The Geek Diary

CentOS / RHEL 6 : How to password protect grub (Password-Protected Booting)

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The GRUB bootloader stores the password in a plaintext file, so any encrypted form of the password is required. To generate an encrypted password we may use grub-crypt command. Until now we have used the command grub-md5-crypt. But now MD5 is widely-considered broken. grub-crypt uses SHA-256 or SHA-512 hashes, which are considered more secure. The general syntax/usage of the grub-crypt command is as shown below :

# grub-crypt --help
Usage: grub-crypt [OPTION]...
Encrypt a password.

  -h, --help              Print this message and exit
  -v, --version           Print the version information and exit
  --md5                   Use MD5 to encrypt the password
  --sha-256               Use SHA-256 to encrypt the password
  --sha-512               Use SHA-512 to encrypt the password (default)

Report bugs to [bug-grub@gnu.org].
EOF

Using SHA-265 or SHA-512 hashes

1. As the root user, use the grub-crypt command to generate password hash. Type the password and re-type password for confirmation.

grub-crypt
Password:
Retype password:
$6$GXGrYVEnbKXAnQoT$p64OkyclNDt4qM2q47GMsgNxJxQaclNs79gvYYsl4h07ReDtJpt5P5kQn1KQ52u2eW8pKHTqcG50ffv0UlRcW0

2. Copy the encrypted password returned in the last line of the output which would look like a long scrambled string. Paste it before the TITLE statement in the /boot/grub/grub.conf file, like this:

# vi /boot/grub/grub.conf
....
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
password --encrypted $6$GXGrYVEnbKXAnQoT$p64OkyclNDt4qM2q47GMsgNxJxQaclNs79gvYYsl4h07ReDtJpt5P5kQn1KQ52u2eW8pKHTqcG50ffv0UlRcW0
title Red Hat Enterprise Linux (2.6.32-358.el6.x86_64)
....
Note : Make sure the permissions of the /boot/grub/grub.conf file are set to read-only to not allow any one to modify it.
# ls -lrt /boot/grub/grub.conf
-rw-------. 1 root root 845 Oct 11 14:43 /boot/grub/grub.conf

3. Once this is done, future boots will require the password before GRUB will permit you to edit boot options.

RHEL 6 set GRUB password

Using plain-text passwords

Although not secure, but if you still want to set a user readable plain-text GRUB password, use the below procedure :

1. Edit /boot/grub/grub.conf in a text editor and add a new “password PASSWORD-GOES-HERE” line prior to the first title stanza, e.g.:

default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
password my-not-so-hidden-password
title Red Hat Enterprise Linux ...

2. Ensure permissions on grub.conf do not allow anyone but root to read it :

# chmod 600 /boot/grub/grub.conf
# ls -l /boot/grub/grub.conf 
-rw------- 1 root root 678 Feb 02 14:12 /boot/grub/grub.conf

Using MD5 hashes

As said earlier in the post, MD5 is widely-considered broken. But if you still want to use them, follow the procedure below :

1. Run grub-md5-crypt to generate the hashed password :

# grub-md5-crypt 
Password: 
Retype password: 
$1$vweqo$CLFlozZ6ELHjGmL.0.37..

Add a new “password –md5 HASH-GOES-HERE” line prior to the first title line, e.g.:

default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
password --md5 $1$vweqo$CLFlozZ6ELHjGmL.0.37..
title Red Hat Enterprise Linux ...

As a final best-practice step, ensure permissions on grub.conf do not allow anyone but root to read it :

# chmod 600 /boot/grub/grub.conf
# ls -l /boot/grub/grub.conf 
-rw------- 1 root root 678 Jan 29 18:27 /boot/grub/grub.conf

Reboot the system and try pressing p to enter the password to unlock and enable next features in grub list.

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