DNS Proxying

While Kubernetes provides DNS resolution for Kubernetes Services out of the box, any custom ServiceEntrys will not be recognized. With this feature, ServiceEntry addresses can be resolved without requiring custom configuration of a DNS server. For Kubernetes Services, the DNS response will be the same, but with reduced load on kube-dns and increased performance.

This functionality is also available for services running outside of Kubernetes. This means that all internal services can be resolved without clunky workarounds to expose Kubernetes DNS entries outside of the cluster.

This feature is not currently enabled by default. To enable it, install Istio with the following settings:

This can also be enabled on a per-pod basis with the .

To try out the DNS capture, first setup a for some external service:

Without the DNS capture, a request to address.internal would likely fail to resolve. Once this is enabled, you should instead get a response back based on the configured address:

In the above example, you had a predefined IP address for the service to which you sent the request. However, it’s common to access external services that do not have stable addresses, and instead rely on DNS. In this case, the DNS proxy will not have enough information to return a response, and will need to forward DNS requests upstream.

This is especially problematic with TCP traffic. Unlike HTTP requests, which are routed based on Host headers, TCP carries much less information; you can only route on the destination IP and port number. Because you don’t have a stable IP for the backend, you cannot route based on that either, leaving only port number, which leads to conflicts when multiple ServiceEntrys for TCP services share the same port.

When this feature is enabled, the DNS response will include a distinct and automatically assigned address for each ServiceEntry. The proxy is then configured to match requests to this IP address, and forward the request to the corresponding ServiceEntry.

Because this feature modifies DNS responses, it may not be compatible with all applications.

To try this out, configure another ServiceEntry:

Now, send a request: