5. Advanced DNS Features

5.1. Notify

DNS NOTIFY is a mechanism that allows primary servers to notify their secondary servers of changes to a zone’s data. In response to a NOTIFY from a primary server, the secondary checks to see that its version of the zone is the current version and, if not, initiates a zone transfer.

For more information about DNS NOTIFY, see the description of the notify option in Boolean Options and the description of the zone option also-notify in Zone Transfers. The NOTIFY protocol is specified in RFC 1996.


As a secondary zone can also be a primary to other secondaries, named, by default, sends NOTIFY messages for every zone it loads. Specifying notify primary-only; causes named to only send NOTIFY for primary zones that it loads.

5.2. Dynamic Update

Dynamic update is a method for adding, replacing, or deleting records in a primary server by sending it a special form of DNS messages. The format and meaning of these messages is specified in RFC 2136.

Dynamic update is enabled by including an allow-update or an update-policy clause in the zone statement.

If the zone’s update-policy is set to local, updates to the zone are permitted for the key local-ddns, which is generated by named at startup. See Dynamic Update Policies for more details.

Dynamic updates using Kerberos-signed requests can be made using the TKEY/GSS protocol, either by setting the tkey-gssapi-keytab option or by setting both the tkey-gssapi-credential and tkey-domain options. Once enabled, Kerberos-signed requests are matched against the update policies for the zone, using the Kerberos principal as the signer for the request.

Updating of secure zones (zones using DNSSEC) follows RFC 3007: RRSIG, NSEC, and NSEC3 records affected by updates are automatically regenerated by the server using an online zone key. Update authorization is based on transaction signatures and an explicit server policy.

5.2.1. The Journal File

All changes made to a zone using dynamic update are stored in the zone’s journal file. This file is automatically created by the server when the first dynamic update takes place. The name of the journal file is formed by appending the extension .jnl to the name of the corresponding zone file unless specifically overridden. The journal file is in a binary format and should not be edited manually.

The server also occasionally writes (“dumps”) the complete contents of the updated zone to its zone file. This is not done immediately after each dynamic update because that would be too slow when a large zone is updated frequently. Instead, the dump is delayed by up to 15 minutes, allowing additional updates to take place. During the dump process, transient files are created with the extensions .jnw and .jbk; under ordinary circumstances, these are removed when the dump is complete, and can be safely ignored.

When a server is restarted after a shutdown or crash, it replays the journal file to incorporate into the zone any updates that took place after the last zone dump.

Changes that result from incoming incremental zone transfers are also journaled in a similar way.

The zone files of dynamic zones cannot normally be edited by hand because they are not guaranteed to contain the most recent dynamic changes; those are only in the journal file. The only way to ensure that the zone file of a dynamic zone is up-to-date is to run rndc stop.

To make changes to a dynamic zone manually, follow these steps: first, disable dynamic updates to the zone using rndc freeze zone. This updates the zone file with the changes stored in its .jnl file. Then, edit the zone file. Finally, run rndc thaw zone to reload the changed zone and re-enable dynamic updates.

rndc sync zone updates the zone file with changes from the journal file without stopping dynamic updates; this may be useful for viewing the current zone state. To remove the .jnl file after updating the zone file, use rndc sync -clean.

5.3. Incremental Zone Transfers (IXFR)

The incremental zone transfer (IXFR) protocol is a way for secondary servers to transfer only changed data, instead of having to transfer an entire zone. The IXFR protocol is specified in RFC 1995. See Proposed Standards.

When acting as a primary server, BIND 9 supports IXFR for those zones where the necessary change history information is available. These include primary zones maintained by dynamic update and secondary zones whose data was obtained by IXFR. For manually maintained primary zones, and for secondary zones obtained by performing a full zone transfer (AXFR), IXFR is supported only if the option ixfr-from-differences is set to yes.

When acting as a secondary server, BIND 9 attempts to use IXFR unless it is explicitly disabled. For more information about disabling IXFR, see the description of the request-ixfr clause of the server statement.

When a secondary server receives a zone via AXFR, it creates a new copy of the zone database and then swaps it into place; during the loading process, queries continue to be served from the old database with no interference. When receiving a zone via IXFR, however, changes are applied to the running zone, which may degrade query performance during the transfer. If a server receiving an IXFR request determines that the response size would be similar in size to an AXFR response, it may wish to send AXFR instead. The threshold at which this determination is made can be configured using the max-ixfr-ratio option.

5.4. Split DNS

Setting up different views of the DNS space to internal and external resolvers is usually referred to as a split DNS setup. There are several reasons an organization might want to set up its DNS this way.

One common reason to use split DNS is to hide “internal” DNS information from “external” clients on the Internet. There is some debate as to whether this is actually useful. Internal DNS information leaks out in many ways (via email headers, for example) and most savvy “attackers” can find the information they need using other means. However, since listing addresses of internal servers that external clients cannot possibly reach can result in connection delays and other annoyances, an organization may choose to use split DNS to present a consistent view of itself to the outside world.

Another common reason for setting up a split DNS system is to allow internal networks that are behind filters or in RFC 1918 space (reserved IP space, as documented in RFC 1918) to resolve DNS on the Internet. Split DNS can also be used to allow mail from outside back into the internal network.

5.4.1. Example Split DNS Setup

Let’s say a company named Example, Inc. (example.com) has several corporate sites that have an internal network with reserved Internet Protocol (IP) space and an external demilitarized zone (DMZ), or “outside” section of a network, that is available to the public.

Example, Inc. wants its internal clients to be able to resolve external hostnames and to exchange mail with people on the outside. The company also wants its internal resolvers to have access to certain internal-only zones that are not available at all outside of the internal network.

To accomplish this, the company sets up two sets of name servers. One set is on the inside network (in the reserved IP space) and the other set is on bastion hosts, which are “proxy” hosts in the DMZ that can talk to both sides of its network.

The internal servers are configured to forward all queries, except queries for site1.internal, site2.internal, site1.example.com, and site2.example.com, to the servers in the DMZ. These internal servers have complete sets of information for site1.example.com, site2.example.com, site1.internal, and site2.internal.

To protect the site1.internal and site2.internal domains, the internal name servers must be configured to disallow all queries to these domains from any external hosts, including the bastion hosts.

The external servers, which are on the bastion hosts, are configured to serve the “public” version of the site1.example.com and site2.example.com zones. This could include things such as the host records for public servers (www.example.com and ftp.example.com) and mail exchange (MX) records (a.mx.example.com and b.mx.example.com).

In addition, the public site1.example.com and site2.example.com zones should have special MX records that contain wildcard (*) records pointing to the bastion hosts. This is needed because external mail servers have no other way of determining how to deliver mail to those internal hosts. With the wildcard records, the mail is delivered to the bastion host, which can then forward it on to internal hosts.

Here’s an example of a wildcard MX record:

*   IN MX 10 external1.example.com.

Now that they accept mail on behalf of anything in the internal network, the bastion hosts need to know how to deliver mail to internal hosts. The resolvers on the bastion hosts need to be configured to point to the internal name servers for DNS resolution.

Queries for internal hostnames are answered by the internal servers, and queries for external hostnames are forwarded back out to the DNS servers on the bastion hosts.

For all of this to work properly, internal clients need to be configured to query only the internal name servers for DNS queries. This could also be enforced via selective filtering on the network.

