Updated: 2015-03-24 05:49 EDT

1 Due Date and Deliverables

Do not print this assignment on paper!

WARNING: Some inattentive students upload Assignment #11 into the Assignment #10 upload area. Don’t make that mistake! Be exact.

2 Purpose of this Assignment

Do not print this assignment on paper! On paper, you cannot follow any of the hyperlink URLs that lead you to hints and course notes relevant to answering a question.

This assignment is based on your weekly Class Notes.

  1. Review account administration commands from the Class Notes.
  2. Practice managing Users and Groups in your own virtual machine.

Remember to READ ALL THE WORDS to work effectively and not waste time.

3 Introduction and Overview

This is an overview of how you are expected to complete this assignment. Read all the words before you start working.

For full marks, follow these directions exactly.

  1. Complete the readings in your weekly Class Notes.
  2. Complete the Tasks listed below, in order.
  3. Verify your own work before running the Checking Program.
  4. Run the Checking Program to help you find errors.
  5. Submit the output of the Checking Program to Blackboard before the due date.
  6. READ ALL THE WORDS to work effectively and not waste time.

3.1 Save your work

You will create some minimal file system structure in your HOME directory on the CLS.

Most work will involve changes in your own Linux Virtual Machine running Centos 6.6. You can use the Checking Program to check your work as you go. You can check your work with the Checking Program as often as you like before you submit your final mark.

When you are finished, leave the files and directories in place on both the CLS and your own CentOS Virtual Machine as part of your deliverables. Do not delete any assignment work until after the term is over! Assignments may be re-marked at any time on the CLS; you must have your term work available on the CLS right until term end.

3.2 Searching the course notes on the CLS

All course notes are available on the Internet and also on the CLS. You can learn about how to read and search these CLS files using the command line on the CLS under the heading Copies of the CST8207 course notes near the bottom of the page Course Linux Server.

3.3 Commands, topics, and features covered

This is a Users and Groups assignment. Review course notes Users and Groups.

See the course notes or use the on-line help (man command) for the commands listed below for more information:

3.4 Correct user, command lines, and command output

3.5 Backup and Recovery on CentOS

  1. Take a snapshot of your virtual machine before you begin each section of this lab so that you can recover back to the snapshot if needed.
    • You can delete the unused snapshots if everything works well.
    • CentOS snapshots are very small and fast compared to your Windows snapshots; you can save lots of them.
  2. You may find it useful to also make a backup copy in a safe place of the /etc/passwd file and its shadow and the /etc/group file and its shadow on your CentOS system.
    • You can compare the old and new files using diff to know what has changed during this lab, and you can sometimes recover these files without reverting everything back to a snapshot.

3.6 Use a remote login, not the VMware console

I recommend that once you have booted your CentOS VM, you connect to it and work using a remote login session (e.g. ssh or PuTTY) where copy-and-paste works and where you can have multiple simultaneous connections into the VM. The VMware console is not friendly.

Note that SSH sessions (and whatever you are doing inside them) do not survive across a VMware suspend. Make sure you save your editor files and exit your SSH session before you pause or suspend your virtual machine. (Editor sessions inside the VMware console do survive across suspend and resume.) (Editor sessions that run inside the VMware console do survive across suspend and resume, since they don’t depend on a network connection.)

Advanced users may look into the various virtual terminal programs such as tmux and screen that do allow you to suspend and resume your sessions even from a remote login.

3.7 CentOS: No root files in non-root accounts

Files saved anywhere under your sysadmin HOME directory in CentOS should be owned by you and in one of your groups, not owned by root or in the root group. (The presence of root files in non-root accounts is often a sign that your machine has been cracked!)

Do not leave root-owner or root-group files in your account. You should change the owner and group to you of anything you create as root in your account. To find files not owned by you or not in your own group in your account:

[abcd0001@abcd0001 ~]$ cd
[abcd0001@abcd0001 ~]$ echo "$USER"
abcd0001                                 # your own userid not abcd0001
[abcd0001@abcd0001 ~]$ find . ! -user "$USER" -ls
[... any non-abcd0001 owner files are listed here ...]
[abcd0001@abcd0001 ~]$ find . ! -group "$USER" -ls
[... any non-abcd0001 group files are listed here ...]

If you find any files, you should use the chown command to fix these files to be your own userid and group. (The command has a recursive option that lets you change everything under a directory.)

