Microsoft KB Archive/198409

= Microsoft DNS Server Registry Parameters, Part 2 of 3 =

Article ID: 198409

Article Last Modified on 10/31/2006

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APPLIES TO


 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q198409



IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.



SUMMARY
This article consists of 3 parts and describes settings for the Microsoft Domain Name Service (DNS) server. You can modify most settings using the DNSADMIN tool, although some settings can only be altered using Registry Editor.

For additional information, please see the following articles in the Microsoft Knowledge Base:

198408 Microsoft DNS Server Registry Parameters, Part 1 of 3

198410 Microsoft DNS Server Registry Parameters, Part 3 of 3



MORE INFORMATION
WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it.

To change these parameters, use the following procedure:  Start Registry Editor (Regedt32.exe).  From the HKEY_LOCAL_MACHINE subtree, go to the following key:

    \SYSTEM\CurrentControlSet\Services  From the Edit menu, click Add Value and add a value to the key described in the appropriate entry below. Type in the value, and use the "Data Type" check box to set the value type. Click OK. Quit Registry Editor. Restart the DNS Server for the above changes to take affect.</li></ol>

Server Parameters
Several registry parameters determine behavior of the entire server. Each of these is a registry value under

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\DNS\Parameters

NOTE: These registry keys are read only at startup. Some may be reset and, in some cases, the server behavior dynamically changed, through the DNS Administrator. But if manually reset, the DNS server must be restarted to pick up the new value.

All of the DNS parameters are registry values located under subkeys of:

<pre class="fixed_text">  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\

SecureResponses
<pre class="fixed_text">  Value:    SecureResponses Added:   SP4 (April 1998) Type:    DWORD (Boolean) Default: NoKey = OFF (Non-secure data is kept) Function: Determines whether server attempts to clean up             responses to avoid cache pollution. With the Service Pack 4 (April, 1998) release, the SecureResponses flag is off. Create the SecureResponses value and set it to 1 to enable it. Responses from other DNS servers may contain records whose validity are open to question.

Examples: DNS server makes MX query for domain.samples.microsoft.com to samples.microsoft.com's DNS server. The samples.microsoft.com DNS server responds but includes A record for A.ROOT-SERVERS.NET giving its own address. The rogue DNS server has then gotten itself set up as a root server in your DNS server's cache. Less malicious, but more common, are referral responses (or direct responses from BIND, see WriteAuthorityNs for discussion) that contain records for the DNS of an ISP: Authority section:

new.samples.microsoft.com NS ns.new.samples.microsoft.com.

new.samples.microsoft.com NS ns.isp.samples.microsoft.com.

Additional section:

ns.new.samples.microsoft.com. A 1.1.1.1

ns.isp.samples.microsoft.com. A 2.2.2.2

NOTE: The address record for the ISP happens to be old\stale. If SecureResponses is on, records that are not in a subtree of the zone queried are eliminated. For example, in the example above, the samples.microsoft.com. DNS server was queried, so the all the samples.microsoft.com records are secure, but the ns.isp.microsoft.com. A record is not in the sample .microsoft.com. subtree, and is not cached or returned by the DNS server.

RecursionRetry
<pre class="fixed_text">  Value:      RecursionRetry Added:   Windows NT 4.0 Type:    DWORD Default: NoKey  (Retry is three seconds) Function: Set the interval before retrying a recursive lookup.

To resolve recursive client queries, the DNS server queries remote DNS servers. When no response comes back, the server must retry (to the same and/or other DNS servers). This key allows override of default retry timeout. If the RecursionRetry key does not exist or is zero, retries are made after three seconds. If the RecursionRetry key is nonzero, its value (in seconds) sets the retry interval. Users should not alter this key.

RecursionTimeout
<pre class="fixed_text">  Value:      RecursionTimeout Added:   Windows NT 4.0 Type:    DWORD Default: NoKey (Timeout is 15 seconds) Function: Set the timeout before DNS server gives up recursive query.

To resolve recursive client queries, the DNS server queries remote DNS servers. When no response comes back, the server from any of the remote servers contacted, the DNS server must eventually give up on the query and report to the client that it was unable to answer it (response code is SERVER_FAILURE). This key allows override of default retry timeout. If the RecursionTimeout key does not exist or is zero, the DNS gives up after 15 seconds. If the RecursionTimeout key is nonzero, its value (in seconds) sets the retry timeout. In general the 15-second timeout allows any outstanding response from anywhere to get back to the DNS server. Users are discouraged from altering this key.

RoundRobin
<pre class="fixed_text">  Value:      RoundRobin Added:   SP4 Type:    DWORD (Boolean) Default: NoKey (Round robin A records) Function: Determine whether server round robins multiple A records.

By default, the Microsoft DNS server round robins A records when multiple A records exists for a name. This is a primitive load balancing mechanism.

If the RoundRobin key does not exist or is nonzero, the DNS server round robins A records. If the RoundRobin key is zero, the DNS server returns A records in a fixed (file load) order.

