Monday, 9 June 2014

Physical Security of Linux Servers

This is part of a series of articles on Red Hat Server Hardening.

The physical security of a Linux Server is the first line of defence against attackers. Who has direct physical access to the server, and should they? Should the server be secured out of hours or while you are away?

BIOS Security

The server's BIOS should be configured to disable booting from CDs/DVDs, floppies, and external devices. If possible, a password should be set to protect these settings.

GRUB Boot Loader Security

The GRUB boot loader should be password protected. The primary reasons for doing this are:
  1. Preventing Access to Single User Mode - If attackers can boot the system into single user mode, they are logged in automatically as root without being prompted for the root password.
  2. Preventing Access to the GRUB Console - If the machine uses GRUB as its boot loader, an attacker can use the use the GRUB editor interface to change its configuration or to gather information using the cat command.
By generating an MD5 hashed password string and including it in the GRUB configuration file, GRUB can be configured to address these issues.

To do this, first decide on a password, then open a shell prompt, log in as root, and type:
This will ask for a password to be entered. Type the new GRUB password twice. This returns an MD5 hash of the password.

Next, edit the GRUB configuration file /boot/grub/grub.conf.

Open the file and below the timeout line in the general section of the document, add the following line:
password --md5 <password-hash>
Replace <password-hash> with the value returned by /sbin/grub-md5-crypt.

Remove the line that contains the hiddenmenu parameter, and save the changes.

The next time the system boots, the GRUB menu does not allow access to the editor or command interface without first pressing p followed by the GRUB password.