DNS Load Balancer

Published April 5, 2023 | Last modified March 20, 2025

Objective

This guide provides instructions on how to set up DNS load balancers and apply them to your DNS zones using F5® Distributed Cloud Services. A DNS load balancer is an ingress controller for the DNS queries made to your DNS servers. The DNS load balancer receives the requests and answers with an IP address from a pool of members based on the configured load balancing rules.

Using this service, you can set up DNS load balancer, add load balancer record, load balancing rule, and pools for your DNS zones.

Overview

A DNS load balancer is composed of load balancing rules that specify what should be returned to requesting DNS clients. You can set these rules to take into account the Geo-Location of the request and send an answer pointing to an IP endpoint member of a pool.

Rules are ordered using a score. If a request matches 2 (or more) rules, the rule with the higher score has precedence. The rule with the lower score is considered as “catch all” for DNS requests that do not match any rule (for example, client belongs to a country not matched in any rule).

The DNS load balancer pools can have one or more members (IP endpoints). Healthchecks can be performed against the endpoints, to exclude members who are down or unreachable. Multiple load balancing algorithms are available for the load balancer pools.

A DNS load balancer must be attached to a zone to activate the load balancer. This is achieved by creating a DNS Load Balancer record inside a zone, and attaching the DNS load balancer to that record.

Prerequisites

The following prerequisites apply:

A Distributed Cloud Services Account. If you do not have an account, see Create an Account.
A DNS domain for your web application. Obtain a domain from the Internet domain registrar.
Name servers for managing your DNS records.
Add the IP addresses mentioned here to your firewall or ACL allow list:

Configuration

Creating DNS load balancer includes configuring or creating Geo-Location sets, creating load balancer pools, and optionally configuring health checks.

You can create DNS load balancer as part of DNS zone creation or separately create and apply it to an existing DNS zone configuration. This example shows creating DNS load balancer separately and applying to an existing DNS zone. Log into the F5 Distributed Cloud Console and perform the instructions shown in the following chapters.

Create Geo-Location Sets

A Geo-Location Set is a grouping of geographical locations identified using either a selector expression or a predefined global Geolocations. Do the following to create a Geo-Location Set:

Step 1: Navigate to DNS Management, and start adding a Geo-Location Set.

Click DNS Management service on the Console home page.

Figure: Navigate to DNS Management

Select Manage > DNS Load Balancer Management > Geo-Location Sets in the primary navigation menu located on the left side of the page.
Select Add Geo-Location Set. Enter a name in the Name field in the metadata section. Optionally, set labels and add a description.

Step 2: Configure Geolocation properties.

Configure the Geolocation Label Selector as per the following guidelines:

Select Global Geolocation to include all predefined Geolocations. This is also populated by default and will include any location.
Select Geolocation Selector to group locations using a selector expression. Select Add Label in the Selector Expression field and do the following:
- Select a key from the drop-down for the Select Key field.
- Choose an expression from the drop-down for the Selector Operator field.
- Start typing a value in the Assign Values box. Select a value from the drop-down.

Figure: Set GeoLocation Selector

Note: To match several countries or cities, do the following:

Select the IN operator.

Select the first country or city of your choice and clear the search.

Find the second country or city of your choice and select it.

Repeat for the other countries or cities.

Select Apply.

Click Save and Exit.

Create Health Check

Health check performs periodic inspection of health for each pool member. Do the following to create health check:

Step 1: Start creating a health check object.

Select Manage > DNS Load Balancer Management > DNS Load Balancer Health Checks in the DNS management service page.
Select Add DNS Load Balancer Health Check.
Enter a name in the Name field in the metadata section.
Optionally, set labels and add a description for the health check.

Step 2: Configure health check and complete creating the health check.

Select an option from the drop-down for Health Check Type and configure as per the following guidelines:

In case of HTTP or HTTPS or TCP or UDP health check choice, do the following:
- Enter a value for the Send String field to send this string in the request to the target.
- Enter a value for the Receive String field to receive this string in the response from the target. This response indicates that the member is healthy.
- Enter a value for the Health Check Port to set the port for performing health checks. You can also use the Common Values option and select from a predefined list of ports.
- Select Save and Exit.
In case of ICMP health check choice, select the ICMP health check and select Save and Exit.

This example creates HTTP health check.

Figure: HTTP Health Check

Note: The health monitor interval is 30 seconds and the timeout value is 90 seconds (i.e. 3x30 seconds). If an endpoint is identified as down, it will be marked as up as soon as it starts to respond correctly again. Configuring health monitor interval and timeout values is currently not supported.