If everything has been set properly, Example, Inc.’s internal clients are now able to:

  • Look up any hostnames in the site1.example.com and site2.example.com zones.

  • Look up any hostnames in the site1.internal and site2.internal domains.

  • Look up any hostnames on the Internet.

  • Exchange mail with both internal and external users.

Hosts on the Internet are able to:

  • Look up any hostnames in the site1.example.com and site2.example.com zones.

  • Exchange mail with anyone in the site1.example.com and site2.example.com zones.

Here is an example configuration for the setup just described above. Note that this is only configuration information; for information on how to configure the zone files, see Sample Configurations.

Internal DNS server config:

acl internals {;; };

acl externals { bastion-ips-go-here; };

options {
    forward only;
    // forward to external servers
    forwarders {
    // sample allow-transfer (no one)
    allow-transfer { none; };
    // restrict query access
    allow-query { internals; externals; };
    // restrict recursion
    allow-recursion { internals; };

// sample primary zone
zone "site1.example.com" {
  type primary;
  file "m/site1.example.com";
  // do normal iterative resolution (do not forward)
  forwarders { };
  allow-query { internals; externals; };
  allow-transfer { internals; };

// sample secondary zone
zone "site2.example.com" {
  type secondary;
  file "s/site2.example.com";
  primaries {; };
  forwarders { };
  allow-query { internals; externals; };
  allow-transfer { internals; };

zone "site1.internal" {
  type primary;
  file "m/site1.internal";
  forwarders { };
  allow-query { internals; };
  allow-transfer { internals; }

zone "site2.internal" {
  type secondary;
  file "s/site2.internal";
  primaries {; };
  forwarders { };
  allow-query { internals };
  allow-transfer { internals; }

External (bastion host) DNS server configuration:

acl internals {;; };

acl externals { bastion-ips-go-here; };

options {
  // sample allow-transfer (no one)
  allow-transfer { none; };
  // default query access
  allow-query { any; };
  // restrict cache access
  allow-query-cache { internals; externals; };
  // restrict recursion
  allow-recursion { internals; externals; };

// sample secondary zone
zone "site1.example.com" {
  type primary;
  file "m/site1.foo.com";
  allow-transfer { internals; externals; };

zone "site2.example.com" {
  type secondary;
  file "s/site2.foo.com";
  masters { another_bastion_host_maybe; };
  allow-transfer { internals; externals; }

In the resolv.conf (or equivalent) on the bastion host(s):

search ...

5.5. TSIG

TSIG (Transaction SIGnatures) is a mechanism for authenticating DNS messages, originally specified in RFC 2845. It allows DNS messages to be cryptographically signed using a shared secret. TSIG can be used in any DNS transaction, as a way to restrict access to certain server functions (e.g., recursive queries) to authorized clients when IP-based access control is insufficient or needs to be overridden, or as a way to ensure message authenticity when it is critical to the integrity of the server, such as with dynamic UPDATE messages or zone transfers from a primary to a secondary server.

This section is a guide to setting up TSIG in BIND. It describes the configuration syntax and the process of creating TSIG keys.

named supports TSIG for server-to-server communication, and some of the tools included with BIND support it for sending messages to named:

5.5.1. Generating a Shared Key

TSIG keys can be generated using the tsig-keygen command; the output of the command is a key directive suitable for inclusion in named.conf. The key name, algorithm, and size can be specified by command-line parameters; the defaults are “tsig-key”, HMAC-SHA256, and 256 bits, respectively.

Any string which is a valid DNS name can be used as a key name. For example, a key to be shared between servers called host1 and host2 could be called “host1-host2.”, and this key can be generated using:

$ tsig-keygen host1-host2. > host1-host2.key

This key may then be copied to both hosts. The key name and secret must be identical on both hosts. (Note: copying a shared secret from one server to another is beyond the scope of the DNS. A secure transport mechanism should be used: secure FTP, SSL, ssh, telephone, encrypted email, etc.)

tsig-keygen can also be run as ddns-confgen, in which case its output includes additional configuration text for setting up dynamic DNS in named. See ddns-confgen - ddns key generation tool for details.

5.5.2. Loading a New Key

For a key shared between servers called host1 and host2, the following could be added to each server’s named.conf file:

key "host1-host2." {
    algorithm hmac-sha256;
    secret "DAopyf1mhCbFVZw7pgmNPBoLUq8wEUT7UuPoLENP2HY=";

(This is the same key generated above using tsig-keygen.)

Since this text contains a secret, it is recommended that either named.conf not be world-readable, or that the key directive be stored in a file which is not world-readable and which is included in named.conf via the include directive.

Once a key has been added to named.conf and the server has been restarted or reconfigured, the server can recognize the key. If the server receives a message signed by the key, it is able to verify the signature. If the signature is valid, the response is signed using the same key.

TSIG keys that are known to a server can be listed using the command rndc tsig-list.

5.5.3. Instructing the Server to Use a Key

A server sending a request to another server must be told whether to use a key, and if so, which key to use.

For example, a key may be specified for each server in the primaries statement in the definition of a secondary zone; in this case, all SOA QUERY messages, NOTIFY messages, and zone transfer requests (AXFR or IXFR) are signed using the specified key. Keys may also be specified in the also-notify statement of a primary or secondary zone, causing NOTIFY messages to be signed using the specified key.

Keys can also be specified in a server directive. Adding the following on host1, if the IP address of host2 is, would cause all requests from host1 to host2, including normal DNS queries, to be signed using the host1-host2. key:

server {
    keys { host1-host2. ;};

Multiple keys may be present in the keys statement, but only the first one is used. As this directive does not contain secrets, it can be used in a world-readable file.

Requests sent by host2 to host1 would not be signed, unless a similar server directive were in host2’s configuration file.

When any server sends a TSIG-signed DNS request, it expects the response to be signed with the same key. If a response is not signed, or if the signature is not valid, the response is rejected.

5.5.4. TSIG-Based Access Control

TSIG keys may be specified in ACL definitions and ACL directives such as allow-query, allow-transfer, and allow-update. The above key would be denoted in an ACL element as key host1-host2.

Here is an example of an allow-update directive using a TSIG key:

allow-update { !{ !localnets; any; }; key host1-host2. ;};

This allows dynamic updates to succeed only if the UPDATE request comes from an address in localnets, and if it is signed using the host1-host2. key.

See Dynamic Update Policies for a discussion of the more flexible update-policy statement.

5.5.5. Errors

Processing of TSIG-signed messages can result in several errors:

  • If a TSIG-aware server receives a message signed by an unknown key, the response will be unsigned, with the TSIG extended error code set to BADKEY.

  • If a TSIG-aware server receives a message from a known key but with an invalid signature, the response will be unsigned, with the TSIG extended error code set to BADSIG.

  • If a TSIG-aware server receives a message with a time outside of the allowed range, the response will be signed but the TSIG extended error code set to BADTIME, and the time values will be adjusted so that the response can be successfully verified.

In all of the above cases, the server returns a response code of NOTAUTH (not authenticated).

5.6. TKEY

TKEY (Transaction KEY) is a mechanism for automatically negotiating a shared secret between two hosts, originally specified in RFC 2930.

There are several TKEY “modes” that specify how a key is to be generated or assigned. BIND 9 implements only one of these modes: Diffie-Hellman key exchange. Both hosts are required to have a KEY record with algorithm DH (though this record is not required to be present in a zone).