Advanced users can modify the above find to send pathnames into sudo running xargs with chown. See Find and Xargs.

4 Tasks

4.1 Set Up – The CLS and the Base Directory on CentOS

You must have successfully completed the CentOS Install and Configure to do this assignment.

  1. Create the CLS directory ~/CST8207-14F/Assignments/assignment11 on the Course Linux Server.

  2. Create the same directory in your sysadmin account on your CentOS VM. This CentOS directory will be the base directory for this assignment.

This CentOS assignment11 directory is the base directory for all pathnames in this assignment. Store your files and answers in this CentOS base assignment11 directory.

Pay careful attention to whether you are working on the CLS or CentOS, and which account you are using! Watch the userid and hostname values in your PS1 prompt string! All answer files in this assignment get stored in the CentOS base directory, not on the CLS.

4.2 CentOS: adduser and useradd are the same

Do the commands in this section using your own sysadmin non-root account. Use sudo only as needed. As mentioned above, all output files must be owned by you and saved on CentOS in the Base Directory for this assignment in your system admin account (not in the root account!).

On CentOS, useradd and adduser are the same command with two different names. On some versions of Linux – Debian, Ubuntu, etc. – adduser is a different command with different options.

  1. On CentOS, try man adduser and then man useradd and see that they are exactly the same thing. (If the man command is not found, you missed a step in a previous assignment. Go back and install the missing package.)

  2. On the CLS (not CentOS), try the same two man command lines and see that the two commands useradd and adduser are different and have different man pages on the CLS. (The CLS runs Ubuntu Linux, which is a Debian-based distribution. CentOS is a Red Hat distribution.)

  3. On CentOS, find the absolute pathnames of the two command names adduser and useradd. (Note: The which command won’t find the location of these two commands when run as a non-root user. Use the other command that locates the pathname and manual pages of a command name. Review the “related commands” in the Search Path notes.) You will need these two absolute pathnames for the next question.

  4. On CentOS, save into output file adduser.txt in your CentOS Base Directory the two lines of output from a long ls listing of the two above absolute pathnames. Your file will be two lines and 20 words.

Look at the ls output lines in adduser.txt:

Run the Fetch and Checking Program to verify your work so far.

4.3 CentOS: The Simpsons Follies: homer and flanders

Review the course notes on Users and Groups.

Reminder: You will need root privileges to run account management commands, but all other commands should be done using your unpriviileged sysadmin account.

Always log in and use your sysadmin account and use sudo only when needed. As mentioned above, all output files must be owned by you and saved on CentOS in the usual place for this assignment in your account (not in the root account!).

For this section you will require two more ordinary user (non-root, non-system) accounts. We will use two names from The Simpsons television series.

  1. Create an account named homer (no options needed) and then give it a simple password.

  2. Create an account named flanders (no options needed) and then give it the same password.

  3. Use some id commands to record the account information for each of the two Simpsons accounts into a simpsons.txt file. Your file will be two lines, six words, 105 characters; one line for each of the two new Simpsons accounts.

  4. Extract the account information lines for the two new accounts from the system password file and save the information in a simpasswd.txt file. Your file will be two lines, two words, 84 characters; one line for each Simpsons account. Each password file line will have seven colon-separated fields in it.

  5. Put the long ls listing of the absolute pathname of each Simpsons account HOME directory (not the contents of the directory – just the directory itself) into a simpath.txt file, one per line. Your file will be two lines and 18 words. If you get permission denied, you aren’t using the correct option to ls to show only the directory, not the contents.

  6. Put the three-digit numeric permissions of either of the above HOME directories in a simperm.txt file. Your file will be one line, one word, four characters. (Remember why three digits in a file is saved as four characters? Review Text file line end differences in File Transfer.)

Run the Fetch and Checking Program to verify your work so far.

4.3.1 Creating a World-Writable Public Directory

The SB Output File: In the next steps, where you are required to save a command line or its output, do the command and then copy and record the command line or its output as a separate line into an output file named simpub.txt in your CentOS Base Directory that will be noted as the SB output file below. If you can’t answer a question, leave a blank line in this output file. (The vim option :set number may be useful to you as you edit.)

If you use redirection to append lines to the output file, instead of using an editor, make sure you back up the file before each try, in case you accidentally over-write the file and lose the previous answers. You could also create snapshots after each question.

