TITLE="BIND 9 Administrator Reference Manual" TITLE="Nameserver Configuration" TITLE="The BIND 9 Lightweight Resolver" >BIND 9 Administrator Reference Manual</
TH>Chapter 4. Advanced Concepts</
A>Incremental Zone Transfers (IXFR)</
A>Dynamic update is the term used for the ability under
certain specified conditions to add, modify or delete records or
RRsets in the master zone files. Dynamic update is fully described
>Dynamic update is enabled on a zone-by-zone basis, by
>Updating of secure zones (zones using DNSSEC) follows
RFC 3007: SIG and NXT 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.</
P>4.1.1. The journal file</
A>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 when the first dynamic update takes place. The name of
the journal file is formed by appending the
name of the corresponding zone file. The journal file is in a
binary format and should not be edited manually.</
P>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 15
minutes, allowing additional updates to take place.</
P>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.</
P>Changes that result from incoming incremental zone transfers are also
journalled in a similar way.</
P>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 <
B>If you have to make changes to a dynamic zone
manually, the following procedure will work: Shut down
sufficient). Wait for the server to exit,
> file, edit the zone file,
and restart the server. Removing the <
TT file is necessary because the manual edits will not be
present in the journal, rendering it inconsistent with the
contents of the zone file.</
PNAME="incremental_zone_transfers" >4.2. Incremental Zone Transfers (IXFR)</
A>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 documented in RFC
>When acting as a master, <
SPAN> 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, but not manually maintained master
zones nor slave zones obtained by performing a full zone transfer
>When acting as a slave, <
SPAN> 9 will attempt to use IXFR unless
it is explicitly disabled. For more information about disabling
IXFR, see the description of the <
B>Setting up different views, or visibility, of DNS space to
internal and external resolvers is usually referred to as a <
I> setup. There are several reasons an organization
would want to set up its DNS this way.</
P>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.</
P>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.</
P>Here is an example of a split DNS setup:</
P>Let's say a company named <
Ihas 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.</
P> 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.</
P>In order to accomplish this, the company will set up two sets
of nameservers. 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.</
P>The internal servers will be configured to forward all queries,
DMZ. These internal servers will have complete sets of information
the internal nameservers must be configured to disallow all queries
to these domains from any external hosts, including the bastion
>The external servers, which are on the bastion hosts, will
be configured to serve the "public" version of the <
TTThis could include things such as the host records for public servers
and mail exchange (MX) records (<
TT>In addition, the public <
TTshould 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
>Here's an example of a wildcard MX record:</
P>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
nameservers for DNS resolution.</
P>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.</
P>In order for all this to work properly, internal clients will
need to be configured to query <
Inameservers for DNS queries. This could also be enforced via selective
filtering on the network.</
P>If everything has been set properly, <
Iinternal clients will now be able to:</
P>Look up any hostnames in the <
TT>Look up any hostnames in the <
TT>Look up any hostnames on the Internet.</
P>Exchange mail with internal AND external people.</
P>Hosts on the Internet will be able to:</
P>Look up any hostnames in the <
TT>Exchange mail with anyone in the <
TT>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 <
A>Internal DNS server config:</
P forwarders { // forward to external servers
allow-transfer { none; }; // sample allow-transfer (no one)
allow-query { internals; externals; }; // restrict query access
allow-recursion { internals; }; // restrict recursion
forwarders { }; // do normal iterative
// resolution (do not forward)
allow-query { internals; externals; };
allow-transfer { internals; };
masters { 172.16.72.3; };
allow-query { internals; externals; };
allow-transfer { internals; };
allow-query { internals; };
allow-transfer { internals; }
masters { 172.16.72.3; };
allow-query { internals };
allow-transfer { internals; }
>External (bastion host) DNS server config:</
Pacl externals { bastion-ips-go-here; };
allow-transfer { none; }; // sample allow-transfer (no one)
allow-query { internals; externals; }; // restrict query access
allow-recursion { internals; externals; }; // restrict recursion
allow-transfer { internals; externals; };
masters { another_bastion_host_maybe; };
allow-transfer { internals; externals; }
>This is a short guide to setting up Transaction SIGnatures
(TSIG) based transaction security in <
SPANto 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 <
SPAN> primarily supports TSIG for server to server communication.
This includes zone transfer, notify, and recursive query messages.