The TKEY process is initiated by a client or server by sending a query of type TKEY to a TKEY-aware server. The query must include an appropriate KEY record in the additional section, and must be signed using either TSIG or SIG(0) with a previously established key. The server’s response, if successful, contains a TKEY record in its answer section. After this transaction, both participants have enough information to calculate a shared secret using Diffie-Hellman key exchange. The shared secret can then be used to sign subsequent transactions between the two servers.

TSIG keys known by the server, including TKEY-negotiated keys, can be listed using rndc tsig-list.

TKEY-negotiated keys can be deleted from a server using rndc tsig-delete. This can also be done via the TKEY protocol itself, by sending an authenticated TKEY query specifying the “key deletion” mode.

5.7. SIG(0)

BIND partially supports DNSSEC SIG(0) transaction signatures as specified in RFC 2535 and RFC 2931. SIG(0) uses public/private keys to authenticate messages. Access control is performed in the same manner as with TSIG keys; privileges can be granted or denied in ACL directives based on the key name.

When a SIG(0) signed message is received, it is only verified if the key is known and trusted by the server. The server does not attempt to recursively fetch or validate the key.

SIG(0) signing of multiple-message TCP streams is not supported.

The only tool shipped with BIND 9 that generates SIG(0) signed messages is nsupdate.


Cryptographic authentication of DNS information is possible through the DNS Security (“DNSSEC-bis”) extensions, defined in RFC 4033, RFC 4034, and RFC 4035. This section describes the creation and use of DNSSEC signed zones.

In order to set up a DNSSEC secure zone, there are a series of steps which must be followed. BIND 9 ships with several tools that are used in this process, which are explained in more detail below. In all cases, the -h option prints a full list of parameters. Note that the DNSSEC tools require the keyset files to be in the working directory or the directory specified by the -d option.

There must also be communication with the administrators of the parent and/or child zone to transmit keys. A zone’s security status must be indicated by the parent zone for a DNSSEC-capable resolver to trust its data. This is done through the presence or absence of a DS record at the delegation point.

For other servers to trust data in this zone, they must be statically configured with either this zone’s zone key or the zone key of another zone above this one in the DNS tree.

5.8.1. Generating Keys

The dnssec-keygen program is used to generate keys.

A secure zone must contain one or more zone keys. The zone keys sign all other records in the zone, as well as the zone keys of any secure delegated zones. Zone keys must have the same name as the zone, have a name type of ZONE, and be usable for authentication. It is recommended that zone keys use a cryptographic algorithm designated as “mandatory to implement” by the IETF. Currently there are two algorithms, RSASHA256 and ECDSAP256SHA256; ECDSAP256SHA256 is recommended for current and future deployments.

The following command generates an ECDSAP256SHA256 key for the child.example zone:

dnssec-keygen -a ECDSAP256SHA256 -n ZONE child.example.

Two output files are produced: Kchild.example.+013+12345.key and Kchild.example.+013+12345.private (where 12345 is an example of a key tag). The key filenames contain the key name (child.example.), the algorithm (5 is RSASHA1, 8 is RSASHA256, 13 is ECDSAP256SHA256, 15 is ED25519, etc.), and the key tag (12345 in this case). The private key (in the .private file) is used to generate signatures, and the public key (in the .key file) is used for signature verification.

To generate another key with the same properties but with a different key tag, repeat the above command.

The dnssec-keyfromlabel program is used to get a key pair from a crypto hardware device and build the key files. Its usage is similar to dnssec-keygen.

The public keys should be inserted into the zone file by including the .key files using $INCLUDE statements.

5.8.2. Signing the Zone

The dnssec-signzone program is used to sign a zone.

Any keyset files corresponding to secure sub-zones should be present. The zone signer generates NSEC, NSEC3, and RRSIG records for the zone, as well as DS for the child zones if -g is specified. If -g is not specified, then DS RRsets for the secure child zones need to be added manually.

By default, all zone keys which have an available private key are used to generate signatures. The following command signs the zone, assuming it is in a file called zone.child.example:

dnssec-signzone -o child.example zone.child.example

One output file is produced: zone.child.example.signed. This file should be referenced by named.conf as the input file for the zone.

dnssec-signzone also produces keyset and dsset files. These are used to provide the parent zone administrators with the DNSKEYs (or their corresponding DS records) that are the secure entry point to the zone.

5.8.3. Configuring Servers for DNSSEC

To enable named to validate answers received from other servers, the dnssec-validation option must be set to either yes or auto.

When dnssec-validation is set to auto, a trust anchor for the DNS root zone is automatically used. This trust anchor is provided as part of BIND and is kept up to date using RFC 5011 key management.

When dnssec-validation is set to yes, DNSSEC validation only occurs if at least one trust anchor has been explicitly configured in named.conf, using a trust-anchors statement (or the managed-keys and trusted-keys statements, both deprecated).

When dnssec-validation is set to no, DNSSEC validation does not occur.

The default is auto unless BIND is built with configure --disable-auto-validation, in which case the default is yes.

The keys specified in trust-anchors are copies of DNSKEY RRs for zones that are used to form the first link in the cryptographic chain of trust. Keys configured with the keyword static-key or static-ds are loaded directly into the table of trust anchors, and can only be changed by altering the configuration. Keys configured with initial-key or initial-ds are used to initialize RFC 5011 trust anchor maintenance, and are kept up-to-date automatically after the first time named runs.

trust-anchors is described in more detail later in this document.

BIND 9 does not verify signatures on load, so zone keys for authoritative zones do not need to be specified in the configuration file.

After DNSSEC is established, a typical DNSSEC configuration looks something like the following. It has one or more public keys for the root, which allows answers from outside the organization to be validated. It also has several keys for parts of the namespace that the organization controls. These are here to ensure that named is immune to compromised security in the DNSSEC components of parent zones.

trust-anchors {
    /* Root Key */
    "." initial-key 257 3 3 "BNY4wrWM1nCfJ+CXd0rVXyYmobt7sEEfK3clRbGaTwS
                 hf+6fElrmLkdaz MQ2OCnACR817DF4BBa7UR/beDHyp
    /* Key for our organization's forward zone */
    example.com. static-ds 54135 5 2 "8EF922C97F1D07B23134440F19682E7519ADDAE180E20B1B1EC52E7F58B2831D"

    /* Key for our reverse zone. */
    2.0.192.IN-ADDRPA.NET. static-key 257 3 5 "AQOnS4xn/IgOUpBPJ3bogzwc

options {
    dnssec-validation yes;


None of the keys listed in this example are valid. In particular, the root key is not valid.

When DNSSEC validation is enabled and properly configured, the resolver rejects any answers from signed, secure zones which fail to validate, and returns SERVFAIL to the client.

Responses may fail to validate for any of several reasons, including missing, expired, or invalid signatures, a key which does not match the DS RRset in the parent zone, or an insecure response from a zone which, according to its parent, should have been secure.


When the validator receives a response from an unsigned zone that has a signed parent, it must confirm with the parent that the zone was intentionally left unsigned. It does this by verifying, via signed and validated NSEC/NSEC3 records, that the parent zone contains no DS records for the child.

If the validator can prove that the zone is insecure, then the response is accepted. However, if it cannot, the validator must assume an insecure response to be a forgery; it rejects the response and logs an error.

