Designing Windows Active Directory DNS (AD DNS) for a multi-site company is a process that should balance a few crucial aspects, including site-to-site replication, DNS refresh and retry intervals, and time to live (TTL) settings. Here are some recommendations to ensure optimal AD DNS design:
1. Site-to-Site Replication: This is crucial in a multi-site company, where you want to make sure that all your DNS zones are up-to-date across all sites. By default, AD DNS uses multi-master replication, which allows updates to the DNS zones from any DNS server. However, to minimize bandwidth usage and the possibility of replication conflicts, it might be better to designate certain DNS servers as primary for a given zone and let the other DNS servers in other sites pull updates from them. This is a decision that should be based on the business requirements, network topology, and the number of changes expected in DNS zones.
2. Time To Live (TTL): The TTL setting is used by DNS to determine how long a resolver should cache a DNS response. Lower TTLs mean that changes propagate more quickly, but at the cost of increased DNS query traffic. A shorter TTL can be beneficial during a Disaster Recovery (DR) scenario, because the DNS changes will propagate faster. However, too short of a TTL can overwhelm your DNS servers and network infrastructure. A TTL value of 5 minutes (300 seconds) is commonly used for balancing rapid changes and avoiding excessive traffic.
3. DNS Refresh and Retry Intervals: These settings are related to the rate at which secondary DNS servers check for updates from the primary DNS server for a given zone. A shorter refresh interval means that changes will propagate more quickly from the primary to the secondary servers. However, similar to the TTL setting, too short of a refresh interval could cause excessive network traffic and load on your servers. A commonly recommended value for the refresh interval is 15 minutes. The retry interval, which is how long the secondary server waits to try again if it can’t reach the primary server, is commonly set to less than the refresh interval, such as 10 minutes.
In the context of DNS (Domain Name System), the Refresh and Retry intervals are part of the SOA (Start of Authority) record, which is an important part of how DNS operates.
Let’s first understand what each of these intervals does:
Refresh Interval: This is the time period that a secondary DNS server waits before querying the primary DNS server to see if any changes have been made to the domain’s zone file. If the primary server has any changes, it will deliver them to the secondary server. This interval is usually set to a value that allows secondary servers to promptly mirror any changes made on the primary server, while minimizing unnecessary traffic if changes are infrequent.
Retry Interval: This is the time period that a secondary DNS server waits, after a failed attempt to reach the primary server, before trying again. This period is usually shorter than the refresh interval, because it applies to a situation where the secondary server has already tried and failed to reach the primary server.
In simple terms, the refresh interval tells the secondary servers when to check for updates, and the retry interval tells them how long to wait before trying again if they can’t reach the primary server
4. Dynamic Updates: Enabling secure dynamic updates can allow your AD DNS clients to automatically register and update their DNS records. This can be beneficial in ensuring that your DNS records are always up-to-date and can be particularly useful in a DR scenario.
5. DNS Scavenging: This is a feature that automatically removes stale DNS records, which can help keep your DNS zones clean and up-to-date. It should be used with caution, though, as incorrectly configured DNS scavenging can potentially remove necessary DNS records.
6. Backup and Recovery Plan: Always have a backup and recovery plan in place for your DNS servers. This should include regular backups of your DNS zones and testing of the restore process.
Remember that while having everything sync with the lowest possible settings may seem like a good idea for DR, it could also lead to excessive network traffic and server load. It’s important to find a balance that ensures rapid propagation of changes without overwhelming your infrastructure. These recommendations should serve as a starting point, but the optimal settings may vary depending on the specifics of your organization’s network and business requirements.
DEFAULTS
Here are the default values and the lowest recommended settings for Site-to-Site Replication, Time To Live (TTL), and DNS Refresh and Retry intervals in a Windows AD DNS setup:
1. Site-to-Site Replication: Active Directory uses multi-master replication, which means that changes made to the directory on any domain controller are replicated to all other domain controllers in the network. The default replication interval within a site is immediate, i.e., changes are replicated as soon as they occur. The default replication interval between sites varies based on your specific configuration, but it is typically every 180 minutes (3 hours). The lowest recommended setting for inter-site replication is 15 minutes, which is the lowest setting allowed by Windows.
2. Time To Live (TTL): The default TTL for a Windows AD DNS zone is 1 hour (3600 seconds). This value is used for all records in the zone that do not have a TTL specified. The lowest recommended setting for TTL in a production environment is generally 5 minutes (300 seconds), but be aware that setting the TTL this low will increase DNS traffic due to more frequent cache invalidations.
3. DNS Refresh and Retry Intervals: The default settings for the refresh and retry intervals in Windows AD DNS are 15 minutes for the refresh interval and 10 minutes for the retry interval. These values determine how often secondary DNS servers will check for updates from the primary DNS server. The lowest recommended settings for these values would be the same as the default, i.e., a refresh interval of 15 minutes and a retry interval of 10 minutes.
Keep in mind that these are the lowest recommended settings, and setting the values this low could cause increased network traffic and server load. It’s important to adjust these values based on your organization’s specific needs and network capacity.
I DON’T WANT TO WAIT, FORCE IT
You can manually force Active Directory replication, and refresh DNS zones. However, TTLs, DNS Refresh, and Retry intervals are more of a waiting period rather than something you can force manually. They represent how long other servers or clients should cache the data before checking for a new copy. Once that data is out there and cached, you can’t force those remote systems to flush their cache. Here’s how to manually force the actions that can be forced:
1. Active Directory Replication: You can use the “repadmin” command-line tool to immediately replicate changes between domain controllers. The command would look like this:
repadmin /replicate {Destination DC} {Source DC} {Directory Partition}
For example:
repadmin /replicate DC02 DC01 dc=domain,dc=com
Here, DC02 is the destination domain controller that you want to replicate changes to, DC01 is the source domain controller that you want to replicate changes from, and “dc=domain,dc=com” is the directory partition that you want to replicate.
2. DNS Zone Refresh: You can also use the “dnscmd” command-line tool to refresh a DNS zone on a domain controller.
The command would look like this:
dnscmd {DC Name} /ZoneRefresh {Zone Name}
For example:
dnscmd DC01 /ZoneRefresh domain.com
Here, DC01 is the name of the domain controller that you want to refresh the zone on, and “domain.com” is the name of the zone that you want to refresh.
Remember that forcibly replicating in this way should generally be done sparingly, such as in cases where you need an urgent change to be propagated. Continuous replication of this sort can put undue strain on your network and domain controllers.
Also, changes made in DNS, particularly changes to TTL, Refresh, and Retry values, will take some time to propagate through the system due to caching. You can’t force a remote system to immediately check for a new copy of the data instead of using its cached copy. You can clear the local DNS cache on a machine with the command `ipconfig /flushdns` but this only works for the local machine and doesn’t affect other systems on the network.
3. TTLs
On a client machine, you cannot force TTL to update per se, but you can manually clear the DNS cache which essentially has the same effect. When the DNS cache is cleared, the client machine will have to request new copies of DNS records, which will come with their updated TTLs.
On a Windows client machine, you can use the `ipconfig /flushdns` command to clear the DNS cache:
1. Open a Command Prompt or PowerShell with administrative privileges.
2. Type the command `ipconfig /flushdns` and press Enter. You should receive a message that the DNS Resolver Cache was successfully flushed.
Remember that this only affects the local machine. If there are other systems on the network that have cached DNS records, they will still hold onto them until their TTL expires.
Additionally, if the client queries a DNS server that has the DNS records cached, it will still receive the cached copy with the old TTL. This is why DNS changes take time to propagate through the network, and why adjusting TTL values is important when you’re planning changes to DNS records.