Chapter 7. BIND 9 Security Considerations

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
// RFC 1918 space and some reserved space, which is
// commonly used in spoofing attacks.
acl bogusnets {
        0.0.0.0/8;  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 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, or ecs elements, which specify a network prefix but are only matched if that prefix matches an EDNS client-subnet option included in the request.

The EDNS Client-Subnet (ECS) option is used by a recursive resolver to inform an authoritative name server of the network address block from which the original query was received, enabling authoritative servers to give different answers to the same resolver for different resolver clients. An ACL containing an element of the form ecs prefix will match if a request arrives in containing an ECS option encoding an address within that prefix. If the request has no ECS option, then "ecs" elements are simply ignored. Addresses in ACLs that are not prefixed with "ecs" are matched only against the source address.

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. When "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 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". (If using the legacy GeoIP API, "netspeed" and "org" databases are also available.)

By default, if a DNS query includes an EDNS Client-Subnet (ECS) option which encodes a non-zero address prefix, then GeoIP ACL elements are matched against that address prefix. Otherwise, they are matched against the source address of the query. To prevent GeoIP ACLs from matching against ECS options, set the geoip-use-ecs to no.

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; !10.0.0.1; } would actually match a query from 10.0.0.1, 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 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.

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

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 update at all.