Chapter 7. BIND 9 Security Considerations

Table of Contents

Access Control Lists
Chroot and Setuid (for UNIX servers)
The chroot Environment
Using the setuid Function
Dynamic Update Security

Access Control Lists

Access Control Lists (ACLs), are address match lists that you can set up and nickname for future use in allow-notify, allow-query, allow-recursion, blackhole, allow-transfer, etc.

Using ACLs allows you to have finer control over who can access your name server, without cluttering up your config files with huge lists of IP addresses.

It is a good idea to use ACLs, and to control access to your server. Limiting access to your server by outside parties can help prevent spoofing and denial of service (DoS) attacks against your server.

Here is an example of how to properly apply ACLs:

// Set up an ACL named "bogusnets" that will block RFC1918 space,
// which is commonly used in spoofing attacks.
acl bogusnets { 0.0.0.0/8; 1.0.0.0/8; 2.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3; 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16; };

// Set up an ACL called our-nets. Replace this with the real IP numbers.
acl our-nets { x.x.x.x/24; x.x.x.x/21; }; 
options {
  ...
  ...
  allow-query { our-nets; };
  allow-recursion { our-nets; };
  ...
  blackhole { bogusnets; };
  ...
};

zone "example.com" {
  type master;
  file "m/example.com";
  allow-query { any; };
};

This allows recursive queries of the server from the outside unless recursion has been previously disabled.

For more information on how to use ACLs to protect your server, see the AUSCERT advisory at ftp://ftp.auscert.org.au/pub/auscert/advisory/AL-1999.004.dns_dos

Chroot and Setuid (for UNIX servers)

On UNIX servers, it is possible to run BIND in a chrooted environment (using the chroot() function) 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 an unprivileged user ( -u user ). We suggest running as an unprivileged 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

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 typically will 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 /etc/localtime.

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 an unprivileged user, it will not be able to bind to new restricted ports if the server is reloaded.

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 can be trivially attacked by sending the update to the slave, which will forward it to the master with its own source IP address causing the master to approve it without question.

For these reasons, we strongly recommend that updates be cryptographically authenticated by means of transaction signatures (TSIG). That is, the allow-update option should list only TSIG key names, not IP addresses or network prefixes. Alternatively, the new update-policy option can be used.

Some sites choose to keep all dynamically-updated DNS data in a subdomain and delegate that subdomain to a separate zone. This way, the top-level zone containing critical data such as the IP addresses of public web and mail servers need not allow dynamic update at all.