Resolvers based on newer versions of <
SPAN>TSIG might be most useful for dynamic update. A primary
server for a dynamic zone should use access control to control
updates, but IP-based access control is insufficient. Key-based
access control is far superior, see <
A program supports TSIG via the <
TT> command line options.</
P>4.4.1. Generate Shared Keys for Each Pair of Hosts</
A>A shared secret is generated to be shared between <
IAn arbitrary key name is chosen: "host1-host2.". The key name must
be the same on both hosts.</
P>4.4.1.1. Automatic Generation</
A>The following command will generate a 128 bit (16 byte) HMAC-MD5
key as described above. Longer keys are better, but shorter keys
are easier to read. Note that the maximum key length is 512 bits;
keys longer than that will be digested with MD5 to produce a 128
>dnssec-keygen -a hmac-md5 -b 128 -n HOST host1-host2.</
B>The key is in the file <
TTNothing directly uses this file, but the base-64 encoded string
can be extracted from the file and used as a shared secret:</
Pbe used as the shared secret.</
P>4.4.1.2. Manual Generation</
A>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.</
P>Also, a known string can be run through <
Ba similar program to generate base-64 encoded data.</
P>4.4.2. Copying the Shared Secret to Both Machines</
A>This is beyond the scope of DNS. A secure transport mechanism
should be used. This could be secure FTP, ssh, telephone, etc.</
P>4.4.3. Informing the Servers of the Key's Existence</
Aboth servers. The following is added to each server's <
TT>The algorithm, hmac-md5, is the only one supported by <
SPANThe secret is the one generated above. Since this is a secret, it
is recommended that either <
TTreadable, or the key directive be added to a non-world readable
file that is included by <
TT>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 succeeds, the response is signed by
>4.4.4. Instructing the Server to Use the Key</
A>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 <
TT>, if the IP address of <
I>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
> sends a message that is a request
to that address, the message will be signed with the specified key. <
Iexpect any responses to signed messages to be signed with the same
>A similar statement must be present in <
Iconfiguration file (with <
Isign request messages to <
I>4.4.5. TSIG Key Based Access Control</
A> allows IP addresses and ranges to be specified in ACL
>allow-{ query | transfer | update }</
BThis has been extended to allow TSIG keys also. The above key would
>An example of an allow-update directive would be:</
P> allow-update { key host1-host2. ;};
>This allows dynamic updates to succeed only if the request
was signed by a key named
>You may want to read about the more
>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 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.</
P>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 is set to
> is a mechanism for automatically
generating a shared secret between two hosts. There are several
> that specify how the key is
generated or assigned. <
SPAN> 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 <
B must use signed messages, signed either by TSIG or SIG(0). The
> is a shared secret that can be
used to sign messages with TSIG. <
B be used to delete shared secrets that it had previously
> process is initiated by a client
or server by sending a signed <
B (including any appropriate KEYs) to a TKEY-aware server. The
server response, if it indicates success, will contain a
> record and any appropriate keys. After
this exchange, both participants have enough information to
determine the shared secret; the exact process depends on the
> mode. When using the Diffie-Hellman
> mode, Diffie-Hellman keys are exchanged,
and the shared secret is derived by both participants.</
P> 9 partially supports DNSSEC SIG(0) transaction
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.</
P>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.</
P>SIG(0) signing of multiple-message TCP streams is not
> 9 does not ship with any tools that generate SIG(0)
>Cryptographic authentication of DNS information is possible
through the DNS Security (<
I defined in RFC 2535. This section describes the creation and use
of DNSSEC signed zones.</
P>In order to set up a DNSSEC secure zone, there are a series
of steps which must be followed. <
SPAN that are used in this process, which are explained in more detail
below. In all cases, the "<
TT full list of parameters. Note that the DNSSEC tools require the
keyset and signedkey files to be in the working directory, and
that the tools shipped with BIND
9.0.x are not fully compatible
with the current ones.</
P>There must also be communication with the administrators of
the parent
and/
or child zone to transmit keys and signatures. A
zone's security status must be indicated by the parent zone for a
DNSSEC capable resolver to trust its data.</
P>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.</
P>4.7.1. Generating Keys</
A>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
>, and must be usable for authentication.
It is recommended that zone keys be mandatory to implement a
cryptographic algorithm; currently the only key mandatory to
implement an algorithm is DSA.</
P>The following command will generate a 768 bit DSA key for
>Two output files will be produced:
12345 is an example of a key tag). The key file names contain
is DSA, 1 is RSA, etc.), and the key tag (12345 in this case).
The private key (in the <
TT used to generate signatures, and the public key (in the
> file) is used for signature
>To generate another key with the same properties (but with
a different key tag), repeat the above command.</
P>The public keys should be inserted into the zone file with
> statements, including the
>4.7.2. Creating a Keyset</
A to create a key set from one or more keys.</
P>Once the zone keys have been generated, a key set must be
built for transmission to the administrator of the parent zone,
so that the parent zone can sign the keys with its own zone key
and correctly indicate the security status of this zone. When
building a key set, the list of keys to be included and the TTL
of the set must be specified, and the desired signature validity
period of the parent's signature may also be specified.</
P>The list of keys to be inserted into the key set may also
included non-zone keys present at the top of the zone.
