Kubernetes is a powerful container orchestration platform that has become increasingly popular for managing containerized applications. One of the key features of Kubernetes is its ability to manage services, which are used to expose applications running inside a cluster to other components within or outside the cluster.
One such feature is ExternalName, which enables a Kubernetes service to act as a proxy to an external resource outside the Kubernetes cluster. This feature is particularly useful for accessing resources such as databases, messaging systems, or other external services that are not hosted within the Kubernetes cluster.
However, while ExternalName is a beneficial feature, it can also lead to a hidden pitfall for DNS resolution. Let's dive deeper into this pitfall and explore how to avoid it.
The Pitfall
Suppose you have a Kubernetes cluster with a service named "default" configured as an ExternalName pointing to www.google.com. If you attempt to access any service in the default namespace without fully qualifying the domain name, the DNS resolution will start failing.
For instance, if you have a service named "serviceA" in the namespace "default," then "serviceA.default" will start resolving to www.google.com instead of resolving to "serviceA.default.svc.cluster.local." This happens because Kubernetes creates a CNAME record for "*.default.default.svc.cluster.local" pointing to the ExternalName resource's DNS name, which in this case is "www.google.com."
This means that any unqualified DNS query for services in the "default" namespace will be redirected to "www.google.com" by default. This can cause unexpected behavior in your applications and make it difficult to diagnose DNS issues.
The Solution
To avoid this pitfall, it is recommended to use a different name for the ExternalName service than "default." This will prevent any unqualified DNS queries from being redirected to the ExternalName resource's DNS name.
Additionally, it's important to note that this pitfall can occur in any namespace, not just the default namespace. So, when configuring ExternalName services, always choose a unique name for the service to avoid any potential conflicts.
Another solution to avoid this pitfall is to fully qualify the domain name when accessing services in the same namespace. This will ensure that the DNS resolution is correct and that your applications can communicate with each other without any issues.
Conclusion
ExternalName is a useful feature in Kubernetes for accessing resources outside the cluster. However, it can cause unexpected behavior for DNS resolution if not used carefully.