The logged error reads “insecurity proof failed” and “got insecure response; parent indicates it should be secure.”

5.9. DNSSEC, Dynamic Zones, and Automatic Signing

5.9.1. Converting From Insecure to Secure

A zone can be changed from insecure to secure in three ways: using a dynamic DNS update, via the auto-dnssec zone option, or by setting a DNSSEC policy for the zone with dnssec-policy.

For any method, named must be configured so that it can see the K* files which contain the public and private parts of the keys that are used to sign the zone. These files are generated by dnssec-keygen, or created when needed by named if dnssec-policy is used. Keys should be placed in the key-directory, as specified in named.conf:

zone example.net {
    type primary;
    update-policy local;
    file "dynamic/example.net/example.net";
    key-directory "dynamic/example.net";

If one KSK and one ZSK DNSKEY key have been generated, this configuration causes all records in the zone to be signed with the ZSK, and the DNSKEY RRset to be signed with the KSK. An NSEC chain is generated as part of the initial signing process.

With dnssec-policy, it is possible to specify which keys should be KSK and/or ZSK. To sign all records with a key, a CSK must be specified. For example:

dnssec-policy csk {
    keys {
        csk lifetime unlimited algorithm 13;

5.9.2. Dynamic DNS Update Method

To insert the keys via dynamic update:

% nsupdate
> ttl 3600
> update add example.net DNSKEY 256 3 7 AwEAAZn17pUF0KpbPA2c7Gz76Vb18v0teKT3EyAGfBfL8eQ8al35zz3Y I1m/SAQBxIqMfLtIwqWPdgthsu36azGQAX8=
> update add example.net DNSKEY 257 3 7 AwEAAd/7odU/64o2LGsifbLtQmtO8dFDtTAZXSX2+X3e/UNlq9IHq3Y0 XtC0Iuawl/qkaKVxXe2lo8Ct+dM6UehyCqk=
> send

While the update request completes almost immediately, the zone is not completely signed until named has had time to “walk” the zone and generate the NSEC and RRSIG records. The NSEC record at the apex is added last, to signal that there is a complete NSEC chain.

To sign using NSEC3 instead of NSEC, add an NSEC3PARAM record to the initial update request. The OPTOUT bit in the NSEC3 chain can be set in the flags field of the NSEC3PARAM record.

% nsupdate
> ttl 3600
> update add example.net DNSKEY 256 3 7 AwEAAZn17pUF0KpbPA2c7Gz76Vb18v0teKT3EyAGfBfL8eQ8al35zz3Y I1m/SAQBxIqMfLtIwqWPdgthsu36azGQAX8=
> update add example.net DNSKEY 257 3 7 AwEAAd/7odU/64o2LGsifbLtQmtO8dFDtTAZXSX2+X3e/UNlq9IHq3Y0 XtC0Iuawl/qkaKVxXe2lo8Ct+dM6UehyCqk=
> update add example.net NSEC3PARAM 1 1 100 1234567890
> send

Again, this update request completes almost immediately; however, the record does not show up until named has had a chance to build/remove the relevant chain. A private type record is created to record the state of the operation (see below for more details), and is removed once the operation completes.

While the initial signing and NSEC/NSEC3 chain generation is happening, other updates are possible as well.

5.9.3. Fully Automatic Zone Signing

To enable automatic signing, set a dnssec-policy or add the auto-dnssec option to the zone statement in named.conf. auto-dnssec has two possible arguments: allow or maintain.

With auto-dnssec allow, named can search the key directory for keys matching the zone, insert them into the zone, and use them to sign the zone. It does so only when it receives an rndc sign <zonename>.

auto-dnssec maintain includes the above functionality, but also automatically adjusts the zone’s DNSKEY records on a schedule according to the keys’ timing metadata. (See dnssec-keygen: DNSSEC key generation tool and dnssec-settime: set the key timing metadata for a DNSSEC key for more information.)

dnssec-policy is similar to auto-dnssec maintain, but dnssec-policy also automatically creates new keys when necessary. In addition, any configuration related to DNSSEC signing is retrieved from the policy, ignoring existing DNSSEC named.conf options.

named periodically searches the key directory for keys matching the zone; if the keys’ metadata indicates that any change should be made to the zone - such as adding, removing, or revoking a key - then that action is carried out. By default, the key directory is checked for changes every 60 minutes; this period can be adjusted with dnssec-loadkeys-interval, up to a maximum of 24 hours. The rndc loadkeys command forces named to check for key updates immediately.

If keys are present in the key directory the first time the zone is loaded, the zone is signed immediately, without waiting for an rndc sign or rndc loadkeys command. Those commands can still be used when there are unscheduled key changes.

When new keys are added to a zone, the TTL is set to match that of any existing DNSKEY RRset. If there is no existing DNSKEY RRset, the TTL is set to the TTL specified when the key was created (using the dnssec-keygen -L option), if any, or to the SOA TTL.

To sign the zone using NSEC3 instead of NSEC, submit an NSEC3PARAM record via dynamic update prior to the scheduled publication and activation of the keys. The OPTOUT bit for the NSEC3 chain can be set in the flags field of the NSEC3PARAM record. The NSEC3PARAM record does not appear in the zone immediately, but it is stored for later reference. When the zone is signed and the NSEC3 chain is completed, the NSEC3PARAM record appears in the zone.

Using the auto-dnssec option requires the zone to be configured to allow dynamic updates, by adding an allow-update or update-policy statement to the zone configuration. If this has not been done, the configuration fails.

5.9.4. Private Type Records

The state of the signing process is signaled by private type records (with a default type value of 65534). When signing is complete, those records with a non-zero initial octet have a non-zero value for the final octet.

If the first octet of a private type record is non-zero, the record indicates either that the zone needs to be signed with the key matching the record, or that all signatures that match the record should be removed. Here are the meanings of the different values of the first octet:

  • algorithm (octet 1)

  • key id in network order (octet 2 and 3)

  • removal flag (octet 4)

  • complete flag (octet 5)

Only records flagged as “complete” can be removed via dynamic update; attempts to remove other private type records are silently ignored.

If the first octet is zero (this is a reserved algorithm number that should never appear in a DNSKEY record), the record indicates that changes to the NSEC3 chains are in progress. The rest of the record contains an NSEC3PARAM record, while the flag field tells what operation to perform based on the flag bits:





5.9.5. DNSKEY Rollovers

As with insecure-to-secure conversions, DNSSEC keyrolls can be done in two ways: using a dynamic DNS update, or via the auto-dnssec zone option.

5.9.6. Dynamic DNS Update Method

To perform key rollovers via a dynamic update, the K* files for the new keys must be added so that named can find them. The new DNSKEY RRs can then be added via dynamic update. named then causes the zone to be signed with the new keys; when the signing is complete, the private type records are updated so that the last octet is non-zero.

If this is for a KSK, the parent and any trust anchor repositories of the new KSK must be informed.

The maximum TTL in the zone must expire before removing the old DNSKEY. If it is a KSK that is being updated, the DS RRset in the parent must also be updated and its TTL allowed to expire. This ensures that all clients are able to verify at least one signature when the old DNSKEY is removed.

The old DNSKEY can be removed via UPDATE, taking care to specify the correct key. named cleans out any signatures generated by the old key after the update completes.

