7. BIND 9 Security Considerations

7.1. Security Assumptions

BIND 9’s design assumes that access to the objects listed below is limited only to trusted parties. An incorrect deployment, which does not follow rules set by this section, cannot be the basis for CVE assignment or special security-sensitive handling of issues.

Unauthorized access can potentially disclose sensitive data, slow down server operation, etc. Unauthorized, unexpected, or incorrect writes to listed objects can potentically cause crashes, incorrect data handling, or corruption.

Certain aspects of the DNS protocol are left unspecified, such as the handling of responses from DNS servers which do not fully conform to the DNS protocol. For such a situation, BIND implements its own safety checks and limits which are subject to change as the protocol and deployment evolve.

7.1.1. Authoritative Servers

By default, zones use intentionally lenient limits (unlimited size, long transfer timeouts, etc.). These defaults can be misused by the source of data (zone transfers or UPDATEs) to exhaust resources on the receiving side.

The impact of malicious zone changes can be limited, to an extent, using configuration options listed in sections Server Resource Limits and Zone Transfers. Limits should also be applied to zones where malicious clients may potentially be authorized to use Dynamic Update.

7.1.2. DNS Resolvers

By definition, DNS resolvers act as traffic amplifiers; during normal operation, a DNS resolver can legitimately generate more outgoing traffic (counted in packets or bytes) than the incoming client traffic that triggered it. The DNS protocol specification does not currently specify limits for this amplification, but BIND implements its own limits to balance interoperability and safety. As a general rule, if a traffic amplification factor for any given scenario is lower than 100 packets, ISC does not handle the given scenario as a security issue. These limits are subject to change as DNS deployment evolves.

All DNS answers received by the DNS resolver are treated as untrusted input and are subject to safety and correctness checks. However, protocol non-conformity might cause unexpected behavior. If such unexpected behavior is limited to DNS domains hosted on non-conformant servers, it is not deemed a security issue in BIND.

7.2. Access Control Lists

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

ACLs give users finer control over who can access the name server, without cluttering up configuration files with huge lists of IP addresses.

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

ACLs match clients on the basis of up to three characteristics: 1) The client’s IP address; 2) the TSIG or SIG(0) key that was used to sign the request, if any; and 3) an address prefix encoded in an EDNS Client-Subnet option, if any.

Here is an example of ACLs based on client addresses:

// Set up an ACL named "bogusnets" that blocks
// RFC1918 space and some reserved space, which is
// commonly used in spoofing attacks.
acl bogusnets {;;;;;;

// 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 primary;
  file "m/example.com";
  allow-query { any; };

This allows authoritative queries for example.com from any address, but recursive queries only from the networks specified in our-nets, and no queries at all from the networks specified in bogusnets.

In addition to network addresses and prefixes, which are matched against the source address of the DNS request, ACLs may include key elements, which specify the name of a TSIG or SIG(0) key.

When BIND 9 is built with GeoIP support, ACLs can also be used for geographic access restrictions. This is done by specifying an ACL element of the form: geoip db database field value.

The field parameter indicates which field to search for a match. Available fields are country, region, city, continent, postal (postal code), metro (metro code), area (area code), tz (timezone), isp, asnum, and domain.

value is the value to search for within the database. A string may be quoted if it contains spaces or other special characters. An asnum search for autonomous system number can be specified using the string “ASNNNN” or the integer NNNN. If a country search is specified with a string that is two characters long, it must be a standard ISO-3166-1 two-letter country code; otherwise, it is interpreted as the full name of the country. Similarly, if region is the search term and the string is two characters long, it is treated as a standard two-letter state or province abbreviation; otherwise, it is treated as the full name of the state or province.

The database field indicates which GeoIP database to search for a match. In most cases this is unnecessary, because most search fields can only be found in a single database. However, searches for continent or country can be answered from either the city or country databases, so for these search types, specifying a database forces the query to be answered from that database and no other. If a database is not specified, these queries are first answered from the city database if it is installed, and then from the country database if it is installed. Valid database names are country, city, asnum, isp, and domain.

Some example GeoIP ACLs:

geoip country US;
geoip country JP;
geoip db country country Canada;
geoip region WA;
geoip city "San Francisco";
geoip region Oklahoma;
geoip postal 95062;
geoip tz "America/Los_Angeles";
geoip org "Internet Systems Consortium";

ACLs use a “first-match” logic rather than “best-match”; if an address prefix matches an ACL element, then that ACL is considered to have matched even if a later element would have matched more specifically. For example, the ACL { 10/8; !; } would actually match a query from, because the first element indicates that the query should be accepted, and the second element is ignored.

When using “nested” ACLs (that is, ACLs included or referenced within other ACLs), a negative match of a nested ACL tells the containing ACL to continue looking for matches. This enables complex ACLs to be constructed, in which multiple client characteristics can be checked at the same time. For example, to construct an ACL which allows a query only when it originates from a particular network and only when it is signed with a particular key, use:

allow-query { !{ !10/8; any; }; key example; };

Within the nested ACL, any address that is not in the 10/8 network prefix is rejected, which terminates the processing of the ACL. Any address that is in the 10/8 network prefix is accepted, but this causes a negative match of the nested ACL, so the containing ACL continues processing. The query is accepted if it is signed by the key example, and rejected otherwise. The ACL, then, only matches when both conditions are true.

7.3. Chroot and Setuid

On Unix servers, it is possible to run BIND in a chrooted environment (using the chroot() function) by specifying the -t option for named. This can help improve system security by placing BIND in a “sandbox,” which limits 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/sbin/named -u 202 -t /var/named

7.3.1. The chroot Environment

For a chroot environment to work properly in a particular directory (for example, /var/named), the environment must include everything BIND needs to run. From BIND’s point of view, /var/named is the root of the filesystem; the values of options like directory and pid-file must be adjusted to account for this.

Unlike with earlier versions of BIND, named does not typically need to be compiled statically, nor do shared libraries need to be installed under the new root. However, depending on the operating system, it may be necessary to set up locations such as /dev/zero, /dev/random, /dev/log, and /etc/localtime.

7.3.2. 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 where BIND should write.


If the named daemon is running as an unprivileged user, it cannot bind to new restricted ports if the server is reloaded.

7.4. 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 secondary server which performs forwarding of dynamic updates, the primary can be trivially attacked by sending the update to the secondary, which forwards it to the primary with its own source IP address - causing the primary 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 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 updates at all.