Making the root filesystem read-only

You need to make your target device able to survive unexpected events, including file corruption, and still be able to boot and achieve at least a minimum level of functionality. Making the root filesystem read-only is a key part of achieving this ambition because it eliminates accidental overwrites. Making it read-only is easy: replace rw with ro on the kernel command line or use an inherently read-only filesystem such as SquashFS. However, you will find that there are a few files and directories that are traditionally writable:

• /etc/resolv.conf: This file is written by network configuration scripts to record the addresses of DNS name servers. The information is volatile, so you simply have to make it a symlink to a temporary directory; for example, /etc/resolv.conf -> /var/run/resolv.conf.
• /etc/passwd: This file, along with /etc/group, /etc/shadow, and
  /etc/gshadow, stores user and group names and passwords. They need to
  be symbolically...