LocalNetPriority
<pre class="fixed_text">  Value:      LocalNetPriority Added:   SP4 Type:    DWORD (Boolean) Default: NoKey (Give local net records priority) Function: Determine the priority of multiple A records given in            response.

By default, the Microsoft DNS server gives priority to the "closest" A record to the client's IP address when there are multiple A records for a name. This is designed so that the client application will attempt to connect to the closest (and fastest) IP available.

Example: www.samples.microsoft.com has three A records: 198.27.1.1, 20.23.25.1, and 131.21.31.7. Client at 131.21.198.25 queries DNS server for www.samples.microsoft.com. Instead of returning in database order or round robin, the DNS server notices that the client's 131.21 address matches the network (class B) portion of the 131.21.31.7 address. The DNS server then reorders the addresses in the response:

<pre class="fixed_text">  131.21.31.7 198.27.1.1  20.23.25.1

If the client query comes from 12.123.248.12, then none of addresses matches in the network portion of the address and NO local net priority reordering is done.

If more than one address matches in the network portion, then the matching addresses are ordered with the closest match first so that the result is most likely to be correct regardless of any subnetting.

Example: www.samples.microsoft.com has four A records: 198.27.1.1, 20.23.25.1, 131.21.31.7, and 131.21.196.8. Client at 131.21.198.25 queries DNS server for www.samples.microsoft.com. Now both 131.21 addresses are returned at the top, but the 131.21.196 address is first because it matches the client's IP address down through the 131.21.192 subnet (that is, it would match even if subnetting down to a 255.255.248 mask). So the response is ordered:

<pre class="fixed_text">  131.21.196.8 131.21.31.7  198.27.1.1   20.23.25.1

NOTE: Having LocalNetPriority supersedes round robining. However, if RoundRobin is on, the records continue to be round-robined, and are returned in the current round-robin order when no LocalNetPriority match can be made.

AddressAnswerLimit
<pre class="fixed_text">  Value:      AddressAnswerLimit Added:   SP3 Type:    DWORD Default: NoKey (No limit) Function: Limits number of A records put in answer to query.

The AddressAnswerLimit registry key puts a limit on the number of A records that are actually written to the answer section in a response to an A record query. This has two benefits:


 * 1) Allows Windows 95 clients to operate correctly, even when a DNS name is configured with more than 28 A records.
 * 2) Keeps remote DNS servers resolving a name from retrying with TCP when they receive a truncated response. (In general, client resolvers will not retry with TCP, whether the truncation bit is set or not.)

If the AddressAnswerLimit registry key is nonzero, it specifies a limit on the number of A records written into an answer for a name. The limit itself is bounded 5 < x < 28. So an AddressAnswerLimit value greater than 28, will result in a limit of 28, and a value 0 < x < 5, will result in a limit of five. (The bottom limit of 5 is there so that, if multiple addresses are available, at least enough is given back so that one computer is likely to be found.) In addition, when AddressAnswerLimit is set, the truncation bit is not set on a response, even if the packet space is exhausted before the limit for writing A records is reached (see benefit 2 above).

NOTE: These restrictions are only for simple A record queries to the name. They do not affect other query types.

If the AddressAnswerLimit registry key does not exist or is zero, A record responses are not limited. All A records for a name are written to the packet. If all the records do not fit in a UDP DNS packet, the truncation bit it set.

LooseWildcarding
<pre class="fixed_text">  Value:      LooseWildcarding Added:   SP4 Type:    DWORD (Boolean) Default: NoKey  (prior to SP4 loose; after SP4 RFC wildcarding) Function: Set server to do wildcarding loosely.

Example:

<pre class="fixed_text">  Zone sample.microsoft.com.

Zone file fragment:

<pre class="fixed_text">     *.sample.microsoft.com. MX  10 mailserver1.sample.microsoft.com. sample.microsoft.com. MX  10 mailserver2.sample.microsoft.com. host.sample.microsoft.com. A  1.1.1.1

Query:

<pre class="fixed_text">     Host.sample.microsoft.com, type MX

According to RFC, the host.samples.microsoft.com.com query is looked up and no MX record is found at host.samples.microsoft.com, but an A record does exist. Hence, no wildcard matching is done. The query responds with an authoritative empty response (no error and no records). Smart mail programs would then requery for an A record for host.samples.microsoft.com and use the A record to send mail directly to host.samples.microsoft.com.

Before Service Pack 4 (SP4), the Microsoft DNS server finds no MX record at host.samples.microsoft.com, notes that MX is a wildcard type, and notes that samples.microsoft.com is the parent of a wildcard name. It checks that wildcard name *.samples.microsoft.com for MX records, finds a match and returns it. The mail program would then send mail to mailserver1.samples.microsoft.com.

The advantages of each approach should be apparent. The RFC approach is easier if your hosts are capable of receiving mail directly. However, if you advertise hosts that are not mail servers, you also need to explicitly add MX records at each host. The Microsoft DNS approach allows you to set up just two MX records and cover queries to the entire zone. However, if you have individual hosts that should receive there own mail, you need to put MX records at each of these hosts.

