Chapter 4. Advanced DNS Features
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## Chapter 4. Advanced DNS Features

## Notify

DNS NOTIFY is a mechanism that allows master servers to notify their slave servers of changes to a zone's data. In response to a NOTIFY from a master server, the slave will check to see that its version of the zone is the current version and, if not, initiate a zone transfer.

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

### Note

As a slave zone can also be a master to other slaves, named, by default, sends NOTIFY messages for every zone it loads. Specifying notify master-only; will cause named to only send NOTIFY for master zones that it loads.

## Dynamic Update

Dynamic Update is a method for adding, replacing or deleting records in a master 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 update-policy clause in the zone statement. The tkey-gssapi-credential and tkey-domain clauses in the options statement enable the server to negotiate keys that can be matched against those in update-policy or allow-update.

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.

### 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 will also occasionally write ("dump") 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 will be created with the extensions `.jnw` and `.jbk`; under ordinary circumstances, these will be removed when the dump is complete, and can be safely ignored.

When a server is restarted after a shutdown or crash, it will replay 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 journalled 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.

If you have to make changes to a dynamic zone manually, the following procedure will work: Disable dynamic updates to the zone using rndc freeze `zone`. This will also remove the zone's `.jnl` file and update the master file. Edit the zone file. Run rndc thaw `zone` to reload the changed zone and re-enable dynamic updates.

## Incremental Zone Transfers (IXFR)

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

When acting as a master, BIND 9 supports IXFR for those zones where the necessary change history information is available. These include master zones maintained by dynamic update and slave zones whose data was obtained by IXFR. For manually maintained master zones, and for slave 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 slave, BIND 9 will attempt 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.

## Split DNS

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

One common reason for setting up a DNS system this way is to hide "internal" DNS information from "external" clients on the Internet. There is some debate as to whether or not 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 a 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 in to the internal network.

### 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.

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

The internal servers will be 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 will 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, will be configured to serve the "public" version of the `site1` 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` 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 do not have any other way of looking up how to deliver mail to those internal hosts. With the wildcard records, the mail will be 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 will need to know how to deliver mail to internal hosts. In order for this to work properly, the resolvers on the bastion hosts will need to be configured to point to the internal name servers for DNS resolution.

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

In order for all this to work properly, internal clients will 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 will now be able to:

- Look up any hostnames in the `site1` 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 people.

Hosts on the Internet will be able to:

- Look up any hostnames in the `site1` and `site2.example.com` zones.
- Exchange mail with anyone in the `site1` and `site2.example.com` zones.

Here is an example configuration for the setup we just described above. Note that this is only configuration information; for information on how to configure your zone files, see the section called “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;
        forwarders {                                // forward to external servers
            bastion-ips-go-here;
        };
        allow-transfer { none; };                   // sample allow-transfer (no one)
        allow-query { internals; externals; };      // restrict query access
        allow-recursion { internals; };             // restrict recursion
        ...
        ...
    };

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

    zone "site2.example.com" {                      // sample slave zone
      type slave;
      file "s/site2.example.com";
      masters { 172.16.72.3; };
      forwarders { };
      allow-query { internals; externals; };
      allow-transfer { internals; };
    };

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

    zone "site2.internal" {
      type slave;
      file "s/site2.internal";
      masters { 172.16.72.3; };
      forwarders { };
      allow-query { internals };
      allow-transfer { internals; }
    };

External (bastion host) DNS server config:

    acl internals { 172.16.72.0/24; 192.168.1.0/24; };

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

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

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

    zone "site2.example.com" {
      type slave;
      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

## TSIG

This is a short guide to setting up Transaction SIGnatures (TSIG) based transaction security in BIND. It describes changes to the configuration file as well as what changes are required for different features, including the process of creating transaction keys and using transaction signatures with BIND.

BIND primarily supports TSIG for server to server communication. This includes zone transfer, notify, and recursive query messages. Resolvers based on newer versions of BIND 8 have limited support for TSIG.

TSIG can also be useful for dynamic update. A primary server for a dynamic zone should control access to the dynamic update service, but IP-based access control is insufficient. The cryptographic access control provided by TSIG is far superior. The nsupdate program supports TSIG via the `-k` and `-y` command line options or inline by use of the key.

### Generate Shared Keys for Each Pair of Hosts

A shared secret is generated to be shared between host1 and host2. An arbitrary key name is chosen: "host1-host2.". The key name must be the same on both hosts.

#### Automatic Generation

The following command will generate a 128-bit (16 byte) HMAC-SHA256 key as described above. Longer keys are better, but shorter keys are easier to read. Note that the maximum key length is the digest length, here 256 bits.

`dnssec-keygen -a hmac-sha256 -b 128 -n HOST host1-host2.`

The key is in the file `Khost1-host2.+163+00000.private`. Nothing directly uses this file, but the base-64 encoded string following "`Key:`" can be extracted from the file and used as a shared secret:

    Key: La/E5CjG9O+os1jq0a2jdA==

The string "`La/E5CjG9O+os1jq0a2jdA==`" can be used as the shared secret.

