KTOR-9554 Add dns config to OkHttp engine

[fru1tworld]

Subsystem
Client / OkHttp engine

Motivation
KTOR-462 / #1678 / #1924

OkHttp exposes a Dns interface but OkHttpConfig does not surface it as a first-class option. Users currently have to drop down to engine { config { dns(...) } } to inject a custom resolver (DNS-over-HTTPS, host overrides for tests, etc.).

Solution
Add OkHttpConfig.dns: Dns? and apply it in OkHttpEngine.createOkHttpClient right after proxy, mirroring the existing pattern. Defaults to null → Dns.SYSTEM is used as before.

HttpClient(OkHttp) {
    engine {
        dns = Dns { hostname -> listOf(InetAddress.getByName("127.0.0.1")) }
    }
}

Scoped to the OkHttp engine; the broader unified resolver abstraction (KTOR-462) is out of scope here. The same per-engine pattern can follow for Apache5 in a separate PR.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds an optional dns property to OkHttpConfig (with public getters/setters), applies it to the OkHttp client builder when set, and adds tests verifying custom and default DNS behavior.

Changes

OkHttp DNS configuration

Layer / File(s)	Summary
API Exposure ktor-client/ktor-client-okhttp/api/ktor-client-okhttp.api	Adds public accessors getDns(): Lokhttp3/Dns and setDns(Lokhttp3/Dns): void for OkHttpConfig.
Configuration ktor-client/ktor-client-okhttp/jvm/src/io/ktor/client/engine/okhttp/OkHttpConfig.kt	Introduces public var dns: Dns? = null with KDoc documenting behavior and defaulting to Dns.SYSTEM when null.
Core Wiring ktor-client/ktor-client-okhttp/jvm/src/io/ktor/client/engine/okhttp/OkHttpEngine.kt	When config.dns is non-null, calls builder.dns(it) while building the OkHttpClient.
Tests ktor-client/ktor-client-okhttp/jvm/test/io/ktor/client/engine/okhttp/OkHttpEngineTest.kt	Adds two tests: one asserting a provided custom Dns instance is applied to the cached OkHttpClient, another asserting default Dns.SYSTEM is preserved when dns is not set.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description follows the required template with all sections properly filled: Subsystem (Client / OkHttp engine), Motivation (issue references and problem statement), and Solution (implementation approach with code example).
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and accurately summarizes the main change: adding DNS configuration support to the OkHttp engine.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In
`@ktor-client/ktor-client-okhttp/jvm/test/io/ktor/client/engine/okhttp/OkHttpEngineTest.kt`:
- Around line 36-53: Tests create OkHttpEngine instances
(OkHttpEngine(OkHttpConfig()...) in the two test methods but never close them,
leaking dispatcher/executor resources; update both tests (`default dns is
preserved when dns config is not set` and the earlier test using customDns) to
ensure the engine is closed when the test finishes — either wrap engine creation
in a try/finally and call engine.close() in finally or use a resource-safe
construct so that the OkHttpEngine instance is closed after extracting
clientCache and performing assertions; reference the OkHttpEngine constructor,
OkHttpConfig usage and the clientCache reflection access when making the change.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 725984e3-f7fe-47d8-8a1a-b0f5488b7c79

📥 Commits

Reviewing files that changed from the base of the PR and between 295e688 and f6c88d9.

📒 Files selected for processing (4)

• ktor-client/ktor-client-okhttp/api/ktor-client-okhttp.api
• ktor-client/ktor-client-okhttp/jvm/src/io/ktor/client/engine/okhttp/OkHttpConfig.kt
• ktor-client/ktor-client-okhttp/jvm/src/io/ktor/client/engine/okhttp/OkHttpEngine.kt
• ktor-client/ktor-client-okhttp/jvm/test/io/ktor/client/engine/okhttp/OkHttpEngineTest.kt

[bjhham]

Thanks for the contribution!

I wouldn't say it covers the ticket, however. The ticket refers to the address resolution in ktor-network which is used in the CIO engine. I'll create a new ticket with reference to supporting DNS configuration in OkHttp that better reflects the work done here.

[bjhham]

Thanks for the contribution!

I wouldn't say it covers the ticket, however. The ticket refers to the address resolution in ktor-network which is used in the CIO engine. I'll create a new ticket with reference to supporting DNS configuration in OkHttp that better reflects the work done here.

[bjhham]

Here's the new ticket to reflect the update to the OkHttp engine:

KTOR-9554 DNS configuration for OkHttp client engine

[fru1tworld]

Thanks for the review!
Updated to reference KTOR-9554 in the commit/title and addressed the resource-leak feedback