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Clarification regarding lab 6.3 (stuck on step 4)

Hello Instructors/Mentors,

While following along the Exercise Lab 6.3, I'm stuck at step-4 i.e. upon 'setfacl -m u:fool:rw /tmp/afile' command, user 'fool' is still unable to write to the file. Screenshot provided below for reference. Only difference with lab screenshot I can notice is mask upon 'getfacl /tmp/acl'.

(I'm running Ubuntu 22.04.1 LTS in a Virtual Machine)

Please provide clarification and help.

Thank you,
Amar

Best Answer

  • coop
    coop Posts: 894
    Answer ✓

    the exams as of now are based on Ubuntu 20.04 and CentOS 8 (Stream). I assume at some point they will go over to 22.04 and Centos 9 but for now that is what it is. This is all in the info on the training web site if you dig enough :)

Answers

  • Hello,

    As an update I tried to complete this lab using Ubuntu 18.04.6 LTS (running as VM) and it worked as expected.

    That brings me to discuss another point (as I'm preparing to take LFCS exam in 2023).

    Which version of Ubuntu should I practice with to prepare for LFCS exam 2023?

    a) with Ubuntu 18.04.6 LTS (as I believe the support for which will come to an end in Apr 2023)

    OR

    b) later versions 20 LTS of 22 LTS

    Thank you,
    Amar

  • Hello Jerry Cooperstein,

    I am still quite new, and getting familiar with the training web site. I appreciate your support.

    Happy Holidays!

    Thank you,
    Amarpreet (amar)

  • Hi @cloud4amar,

    I made a testcase on Ubuntu 20.04.5 LTS and it worked as expected.

    So, the only thing I did different from you, is how I connected to the second account. When being in the first terminal, I just did 'su cata', with 'cata' as my second account name.

    This is it:

    [email protected]:~$ ls -l
    total 56
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Desktop
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Documents
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Downloads
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Music
    -rw-rw-r-- 1 luis luis 0 Sak 27 11:50 myfile
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Pictures
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Public
    -rwxrwxr-x 1 luis luis 16880 Yag 22 10:41 signal
    -rwxrwxrwx 1 luis luis 481 Yag 22 10:23 signal_int.c
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Templates
    drwxr-xr-x 2 luis luis 4096 Teg 28 2021 Videos

    [email protected]:~$ getfacl myfile

    file: myfile

    owner: luis

    group: luis

    user::rw-
    group::rw-
    other::r--

    [email protected]:~$ setfacl -m u:cata:rw myfile

    [email protected]:~$ getfacl myfile

    file: myfile

    owner: luis

    group: luis

    user::rw-
    user:cata:rw-
    group::rw-
    mask::rw-
    other::r--

    [email protected]:~$ su cata
    Password:
    [email protected]:/home/luis$ echo "another line" > myfile
    [email protected]:/home/luis$
    [email protected]:/home/luis$ cat myfile
    another line

    That's it :)

    Regards,
    Luis.

  • Hello @luisviveropena,

    I tried the lab again, this time, with Ubuntu 20.04.5 LTS and I encountered the same issue as with Ubuntu 22.04.1 LTS.

    Here's the screenshot for reference:

    Please provide feedback to further diagnose and resolve the issue.

    Thank you,
    Amarpreet (amar)

  • Hello @luisviveropena,

    In an effort to further diagnose and improve my understanding on file permissions, I noticed that even when I 'chmod' of the file 'myfile' to 666 I am still unable to append the file as ('other' / world) user 'john'.

    And to the best of my understanding, I believe john should fall within 'other'. Please correct me if I am wrong.

    Below is the screenshot for reference:

    Thank you,
    Amarpreet (amar)

  • Hi Amar

    I was also having trouble with this exercise but I now believe I know what is causing the issue.

    This issue is that the "/tmp" directory has the "sticky bit" set as you can see by the "t" at the end of the permissions:

    I believe this is preventing any other user from writing to that file.

    Read more about the sticky bit here:
    https://www.linuxfoundation.org/blog/blog/classic-sysadmin-understanding-linux-file-permissions#:~:text=Sticky Bit Special,is t.

    SOLUTION
    So, instead of creating the file in the "/tmp" directory, simply create the file in the home directory of your "amar" user. Please see my example below:

    As you can see from above, I was able to read and write to the file once john had "rw" permissions. Then I was able to write but not read when john only had "w" permission.

    I hope this helps you and our other fellow students.

    Cheers
    Andy

  • Hi all,

    I made a test case on Ubuntu 18 LTS and 20 LTS, and I can confirm that the issue is regarding what @anewberry mentioned about the permissions on /tmp. So I made a test case on /mnt, with a directory owned by the first user, an ACL giving rw permissions to the second user, and it can write on the file.

    Regards,
    Luis.

  • @luisviveropena said:
    Hi all,

    I made a test case on Ubuntu 18 LTS and 20 LTS, and I can confirm that the issue is regarding what @anewberry mentioned about the permissions on /tmp. So I made a test case on /mnt, with a directory owned by the first user, an ACL giving rw permissions to the second user, and it can write on the file.

    Regards,
    Luis.

    Thanks @luisviveropena .... I appreciate you taking the time to conduct tests on both Ubuntu 18 LTS and 20 LTS to confirm that theory.
    :)

    Hopefully the course material will be updated in due course ... it would be a good opportunity to teach future students about the sticky bit on directories (like /tmp).

  • Hi @anewberry,

    Yes, the course material will be updated, so students will avoid the issue.

    Regards,
    Luis.

  • Hello @luisviveropena and @anewberry,

    I tried the lab again in "/home" instead of "/tmp," and it worked as expected.
    Thank you for taking the time to verify and provide clarification.

    Amar

  • Hi @cloud4amar, it's a pleasure! I'm glad this worked for you :)

    Regards,
    Luis.

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