> may also be used at other
>The following command generates a key set containing the
above key and another key similarly generated, with a TTL of
3600 and a signature validity period of 10 days starting from
>One output file is produced:
transmitted to the parent to be signed. It includes the keys,
as well as signatures over the key set generated by the zone
keys themselves, which are used to prove ownership of the
private keys and encode the desired validity period.</
P>4.7.3. Signing the Child's Keyset</
A sign one child's keyset.</
P delegations which are secure, for example,
> administrator should receive
keyset files for each secure subzone. These keys must be signed
by this zone's zone keys.</
P>The following command signs the child's key set with the
>One output file is produced:
should be both transmitted back to the child and retained. It
includes all keys (the child's keys) from the keyset file and
signatures generated by this zone's zone keys.</
P>4.7.4. Signing the Zone</
A secure subzones should be present, as well as a
> file for this zone generated by
the parent (if there is one). The zone signer will generate
the zone, as well as incorporate the zone key signature from the
parent and indicate the security status at all delegation
>The following command signs the zone, assuming it is in a
default, all zone keys which have an available private key are
used to generate signatures.</
P>One output file is produced:
should be referenced by <
TT input file for the zone.</
P>4.7.5. Configuring Servers</
A> 8, data is not verified on load in <
SPAN so zone keys for authoritative zones do not need to be specified
in the configuration file.</
P>The public key for any security root must be present in
the configuration file's <
B statement, as described later in this document. </
P>4.8. IPv6 Support in <
SPAN> 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
>For forward lookups, <
SPAN> 9 supports both A6 and AAAA
records. The use of AAAA records is deprecated, but it is still
useful for hosts to have both AAAA and A6 records to maintain
backward compatibility with installations where AAAA records are
still used. In fact, the stub resolvers currently shipped with
most operating system support only AAAA lookups, because following
A6 chains is much harder than doing A or AAAA lookups.</
P>For IPv6 reverse lookups, <
SPAN "bitstring" format used in the <
I domain, as well as the older, deprecated "nibble" format used in
> 9 includes a new lightweight resolver library and
resolver daemon which new applications may choose to use to avoid
the complexities of A6 chain following and bitstring labels, see <
A>For an overview of the format and structure of IPv6 addresses,
>4.8.1. Address Lookups Using AAAA Records</
A>The AAAA record is a parallel to the IPv4 A record. It
specifies the entire address in a single record. For
host 3600 IN AAAA 3ffe:8050:201:1860:42::1
>While their use is deprecated, they are useful to support
older IPv6 applications. They should not be added where they
are not absolutely necessary.</
P>4.8.2. Address Lookups Using A6 Records</
A>The A6 record is more flexible than the AAAA record, and
is therefore more complicated. The A6 record can be used to
form a chain of A6 records, each specifying part of the IPv6
address. It can also be used to specify the entire record as
well. For example, this record supplies the same data as the
AAAA record in the previous example:</
Phost 3600 IN A6 0 3ffe:8050:201:1860:42::1
>A6 records are designed to allow network
renumbering. This works when an A6 record only specifies the
part of the address space the domain owner controls. For
example, a host may be at a company named "company." It has
two ISPs which provide IPv6 address space for it. These two
ISPs fully specify the IPv6 prefix they supply.</
P>In the company's address space:</
Pcompany 3600 IN A6 0 3ffe:8050:201:1860::
company 3600 IN A6 0 1234:5678:90ab:fffa::
the resolver (in the resolver daemon or caching name server)
will find two partial A6 records, and will use the additional
name to find the remainder of the data.</
P>4.8.2.2. A6 Records for DNS Servers</
A>When an A6 record specifies the address of a name
server, it should use the full address rather than specifying
a partial address. For example:</
Pns0 14400 IN A6 0 3ffe:8050:201:1860:42::1
ns1 14400 IN A 192.168.42.1
>It is recommended that IPv4-in-IPv6 mapped addresses not
be used. If a host has an IPv4 address, use an A record, not
>4.8.3. Address to Name Lookups Using Nibble Format</
A>While the use of nibble format to look up names is
deprecated, it is supported for backwards compatiblity with
existing IPv6 applications.</
P>When looking up an address in nibble format, the address
components are simply reversed, just as in IPv4, and
> is appended to the resulting name.
For example, the following would provide reverse name lookup for
>3ffe:8050:201:1860:42::1</
TT>4.8.4. Address to Name Lookups Using Bitstring Format</
A>Bitstring labels can start and end on any bit boundary,
rather than on a multiple of 4 bits as in the nibble
>To replicate the previous example using bitstrings:</
P>4.8.5. Using DNAME for Delegation of IPv6 Reverse Addresses</
A>In IPV6, the same host may have many addresses from many
network providers. Since the trailing portion of the address
usually remains constant, <
B reduce the number of zone files used for reverse mapping that
need to be maintained.</
P>For example, consider a host which has two providers
therefore two IPv6 addresses. Since the host chooses its own 64
bit host address portion, the provider address is the only part
ipv6net IN A6 0 aa:bb:cccc::
ipv6net2 IN A6 0 6666:5555:4::
>This sets up forward lookups. To handle the reverse lookups,
needs only one zone file to handle both of these reverse
>Nameserver Configuration</
TD> 9 Lightweight Resolver</
TD