/group /etc/shadow /etc/skel

INVESTIGATION 1: User/Group Management

Part 1: The /etc/passwd file

1. Look at the /etc/passwd file.
2. Make note of the contents of that file.
3. Read about the file: http://man7.org/linux/man-pages/man5/passwd.5.html
4. Make sure you know what information each field contains.
5. Why do you think there are so many users?
6. Look at the names of the users. What do you think these user names represent? Are they people?
7. What is the numeric user ID (UID) of the root user?
8. The user IDs of real users (people) are different from the user IDs of system accounts. What is the pattern?

Answer the Part 1 observations / questions in your lab log book.

Part 2: Adding users

1. Perform this part in your centos1 VM.
2. Read the man page for the useradd command.
3. Create three fictitious users (make-up their userids and full names. Give each of these newly-created users a password.
4. Grep the /etc/passwd file for each of the new users.
   • What is the home directory of each user?
   • What group is each user in?
   • What else do you know about each user?
   • Where are the passwords stored?
5. Look at the man page for /etc/shadow using the command man 5 shadow
   • Grep the /etc/shadow file for each of the new users.
   • Make note of this information.
6. Create two new dummy users, ops235_1 and ops235_2.
7. Investigate the home directory of one of your new users.
   • What files are there? Be sure to include hidden files.
   • What do you think these files are used for?
   • How does the operating system determine which files are created in a new home account? The answer can be found here:
     http://www.linuxhowtos.org/Tips%20and%20Tricks/using_skel.htm
   • Look at the files (including hidden files) in the template directory referred to in the article. Compare them to what is in a home directory for a new user. What do you notice?
   • Create a new file in this directory with the following command: touch foo
   • Create a new user named foobar, with the option to automatically create a home directory.
   • Look at the contents of foobar's home directory. What do you notice?
8. Be sure to record your observations in your lab notes.

1. Perform this part in your centos3 VM.
2. Add your matrix account user to centos3.

Answer the Part 2 observations / questions in your lab log book.

Part 3: Managing Groups

1. Perform this part in your centos1 VM.
2. Read the man page for the groupadd and groupdel commands.
3. Note which option allows you to set the Group ID number (GID) when you create a new group.
4. Examine the file /etc/group
   • Which values of GID are reserved for system accounts?
   • Which values of GID are reserved for non-system user accounts?
   • What is the lowest available GID number for non-system users?
   • What is the default group name of a new user?
   • Add a new group named ops235 with a GID of 600.
   • You are angry at some irresponsible users on your system.
     • Add a new group named idiots.
     • Look at /etc/group and note the GID of idiots.
     • What GID is given to a new group if if you do not specify it?
     • Your anger has subsided. Delete the idiots group.
     • Look at /etc/group again and note the change.

Answer the Part 3 observations / questions in your lab log book.

Part 4: Deleting / Modifying Users

1. Perform this part in your centos1 VM.
2. Read the man page for the userdel command. Note which option automatically removes the users home directory when that user is deleted.

1. Delete the user ops235_1 using the command userdel ops235_1
2. Delete the user ops235_2 using the same command with the option which removes the home directory of the user.
3. Check the contents of the /home directory. What do you notice?
4. Check the contents of the /etc/group file. What do you notice?
5. Read the man page for the usermod command. Note which options change the user's full name, primary group, supplementary groups, and shell.

1. Add each of your new users to the group ops235 (in other words, add ops235 to each user as a supplementary group).
2. Examine /etc/group. What has changed?
3. Use the usermod command to associate each of your pod mates' full name to their user name. With each change, examine their entries in the /etc/passwd file. What has changed?
4. Be sure to record your observations in your lab notes.

Answer the Part 4 observations / questions in your lab log book.

INVESTIGATION 2: Managing Run-Levels and System Services

Part 1: How do we Manage Runlevels?

1. Perform this part in both your centos2 and centos3 VMs.

The runlevel command is now deprecated in Fedora, and will likely be deprecated in RHEL/CentOS at some point as well, but for now this is what the industry is using.

