30.4. Creating and Manipulating Resource Records

Resource records are the basic unit of information in DNS. A DNS server's primary job is to respond to queries for resource records . Most people don't realize they are generating resource record queries with nearly every network-based operation they do, including accessing a website, pinging a host via its Fully Qualified Domain Name (FQDN), or logging into Active Directory.

Resource records come in many different flavors or types. Each type corresponds to a certain type of name or address lookup. Each record type also has additional information encoded with the record that represents things such as the time to live of the record. The following is a textual example of what a CNAME record looks like:

www.mycorp.com.  1800  IN  CNAME  www1.mycorp.com.

Or, more generically:

Owner  TTL  Class  Type  RR-Data

Now let's break down the record into its individual parts:

Owner

The owner of the resource record. This field is typically what is specified during a query for the particular type.

TTL

The time to live, or length of time a nonauthoritative DNS server should cache the record. After the TTL expires, a nonauthoritative server should requery for an authoritative answer.

Class

Resource record classification. In nearly all cases, this will be "IN" for Internet.

Type

Name of the resource record type. Each type has a standard name that is used in zones (e.g., CNAME, A, PTR, SRV).

RR-Data

Resource record specific data. When you perform a query, you are typically looking for the information returned as part of the RR-Data.

The WMI DNS provider fully supports querying and manipulating resource records. In Tables 30-5 and 30-6, the supported properties and methods are listed for the MicrosoftDNS_ResourceRecord class, which implements a generic interface for resource records.

Table 30-5. MicrosoftDNS_ResourceRecord class properties

Property name	Property description
ContainerName	Name of container (e.g., zone name) that holds the RR
DnsServerName	FQDN of the server that contains the RR
DomainName	FQDN of the domain that contains the RR
OwnerName	Owner of the RR
RecordClass	Class of the RR; 1 represents IN
RecordData	Resource record data
Textrepresentation	Textual representation of the RRe.g., www.mycorp.com. 1800 IN CNAME www1.mycorp.com
Timestamp	Time RR was last refreshed
TTL	Time to live or maximum time a DNS server is supposed to cache the RR

Table 30-6. MicrosoftDNS_ResourceRecord class methods

Method name	Method description
CreateInstanceFromTextRepresentation	Creates a new instance of a MicrosoftDNS_ResourceRecord subclass based on the textual representation of the resource record, server name, and container or zone name. A reference to the new object is returned as an out parameter.
GetObjectByTextRepresentation	Gets an instance of the appropriate MicrosoftDNS_ResourceRecord subclass as specified by the textual representation of the resource record, server name, and container or zone name.

The MicrosoftDNS_ResourceRecord class by itself is not enough. There are over two dozen different types of resource records with many having additional fields that would not have corresponding methods in the generic interface. To solve this problem, subclasses of MicrosoftDNS_ResourceRecord were created for each supported record type. Each subclass provides specific methods to access any field supported by the resource record type. Each supported resource record has a subclass with a name in the format of MicrosoftDNS_<RR Type>Type.

To show just how different resource records can be, let's take a look at an A record:

www.mycorp.com.  1800  IN  A  192.10.4.5

Now let's compare that with an SRV record:

_ldap._tcp.dc._msdcs.mycorp.com 1800  IN  SRV  0 100 389 dc1.mycorp.com.

As you can see, the SRV record has several additional fields. By using the Microsoft-DNS_SRVType subclass, we can access each field with methods provided by the class.

The complete list of supported resource record types is provided in Table 30-7.

Table 30-7. DNS provider supported resource records

Resource record type	DNS provider class	RFC	Description
A	MicrosoftDNS_Atype	RFC1035	Name-to-IPv4 address mapping
AAAA	MicrosoftDNS_AAAAType	RFC1886	Name-to-IPv6 address mapping
AFSDB	MicrosoftDNS_AFSDBType	RFC1183	Andrew File System (AFS) Database Server record
ATMA	MicrosoftDNS_ATMAType	N/A	ATM-address-to-name mapping
CNAME	MicrosoftDNS_CNAMEType	RFC1035	Canonical (alias) name
HINFO	MicrosoftDNS_HINFOType	RFC1035	Host information
ISDN	MicrosoftDNS_ISDNType	RFC1183	Integrated services digital network (ISDN) record
KEY	MicrosoftDNS_KEYType	RFC2535	KEY record
MB	MicrosoftDNS_MBType	RFC1035	Mailbox record
MD	MicrosoftDNS_MDType	RFC1035	Mail agent
MF	MicrosoftDNS_MFType	RFC1035	Mail forwarding agent
MG	MicrosoftDNS_MGType	RFC1035	Mail group record
MINFO	MicrosoftDNS_MINFOType	RFC1035	Mail information record
MR	MicrosoftDNS_MRType	RFC1035	Mailbox rename record
MX	MicrosoftDNS_MXType	RFC1035	Mail exchanger
NS	MicrosoftDNS_NSType	RFC1035	Name server
NXT	MicrosoftDNS_NXTType	RFC2535	Next record
PTR	MicrosoftDNS_PTRType	RFC1035	Address-to-name mapping record
RP	MicrosoftDNS_RPTType	RFC1183	Responsible person
RT	MicrosoftDNS_RTType	RFC1183	Route through record
SIG	MicrosoftDNS_SIGType	RFC2535	Signature record
SOA	MicrosoftDNS_SOAType	RFC1035	Start of authority
SRV	MicrosoftDNS_SRVType	RFC2052	Service record
TXT	MicrosoftDNS_TXTType	RFC1035	Text record
WINS	MicrosoftDNS_WINSType	N/A	WINS server
WINSR	MicrosoftDNS_WINSRType	N/A	WINS reverse-lookup
WKS	MicrosoftDNS_WKSType	RFC1035	Well-known services
X25	MicrosoftDNS_X25Type	RFC1183	X.121 Address-to-name mapping

