[BUG] com.azure.core.http.HttpClient resolves proxy's DNS name only once and caches the IP address forever regardless of TTL.

[lukolszewski]

Describe the bug
com.azure.core.http.HttpClient when built with proxy name set, appears to perform only one DNS to IP address resolution during the build(). Later only the IP address is used for the proxy disregarding DNS TTL. When the DNS IP of the proxy changes the client still tries to access the old one causing connectivity issues unless it is destroyed and recreated.

To Reproduce
Create a com.azure.core.http.HttpClient, set proxy configuration using DNS name that resolves to one or more IP addresses with DNS A records. Build the client and use it, then change one of the proxy's DNS records to point to another IP. It will still try to connect to the old proxy IP.

Code Snippet

From azure-sdk-for-java/blob/main/sdk/core/azure-core-http-netty/src/main/java/com/azure/core/http/netty/NettyAsyncHttpClientBuilder.java line starting at 264

nettyHttpClient = nettyHttpClient.doOnChannelInit((connectionObserver, channel, socketAddress) -> { if (shouldApplyProxy(socketAddress, nonProxyHostsPattern)) { channel.pipeline() .addFirst(NettyPipeline.ProxyHandler, new HttpProxyHandler(AddressUtils.replaceWithResolved(buildProxyOptions.getAddress()), handler, proxyChallengeHolder)); } });

Expected behavior
Repeated address resolutions either in accordance with the DNS TTL, or some configuration variable.

Setup (please complete the following information):

• OS: [e.g. iOS] not OS specific
• IDE: [e.g. IntelliJ] nod IDE specific
• Library/Libraries: [e.g. com.azure:azure-core:1.44.1 (groupId:artifactId:version)] (also present in latest)
• Java version: [e.g. 8] 11
• App Server/Environment: [e.g. Tomcat, WildFly, Azure Function, Apache Spark, Databricks, IDE plugin or anything special] Jenkins
• Frameworks: [e.g. Spring Boot, Micronaut, Quarkus, etc] N/A

Additional context
Another way to reproduce in the actual environment is:
Use software that uses azure-sdk-for-java for connectivity to azure that uses HttpClient with a proxy configuration. Change the DNS IP of the proxy. Observe communication being broken.

For example: There is a Jenkins plugin called azure-ad-plugin. That plugin uses com.azure.core.http.HttpClient as a deep dependency building it on server start. After proxy's DNS IP (or one of its IPs) changes it still attempts to connect to the old IP until entire server is restarted logging io.netty.channel.ConnectTimeoutException

Information Checklist
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[joshfree]

Thank you for reporting this issue, @lukolszewski. @alzimmermsft could you take a look at fixing this TTL problem?

[alzimmermsft]

Thanks for filing this issue @lukolszewski.

Have a quick question before I dive further into this. Does the proxy that you're using have an authentication requirement? If it does, could you check what happens if that is disabled for a test run that replicates the same flow when using proxy authentication.