10 DNS Scavenging Best Practices

DNS scavenging is the process of removing unused DNS records from a DNS zone. This is done to reduce the size of the DNS zone and to prevent DNS records from being used to attack the DNS system.

DNS scavenging is a best practice for DNS administrators. It helps to keep the DNS system clean and running efficiently.

1. Set the Scavenging Period

The scavenging period is the amount of time between when a DNS record is created and when it’s eligible to be scavenged. By default, this period is seven days.

This setting is important because it determines how long you have to manually delete a DNS record before it’s automatically deleted by the scavenger. If you set the scavenging period to seven days and accidentally create a DNS record that you don’t want, you have seven days to delete it before it’s gone forever.

You can change the scavenging period by opening the DNS Manager, right-clicking on the DNS server, and selecting Properties. In the Properties window, select the Advanced tab and then click the Change button next to the Scavenging Period setting.

2. Enable DNS Aging and Scavenging on all zones

When you enable DNS scavenging on a zone, the DNS server will automatically remove any DNS records that have not been used for a certain period of time. This is known as “aging” the DNS record.

The problem is that if you only enable DNS scavenging on a small subset of your zones, then you may end up with “stale” DNS records in other zones. These stale records can cause all sorts of problems, such as:

– Clients being unable to resolve hostnames
– Emails being delivered to the wrong recipient
– Websites being inaccessible

Therefore, it’s important to ensure that DNS scavenging is enabled on all zones. This way, you can be confident that all DNS records are accurate and up-to-date.

3. Use a single DNS server for scavenging

When you have multiple DNS servers, each server will have its own scavenging process. This can lead to inconsistencies in your DNS data, which can cause problems.

It’s much simpler and more reliable to use a single DNS server for scavenging. This way, you only have to configure the scavenging process once, and you can be confident that all of your DNS servers are using the same data.

4. Configure your DHCP servers to update client records dynamically

When a DHCP server leases an IP address to a client, it will also attempt to update the DNS record for that client. If the DHCP server is unable to update the DNS record, the lease will still be valid and the client will continue to use the IP address, but the DNS record will not be updated.

Over time, this can lead to a large number of stale DNS records, which can cause problems when you try to scavenge them. By configuring your DHCP servers to update DNS records dynamically, you can ensure that all of your DNS records are up-to-date, which will make scavenging much easier.

5. Monitor your DNS environment regularly

If you’re not monitoring your DNS environment, you won’t know when records have been scavenged. This can lead to a number of problems, such as:

– Users not being able to access resources they need
– Emails not being delivered
– Websites not loading

All of these problems can be avoided by simply monitoring your DNS environment on a regular basis. There are a number of tools available that can help you do this, such as Microsoft’s DNS Management Pack.

Additionally, it’s a good idea to configure your DNS servers to send scavenging logs to a central location. This will allow you to track when records are scavenged and quickly identify any problems that may arise.

6. Review event logs periodically

Event logs can give you a wealth of information about what’s going on with your DNS servers. By default, the DNS Server service will log all scavenging-related events to the Application log. You can also enable debug logging, which will provide even more information.

Reviewing these logs regularly will help you catch any potential problems with your scavenging configuration. For example, if you see that records are being deleted when they shouldn’t be, you can adjust your scavenging settings accordingly.

Additionally, event logs can be helpful in troubleshooting scavenging issues. If you see that records are not being deleted when they should be, checking the event logs can help you identify the problem.

7. Clean up stale resource records manually

When you enable DNS scavenging on a zone, the DNS server will automatically delete any resource records that it thinks are stale. However, there’s always a chance that the DNS server will delete a resource record that’s still in use.

To avoid this, it’s best to clean up stale resource records manually. That way, you can be sure that only the resource records that you want to delete are actually deleted.

To do this, you can use the dnscmd command-line tool. To find all of the stale resource records in a zone, you can use the following command:

dnscmd /zoneinfo /v

This will list all of the resource records in the zone, along with their time-to-live (TTL) values. Any resource records with a TTL of 0 are stale and can be safely deleted.

8. Disable dynamic updates for static records

When dynamic updates are enabled, any client can update any record in the zone, regardless of whether the record is static or not. This means that a malicious user could potentially change the IP address of a static record to point to a malicious server.

To prevent this from happening, you should disable dynamic updates for static records. This will ensure that only the administrator can update static records, and that all other clients are unable to make any changes.

It’s also a good idea to enable scavenging for all zones, as this will help to keep your DNS records clean and up-to-date.

9. Create reverse lookup zones for IPv4 and IPv6 addresses

Reverse lookup zones are used to resolve IP addresses to hostnames, and they’re essential for proper DNS scavenging. Without them, the DNS server won’t be able to properly identify which records are no longer in use and should be scavenged.

Creating reverse lookup zones is a simple process, and it’s well worth the effort to ensure that your DNS scavenging is effective.

10. Don’t use aging and scavenging on Active Directory-integrated zones

When a DNS server that is configured to use an Active Directory-integrated zone receives a dynamic update for a resource record, it does not update the timestamp of the associated resource record in the zone. As a result, if aging and scavenging are enabled on the zone, the DNS server will incorrectly determine that the resource record is stale and delete it.

To avoid this problem, make sure that you do not enable aging and scavenging on Active Directory-integrated zones. If you have already done so, you can disable scavenging by setting the NoRefreshInterval and RefreshInterval registry entries to the same value.