Monitoring Load Balancer DNS Records

DNS load balancers respond to queries with one or more of the DNS records that they are configured to split the load between. This is done to distribute the load between multiple servers, provide redundancy, and reduce latency.

DNS Check supports monitoring A record (IPv4), AAAA record (IPv6), and CNAME record (IPv4 and IPv6) based load balancers. It notifies you if a record becomes unresolvable or starts resolving to the wrong addresses.

This functionality is available for paid accounts only.

Load Balancer Logic

DNS Check compares the list of IP addresses or domain names that you enter to the records returned in response to a DNS query. If the records returned are either a subset or an exact match of those that were entered, the check passes.

If any of the records returned in response to the DNS query were not entered into DNS Check, the check fails.

The order in which records are entered into DNS Check and returned in response to queries does not matter.

Fields

Here are the fields that make up a DNS load balancer record:

Field	Description	Example
Name	A fully qualified domain name (FQDN).	www.dnscheck.co.
Type	The DNS record type. Set to "A" for IPv4, "AAAA" for IPv6, or "CNAME".	A
Addresses	A comma or newline-delimited set of IP addresses, CIDR ranges (e.g., 104.16.0.0/13), or domain names. A minimum of 1 and a maximum of 30 entries are supported per monitored load balancer. Each CIDR range counts as one entry.	104.131.72.189, 143.198.237.244, 52.48.61.155

DNS Zone File Examples

Here's an example of how an A record load balancer could look in a DNS zone file:

; Name                   Type  Address
nameservers.dnscheck.co. A     104.131.72.189
nameservers.dnscheck.co. A     143.198.237.244
nameservers.dnscheck.co. A     52.48.61.155

If the above zone file is imported into the BIND DNS server, then it will respond to queries for the nameservers.dnscheck.co DNS record with all three records shown above, sorted in random order.

DNS servers can also be configured to use different load balancing logic. For example, a load balancer might be configured only to return records that point to healthy servers, randomly return one of its records for each request, or deterministically return records based on the querying IP address' geographic location.

CIDR Range Monitoring

Some providers, such as Cloudflare, serve content from large IP pools. Instead of listing every possible IP address, you can use CIDR notation to monitor that DNS responses fall within the expected range.

For example, Cloudflare publishes its IP ranges at cloudflare.com/ips. You can enter these ranges as load balancer addresses to monitor a Cloudflare-proxied domain:

If a DNS query for the domain returns IP address(es) within any of the specified CIDR ranges, the check passes. If it returns an IP outside the ranges, the check fails.

You can paste IP ranges directly from a provider's website. For example, you can copy the ranges listed at cloudflare.com/ips and paste them into the addresses field as a newline-delimited list. The newlines are automatically converted to commas when the record is saved.

Additional Resources

• HOWTO - Configure Load Balancing - a section from Pro DNS and BIND that discusses load balancers in the BIND name server.
• Choosing a Routing Policy - documentation on some of the DNS based load balancing options available in Amazon Route 53.