Table of Contents
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 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.
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.
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. 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.
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 unfreeze zone
to reload the changed zone and re-enable dynamic updates.
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.
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:
site1
and
site2.example.com
zones.site1.internal
and
site2.internal
domains.Hosts on the Internet will be able to:
site1
and
site2.example.com
zones.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 serversbastion-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
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.
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.
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 bit key.
dnssec-keygen -a hmac-md5 -b 128 -n HOST host1-host2.
The key is in the file Khost1-host2.+157+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.
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.
This is beyond the scope of DNS. A secure transport mechanism should be used. This could be secure FTP, ssh, telephone, etc.
Imagine host1 and host 2 are
both servers. The following is added to each server's named.conf
file:
key host1-host2. { algorithm hmac-md5; secret "La/E5CjG9O+os1jq0a2jdA=="; };
The algorithm, hmac-md5, is the only one supported by BIND.
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.
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.
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”.
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.
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 NOTAUTH.
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.
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.
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.
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 file names 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 public keys should be inserted into the zone file by
including the .key
files using
$INCLUDE statements.
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
and RRSIG
records for the zone, as well as DS
for
the child zones if '-d'
is specified.
If '-d'
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 administators with the
DNSKEYs
(or their corresponding DS
records) that are the secure entry point to the zone.
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.
The public key for any security root must be present in the configuration file's trusted-keys statement, as described later in this document.
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 the section called “IPv6 addresses (AAAA)”.
The AAAA record is a parallel to the IPv4 A record. It specifies the entire address in a single record. For example,
$ORIGIN example.com. host 3600 IN AAAA 2001:db8::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
a AAAA, with ::ffff:192.168.42.1
as the
address.
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.