cis-bind-v100-2-9

CIS 2.9 — Isolate BIND with chroot'ed Subdirectory

Profile Applicability

• Caching Only Name Server Level 2
• Authoritative Name Server Level 2

Description

The chroot() system call causes an application to run with limited file system access so that a subdirectory becomes the root directory for the application environment. When this is done, the application is "jailed" and no longer has access to the entire file structure but is limited to the given subdirectory.

The chroot'd subdirectory and the recommendations in section "Enable SELinux to Restrict BIND Processes" provide similar controls, in that the DNS service is prevented from accessing and modifying inappropriate files. SELinux goes well beyond what the chroot is able to prevent, however for audit purposes either control, the chroot'd subdirectory or the SELinux in enforcing mode is sufficient.

Rationale

Although there are ways that a chroot jail can be broken, most methods require that a process be running as root in order to escape. Since BIND should be run as a different user than root, a chroot is an effective defense, to limit access to sensitive system configuration files. In the event that BIND has a vulnerability that allows code execution, the attack will not have access to the real system files such as /etc/password, but will be limited to the files placed in the chroot subdirectory.

Impact

Not specified.

Audit Procedure

Run the following two commands to find the root directory of the currently running named process. If the named process is chroot'ed, then the listing will show a symbolic link to the chroot subdirectory. If process is not chroot'ed, then the symbolic link will point to the real root directory /.

# NAMEDPID=$(pidof named)
# ls -ld /proc/$NAMEDPID/root
lrwxrwxrwx 1 named named 0 Sep 10 13:21 /proc/423/root -> /var/named/chroot

Note: SELinux in enforcing mode may be used as an audit alternative to the chroot'd subdirectory.

Remediation

Perform the following:

• Stop the named service and install the bind-chroot package to provide the chroot directories.

# systemctl stop named.service
# yum install bind-chroot

• Edit the /etc/sysconfig/named configuration file to have a line similar to the one shown below that sets the ROOTDIR environment variable.

ROOTDIR="/var/named/chroot"

• Move all the configuration files and any master zone files into their respective directions under the subdirectory /var/named/chroot/
• It may be helpful to create symbolic links from a couple of system /etc files such as /etc/named.conf and /etc/rndc.key to the real files in the chroot'ed subdirectory, so that utilities like rndc will work as expected. Do not create symbolic links or hard links from inside the chroot to external resources! Instead use symbolic links to point from the outside resources into the chroot.
• Restart the named service and test the configuration.

# systemctl start named.service

Default Value

The BIND service is not chroot'ed by default.

References

None listed.

CIS Controls

MITRE ATT&CK Mappings

Profile

• Level 2 - Caching Only Name Server
• Level 2 - Authoritative Name Server