Question:
How can I troubleshoot DNS replication issues between a read-only domain controller (RODC) and two writable domain controllers (DCs) in the same domain? >
> I have a domain with two DCs that host multiple DNS zones. I installed a RODC with the DNS role, but the DNS records on the RODC are incomplete and do not match the ones on the DCs. When I create a new record on the DCs, it does not replicate to the RODC. What could be the cause of this problem and how can I fix it?
Answer:
How to Troubleshoot DNS Replication Issues between a RODC and DCs
DNS replication is a vital process for ensuring the consistency and availability of DNS data across domain controllers (DCs) in an Active Directory domain. However, sometimes DNS replication may fail or encounter errors, resulting in outdated or missing DNS records on some DCs. This can cause various problems for clients and servers that rely on DNS for name resolution and service discovery.
One scenario where DNS replication issues may occur is when a read-only domain controller (RODC) is deployed in a branch office or a remote site. An RODC is a special type of DC that hosts a read-only copy of the Active Directory database and the DNS zones. An RODC can improve security, performance, and availability in scenarios where a writable DC is not feasible or desirable. However, an RODC also introduces some challenges and limitations for DNS replication, as it cannot initiate or accept inbound or outbound replication requests.
In this article, we will explain how DNS replication works between a RODC and writable DCs, what are the common causes of DNS replication issues, and how to troubleshoot and resolve them.
DNS replication between a RODC and writable DCs is different from DNS replication between writable DCs. The main difference is that a RODC cannot perform any write operations on its DNS zones, and it relies on writable DCs to update its DNS data. Therefore, a RODC cannot make any DNS changes, such as creating, modifying, or deleting DNS records, or transferring DNS zones.
When a client attempts a dynamic update, it sends a query for an authoritative server to its configured DNS server, which would be the RODC in this scenario. The RODC then forwards the query to a writable DC that hosts a writable copy of the DNS zone. The writable DC performs the update and sends a response back to the RODC, which then sends it back to the client. The writable DC also replicates the update to other writable DCs in the domain, according to the replication scope and schedule of the DNS zone.
However, the update is not immediately replicated to the RODC, as the RODC cannot initiate or accept inbound replication requests. Instead, the RODC performs a special operation called Replica Single Object (RSO), which allows it to request a specific object from a writable DC. The RODC periodically performs an RSO operation for each DNS record that has been updated on a writable DC, and updates its own copy of the DNS zone accordingly. The RSO operation is triggered by a timer that is set to 15 minutes by default, but can be configured using the `msDS-ReplValueMetaDatatimeStamps` attribute on the RODC computer object.
The RSO operation requires that the writable DC that hosts the DNS zone is running Windows Server 2008 or later, and that the DNS zone is stored in the Active Directory database. If the DNS zone is stored in a file, or if the writable DC is running an earlier version of Windows Server, the RSO operation will fail, and the DNS record update will not be replicated to the RODC until the next scheduled replication cycle.
Common Causes of DNS Replication Issues between a RODC and DCs
There are several factors that can cause DNS replication issues between a RODC and writable DCs, such as:
- Network connectivity issues: If the RODC cannot communicate with the writable DCs, it cannot forward dynamic update requests, perform RSO operations, or receive scheduled replication updates. This can be caused by network outages, firewall rules, VPN configurations, or DNS name resolution failures.
- DNS configuration issues: If the DNS zones or records are not configured correctly on the writable DCs or the RODC, it can prevent DNS replication from working properly. For example, if the DNS zones are not replicated to the RODC, or if the DNS records have incorrect or conflicting values, such as TTL, timestamp, or security settings.
- Active Directory replication issues: If the Active Directory replication between the writable DCs or between the writable DCs and the RODC is not working correctly, it can affect the DNS replication as well. For example, if the RODC computer object, the DNS application partition, or the DNS zone objects are not replicated to the RODC, or if they have replication errors or conflicts.
- RODC configuration issues: If the RODC is not configured correctly, it can cause DNS replication issues. For example, if the RODC is not authorized to host the DNS zones, or if the RODC has incorrect or missing credentials, or if the RODC has a different domain functional level than the writable DCs.
How to Troubleshoot and Resolve DNS Replication Issues between a RODC and DCs
To troubleshoot and resolve DNS replication issues between a RODC and writable DCs, you can use the following steps:
1. Verify the network connectivity between the RODC and the writable DCs: You can use tools such as `ping`, `tracert`, `telnet`, or `PortQry` to test the network connectivity and the availability of the required ports and protocols. You can also use the `dcdiag /test:connectivity` command to check the basic connectivity and DNS settings of the RODC and the writable DCs.
2. Verify the DNS configuration on the writable DCs and the RODC: You can use tools such as `dnscmd`, `nslookup`, or `DNS Manager` to check the DNS zones and records on the writable DCs and the RODC. You can also use the `dcdiag /test:dns` command to check the DNS health and consistency of the RODC and the writable DCs. You should make sure that the DNS zones are replicated to the RODC, and that the DNS records have the correct values and settings.
3. Verify the Active Directory replication between the writable DCs and the RODC: You can use tools such as `repadmin`, `Replmon`, or `Active Directory Sites and Services` to check the Active Directory replication status and errors between the writable DCs and the RODC. You can also use the `dcdiag /test:replications` command to check the replication health and consistency of the RODC and the writable DCs. You should make sure that the RODC computer object, the DNS application partition, and the DNS zone objects are replicated to the RODC, and that they do not have any replication errors or conflicts.
4. Verify the RODC configuration and credentials: You can use tools such as `RODCprep`, `RODC Filter`, or `Active Directory Users and Computers` to check the RODC configuration and credentials. You should make sure that the RODC is authorized to host the DNS zones, and that it has the correct credentials to communicate with the writable DCs. You should also make sure that the RODC has the same domain functional level as the writable DCs, and that it has the latest updates and patches installed.
If
you follow these steps, you should be able to troubleshoot and resolve most of the DNS replication issues between a RODC and writable DCs. However, if you still encounter problems, you can contact Microsoft support or consult the following resources for more information and guidance:
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