In my last article I shared the steps to harden security of your Linux node by encrypting the hard disk partition using LUKS. Now let me show some more tweaks to increase security of your Linux setup.
Now in this article I will show you steps to prevent or restrict access of root user to access certain files or directories. Now by default root is the super user who has access to all the files and directories available on the Linux node but it is also possible to restrict even a root user from accessing and modifying the content of a file or directory.
You can restrict root user from accessing and modifying a file or directory using extended file attributes. We will be dealing with
lsattr to achieve this in our demonstration.
chattr - change file attributes on a Linux file system, supports multiple options but we will be concentrating only on the options which can help restrict root user access on certain files and directories.
To get the complete list of options supported with
chattr you can view the man page of
chattr using below command
# man chattr
We will work with two attributes
- Append text to a file
- Can’t overwrite
- Makes a file immutable
- Can’t be deleted or changed in any way
Create a secret file and directory
Now before we start we must have a top secret file which needs protection from root user. I have created a secret_file with below text
[root@node1 ~]# cat /tmp/deepak/secret_file This is a secret file
Check the assigned attributes
By default when we create a file or directory, it does not has any extended attributes other than “e” which means extent format i.e. these files support extended attributes
[root@node1 ~]# lsattr /tmp/ -------------e-- /tmp/tracker-extract-files.0 -------------e-- /tmp/yum_save_tx.2019-03-22.22-16.7ocUW8.yumtx -------------e-- /tmp/systemd-private-1ad03926d17f4de68a8fdfdd0449c980-chronyd.service-FhlC0B -------------e-- /tmp/systemd-private-1ad03926d17f4de68a8fdfdd0449c980-bolt.service-2Oomt7 -------------e-- /tmp/systemd-private-1ad03926d17f4de68a8fdfdd0449c980-rtkit-daemon.service-TEwKlB -------------e-- /tmp/deepak -------------e-- /tmp/systemd-private-1ad03926d17f4de68a8fdfdd0449c980-colord.service-cUfgTm -------------e-- /tmp/yum_save_tx.2019-03-22.22-16.ZCjaVi.yumtx -------------e-- /tmp/systemd-private-1ad03926d17f4de68a8fdfdd0449c980-cups.service-5yacYU
Restrict access and allow only to append content
Now we will use “
+a” to allow root user to append some data to our secret file but root won’t be allowed to overwrite the file.
[root@node1 ~]# chattr +a /tmp/deepak/secret_file
Check the assigned attributes and as you see now we have “
a” also assigned to our
[root@node1 ~]# lsattr /tmp/deepak/ -----a-------e-- /tmp/deepak/secret_file
Next try to append some data to this file
[root@node1 ~]# echo "I am appending some more content" >> /tmp/deepak/secret_file
Looks like it worked as expected, verify the same
[root@node1 ~]# cat /tmp/deepak/secret_file This is a secret file I am appending some more content
So, as you see now our secret file has some more content.
Let us try to overwrite the data
[root@node1 ~]# echo "I am trying to overwrite the content" > /tmp/deepak/secret_file -bash: /tmp/deepak/secret_file: Operation not permitted
As expected the extended attributes didn’t allowed me to overwrite the data.
Make the file immutable (restrict all activity)
Now let us make the file immutable so no change at all can be made to this file.
[root@node1 ~]# chattr +i /tmp/deepak/secret_file
Check the applied attributes
[root@node1 ~]# lsattr /tmp/deepak/secret_file ----ia-------e-- /tmp/deepak/secret_file
As you see both “
a” and “
i” are applied to our secret file but since “
i” serves our purpose we do not need “
a” here so we will remove the “
[root@node1 ~]# chattr -a /tmp/deepak/secret_file
Next verify the applied attributes again
[root@node1 ~]# lsattr /tmp/deepak/secret_file ----i--------e-- /tmp/deepak/secret_file
Next I will try to overwrite the data of this file and will also attempt to remove this file
[root@node1 ~]# echo "I am trying to overwrite the content" > /tmp/deepak/secret_file -bash: /tmp/deepak/secret_file: Permission denied [root@node1 ~]# rm -f /tmp/deepak/secret_file rm: cannot remove ‘/tmp/deepak/secret_file’: Operation not permitted
But as you see due to the extended attributes the system does not allows root user to perform any activity on this file.
Instead of file you can also apply these attributes at directory level to protect all the files under the respective directory.
Remove extended attributes
To remove an extended attributes as I also showed in above step use minus sign along with the option
# chattr -a <file/directory> # chattr -i <file/directory>
Lastly I hope the steps from the article to prevent or restrict root user access on files and directories on Linux was helpful. So, let me know your suggestions and feedback using the comment section.