Below, we will create a /public directory in the system ROOT directory in which any user can create files. The directory will allow any user to create names in it (or remove names, or rename). Recall that the permissions on a directory are not the same as the permissions on the things named in the directory. Permission to change file names does not grant permission to change file content.

You will need to use a command that lets you start a subshell as another user. Review Users and Groups.

In this section you will need to run commands as different users. You may find it helpful to have separate windows open into your CentOS VM, with subshells in each window running as a different user. Then, you can simply change windows to change users instead of having to exit and enter subshells in the same window. Be lazy! (You could also use the suspend command and fg to change sub-shells.)

  1. Use appropriate privileges to create a directory called /public under the top-level ROOT directory. The exact name is /public (NOT /root/public and NOT ./public and NOT public!).

  2. Now, give the /public directory full access permissions for everybody.

  3. You should be able to touch and remove a file in this new /public directory as any non-root account. Test this (not as root).

  4. Start up a subshell that runs as the flanders user.
    • Optionally use a new window for this, so you have two windows.
    • Use the command that displays who you are to make sure it says flanders!
  5. As the flanders user, redirect the current date into a new flanfile file in the above /public directory. The file will be one line, 6 words, 29 characters.
    • If this fails, you didn’t set the correct directory access permissions on the directory, above.
  6. As your sysadmin user, record just the owner and group of the flanfile file as Line 1 in the SB output file in your CentOS Base Directory (two words only).
    • See why having two windows is useful here? You could be executing commands as one userid in one window and recording the results using your sysadmin account in the other window.

  7. Record the current numeric permissions of the flanfile file as Line 2 in the SB output file (three digits only).

4.3.2 Removing (only) other permissions from a file

  1. Remove only all other permissions from flanfile and use the syntax that does not change any other permissions, no matter what they might be.
    • Record the exact command line used as Line 3 in the SB output file.
    • Hint: You can’t use numeric permissions, since numeric permissions set all permissions, not just other permissions. Review Permissions especially Changing Permissions.
  2. Record the new resulting numeric permissions for the flanfile file as Line 4 in the SB output file (three digits only).

4.3.3 Renaming and deleting other users’ files

  1. Start a subshell that runs as the homer user.
    • Optionally use a new window for this, so you have three windows.
    • Use the command that displays who you are to make sure it says homer!
  2. As the homer user, try to display the contents of the flanfile.
    • You should get a “Permission denied” error message; homer does not have permission to read the file content.
    • If you didn’t get an error message, you didn’t correctly remove other permissions from the file.
    • If you get an error “No such file”, you typed the wrong pathname.
  3. As the homer user, rename the flanfile file (still owned by flanders) to have the new name homfile (in the same directory as flanfile).
    • No error message should print.
  4. Do a long ls listing of that new name confirming that the renamed file is still owned by flanders.
    • Record the long ls output as Line 5 in the SB output file.
  5. As the homer user, remove the homfile name.
    • This name is the name of a file still owned by flanders.
    • You will need to answer “yes” to a question about write-protection.
    • The file had only one name; removing this one name means the data in the file (data owned by flanders) is now gone forever.
    • Verify that the /public directory is now completely empty.

Understand clearly why homer can rename and then delete a file that he doesn’t own and can’t read. Review Links and Inodes.

Run the Fetch and Checking Program to verify your work so far.

4.3.4 One file multiple write permissions

In this section you will need to run commands as different users. You may find it helpful to have separate windows open into your CentOS VM, with subshells in each window running as a different user. Then, you can simply change windows to change users instead of having to exit and enter subshells in the same window. Be lazy! (You could also use the suspend command and fg to change sub-shells.)

  1. Create a new empty file /public/homflan that has the owner and group of homer and homer.
    • Hint: You could log in as homer to create this file, or you could create the empty file as any user first, then use chown as root to change the owner and group.
    • Do a long ls listing of the file and verify that both the owner and group are homer.

  2. As the flanders user try (and fail) to append the date to the /public/homflan file. You should get Permission denied.

  3. As the homer user try (and succeed) to append the date to the /public/homflan file. There should be no error message.

  4. Change only the file group and group permissions of the homflan file so that both homer and flanders can read and write the homflan file.

    Hint: homer, the owner of the file, will read and write the file using the owner permissions. Arrange that the file will be in one of the groups of the non-owner flanders and so group permissions will apply to flanders, allowing flanders to read and write the file using the group permissions. Other users will be neither the owner of the file nor in the group of the file, so other permissions will apply to them and they cannot write the file.