Create DNS Load Balancer Pool

Do the following to create a group of members:

Step 1: Start creating a pool.

Select Manage > DNS Load Balancer Management > DNS Load Balancer Pools in the DNS management service page.
Select Add DNS Load Balancer Pool.
Enter a name in the Name field in the metadata section.
Optionally, set labels and add a description for your pool.

Step 2: Configure pool members.

Select an option for the Pool Type field and use Add Item button to open pool member configuration page.
Enter an IP address in the Public IP field.
Enter a name for this pool.
Enter values for the Load Balancing Ratio and Load Balancing Priority fields. The values 1 and 10 are populated by default for these fields respectively.

Note: The ratio and priority determine how the load balancing happens when the load balancing mode is based on the ratio and priority. The mode is set in subsequent steps.

Figure: DNS LB Pool Endpoint

Select Apply.

Note: You can use Add Item button to add more than one member.

Step 3: Configure health check.

Select Enable Health Check for the DNS Load Balancer Health Check field.
Select a health check object from the drop-down of the Enable Health Check field. You can also use the Add Item option in the drop-down to create a new health check object and apply to pool type settings.

Note: The healcheck is disabled by default and in this mode, all pool members are assumed to be healthy.

Optionally, enter a value for the Maximum Answers field to set the maximum number of resource records included in the responses.

Step 4: Configure load balancing mode and TTL.

The Static Persist load balancing method uses the persist mask, with the source IP address of the Local Domain Name Sever (LDNS), in a deterministic algorithm to send requests to a specific pool member. If the DNS resolver passes ECS (EDNS-Client-Subnet) information, then a hash of it will be used, to send the client to the same pool member.

Select a choice from the drop-down for the Load Balancing Mode field as per the following guidelines:
- Select Round-Robin to enable equal distribution of load among the pool members.
- Select Ratio-Member to distribute requests based on the ratio set to pool members.
- Select Statc-Persist to distribute requests persistently to specific members. Using this, when a request is answered by a member, subsequent such requests are persistently forwarded to that member.
- Select Priority to distribute requests based on the priority assigned to pool members. Lower number for the priority field for a member means higher priority for that member.

Figure: DNS Load Balancer Pool Settings

Select a Time-To-Live (TTL) choice for the TTL fied as per the following guidelines:
- Select Use RRSet TTL to use the TTL value set in the load balancer record associated with this pool.
- Select TTL to enforce a custom TTL and enter a value in seconds in the TTL(s) field. It is recommended you use a lower value for this setting.

Step 5: Complete creating the load balancer pool.

Select Save and Exit to complete creating the pool.

Create DNS Load Balancer

Do the following to create DNS load balancer:

Step 1: Start creating a DNS load balancer.

Select Manage > DNS Load Balancer Management > DNS Load Balancers in the DNS management service page.
Select Add DNS Load Balancer.
Enter a name in the Name field in the metadata section.
Optionally, set labels and add a description for your load balancer.

Step 2: Configure record type and load balancing rules.

Select an option for the Record Type field.
Select Configure for the Load Balancing Rules field to open the load balancing rules page and use the + Add Item button on the page to start adding a rule.
Select Client Selection drop-down option for the Geo Location Selection, and configure as per the following guidelines:
- Select Geo Locations by label selector and Select Add Label in the Selector Expression field. Select a key from the drop-down for the Select Key field, choose an expression from the drop-down for the Selector Operator field. Start typing a value in the Assign Values box and select a value from the drop-down.
- Select Geo Location Set selector and select a Geo-Location set from the drop-down of the selector field. You can also use the Add Item button to create a new Geo-Location set and apply it to the rule.
- Select ASN List and enter an ASN value in the AS Numbers field. You can add more than one ASN using Add Item option and also drag and drop the row entries to change the ASN match order.
- Select BGP ASN Sets and select or create new BGP ASN sets from the drop-down. You can add more than one BGP ASN set using the Add Item option and also drag and drop the row entries to change the ASN match order.
- Select IP Prefix List and select + Add Item button for IPv4 Prefix List and IPv6 Prefix List sections to enter prefixes.
- Select IP Prefix Sets and select or create new IP Prefix sets from the drop-down. You can add more than one IP Prefix set using the Add Item option and also drag and drop the row entries to change the IP Prefix match order.
Select Use DNS Load Balancer pool for the Action field and select a pool from the drop-down.
Select Use DNS Load Balancer pool drop-down menu. You can also use the Add Item button to create a new pool and apply it to the rule.
Optionally, enter a Score for this rule. When a number of rules match a specific query, rule with the highest score is used to process the query.
Select Apply to add the rule to the list of rules.