2. Issue the following Linux command:
   • runlevel
3. Note the difference in output between centos2 and centos3.
4. You can use the init command to change the current runlevel. See a list of runlevels here. Use the man command to learn how to use the init command. Use this command to change the current runlevel in centos2 to 3. What happened? What happens after your reboot?
5. Change the default runlevel on centos2 to 3. What happens now after you reboot?
6. Issue the following Linux command:
   • startx
7. What happens?
8. Log-off your graphical system. You should return to your shell prompt.
9. Change the default runlevel for centos2 back to 5 and reboot to make sure it works.

Answer Part 1 observations / questions in your lab log book.

Part 2: How do we Manage System Services?

We have seen that maintaining unneeded packages can be a security risk due to the unnecessary increase in the complexity of your system. Similarly, it is also unnecessarily hazardous, and even more so, to leave unneeded services running. In this investigation, we will learn how to control services, and turn off those services that we think are not necessary to help reduce security risks.}}

1. For this part, remain in your centos2 VM.

2. Issue the following Linux command:
   • service --status-all
3. Note the services that are currently running.
4. Use the service command to stop the service named iptables
5. Issue the service command to verify that this service has stopped.
6. If you reboot now - the iptables service will be turned back on. We don't want it on though, it causes students headaches. To turn it off permanently we need to use the chkconfig command:
   chkconfig iptables off
7. Reboot and confirm that it's no longer running.

Answer Part 2 observations / questions in your lab log book.

INVESTIGATION 3: LOOKING AHEAD

Automating Routine Tasks (Shell Scripting)

Bash Shell Scripting Tips: The case statement: The case statement is a control-flow statement that works in a similar way as the if-elif-else statement (but is more concise). This statement presents scenerios or "cases" based on values or regular expressions (not ranges of values like if-elif-else statements). After action(s) are taken for a particular scenerio (or "case"), a break statement (;;) is used to "break-out" of the statement (and not perform other actions). A default case (*) is also used to catch exceptions. Examples (try in shell script): read -p "pick a door (1 or 2): " pick case $pick in   1) echo "You win a car!" ;;   2) echo "You win a bag of dirt!" ;;   *) echo "Not a valid entry"      exit 1 ;; esac read -p "enter a single digit: " digit case $digit in   [0-9]) echo "Your single digit is: $digit" ;;          *) echo "not a valid single digit"              exit 1 ;; esac The getopts function: The getopts function allows the shell scripter to create scripts that accept options (like options for Linux commands). This provides the Linux administrator with scripts that provide more flexibility and versatility. A built-in function called getopts (i.e. get command options) is used in conjunction with a while loop and a case statement to carry out actions based on if certain options are present when the shell script is run. The variable $OPTARG can be used if an option accepts text (denoted in the getopts function with an option letter followed by a colon. Case statement exceptions use the :) and \?) cases for error handling. Example of getopts (try in script and run with options) while getopts abc: name do   case $name in     a) echo "Action for option \"a\"" ;;     b) echo "Action for option \"b\"" ;;     c) echo "Action for option \"c\""         echo Value is: $OPTARG" ;;     :) echo "Error: You need text after -c option"         exit 1 ;;     \?) echo "Error: Incorrect option"         exit 1 ;; esac

We will now use shell scripting to help automate the task for a Linux adminstrator to create regular user accounts.

1. Use the wget command to download, study, and run the following shell scripts on-line:

   https://scs.senecac.on.ca/~murray.saul/user-create.bash

2. Try to understand what these Bash Shell scripts do, and then run the script as root. After running the shell script, view the contents of the /home directory to confirm.

Although the zenity command is a "user-friendly" way to run shell scripts, Linux administrators usually create shell scripts that resemble common Linux commands. In this lab, you will learn to create a shell script using the getopts function to make your shell script behave more like actual Linux commands (including the use of options). Refer to the notes section on the right-hand-side for reference about the case statement and the getopts function.