    • Do not change anything in the system except the group and the group permissions of the file!
    • Do not enable other write permissions!
    • Do not create any groups or do anything other than change the file group and group permissions.

  5. Test it as three users. Both homer and flanders should be able to append data to the file, while your sysadmin (non-root) account should not be able to append data.

  6. Exit from all the Simpsons user subshells and return to your sysadmin account shell.

Run the Fetch and Checking Program to verify your work so far.

4.4 CentOS: Star Wars account management

The WA Output File: In the next steps, where you are required to save a command line or its output, do the command and then copy and record the command line or its output as a separate line into an output file name starwars.txt in your CentOS Base Directory that will be noted as the WA output file below. If you can’t answer a question, leave a blank line in this output file. (The vim option :set number may be useful to you as you edit.)

Take a snapshot of your virtual machine. Review Users and Groups.

  1. Create an account named luke (all lower-case; no options needed).

  2. Give the new luke account a simple password.

  3. Search for lines containing luke in the four system accounting files (in the password and password shadow files and in the group and group shadow files – four files in total). When you see four lines of output, put them into a new luke.txt file. That file should contain the four lines that contain luke in each line.

  4. As your sysadmin account user (not root), run a long ls listing on the new HOME directory (just the directory, not the contents) of the new luke account. Only one line should output.
    • If you get permission denied, you aren’t using the correct option to ls to show only the directory, not the contents.
    • You should see one line of detail information about the directory itself (permissions, owner, etc.). Save the one line as Line 1 in the WA output file in your CentOS Base Directory.

4.4.1 Modifying a user account with usermod

The usermod command modifies user account attributes, as recorded in the password file. Use only this command to make the following account change.

Do not use any other commands to make these changes unless told to do so. Do not text-edit any account files; use the commands designed for the purpose.

  1. Use a command to modify the login name of the luke account to be darth
    • Record the exact command line used as Line 2 in the WA output file.
    • Confirm the change by looking in the system password file: The luke user should be renamed to darth.
    • Run id luke and confirm that it fails with “No such user”
  2. Run id darth and confirm that darth exists, but the account is still in the group named luke.
    • Save the output as Line 3 in the WA output file.
  3. Run ls -ld on the HOME directory for this account (which hasn’t changed) and note that the owner has changed, but not the group (it’s still luke). The name of the directory hasn’t changed, only the owner name.

If you didn’t get the above results, restore the snapshot and try again.

4.4.2 Modifying a group with groupmod

The groupmod command modifies the group attributes, as recorded in the group file. Use only this command to make the following account change.

Do not use any other commands to make these changes unless told to do so. Do not text-edit any account files; use the commands designed for the purpose.

  1. Use a command to modify the group name of the luke group to be darth
    • Record the exact command line used as Line 4 in the WA output file.
    • Confirm the change by looking in the system group file: The luke group should be renamed to darth.
    • Run id darth and confirm that the account is now in the group named darth.
    • Run ls -ld on the HOME directory for this account (which still hasn’t changed) and note that both the owner and group have been changed (but not the name of the directory).

If you didn’t get the above results, restore the snapshot and try again.

4.4.3 Moving a HOME directory with usermod

  1. The new darth account still uses an unchanged home directory of/home/luke. Use a command to change and move (in one command line) this old home directory from its current luke name to the new name sith in the same parent directory as luke:
    • Record the exact command line used as Line 5 in the WA output file.
    Hints:
    • Use both options -d and -m strictly according to the course notes (or read the man page).
    • Do not insert anything between -d and its following argument.
    • Use the correct absolute path for the new HOME directory.
  2. Check your work:
    • Run ls -ld on the new changed and moved sith HOME directory for this account and confirm that both the owner and group are still darth.
    • Use a command to search in the four system accounting files and make sure the word luke does not appear anywhere in those four files.
    • Start a login subshell in the darth account using su with the correct option. There should be no errors and the current directory should be the sith HOME directory.
    • Exit the darth subshell and return to your sysadmin account shell.

If you didn’t get the above results, restore the snapshot and try again.

Run the Fetch and Checking Program to verify your work so far.

4.4.4 Changing shells with chsh

  1. Use the appropriate option to the chsh command to print the list of possible shells (four lines).

  2. Change the shell for darth to be the one that prevents logins. (Choose the shell pathname that allows “no logins”.)

  3. Try to start a subshell in the darth account using su. You will be rejected with This account is currently not available.