5.9.7. Automatic Key Rollovers

When a new key reaches its activation date (as set by dnssec-keygen or dnssec-settime), and if the auto-dnssec zone option is set to maintain, named automatically carries out the key rollover. If the key’s algorithm has not previously been used to sign the zone, then the zone is fully signed as quickly as possible. However, if the new key replaces an existing key of the same algorithm, the zone is re-signed incrementally, with signatures from the old key replaced with signatures from the new key as their signature validity periods expire. By default, this rollover completes in 30 days, after which it is safe to remove the old key from the DNSKEY RRset.

5.9.8. NSEC3PARAM Rollovers via UPDATE

The new NSEC3PARAM record can be added via dynamic update. When the new NSEC3 chain has been generated, the NSEC3PARAM flag field is set to zero. At that point, the old NSEC3PARAM record can be removed. The old chain is removed after the update request completes.

5.9.9. Converting From NSEC to NSEC3

Add a nsec3param option to your dnssec-policy and run rndc reconfig.

Or use nsupdate to add an NSEC3PARAM record.

In both cases, the NSEC3 chain is generated and the NSEC3PARAM record is added before the NSEC chain is destroyed.

5.9.10. Converting From NSEC3 to NSEC

To do this, remove the nsec3param option from the dnssec-policy and run rndc reconfig.

Or use nsupdate to remove all NSEC3PARAM records with a zero flag field. The NSEC chain is generated before the NSEC3 chain is removed.

5.9.11. Converting From Secure to Insecure

To convert a signed zone to unsigned using dynamic DNS, delete all the DNSKEY records from the zone apex using nsupdate. All signatures, NSEC or NSEC3 chains, and associated NSEC3PARAM records are removed automatically. This takes place after the update request completes.

This requires the dnssec-secure-to-insecure option to be set to yes in named.conf.

In addition, if the auto-dnssec maintain zone statement is used, it should be removed or changed to allow instead; otherwise it will re-sign.

5.9.12. Periodic Re-signing

In any secure zone which supports dynamic updates, named periodically re-signs RRsets which have not been re-signed as a result of some update action. The signature lifetimes are adjusted to spread the re-sign load over time rather than all at once.

5.9.13. NSEC3 and OPTOUT

named only supports creating new NSEC3 chains where all the NSEC3 records in the zone have the same OPTOUT state. named supports UPDATES to zones where the NSEC3 records in the chain have mixed OPTOUT state. named does not support changing the OPTOUT state of an individual NSEC3 record; if the OPTOUT state of an individual NSEC3 needs to be changed, the entire chain must be changed.

5.10. Dynamic Trust Anchor Management

BIND is able to maintain DNSSEC trust anchors using RFC 5011 key management. This feature allows named to keep track of changes to critical DNSSEC keys without any need for the operator to make changes to configuration files.

5.10.1. Validating Resolver

To configure a validating resolver to use RFC 5011 to maintain a trust anchor, configure the trust anchor using a trust-anchors statement and the initial-key keyword. Information about this can be found in trust-anchors Statement Definition and Usage.

5.10.2. Authoritative Server

To set up an authoritative zone for RFC 5011 trust anchor maintenance, generate two (or more) key signing keys (KSKs) for the zone. Sign the zone with one of them; this is the “active” KSK. All KSKs which do not sign the zone are “stand-by” keys.

Any validating resolver which is configured to use the active KSK as an RFC 5011-managed trust anchor takes note of the stand-by KSKs in the zone’s DNSKEY RRset, and stores them for future reference. The resolver rechecks the zone periodically; after 30 days, if the new key is still there, the key is accepted by the resolver as a valid trust anchor for the zone. Anytime after this 30-day acceptance timer has completed, the active KSK can be revoked, and the zone can be “rolled over” to the newly accepted key.

The easiest way to place a stand-by key in a zone is to use the “smart signing” features of dnssec-keygen and dnssec-signzone. If a key exists with a publication date in the past, but an activation date which is unset or in the future, dnssec-signzone -S includes the DNSKEY record in the zone but does not sign with it:

$ dnssec-keygen -K keys -f KSK -P now -A now+2y example.net
$ dnssec-signzone -S -K keys example.net

To revoke a key, use the command dnssec-revoke. This adds the REVOKED bit to the key flags and regenerates the K*.key and K*.private files.

After revoking the active key, the zone must be signed with both the revoked KSK and the new active KSK. Smart signing takes care of this automatically.

Once a key has been revoked and used to sign the DNSKEY RRset in which it appears, that key is never again accepted as a valid trust anchor by the resolver. However, validation can proceed using the new active key, which was accepted by the resolver when it was a stand-by key.

See RFC 5011 for more details on key rollover scenarios.

When a key has been revoked, its key ID changes, increasing by 128 and wrapping around at 65535. So, for example, the key “Kexample.com.+005+10000” becomes “Kexample.com.+005+10128”.

If two keys have IDs exactly 128 apart and one is revoked, the two key IDs will collide, causing several problems. To prevent this, dnssec-keygen does not generate a new key if another key which may collide is present. This checking only occurs if the new keys are written to the same directory that holds all other keys in use for that zone.

Older versions of BIND 9 did not have this protection. Exercise caution if using key revocation on keys that were generated by previous releases, or if using keys stored in multiple directories or on multiple machines.

It is expected that a future release of BIND 9 will address this problem in a different way, by storing revoked keys with their original unrevoked key IDs.

5.11. PKCS#11 (Cryptoki) Support

Public Key Cryptography Standard #11 (PKCS#11) defines a platform-independent API for the control of hardware security modules (HSMs) and other cryptographic support devices.

BIND 9 is known to work with three HSMs: the AEP Keyper, which has been tested with Debian Linux, Solaris x86, and Windows Server 2003; the Thales nShield, tested with Debian Linux; and the Sun SCA 6000 cryptographic acceleration board, tested with Solaris x86. In addition, BIND can be used with all current versions of SoftHSM, a software-based HSM simulator library produced by the OpenDNSSEC project.

PKCS#11 uses a “provider library”: a dynamically loadable library which provides a low-level PKCS#11 interface to drive the HSM hardware. The PKCS#11 provider library comes from the HSM vendor, and it is specific to the HSM to be controlled.

There are two available mechanisms for PKCS#11 support in BIND 9: OpenSSL-based PKCS#11 and native PKCS#11. With OpenSSL-based PKCS#11, BIND uses a modified version of OpenSSL, which loads the provider library and operates the HSM indirectly; any cryptographic operations not supported by the HSM can be carried out by OpenSSL instead. Native PKCS#11 enables BIND to bypass OpenSSL completely; BIND loads the provider library itself, and uses the PKCS#11 API to drive the HSM directly.

5.11.1. Prerequisites

See the documentation provided by the HSM vendor for information about installing, initializing, testing, and troubleshooting the HSM.

5.11.2. Native PKCS#11

Native PKCS#11 mode only works with an HSM capable of carrying out every cryptographic operation BIND 9 may need. The HSM’s provider library must have a complete implementation of the PKCS#11 API, so that all these functions are accessible. As of this writing, only the Thales nShield HSM and SoftHSMv2 can be used in this fashion. For other HSMs, including the AEP Keyper, Sun SCA 6000, and older versions of SoftHSM, use OpenSSL-based PKCS#11. (Note: Eventually, when more HSMs become capable of supporting native PKCS#11, it is expected that OpenSSL-based PKCS#11 will be deprecated.)