If the LooseWildcarding key does not exist or is zero, the server (after SP4) will follow the RFC specified wildcarding behavior. The administrator is advised to put MX records in for all hosts that are not capable of receiving mail.

If the LooseWildcarding key is nonzero, the server aggressively seek out the closest wildcard node (equivalent to shipped Windows NT 4.0 behavior). The administrator should put MX records at both the zone root and in a wildcard node (*) directly below the zone root. The administrator should also put self-referent MX records on hosts that are to receive their own mail.

BindSecondaries
<pre class="fixed_text">  Value:      BindSecondaries Added:   Windows NT 4.0 Type:    DWORD (Boolean) Default: NoKey (Send uncompressed transfers to non-MS secondaries) In Admin: No.  Function: Determine AXFR message format when sending to non-MS DNS secondary.

BIND did not implement the RFC specified zone transfer protocol. Specifically, BIND sends one zone record in every DNS message. This adds the unnecessary overhead of a message header with every record; it also eliminates any possibility of name compression. BIND servers prior to 4.9.4 will refuse zone transfer messages that properly buffer multiple records in each DNS message, and some implementations may fault. More recent BIND versions (4.9.4 and later) will both accept and send the faster compressed format, but may be configured with a list of servers that must receive the old format.

The Microsoft DNS server can send (and receive) messages in either format. To allow Microsoft to Microsoft transfers to use the faster format, a Microsoft DNS secondary appends two characters (MS) to its AXFR request packet. When a Microsoft master receives a request with this signal, it then sends the transfer using the fast compressed format. If the request does not contain this signal, the transfer is sent using the slower one record per message format, to avoid causing problems if the secondary is an old BIND implementation.

If the BindSecondaries registry key does not exist or is nonzero, the server uses the paradigm described above and will always send transfers to non-Microsoft DNS secondaries in the uncompressed BIND compatible format. If the BindSecondaries registry key is zero, the server will send all transfers in the fast format. Note that the default behavior is adequate to transfer to any DNS server. The only reason to use the BindSecondaries key is when you have NEW BIND servers (or non-BIND, non-Microsoft servers) that are secondaries to an Microsoft DNS server and you want to get the fastest possible transfer performance.

As the fixed BIND servers replace older versions, the defaults to this key may be altered so that the efficient transfer is the default even to non- Microsoft servers.

WriteAuthorityNs
<pre class="fixed_text">  Value:      WriteAuthorityNs Added:   SP3 Type:    DWORD (Boolean) Default: NoKey (Do not write unnecessary NS records) Function: Write NS records to authority section on successful response.

By default, Microsoft DNS server uses the authority section only as outlined by RFC1034:


 * For NS records when making a referral.
 * For an SOA record to allow caching of a NAME_ERROR response. (In compliance with dnsind clarify draft.)

If the WriteAuthorityNs registry key is nonzero, the server will write NS records for the zone into the Authority section when making a successful authoritative response. If the WriteAuthorityNs registry key does not exist or is zero, the server use the default RFC compliant behavior and only write to the Authority section in the two cases noted above.

NOTE: This key exists only for those who need Authority data for some programmatic reason.

WriteAuthoritySoa
<pre class="fixed_text">  Value:      WriteAuthoritySoa Added:   SP3 Removed: SP4 Type:    DWORD (Boolean) Default: NoKey (Do not write unnecessary SOA record) Function: Write NS records to authority section on successful response.

The use of this key is deprecated. Use WriteAuthorityNs (see above) if mimicking of BIND authority section behavior is desired.

ListenAddresses
<pre class="fixed_text">  Value:      ListenAddresses Added:   Windows NT 4.0 Type:    BINARY Default: NoKey  (Use all IP interfaces) Function: Determines IP addresses bound to DNS server.

Some DNS resolvers (including Windows 95) require that the IP source address of a DNS response be the same as the IP address they sent the query to, or they reject the response. Accommodating these resolvers means that the DNS server must explictly bind sockets to IP addresses on the computer.

The ListenAddresses key is a list of IP addresses for the DNS server to listen on. The list is not dotted IP strings, but a counted array of raw addresses in net byte order. It should be configured through the Server Properties, Interfaces dialog box in the Administrator tool. Editing the registry key is discouraged. If the ListenAddresses key does not exist, the DNS server attempts to bind to every IP address on the computer. This is, in general, desirable behavior.

However, there are a few reason why using the listen address key may be desirable:


 * For administrative reasons, you do not want to use some interfaces.
 * Computer contains lots of IP interfaces, binding to all of them is expensive.
 * If greater than 35 IP interfaces, the DNS server will not detect and bind to all IP interfaces properly. Because of limitations in Winsock's GetAddressByName, only the first 35 interfaces are returned to the DNS server. If your DNS should be bound to any addresses beyond the first 35, whether all the addresses on the computer or some subset, ListenAddresses is required to get the correct binding.

Keywords: kbinfo KB198409

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