4.1. 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.

DNS For more information about NOTIFY, see the description of the notify option in Section 6.2.16.1 and the description of the zone option also-notify in Section 6.2.16.7. The NOTIFY protocol is specified in RFC 1996.

4.2. 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 on a zone-by-zone basis, by including an allow-update or update-policy clause in the zone statement.

Updating of secure zones (zones using DNSSEC) follows RFC 3007: RRSIG and NSEC 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.

4.3. 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.

4.4. 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.

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.

Here is an example of a 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 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 Section 3.1

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 { internals; externals; };        // restrict query access
  allow-recursion { internals; externals; };    // restrict recursion
  ...
  ...
};

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

zone "site2.example.com" {
  type slave;
  file "s/site2.foo.com";
  masters { another_bastion_host_maybe; };
  allow-query { any; };
  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

4.5. 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 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. The cryptographic access control provided by TSIG is far superior. The nsupdate program supports TSIG via the -k and -y command line options.

Key: La/E5CjG9O+os1jq0a2jdA==

key host1-host2. {
  algorithm hmac-md5;
  secret "La/E5CjG9O+os1jq0a2jdA==";
};

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

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

4.6. 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.

4.7. SIG(0)

BIND 9 partially supports DNSSEC SIG(0) transaction signatures as specified in RFC 2535 and RFC2931. 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.

4.8. DNSSEC

Cryptographic authentication of DNS information is possible through the DNS Security (DNSSEC-bis) extensions, defined in RFC <TBA>. 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 -h 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 presense 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.

4.9. 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. The use of A6 records is deprecated by RFC 3363, and the support for forward lookups in BIND 9 is removed accordingly. 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. BIND 9 formerly supported the "binary label" (also known as "bitstring") format. The support of binary labels, however, is now completely removed according to the changes in RFC 3363. Any applications in BIND 9 do not understand the format any more, and will return an error if given. In particular, an authoritative BIND 9 name server rejects to load a zone file containing binary labels.

For an overview of the format and structure of IPv6 addresses, see Section A.2.1.

$ORIGIN example.com.
host            3600    IN      AAAA    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.