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DNS lookups should not block the event loop #1724
Description
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@n-miles Current maintainer. If you'd like we can further discuss this - a proper spec is in order. Let me know if this is still relevant and we can develop the idea further (I don't see why allowing for less blocking is a bad idea, but would have to dig deeper to give a smarter answer).
The main issue with doing this is that there just isn't a mechanism in Java to do async lookups. Netty has resolvers that will hit DNS servers in an async fashion, but presumably most users want to use the normal OS mechanism. The closest approximation I can come up with is a threadpool doing blocking DNS lookups and just not blocking the calling (Netty) threads.
The APIs are there in Linux and Windows at the OS layer, yes. But I don't believe those APIs exist on all OSes that Java supports, so there is not a mechanism to do that in Java.
If you figure out how to do non-blocking DNS lookups using the OS mechanism on the JVM please tell me how you did it
- added a commit that references this issue
on Apr 12, 2026
The Problem
The default name resolver for AHC is:
This does DNS lookups by using the calling thread (a Netty event loop thread) to do a blocking DNS lookup via the OS mechanism. This is a bad default behavior.
Proposed Solution
I propose that we move the blocking DNS lookups into a separate event loop. There does not appear to be a resolver in Netty that will do this, so we would likely need to create one.
Thoughts?
I am willing to do the development for this if we want this feature.