Making files immutable by Linux 'chattr' command

After all those years working in Linux env and studying on application hardening, I don't know how did I miss "chattr" command.

chattr - change file attributes on a Linux file system

Specially the attribute "i" is interesting for me, as it allows you to make a file immutable.

"A file with the 'i' attribute cannot be modified: it cannot be deleted or renamed,  no  link  can  be created  to  this  file and no data can be written to the file.  Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute."

From man page of chattr, other attributes you can change are,
 -    append  only  (a),
 -    no  atime updates  (A),
 -    compressed  (c),
 -    no copy on write (C),
 -    no dump (d),
 -    synchronous directory updates (D),
 -    extent format (e),
 -    immutable (i),
 -    data journalling (j),
 -    secure deletion (s),
 -    synchronous updates (S),
 -    no tail-merging (t),
 -    top of directory hierarchy (T), and
 -    undeletable (u).


Following article explains many interesting usage of this command (including disabling user creation by making /etc/passwd file immutable)

https://www.tecmint.com/chattr-command-examples/


You can use lsattr command to see the attributes of the files/folders.