Consul’s DNS interface is not just a simple DNS server; it’s a dynamic, real-time service discovery mechanism that reflects the current state of your services.
Let’s see it in action. Imagine you have a redis service registered in Consul. To query its DNS address, you’d typically use dig or nslookup pointing to your Consul agent’s DNS port (default is 8600).
dig @127.0.0.1 -p 8600 redis.service.consul TXT
This command, when redis is registered, might return something like:
;; ANSWER SECTION:
redis.service.consul. 0 IN TXT "prod" "us-east-1" "db"
The TXT record is useful for metadata. But the real power is in the A record, which resolves to the IP addresses of healthy redis instances.
dig @127.0.0.1 -p 8600 redis.service.consul A
This would return something like:
;; ANSWER SECTION:
redis.service.consul. 0 IN A 10.0.1.10
redis.service.consul. 0 IN A 10.0.1.11
These are the actual IP addresses of healthy redis instances. If an instance fails health checks, its IP address will disappear from the DNS response. This dynamic nature is key.
Consul’s DNS interface works by having each Consul agent run a DNS server. When a DNS query arrives, the agent consults its local catalog of services and health checks. For a query like redis.service.consul, it looks for services named redis within the service.consul domain. It then filters these results based on health status. If the query includes a node name, like redis.service.us-east-1a.consul, it will only return instances running in that specific availability zone. The consul top-level domain is reserved for Consul’s internal use, and service is the default subdomain for service lookups.
You can also query specific instances by appending the instance name, like redis-a.service.consul. This allows for more granular control and routing to individual service instances. The TTL (Time To Live) for DNS records is typically 0, meaning clients should re-query Consul frequently, ensuring they always get the most up-to-date information. This is crucial for dynamic environments where services scale up and down or move between hosts.
The real magic happens when you combine service discovery with Consul’s health checking. Consul agents periodically run health checks defined for each service. These checks can be HTTP endpoints, TCP connections, script executions, or TTL-based checks. Only services passing their health checks are advertised via DNS. This means your applications querying Consul’s DNS will automatically avoid unhealthy instances, preventing cascading failures.
Consider a scenario where you have multiple tiers of a service, say web and api. A web service might query api.service.consul. Consul’s DNS will then resolve api.service.consul to the IPs of only the healthy api instances. If an api instance becomes unhealthy, Consul stops advertising its IP address via DNS, and the web service will seamlessly connect to the remaining healthy api instances. This automatic failover is a cornerstone of resilient microservice architectures.
The ability to query using SRV records is also supported, which can provide port information along with the IP addresses.
dig @127.0.0.1 -p 8600 _http._tcp.web.service.consul SRV
This would return information including the port the web service is listening on, useful for applications that don’t have hardcoded port assumptions.
The most surprising thing about Consul’s DNS interface is that it’s not just a passive resolver. When you make a query, the Consul agent is actively communicating with other agents in the cluster, synchronizing its local view of the service catalog and health status to give you the most accurate, real-time picture. It’s not just looking up DNS records; it’s querying a distributed state.
If you need to resolve services outside of the service.consul domain, or if you want to integrate Consul’s DNS with your existing DNS infrastructure, you can configure Consul as a DNS forwarder. This allows your internal DNS server to forward queries for *.consul to the Consul agent, while Consul forwards other queries to upstream DNS servers.
The next step is often integrating this DNS-based service discovery directly into your applications, where they can automatically resolve service endpoints without manual configuration.