3. Open a Bash shell terminal and login as root.
4. Use the wget command to download the input file called user-data.txt
5. View the contents on the user-data.txt file to confirm there are 3 fields (username, fullname, and e-mail address).
7. Use a text editor (such as vi or nano) to create a Bash Shell script called: createUsers.bash in /root's home directory.
8. Enter the following text content into your text-editing session:


#!/bin/bash

# createUsers.bash
# Purpose: Generates a batch of user accounts (user data stored in a text file)
#
# USAGE:
#
# /root/createUsers.bash [-i {input-path}]
#
# Author: *** INSERT YOUR NAME ***
# Date: *** CURRENT DATE ***

if [ $HOME != "root" ] # only runs if logged in as root
then
 echo "You must be logged in as root." >&2
 exit 1
fi


6. Save your editing session, but remain in the text editor.
7. The code displayed below uses the getopt function set the input file pathname or check for invalid options or missing option text. Add the following code


outputFlag="n"
while getopts i:n:a:g:o: name
do
 case $name in
   i) inputFile=$OPTARG ;;
   :) echo "Error: You need text after options requiring text"
       exit 1 ;;
   \?) echo "Error: Incorrect option"
        exit 1 ;;
 esac


6. Save your editing session, but remain in the text editor.
7. The code displayed below uses logic to exit the script if the input file does not exist. Command substitution is used to store each line of the input file as a positional parameter. There is one subtle problem here: The full names of the users contain spaces which can create havoc when trying to set each line as a separate positional parameter. In this case the sed command is used to convert spaces to plus signs (+), which will be converted back later. Finally, a for loop is used to create each account (useradd) and mail the user their account information (mail). Add the following code:


if [ ! -f $inputFile ]
then
  echo "The file pathname \"$inputFile\" is empty or does not exist" >&2
  exit 2
fi

set $(sed 's/ /+/g' $inputfile) # temporarily convert spaces to + for storing lines as positional parameters

for x
do
    useradd -m -c "$(echo $x | cut -d":" -f2 | sed 's/+/ /g')" -p $(date | md5sum | cut -d" " -f1) $(echo $x | cut -d":" -f1)
    mail -s "Server Account Information" $(echo $x | cut -d":" -f3) <<+
    Here is your server account information:
    servername: myserver.senecac.on.ca
    username: $(echo $x | cut -d":" -f1)
    password: $(date | md5sum | cut -d" " -f1)
    Regards,
    IT Department
+
done

echo -e "\n\nAccounts have been created\n\n"
exit 0


8. Save, set permissions, and then run that shell script for the input text file user-data.txt. Did it work? Try running the script without an argument - What did it do?
9. You have completed lab4. Proceed to Completing The Lab, and follow the instructions for "lab sign-off".

Answer Investigation 3 observations / questions in your lab log book.

LAB 4 SIGN-OFF (SHOW INSTRUCTOR)

Time for a new backup!
If you have successfully completed this lab, make a new backup of your virtual machines. Remember to also make a backup of the new second virtual disk drive on centos1 -- you now have two virtual disks on centos1, and therefore two image files, and therefore will need two backup files.

Arrange proof of the following on the screen:

✓ Account created on centos3 matching your Matrix account.

✓ List contents of /etc/group file (ops235 group).

✓ List contents of /etc/passwd file (created accounts).

✓ Creation of your bash shell script called createUsers.bash

Preparing for the Quizzes

1. Describe all of the field in /etc/passwd
2. What is the command to create a user? What option to create a home directory for that user?
3. What is the command to change the full name of an already-created user?
4. What is the command to delete a user account? What option allows for the user's home directory to be removed as well?
5. What is the command to create a group? What is the command (or steps) to include a user in a newly-created group?
6. What is the purpose of /etc/shadow?
7. What is the purpose of /etc/skel?
8. What does the term run-level mean?
9. How to set the run-level of a Linux system to text-based only? How to set to graphical mode?
10. What is the command to view the status of running services?
11. What is the command to start a service (like httpd, or sshd)?
12. What is the command to start a service?
13. Can a service be stopped and started by issuing just one command?