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© Rakhesh Sasidharan

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SRV records and AD

Example of an SRV record:

Format is:

Where:

  • class is always IN
  • TTL is in seconds 
  • service is the name of the service (example: LDAP, GC, DC, Kerberos). It is preceded by an underscore (prevents collisions with any existing names). 
  • protocol is the protocol over which the service is offered (example: TCP, UDP). It is preceded by an underscore (prevents collisions with any existing names). 
  • domain is the DNS name of the domain for which the service is offered. 
  • host is the machine that will provide this service. 
  • SRV is the text SRV. Indicates that this is an SRV record. 
  • priority is the priority of the host. Similar to SMTP MX record priorities. Host with lower number has higher preference (similar to SMTP records). 
  • weight allows for load balancing between hosts of the same priority. Host with higher number has higher preference (intuitive: higher weight wins).

Similar to MX records, the host of an SRV record must be a name with an A or AAAA record. It cannot point to a CNAME record. 

A machine starting up in a domain can easily query for DCs of that domain via DNS. DCs provide the LDAP service, so a query for _ldap._tcp.dnsdomain will return a list of DCs the machine can use. 

On DCs, the netlogon service registers many records relevant to the services offered by the DC. The A records of the DC are registered by the DNS client service (for Server 2008 and above) but the other records are taken care of by the netlogon service. A copy of the registered records is also stored in the %systemroot%\system32\config\netlogon.dns file in case it needs to be imported manually or compared for troubleshooting. Note that each DC only adds the records for itself. That is, WIN-DC01 for instance, will add a record like this:

It will not add records for WIN-DC02 and others as it does not know of them. The records added by each DC will be combined together in the DNS database as it replicates and gets records from all the DCs, so when a client queries for the above record, for instance, it will get records added by all DCs. 

AD creates a sub-domain called _msdcs.domainname to hold items pertaining to AD for that domain. (MSDCS = Microsoft Domain Controller Services) This sub-domain is created for each domain (including child-domains) of the forest. The _msdcs sub-domain belonging to the forest root domain is special in that it is a separate zone that is replicated to all DNS servers in the forest. The other _msdcs sub-domains are part of the parent zone itself. For instance below are the _msdcs sub-domains for a forest root domain (rakhesh.local) and the _msdcs sub-domain for a child domain (anoushka.rakhesh.local). Notice how the former is a separate zone with a delegation to it, while the latter is a part of the parent zone. 

For forest-root domain

For forest-root domain

For child-domain

For child-domain

Under the _msdcs sub-domain a convention such as the following is used:

Here DcType is one of dc (domain controller), gc (global catalog), pdc (primary domain controller), or domains (GUIDs of the domains).

The various SRV records registered by netlogon can be found at this link. Note that SRV records are created under both the domain/ child-domain and the forest root domain (the table in the link marks these accordingly). Below are some of the entries added by netlogon for DCs – WIN-DC04 and WIN-DC05 – in a site called KOTTAYAM for my domain anoushka.rakhesh.local. Of these WIN-DC05 is also a GC. 

Advertise that the DCs offer the LDAP service over TCP protocol for the anoushka.rakhesh.local domain:

Advertise that the DCs offer the LDAP service over TCP protocol for the anoushka.rakhesh.local domain for both sites of the forest (even though the DCs themselves are only in a single site – this way clients in any site can get these DC names when querying DNS SRV records):

WIN-DC05 has a few additional records compared to WIN-DC04 because it is a GC. 

Notice all of them are specific to its site and are created in the forest root domain zone/ _msdcs sub-domain of the forest root domain. This is because GCs are used forest-wide also. In contrast, similar records advertising the DC service are created for both sites and in the _msdcs sub-domain of the child zone:

To re-register SRV records, either restart the netlogon service of a DC or use nltest as below:

 

Useful ldp queries

Just dumping useful ldp.exe queries as I come across/ think of them. Better to keep them here in one consolidated post for future reference …

Find all GCs in the forest

If a DC is a GC, its NTDS Setting object has an attribute called options whose value is 0x1. Not sure if a DC is a GC and something else, whether the value changes, but for now I’ll assume it doesn’t and so one can quickly search for all GCs in the forest by connecting to the Configuration partition and filtering by the following:

  • Filter: (&(cn=NTDS Settings)(options=1))
  • Base DN: CN=Configuration,DC=domainname
  • Attributes: distinguishedName
  • Scope: Subtree