To build BIND with native PKCS#11, configure it as follows:

$ cd bind9
$ ./configure --enable-native-pkcs11 \

This causes all BIND tools, including named and the dnssec-* and pkcs11-* tools, to use the PKCS#11 provider library specified in provider-library-path for cryptography. (The provider library path can be overridden using the -E argument in named and the dnssec-* tools, or the -m argument in the pkcs11-* tools.) Building SoftHSMv2

SoftHSMv2, the latest development version of SoftHSM, is available from https://github.com/opendnssec/SoftHSMv2. It is a software library developed by the OpenDNSSEC project (https://www.opendnssec.org) which provides a PKCS#11 interface to a virtual HSM, implemented in the form of an SQLite3 database on the local filesystem. It provides less security than a true HSM, but it allows users to experiment with native PKCS#11 when an HSM is not available. SoftHSMv2 can be configured to use either OpenSSL or the Botan library to perform cryptographic functions, but when using it for native PKCS#11 in BIND, OpenSSL is required.

By default, the SoftHSMv2 configuration file is prefix/etc/softhsm2.conf (where prefix is configured at compile time). This location can be overridden by the SOFTHSM2_CONF environment variable. The SoftHSMv2 cryptographic store must be installed and initialized before using it with BIND.

$  cd SoftHSMv2
$  configure --with-crypto-backend=openssl --prefix=/opt/pkcs11/usr
$  make
$  make install
$  /opt/pkcs11/usr/bin/softhsm-util --init-token 0 --slot 0 --label softhsmv2

5.11.3. OpenSSL-based PKCS#11

OpenSSL-based PKCS#11 uses engine_pkcs11 OpenSSL engine from libp11 project.

For more information, see https://gitlab.isc.org/isc-projects/bind9/-/wikis/BIND-9-PKCS11

5.11.4. PKCS#11 Tools

BIND 9 includes a minimal set of tools to operate the HSM, including pkcs11-keygen to generate a new key pair within the HSM, pkcs11-list to list objects currently available, pkcs11-destroy to remove objects, and pkcs11-tokens to list available tokens.

In UNIX/Linux builds, these tools are built only if BIND 9 is configured with the --with-pkcs11 option. (Note: If --with-pkcs11 is set to yes, rather than to the path of the PKCS#11 provider, the tools are built but the provider is left undefined. Use the -m option or the PKCS11_PROVIDER environment variable to specify the path to the provider.)

5.11.5. Using the HSM

For OpenSSL-based PKCS#11, the runtime environment must first be set up so the OpenSSL and PKCS#11 libraries can be loaded:

$ export LD_LIBRARY_PATH=/opt/pkcs11/usr/lib:${LD_LIBRARY_PATH}

This causes named and other binaries to load the OpenSSL library from /opt/pkcs11/usr/lib, rather than from the default location. This step is not necessary when using native PKCS#11.

Some HSMs require other environment variables to be set. For example, when operating an AEP Keyper, the location of the “machine” file, which stores information about the Keyper for use by the provider library, must be specified. If the machine file is in /opt/Keyper/PKCS11Provider/machine, use:

$ export KEYPER_LIBRARY_PATH=/opt/Keyper/PKCS11Provider

Such environment variables must be set when running any tool that uses the HSM, including pkcs11-keygen, pkcs11-list, pkcs11-destroy, dnssec-keyfromlabel, dnssec-signzone, dnssec-keygen, and named.

HSM keys can now be created and used. In this case, we will create a 2048-bit key and give it the label “sample-ksk”:

$ pkcs11-keygen -b 2048 -l sample-ksk

To confirm that the key exists:

$ pkcs11-list
Enter PIN:
object[0]: handle 2147483658 class 3 label[8] 'sample-ksk' id[0]
object[1]: handle 2147483657 class 2 label[8] 'sample-ksk' id[0]

Before using this key to sign a zone, we must create a pair of BIND 9 key files. The dnssec-keyfromlabel utility does this. In this case, we are using the HSM key “sample-ksk” as the key-signing key for “example.net”:

$ dnssec-keyfromlabel -l sample-ksk -f KSK example.net

The resulting K*.key and K*.private files can now be used to sign the zone. Unlike normal K* files, which contain both public and private key data, these files contain only the public key data, plus an identifier for the private key which remains stored within the HSM. Signing with the private key takes place inside the HSM.

To generate a second key in the HSM for use as a zone-signing key, follow the same procedure above, using a different keylabel, a smaller key size, and omitting -f KSK from the dnssec-keyfromlabel arguments:

$ pkcs11-keygen -b 1024 -l sample-zsk
$ dnssec-keyfromlabel -l sample-zsk example.net

Alternatively, a conventional on-disk key can be generated using dnssec-keygen:

$ dnssec-keygen example.net

This provides less security than an HSM key, but since HSMs can be slow or cumbersome to use for security reasons, it may be more efficient to reserve HSM keys for use in the less frequent key-signing operation. The zone-signing key can be rolled more frequently, if desired, to compensate for a reduction in key security. (Note: When using native PKCS#11, there is no speed advantage to using on-disk keys, as cryptographic operations are done by the HSM.)

Now the zone can be signed. Please note that, if the -S option is not used for dnssec-signzone, the contents of both K*.key files must be added to the zone master file before signing it.

$ dnssec-signzone -S example.net
Enter PIN:
Verifying the zone using the following algorithms:
Zone signing complete:
Algorithm: NSEC3RSASHA1: ZSKs: 1, KSKs: 1 active, 0 revoked, 0 stand-by

5.11.6. Specifying the Engine on the Command Line

When using OpenSSL-based PKCS#11, the “engine” to be used by OpenSSL can be specified in named and all of the BIND dnssec-* tools by using the -E <engine> command line option. If BIND 9 is built with the --with-pkcs11 option, this option defaults to “pkcs11”. Specifying the engine is generally not necessary unless a different OpenSSL engine is used.

To disable use of the “pkcs11” engine - for troubleshooting purposes, or because the HSM is unavailable - set the engine to the empty string. For example:

$ dnssec-signzone -E '' -S example.net

This causes dnssec-signzone to run as if it were compiled without the --with-pkcs11 option.

When built with native PKCS#11 mode, the “engine” option has a different meaning: it specifies the path to the PKCS#11 provider library. This may be useful when testing a new provider library.

5.11.7. Running named With Automatic Zone Re-signing

For named to dynamically re-sign zones using HSM keys, and/or to sign new records inserted via nsupdate, named must have access to the HSM PIN. In OpenSSL-based PKCS#11, this is accomplished by placing the PIN into the openssl.cnf file (in the above examples, /opt/pkcs11/usr/ssl/openssl.cnf).

The location of the openssl.cnf file can be overridden by setting the OPENSSL_CONF environment variable before running named.

Here is a sample openssl.cnf:

openssl_conf = openssl_def
[ openssl_def ]
engines = engine_section
[ engine_section ]
pkcs11 = pkcs11_section
[ pkcs11_section ]

This also allows the dnssec-\* tools to access the HSM without PIN entry. (The pkcs11-\* tools access the HSM directly, not via OpenSSL, so a PIN is still required to use them.)

In native PKCS#11 mode, the PIN can be provided in a file specified as an attribute of the key’s label. For example, if a key had the label pkcs11:object=local-zsk;pin-source=/etc/hsmpin, then the PIN would be read from the file /etc/hsmpin.