#### Manual Generation

The shared secret is simply a random sequence of bits, encoded in base-64. Most ASCII strings are valid base-64 strings (assuming the length is a multiple of 4 and only valid characters are used), so the shared secret can be manually generated.

Also, a known string can be run through mmencode or a similar program to generate base-64 encoded data.

### Copying the Shared Secret to Both Machines

This is beyond the scope of DNS. A secure transport mechanism should be used. This could be secure FTP, ssh, telephone, etc.

### Informing the Servers of the Key's Existence

Imagine host1 and host 2 are both servers. The following is added to each server's `named.conf` file:

    key host1-host2. {
      algorithm hmac-sha256;
      secret "La/E5CjG9O+os1jq0a2jdA==";
    };

The secret is the one generated above. Since this is a secret, it is recommended that either `named.conf` be non-world readable, or the key directive be added to a non-world readable file that is included by `named.conf`.

At this point, the key is recognized. This means that if the server receives a message signed by this key, it can verify the signature. If the signature is successfully verified, the response is signed by the same key.

### Instructing the Server to Use the Key

Since keys are shared between two hosts only, the server must be told when keys are to be used. The following is added to the `named.conf` file for host1, if the IP address of host2 is 10.1.2.3:

    server 10.1.2.3 {
      keys { host1-host2. ;};
    };

Multiple keys may be present, but only the first is used. This directive does not contain any secrets, so it may be in a world-readable file.

If host1 sends a message that is a request to that address, the message will be signed with the specified key. host1 will expect any responses to signed messages to be signed with the same key.

A similar statement must be present in host2's configuration file (with host1's address) for host2 to sign request messages to host1.

### TSIG Key Based Access Control

BIND allows IP addresses and ranges to be specified in ACL definitions and allow-{ query | transfer | update } directives. This has been extended to allow TSIG keys also. The above key would be denoted key host1-host2.

An example of an allow-update directive would be:

    allow-update { key host1-host2. ;};

This allows dynamic updates to succeed only if the request was signed by a key named "host1-host2.".

You may want to read about the more powerful update-policy statement in the section called “Dynamic Update Policies”.

### Errors

The processing of TSIG signed messages can result in several errors. If a signed message is sent to a non-TSIG aware server, a FORMERR (format error) will be returned, since the server will not understand the record. This is a result of misconfiguration, since the server must be explicitly configured to send a TSIG signed message to a specific server.

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 with a signature that does not validate, 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 with the TSIG extended error code set to BADTIME, and the time values will be adjusted so that the response can be successfully verified. In any of these cases, the message's rcode (response code) is set to NOTAUTH (not authenticated).

## TKEY

TKEY is a mechanism for automatically generating a shared secret between two hosts. There are several "modes" of TKEY that specify how the key is generated or assigned. BIND 9 implements only one of these modes, the Diffie-Hellman key exchange. Both hosts are required to have a Diffie-Hellman KEY record (although this record is not required to be present in a zone). The TKEY process must use signed messages, signed either by TSIG or SIG(0). The result of TKEY is a shared secret that can be used to sign messages with TSIG. TKEY can also be used to delete shared secrets that it had previously generated.

The TKEY process is initiated by a client or server by sending a signed TKEY query (including any appropriate KEYs) to a TKEY-aware server. The server response, if it indicates success, will contain a TKEY record and any appropriate keys. After this exchange, both participants have enough information to determine the shared secret; the exact process depends on the TKEY mode. When using the Diffie-Hellman TKEY mode, Diffie-Hellman keys are exchanged, and the shared secret is derived by both participants.

## SIG(0)

BIND 9 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 TSIG keys; privileges can be granted or denied based on the key name.

When a SIG(0) signed message is received, it will only be verified if the key is known and trusted by the server; the server will not attempt to locate and/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.

## DNSSEC

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, and that the tools shipped with BIND 9.2.x and earlier are not compatible with the current ones.

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 either be statically configured with this zone's zone key or the zone key of another zone above this one in the DNS tree.

### Generating Keys

The dnssec-keygen program is used to generate keys.

A secure zone must contain one or more zone keys. The zone keys will 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, a name type of ZONE, and must be usable for authentication. It is recommended that zone keys use a cryptographic algorithm designated as "mandatory to implement" by the IETF; currently the only one is RSASHA1.

The following command will generate a 768-bit RSASHA1 key for the `child.example` zone:

`dnssec-keygen -a RSASHA1 -b 768 -n ZONE child.example.`

Two output files will be produced: `Kchild.example.+005+12345.key` and `Kchild.example.+005+12345.private` (where 12345 is an example of a key tag). The key filenames contain the key name (`child.example.`), algorithm (3 is DSA, 1 is RSAMD5, 5 is RSASHA1, 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 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.