30.4.1. Finding Resource Records in a Zone

With the marriage of DNS and WMI, querying DNS has never been so easy. By using WQL, you can write complex query routines that would not have been possible previously. To list all of the resource records on a server, you simply need to execute the WQL query select * from MicrosoftDNS_ResourceRecord against the target server. The following example shows what this would look like if the script is run on a DNS server:

Set objDNS = GetObject("winMgmts:root\MicrosoftDNS")
Set objRR = objDNS.ExecQuery("Select * from MicrosoftDNS_ResourceRecord ")

For Each objInst In objRR
   WScript.Echo objInst.TextRepresentation 

Next
WScript.Echo "The script has completed successfully."

The TexTRepresentation method is available to all resource record types since it is defined in MicrosoftDNS_ResourceRecord. It will return a text string representing the resource record, such as the following:

www.mycorp.com. IN  A  192.10.4.5

If you want to limit the query to only a specific zone, change the WQL query to include criteria for ContainerName, such as the following:

Select * from MicrosoftDNS_ResourceRecord Where ContainerName = 'ZoneName'

Since Active Directory uses DNS to store all of the Global Catalog servers in a forest and domain controllers in a domain, you can write scripts to utilize DNS to access this information and integrate it into your applications. The following example does exactly this by selecting all SRV records with a particular OwnerName. To find all Global Catalog servers in a forest, you can simply query _ldap._tcp.gc._msdcs.<ForestDNSName>, and to find all domain controllers in a domain, query _ldap._tcp.dc._msdcs.<DomainDNSName>.

Option Explicit

Dim strDomain
strDomain = "mycorp.com"

Dim objDNS, objRRs, objRR
Set objDNS = GetObject("winMgmts:root\MicrosoftDNS")
Set objRRs = objDNS.ExecQuery("Select * from MicrosoftDNS_SRVType " & _
                             " Where OwnerName = '_ldap._tcp.gc._msdcs." & _
                             strDomain & "'")
WScript.Echo "Global Catalogs for " & strDomain
For Each objRR In objRRs
   WScript.Echo " " & objRR.DomainName
Next

WScript.Echo

Set objRRs = objDNS.ExecQuery("Select * from MicrosoftDNS_SRVType " & _
                             " Where OwnerName = '_ldap._tcp.dc._msdcs." & _
                             strDomain & "'")
WScript.Echo "Domain Controllers for " & strDomain
For Each objRR In objRRs
   WScript.Echo " " & objRR.DomainName
Next
WScript.Echo "The script has completed successfully."

30.4.2. Creating Resource Records

With the DNS provider, creating resource records is also very easy to do. The MicrosoftDNS_ResourceRecord::CreateInstanceFromTextRepresentation method takes the server name to create the record on, the domain name, and the text representation of the resource record as in parameters. It also provides an out parameter, which will be an object representing the newly created record.

Example 30-3 goes through the process of creating both A and PTR records. Both records are typically necessary when adding a new host to DNS.

Example 30-3. Creating A and PTR resource records

Option Explicit Dim strRR, strReverseRR, strDomain, strReverseDomain ' A record to add strRR = "testb.mycorp.com. IN A 192.32.64.13" strDomain = "mycorp.com" ' PTR record to add strReverseRR = "13.64.32.192.in-addr.arpa IN PTR testb.mycorp.com" strReverseDomain = "192.in-addr.arpa." Dim objDNS, objRR, objDNSServer, objRR2, objOutParam Set objDNS = GetObject("winMgmts:root\MicrosoftDNS") Set objRR = objDNS.Get("MicrosoftDNS_ResourceRecord") Set objDNSServer = objDNS.Get("MicrosoftDNS_Server.Name="".""") ' Create the A record Dim strNull strNull = objRR.CreateInstanceFromTextRepresentation( _ objDNSServer.Name, _ strDomain, _ strRR, _ objOutParam) Set objRR2 = objDNS.Get(objOutParam) WScript.Echo "Created Record: " & objRR2.TextRepresentation Set objOutParam = Nothing ' Create the PTR record strNull = objRR.CreateInstanceFromTextRepresentation( _ objDNSServer.Name, _ strReverseDomain, _ strReverseRR, _ objOutParam) Set objRR2 = objDNS.Get(objOutParam) WScript.Echo "Created Record: " & objRR2.TextRepresentation