Bind 9 Administrator Reference Manual

Chapter 7. BIND 9 Security Considerations
7.2. chroot and setuid (for UNIX servers)
On UNIX servers, it is possible to run BIND in a chrooted environment (chroot()) by specifying the
"-t" option. This can help improve system security by placing BIND in a "sandbox," which will limit
the damage done if a server is compromised.
Another useful feature in the UNIX version of BIND is the ability to run the daemon as a nonprivileged
user ( -u user ). We suggest running as a nonprivileged user when using the chroot feature.
Here is an example command line to load BIND in a chroot() sandbox, /var/named, and to run named
setuid to user 202:
/usr/local/bin/named -u 202 -t /var/named
7.2.1. The chroot Environment
In order for a chroot() environment to work properly in a particular directory (for example,
/var/named), you will need to set up an environment that includes everything BIND needs to run. From
BIND’s point of view, /var/named is the root of the filesystem. You will need to adjust the values of
options like like directory and pid-file to account for this.
Unlike with earlier versions of BIND, you will typically not need to compile named statically nor install
shared libraries under the new root. However, depending on your operating system, you may need to set
up things like /dev/zero, /dev/random, /dev/log, and/or /etc/localtime.
7.2.2. Using the setuid Function
Prior to running the named daemon, use the touch utility (to change file access and modification times)
or the chown utility (to set the user id and/or group id) on files to which you want BIND to write. Note
that if the named daemon is running as a nonprivileged user, it will not be able to bind to new restricted
ports if the server is reloaded.
7.3. Dynamic Update Security
Access to the dynamic update facility should be strictly limited. In earlier versions of BIND the only way
to do this was based on the IP address of the host requesting the update, by listing an IP address or
network prefix in the allow-update zone option. This method is insecure since the source address of the
update UDP packet is easily forged. Also note that if the IP addresses allowed by the allow-update
option include the address of a slave server which performs forwarding of dynamic updates, the master
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