Placing the HSM’s PIN in a text file in this manner may reduce the security advantage of using an HSM. Use caution when configuring the system in this way.

5.12. Dynamically Loadable Zones (DLZ)

Dynamically Loadable Zones (DLZ) are an extension to BIND 9 that allows zone data to be retrieved directly from an external database. There is no required format or schema. DLZ drivers exist for several different database backends, including PostgreSQL, MySQL, and LDAP, and can be written for any other.

Historically, DLZ drivers had to be statically linked with the named binary and were turned on via a configure option at compile time (for example, configure --with-dlz-ldap). The drivers provided in the BIND 9 tarball in contrib/dlz/drivers are still linked this way.

In BIND 9.8 and higher, it is possible to link some DLZ modules dynamically at runtime, via the DLZ “dlopen” driver, which acts as a generic wrapper around a shared object implementing the DLZ API. The “dlopen” driver is linked into named by default, so configure options are no longer necessary when using these dynamically linkable drivers; they are still needed for the older drivers in contrib/dlz/drivers.

The DLZ module provides data to named in text format, which is then converted to DNS wire format by named. This conversion, and the lack of any internal caching, places significant limits on the query performance of DLZ modules. Consequently, DLZ is not recommended for use on high-volume servers. However, it can be used in a hidden primary configuration, with secondaries retrieving zone updates via AXFR. Note, however, that DLZ has no built-in support for DNS notify; secondary servers are not automatically informed of changes to the zones in the database.

5.12.1. Configuring DLZ

A DLZ database is configured with a dlz statement in named.conf:

dlz example {
database "dlopen driver.so args";
search yes;

This specifies a DLZ module to search when answering queries; the module is implemented in driver.so and is loaded at runtime by the dlopen DLZ driver. Multiple dlz statements can be specified; when answering a query, all DLZ modules with search set to yes are queried to see whether they contain an answer for the query name. The best available answer is returned to the client.

The search option in the above example can be omitted, because yes is the default value.

If search is set to no, this DLZ module is not searched for the best match when a query is received. Instead, zones in this DLZ must be separately specified in a zone statement. This allows users to configure a zone normally using standard zone-option semantics, but specify a different database backend for storage of the zone’s data. For example, to implement NXDOMAIN redirection using a DLZ module for backend storage of redirection rules:

dlz other {
       database "dlopen driver.so args";
       search no;

zone "." {
       type redirect;
       dlz other;

5.12.2. Sample DLZ Driver

For guidance in the implementation of DLZ modules, the directory contrib/dlz/example contains a basic dynamically linkable DLZ module - i.e., one which can be loaded at runtime by the “dlopen” DLZ driver. The example sets up a single zone, whose name is passed to the module as an argument in the dlz statement:

dlz other {
       database "dlopen driver.so example.nil";

In the above example, the module is configured to create a zone “example.nil”, which can answer queries and AXFR requests and accept DDNS updates. At runtime, prior to any updates, the zone contains an SOA, NS, and a single A record at the apex:

example.nil.  3600    IN      SOA     example.nil. hostmaster.example.nil. (
                          123 900 600 86400 3600
example.nil.  3600    IN      NS      example.nil.
example.nil.  1800    IN      A

The sample driver can retrieve information about the querying client and alter its response on the basis of this information. To demonstrate this feature, the example driver responds to queries for “source-addr.``zonename``>/TXT” with the source address of the query. Note, however, that this record will not be included in AXFR or ANY responses. Normally, this feature is used to alter responses in some other fashion, e.g., by providing different address records for a particular name depending on the network from which the query arrived.

Documentation of the DLZ module API can be found in contrib/dlz/example/README. This directory also contains the header file dlz_minimal.h, which defines the API and should be included by any dynamically linkable DLZ module.

5.13. Dynamic Database (DynDB)

Dynamic Database, or DynDB, is an extension to BIND 9 which, like DLZ (see Dynamically Loadable Zones (DLZ)), allows zone data to be retrieved from an external database. Unlike DLZ, a DynDB module provides a full-featured BIND zone database interface. Where DLZ translates DNS queries into real-time database lookups, resulting in relatively poor query performance, and is unable to handle DNSSEC-signed data due to its limited API, a DynDB module can pre-load an in-memory database from the external data source, providing the same performance and functionality as zones served natively by BIND.

A DynDB module supporting LDAP has been created by Red Hat and is available from https://pagure.io/bind-dyndb-ldap.

A sample DynDB module for testing and developer guidance is included with the BIND source code, in the directory bin/tests/system/dyndb/driver.

5.13.1. Configuring DynDB

A DynDB database is configured with a dyndb statement in named.conf:

dyndb example "driver.so" {

The file driver.so is a DynDB module which implements the full DNS database API. Multiple dyndb statements can be specified, to load different drivers or multiple instances of the same driver. Zones provided by a DynDB module are added to the view’s zone table, and are treated as normal authoritative zones when BIND responds to queries. Zone configuration is handled internally by the DynDB module.

The parameters are passed as an opaque string to the DynDB module’s initialization routine. Configuration syntax differs depending on the driver.

5.13.2. Sample DynDB Module

For guidance in the implementation of DynDB modules, the directory bin/tests/system/dyndb/driver contains a basic DynDB module. The example sets up two zones, whose names are passed to the module as arguments in the dyndb statement:

dyndb sample "sample.so" { example.nil. arpa. };

In the above example, the module is configured to create a zone, “example.nil”, which can answer queries and AXFR requests and accept DDNS updates. At runtime, prior to any updates, the zone contains an SOA, NS, and a single A record at the apex:

example.nil.  86400    IN      SOA     example.nil. example.nil. (
                                              0 28800 7200 604800 86400
example.nil.  86400    IN      NS      example.nil.
example.nil.  86400    IN      A

When the zone is updated dynamically, the DynDB module determines whether the updated RR is an address (i.e., type A or AAAA); if so, it automatically updates the corresponding PTR record in a reverse zone. Note that updates are not stored permanently; all updates are lost when the server is restarted.

5.14. Catalog Zones

A “catalog zone” is a special DNS zone that contains a list of other zones to be served, along with their configuration parameters. Zones listed in a catalog zone are called “member zones.” When a catalog zone is loaded or transferred to a secondary server which supports this functionality, the secondary server creates the member zones automatically. When the catalog zone is updated (for example, to add or delete member zones, or change their configuration parameters), those changes are immediately put into effect. Because the catalog zone is a normal DNS zone, these configuration changes can be propagated using the standard AXFR/IXFR zone transfer mechanism.

Catalog zones’ format and behavior are specified as an Internet draft for interoperability among DNS implementations. The latest revision of the DNS catalog zones draft can be found here: https://datatracker.ietf.org/doc/draft-toorop-dnsop-dns-catalog-zones/ .

5.14.1. Principle of Operation

Normally, if a zone is to be served by a secondary server, the named.conf file on the server must list the zone, or the zone must be added using rndc addzone. In environments with a large number of secondary servers, and/or where the zones being served are changing frequently, the overhead involved in maintaining consistent zone configuration on all the secondary servers can be significant.

A catalog zone is a way to ease this administrative burden: it is a DNS zone that lists member zones that should be served by secondary servers. When a secondary server receives an update to the catalog zone, it adds, removes, or reconfigures member zones based on the data received.