Figure: DNS Load Balancer Rule Settings

Note: You can use Add Item button to add more than one rule.

Select Apply button to add rules to the load balancer configuration.

Step 3: Configure response cache settings.

Select an option for the Response Cache Parameters Choice and configure as per the following guidelines:

Select Disable Response Cache to disable caching of response.
Select Default Response Cache Parameters to apply default settings for response caching.
Select Response Cache Parameters to apply custom caching and do the following:
- Enter a value for Length of IPv4 CIDR masks to group IPv4 clients into subnets.
- Enter a value for Length of IPv6 CIDR masks to group IPv6 clients into subnets.
- Enter a value for TTL(s) to set TTL value for response cache. The cache gets cleared and responses are recomputed after the expiry of TTL.

Note: Exercise caution when using IP prefix load balancing rules in conjunction with the Response Cache. For example, if prefix list has CIDR with /28 subnet mask and response cache parameters has subnet mask as /24 then the response cache would be used even if the src prefix doesn't match the rule.

Figure: DNS Load Balancer Settings

Optionally. add a fallback pool by selecting a pool from the Fallback Pool field. If no pool matches, then this pool will be the fallback pool.

Step 4: Complete creating the load balancer.

Select Save and Exit to complete creating the pool.

Add DNS Load Balancer to DNS Zone

After creating a DNS load balancer, it is required that you add the load balancer record to a DNS zone.

Note: Subdomains are not supported for DNS load balancer record names. In case you want to use a subdomain, create another DNS zone with the subdomain included in the zone name and add the load balancer record to that zone.

Step 1: Navigate to zone management and start adding a zone.

Select DNS Zone Management option in the primary navigation menu located on the left side of the page.
Select ... > Manage Configuration for your zone object among the list of displayed zones.
Select Edit Configuration located on the top right in the manage configuration page.

Step 2: Add load balancer record for the resource records.

Go to Resource Record Sets section and click Add Item. The resource record sets configuration form opens.
Enter a value for the Time to live field.
Select DNS Load Balancer for the Record Set field, enter your record name in the Record Name field, and set the fields as per your record type selection.
Select a DNS load balancer from the drop-down for the DNS Load Balancer Record field. You can also use the Add Item button to create a new load balancer and apply.
Select Apply to add the load balancer record to the zone configuration.
Select Save and Exit in the main zone configuration form to save changes to the zone configuration.

Note: You can inspect DNS load balancer status in the Overview > DNS Load Balancers page. However, at any given time, the health status value displayed is valid only for pools currently associated with an active DNS load balancer applied to a DNS zone.

You can use the ... > Disable Object in the Actions column of any DNS load balancer, pool, and healthcheck objects to disable them in case you want to perform troubleshooting activities. Also, a disabled object can be enabled using the ... > Enable Object option.

Monitor DNS Load Balancer

In the DNS Management service, click Overview > DNS Load Balancer to see a list of DNS load balancers.

Figure: DNS Load Balancers

Click the name of the load balancer you want to observe.

Figure: DNS Zone Dashboard

Click the Refresh button next in the upper right to update the contents of the dashboard.
The Health section shows the overall health of the load balancer.
The Pools Overview section provides overview health information for the pools in the load balancer.
The Pools list provides some details for each pool.
- Enter a string into the Search field to only show pool names containing that string.
- Click the gear icon ( ⚙ ) to change the columns shown in the pools table.
- Click on a column name to sort the table by that column.
- Hover over a column name to see the drag icon ( ⠿ ). Click and drag the icon to move the column.
- Click on a pool name to get health information for individual members (IP endpoints) in that pool.
  Figure: Pool Member Health

Note: Health checks are triggered every 30s.

A pool member will be marked as unhealthy only when the third check fails.

A pool member will be marked as healthy upon the first successful health check. In both cases, visibility will be reflected in the Console 60 to 90 seconds later.

See the following table for expected time taken for pool events:

Member	Remove from Pool	Added to Pool	Console Reflection
Unhealthy	1m 30s	N/A	2m 30s to 3m
Healthy	N/A	30s	2m to 2m 30s