The Read, Write, Execute, and Setuid fields are common to all Linux file systems. The Extended File Systems (ext2, ext3, and ext4) support more attributes.
One of the extended attributes makes files immutable. When a file is made immutable, any user or super user cannot remove the file until the immutable attribute is removed from the file. You can determine the type of filesystem with the df -T command, or by looking at the /etc/mtab file. The first column of the file specifies the partition device path (for example, /dev/sda5) and the third column specifies the filesystem type (for example, ext3).
Making a file immutable is one method for securing files from modification. One example is to make the /etc/resolv.conf file immutable. The resolv.conf file stores a list of DNS servers, which convert domain names (such as packtpub.com) to IP addresses. The DNS server is usually your ISP's...
