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, 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 the record down 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 re-query for a 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 Table 27-5 and Table 27-6, the supported properties and methods are
listed for the MicrosoftDNS_ResourceRecord class,
which implements a generic interface for resource records.
Table 27-5. MicrosoftDNS_ResourceRecord class properties
|
ContainerName
|
Name of container (e.g., zone name) that holds the RR
|
|
DomainName
|
FQDN of the domain that contains the RR
|
|
DnsServerName
|
FQDN of the server 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 RR, e.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 27-6. MicrosoftDNS_ResourceRecord class methods
|
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 MicrosoftDNS_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 27-7.
Table 27-7. DNS provider supported resource records
|
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
|
27.4.1 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
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
27.4.2 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 27-3 goes through the process of creating
both A and PTR records. Both records are typically necessary when
adding a new host to DNS.
Example 27-3. 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