Run the Fetch and Checking Program to verify your work so far.

4.5 CentOS: Group management: The Megadeth Project

4.5.1 Requirements for Megadeth Group Management

The four-person band Megadeth (note the unusual spelling of Megadeth) uses the following work approach and has the following Requirements:

  1. Song files can be created by one band member who is the single group administrator account. Only this one group administrator account can create, delete, modify and write song files.

  2. Files must only be readable (not writable or removable) by the other three non-administrator group (band) members. These three ordinary band members must only be able to read the song files, not change or remove or rename them.

  3. Anybody who is not a band member is not allowed to view song files. You must prevent all public access of any kind to the song files. These people do not share their songs for free.

  • Follow the directions below to create accounts and directories that implement the above Requirements. Some of the work will need to be done as the root super-user. (Only the root user can create new accounts.) Most group maintenance work can be done as the band member who is assigned to be the group administrator.

  • The four Megadeth band members are (get the name and account spellings correct when you create these accounts, below! Case matters):

    • Dave Mustaine – login name: mustaid
    • David Ellefson – login name: ellefsd
    • Chris Broderick – login name: broderc
    • Shawn Drover – login name: drovers

4.5.2 Creating and configuring the Megadeth Working Group

  1. Take a VM snapshot before you begin this section, so you can return here if you make many mistakes. Keep a separate record of the exact commands you use in this section, for study purposes.

  2. Create the four new band accounts. The only option you need is the correct option to include each band member’s full name in each account you create. That is all.
    • Remember to quote shell arguments containing blanks.
    • Don’t worry about any special groups yet.
    • If you forget to include the full names when you create the accounts, you can modify the information afterword with the correct command.

  3. Give all four accounts some simple passwords.

  4. Use a command to add a new group named megadeth (spelled all lower case) to the system accounting files.
    • Do not make this megadeth group a system group. Keep it ordinary.
    • Verify that the new megadeth group is now in the system group file and has a numeric GID larger than GID_MIN. (Recall looking up this value when you created your own sysadmin account.)
    • If not, delete it and try again.
  5. Use a command to set band member Chris Broderick as the only group administrator of the new megadeth group.
    • Verify that the megadeth group in the group shadow file lists Chris’ account userid in the third field.
    • If not, delete it and try again.
  6. Log in (start a subshell) as the megadeth group administrator. Using his account, use a command four times to add each of the four band members to the megadeth group, one-by-one.
    • The group administrator can add members to the group.
    • The group administrator can only add users one-at-a-time.
    • The message Adding user will print each time.

  7. Confirm that the system group file contains the new megadeth group with all four band members listed beside it.

  8. Exit the Chris subshell when you are done adding four members.

  9. Create a new song directory named /home/music (lower-case).

  10. The band wants to store its song files under this music directory, with permissions that implement the Requirements given above. Set ownership and permissions for the music directory to implement the given Requirements:
    1. Which band account should be set as the owner of directory music?
      • Make it so.
    2. Which group should be set as the group of directory music?
      • Make it so.
    3. Which permissions should be set on directory music?
      • Make it so, so that the Requirements are met.

4.5.3 Test Plan for the Megadeth Group Project

You need to verify that the requirements have been met using a Test Plan. Here it is:

  1. Become the group administrator and do these read/write tests:
    • Redirect the current date into a new file named test in the music directory. It should be possible without any errors.
    • Display the file on your screen to make sure the file has content you can read and write while you are logged in as the group administrator.
    • Exit the group administrator account when you are done testing.
  2. Become a non-group-administrator band member and do these read-only tests:.
    • Make sure the member can read the test file, but can not change it, rename it, or remove it.
  3. Become any other ordinary user (e.g. your own sysadmin account) and test for no access:
    • Make sure that the music directory cannot be listed or entered by any user who is not a band member.

4.6 When you are done

That is all the tasks you need to do.

Log in to the CLS and submit your mark from the CLS following the Checking Program instructions below.

Optional: Keeping your main configuration snapshots, remove any intermediate snapshots you no longer require, to free up disk space. - Be careful not to remove your current work!

5 Checking, Marking, and Submitting your Work

See CentOS: Remote Checking, Marking, and Submitting your Work.

Author: 
| Ian! D. Allen  -  idallen@idallen.ca  -  Ottawa, Ontario, Canada
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