6. Advanced DNS Features
6.1. 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.
Note
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.
6.2. 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.
6.2.1. 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.
6.3. 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.
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.
6.4. 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.
6.4.1. 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.comandsite2.example.comzones.Look up any hostnames in the
site1.internalandsite2.internaldomains.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.comandsite2.example.comzones.Exchange mail with anyone in the
site1.example.comandsite2.example.comzones.
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 { 172.16.72.0/24; 192.168.1.0/24; };
acl externals { bastion-ips-go-here; };
options {
...
...
forward only;
// forward to external servers
forwarders {
bastion-ips-go-here;
};
// 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 { 172.16.72.3; };
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 { 172.16.72.3; };
forwarders { };
allow-query { internals };
allow-transfer { internals; }
};
External (bastion host) DNS server configuration:
acl internals { 172.16.72.0/24; 192.168.1.0/24; };
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 ...
nameserver 172.16.72.2
nameserver 172.16.72.3
nameserver 172.16.72.4
6.5. 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:
nsupdate - dynamic DNS update utility supports TSIG via the
-k,-l, and-ycommand-line options, or via thekeycommand when running interactively.dig - DNS lookup utility supports TSIG via the
-kand-ycommand-line options.
6.5.2. 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.
6.5.3. 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 10.1.2.3, would
cause all requests from host1 to host2, including normal DNS
queries, to be signed using the host1-host2. key:
server 10.1.2.3 {
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.
6.5.4. 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.
6.5.5. 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).
6.6. 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.
6.7. 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.
6.8. 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.
6.8.1. 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.
6.9. 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.
6.9.1. Prerequisites
See the documentation provided by the HSM vendor for information about installing, initializing, testing, and troubleshooting the HSM.
6.9.2. 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 \
--with-pkcs11=provider-library-path
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.)
6.9.2.1. 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
6.9.3. 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
6.9.4. 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.)
6.9.5. 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:
NSEC3RSASHA1.
Zone signing complete:
Algorithm: NSEC3RSASHA1: ZSKs: 1, KSKs: 1 active, 0 revoked, 0 stand-by
example.net.signed
6.9.6. 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.
6.9.7. 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 ]
PIN = <PLACE PIN HERE>
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.
Warning
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.
6.10. 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.
6.10.1. 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;
};
6.10.2. 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 10.53.0.1
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.
6.11. 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.
6.11.1. Configuring DynDB
A DynDB database is configured with a dyndb statement in
named.conf:
dyndb example "driver.so" {
parameters
};
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.
6.11.2. 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 127.0.0.1
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.
6.12. 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/ .
6.12.1. 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.confor by runningrndc 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 usingnsupdate.
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 masters 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.
6.12.2. 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-masters { 10.53.0.1; }
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.
default-mastersThis 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.
in-memoryThis option, if set to
yes, causes member zones to be stored only in memory. This is functionally equivalent to configuring a secondary zone without afileoption. The default isno; 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.zone-directoryThis option causes local copies of member zones’ zone files to be stored in the specified directory, if
in-memoryis not set toyes. The default is to store zone files in the server’s working directory. A non-absolute pathname inzone-directoryis assumed to be relative to the working directory.min-update-intervalThis 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-intervalseconds 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.
6.12.3. 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.:
masters.catalog.example. IN AAAA 2001:db8::1
BIND currently supports the following options:
A simple
mastersdefinition:masters.catalog.example. IN A 192.0.2.1
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
masterswith a TSIG key defined:label.masters.catalog.example. IN A 192.0.2.2 label.masters.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.
labelcan be any valid DNS label.allow-queryandallow-transferACLs:allow-query.catalog.example. IN APL 1:10.0.0.1/24 allow-transfer.catalog.example. IN APL !1:10.0.0.1/32 1:10.0.0.0/24
These options are the equivalents of
allow-queryandallow-transferin a zone declaration in thenamed.confconfiguration 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:
masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN A 192.0.2.2
label.masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN AAAA 2001:db8::2
label.masters.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN TXT "tsig_key"
allow-query.5960775ba382e7a4e09263fc06e7c00569b6a05c.zones.catalog.example. IN APL 1:10.0.0.0/24
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.
6.13. 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.
6.13.1. 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:192.168.42.1 as the address.
6.13.2. 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 0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0 14400 IN PTR (
host.example.com. )