To use a catalog zone, it must first be set up as a normal zone on both the primary and secondary servers that are configured to use it. It must also be added to a catalog-zones list in the options or view statement in named.conf. This is comparable to the way a policy zone is configured as a normal zone and also listed in a response-policy statement.

To use the catalog zone feature to serve a new member zone:

  • Set up the member zone to be served on the primary as normal. This can be done by editing named.conf or by running rndc addzone.

  • Add an entry to the catalog zone for the new member zone. This can be done by editing the catalog zone’s zone file and running rndc reload, or by updating the zone using nsupdate.

The change to the catalog zone is propagated from the primary to all secondaries using the normal AXFR/IXFR mechanism. When the secondary receives the update to the catalog zone, it detects the entry for the new member zone, creates an instance of that zone on the secondary server, and points that instance to the primaries specified in the catalog zone data. The newly created member zone is a normal secondary zone, so BIND immediately initiates a transfer of zone contents from the primary. Once complete, the secondary starts serving the member zone.

Removing a member zone from a secondary server requires only deleting the member zone’s entry in the catalog zone; the change to the catalog zone is propagated to the secondary server using the normal AXFR/IXFR transfer mechanism. The secondary server, on processing the update, notices that the member zone has been removed, stops serving the zone, and removes it from its list of configured zones. However, removing the member zone from the primary server must be done by editing the configuration file or running rndc delzone.

5.14.2. Configuring Catalog Zones

Catalog zones are configured with a catalog-zones statement in the options or view section of named.conf. For example:

catalog-zones {
    zone "catalog.example"
         default-primaries {; }
         in-memory no
         zone-directory "catzones"
         min-update-interval 10;

This statement specifies that the zone catalog.example is a catalog zone. This zone must be properly configured in the same view. In most configurations, it would be a secondary zone.

The options following the zone name are not required, and may be specified in any order.


This option defines the default primaries for member zones listed in a catalog zone, and can be overridden by options within a catalog zone. If no such options are included, then member zones transfer their contents from the servers listed in this option.


This option, if set to yes, causes member zones to be stored only in memory. This is functionally equivalent to configuring a secondary zone without a file option. The default is no; member zones’ content is stored locally in a file whose name is automatically generated from the view name, catalog zone name, and member zone name.


This option causes local copies of member zones’ zone files to be stored in the specified directory, if in-memory is not set to yes. The default is to store zone files in the server’s working directory. A non-absolute pathname in zone-directory is assumed to be relative to the working directory.


This option sets the minimum interval between updates to catalog zones, in seconds. If an update to a catalog zone (for example, via IXFR) happens less than min-update-interval seconds after the most recent update, the changes are not carried out until this interval has elapsed. The default is 5 seconds.

Catalog zones are defined on a per-view basis. Configuring a non-empty catalog-zones statement in a view automatically turns on allow-new-zones for that view. This means that rndc addzone and rndc delzone also work in any view that supports catalog zones.

5.14.3. Catalog Zone Format

A catalog zone is a regular DNS zone; therefore, it must have a single SOA and at least one NS record.

A record stating the version of the catalog zone format is also required. If the version number listed is not supported by the server, then a catalog zone may not be used by that server.

catalog.example.    IN SOA . . 2016022901 900 600 86400 1
catalog.example.    IN NS nsexample.
version.catalog.example.    IN TXT "1"

Note that this record must have the domain name version.catalog-zone-name. The data stored in a catalog zone is indicated by the domain name label immediately before the catalog zone domain.

Catalog zone options can be set either globally for the whole catalog zone or for a single member zone. Global options override the settings in the configuration file, and member zone options override global options.

Global options are set at the apex of the catalog zone, e.g.:

primaries.catalog.example.    IN AAAA 2001:db8::1

BIND currently supports the following options:

  • A simple primaries definition:

    primaries.catalog.example.    IN A

    This option defines a primary server for the member zones, which can be either an A or AAAA record. If multiple primaries are set, the order in which they are used is random.

  • A primaries with a TSIG key defined:

    label.primaries.catalog.example.     IN A
    label.primaries.catalog.example.     IN TXT "tsig_key_name"

    This option defines a primary server for the member zone with a TSIG key set. The TSIG key must be configured in the configuration file. label can be any valid DNS label.

  • allow-query and allow-transfer ACLs:

    allow-query.catalog.example.   IN APL 1:
    allow-transfer.catalog.example.    IN APL !1: 1:

    These options are the equivalents of allow-query and allow-transfer in a zone declaration in the named.conf configuration file. The ACL is processed in order; if there is no match to any rule, the default policy is to deny access. For the syntax of the APL RR, see RFC 3123.

A member zone is added by including a PTR resource record in the zones sub-domain of the catalog zone. The record label is a SHA-1 hash of the member zone name in wire format. The target of the PTR record is the member zone name. For example, to add the member zone domain.example:

5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN PTR domain.example.

The hash is necessary to identify options for a specific member zone. The member zone-specific options are defined the same way as global options, but in the member zone subdomain:

primaries.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN A
label.primaries.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN AAAA 2001:db8::2
label.primaries.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN TXT "tsig_key"
allow-query.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN APL 1:

Options defined for a specific zone override the global options defined in the catalog zone. These in turn override the global options defined in the catalog-zones statement in the configuration file.

Note that none of the global records for an option are inherited if any records are defined for that option for the specific zone. For example, if the zone had a masters record of type A but not AAAA, it would not inherit the type AAAA record from the global option.

5.15. IPv6 Support in BIND 9

BIND 9 fully supports all currently defined forms of IPv6 name-to-address and address-to-name lookups. It also uses IPv6 addresses to make queries when running on an IPv6-capable system.

For forward lookups, BIND 9 supports only AAAA records. RFC 3363 deprecated the use of A6 records, and client-side support for A6 records was accordingly removed from BIND 9. However, authoritative BIND 9 name servers still load zone files containing A6 records correctly, answer queries for A6 records, and accept zone transfer for a zone containing A6 records.

For IPv6 reverse lookups, BIND 9 supports the traditional “nibble” format used in the ip6.arpa domain, as well as the older, deprecated ip6.int domain. Older versions of BIND 9 supported the “binary label” (also known as “bitstring”) format, but support of binary labels has been completely removed per RFC 3363. Many applications in BIND 9 do not understand the binary label format at all anymore, and return an error if one is given. In particular, an authoritative BIND 9 name server will not load a zone file containing binary labels.

For an overview of the format and structure of IPv6 addresses, see IPv6 Addresses (AAAA).

5.15.1. Address Lookups Using AAAA Records

The IPv6 AAAA record is a parallel to the IPv4 A record, and, unlike the deprecated A6 record, specifies the entire IPv6 address in a single record. For example:

$ORIGIN example.com.
host            3600    IN      AAAA    2001:db8::1

Use of IPv4-in-IPv6 mapped addresses is not recommended. If a host has an IPv4 address, use an A record, not a AAAA, with ::ffff: as the address.

5.15.2. Address-to-Name Lookups Using Nibble Format

When looking up an address in nibble format, the address components are simply reversed, just as in IPv4, and ip6.arpa. is appended to the resulting name. For example, the following commands produce a reverse name lookup for a host with address 2001:db8::1:

$ORIGIN  14400   IN    PTR    (
                    host.example.com. )