### Signing the Zone

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

Any `keyset` files corresponding to secure subzones should be present. The zone signer will generate `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.

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

`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 will also produce a keyset and dsset files and optionally a dlvset file. 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.

### Configuring Servers

To enable named to respond appropriately to DNS requests from DNSSEC aware clients, dnssec-enable must be set to yes. (This is the default setting.)

To enable named to validate answers from other servers, the dnssec-enable and dnssec-validation options must both be set to yes (the default setting in BIND 9.5 and later), and at least one trust anchor must be configured with a trusted-keys statement in `named.conf`.

trusted-keys are copies of DNSKEY RRs for zones that are used to form the first link in the cryptographic chain of trust. All keys listed in trusted-keys (and corresponding zones) are deemed to exist and only the listed keys will be used to validated the DNSKEY RRset that they are from.

trusted-keys are described in more detail later in this document.

Unlike BIND 8, 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 gets established, a typical DNSSEC configuration will look something like the following. It has a one or more public keys for the root. This allows answers from outside the organization to be validated. It will also have several keys for parts of the namespace the organization controls. These are here to ensure that named is immune to compromises in the DNSSEC components of the security of parent zones.

    trusted-keys {

            /* Root Key */
    "." 257 3 3 "BNY4wrWM1nCfJ+CXd0rVXyYmobt7sEEfK3clRbGaTwSJxrGkxJWoZu6I7PzJu/
                 E9gx4UC1zGAHlXKdE4zYIpRhaBKnvcC2U9mZhkdUpd1Vso/HAdjNe8LmMlnzY3
                 zy2Xy4klWOADTPzSv9eamj8V18PHGjBLaVtYvk/ln5ZApjYghf+6fElrmLkdaz
                 MQ2OCnACR817DF4BBa7UR/beDHyp5iWTXWSi6XmoJLbG9Scqc7l70KDqlvXR3M
                 /lUUVRbkeg1IPJSidmK3ZyCllh4XSKbje/45SKucHgnwU5jefMtq66gKodQj+M
                 iA21AfUVe7u99WzTLzY3qlxDhxYQQ20FQ97S+LKUTpQcq27R7AT3/V5hRQxScI
                 Nqwcz4jYqZD2fQdgxbcDTClU0CRBdiieyLMNzXG3";

    /* Key for our organization's forward zone */
    example.com. 257 3 5 "AwEAAaxPMcR2x0HbQV4WeZB6oEDX+r0QM65KbhTjrW1ZaARmPhEZZe
                          3Y9ifgEuq7vZ/zGZUdEGNWy+JZzus0lUptwgjGwhUS1558Hb4JKUbb
                          OTcM8pwXlj0EiX3oDFVmjHO444gLkBO UKUf/mC7HvfwYH/Be22GnC
                          lrinKJp1Og4ywzO9WglMk7jbfW33gUKvirTHr25GL7STQUzBb5Usxt
                          8lgnyTUHs1t3JwCY5hKZ6CqFxmAVZP20igTixin/1LcrgX/KMEGd/b
                          iuvF4qJCyduieHukuY3H4XMAcR+xia2 nIUPvm/oyWR8BW/hWdzOvn
                          SCThlHf3xiYleDbt/o1OTQ09A0=";

    /* Key for our reverse zone. */
    2.0.192.IN-ADDRPA.NET. 257 3 5 "AQOnS4xn/IgOUpBPJ3bogzwcxOdNax071L18QqZnQQQA
                                    VVr+iLhGTnNGp3HoWQLUIzKrJVZ3zggy3WwNT6kZo6c0
                                    tszYqbtvchmgQC8CzKojM/W16i6MG/ea fGU3siaOdS0
                                    yOI6BgPsw+YZdzlYMaIJGf4M4dyoKIhzdZyQ2bYQrjyQ
                                    4LB0lC7aOnsMyYKHHYeRv PxjIQXmdqgOJGq+vsevG06
                                    zW+1xgYJh9rCIfnm1GX/KMgxLPG2vXTD/RnLX+D3T3UL
                                    7HJYHJhAZD5L59VvjSPsZJHeDCUyWYrvPZesZDIRvhDD
                                    52SKvbheeTJUm6EhkzytNN2SN96QRk8j/iI8ib";
    };

    options {
            ...
            dnssec-enable yes;
            dnssec-validation yes;
    };

### Note

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 will reject any answers from signed, secure zones which fail to validate, and will return 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.

### Note

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, then it 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". (Prior to BIND 9.7, the logged error was "not insecure". This referred to the zone, not the response.)

## IPv6 Support in BIND 9

BIND 9 fully supports all currently defined forms of IPv6 name to address and address to name lookups. It will also use 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 any more, and will return an error if 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 the section called “IPv6 addresses (AAAA)”.

### 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.

### 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 would provide 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.

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