WebClient DNS caching behavior and TTL inconsistency

Short answer: No. Spring WebClient (via Reactor Netty) does not honor JVM-wide DNS TTL properties such as sun.net.inetaddr.ttl. The transport uses Netty’s DnsNameResolver, which maintains an independent cache. JVM properties only affect java.net.InetAddress lookups, not the Netty resolver that WebClient uses for outbound connections.

Likely explanation vs. confirmed behavior

If you set -Dsun.net.inetaddr.ttl=30 and still see stale IPs, the cause is almost certainly that the Netty resolver is caching addresses with its own TTL policy. In current Reactor Netty versions, the default positive-cache TTL is effectively unbounded (often documented as Integer.MAX_VALUE) unless explicitly configured. Negative caching (NXDOMAIN) defaults to zero seconds. These are resolver-level defaults, not JVM-wide settings. Confirm your exact Reactor Netty version, because resolver defaults and configuration APIs can change across releases.

How to override DNS caching for WebClient only

You can scope the cache policy to a single HttpClient without touching JVM system properties.

Programmatic configuration (all Spring Boot versions)

HttpClient httpClient = HttpClient.create()
    .resolver(spec -> spec
        .queryTimeout(Duration.ofSeconds(5))
        .cache(Duration.ofMinutes(1), Duration.ofSeconds(0)));

WebClient webClient = WebClient.builder()
    .clientConnector(new ReactorClientHttpConnector(httpClient))
    .build();

Here cache(maxTtl, minTtl) controls how long successful lookups are retained. The queryTimeout is a separate DNS query timeout, not a cache TTL.

Spring Boot 3.x property shortcut

If you are using Spring Boot 3.x (Reactor Netty 1.1+), auto-configured WebClient builders can pick up global DNS cache settings via properties such as spring.reactor.netty.http.client.dns-cache-ttl and spring.reactor.netty.http.client.dns-cache-negative-ttl. These apply to auto-configured clients, not to manually constructed HttpClient instances. Verify the exact property names against your Boot version’s reference documentation.

Practical verification

• Enable Netty DNS trace logging with -Dio.netty.resolver.dns.trace=true to see query and cache activity.
• Set a short cache TTL (e.g., 1 second), resolve a test hostname, change its A record, and confirm the new IP is used after the TTL expires without restarting the JVM.
• Inspect the resolver configuration in a debugger or log to ensure the cache TTL was applied to the HttpClient actually used by WebClient.

One missing detail

Are you running Spring Boot 3.x with auto-configured WebClient, or are you constructing HttpClient manually? That determines whether the property shortcut is available or whether you must use the programmatic resolver spec.