JVM의 DNS Cache 설정에 따른 갱신시간 테스트

오규석

Published in

NAVER Pay Dev Blog

56 min readMar 7, 2023

안녕하세요. 네이버파이낸셜 카드결제개발팀 오규석입니다.

이번 포스팅에서는 JVM 에서의 DNS Cache가 무엇인지, 그리고 DNS Cache 설정값을 언제 어떤 상황에서 변경해야 하는지에 대해 테스트 및 리서치를 통해 알아본 결과를 공유하고자 합니다.

1. 테스트 동기

네이버파이낸셜에서 어떠한 장애 상황이 발생하더라도 카드결제 관련 서비스를 안정적으로 제공하기 위해 기존 서버 이중화를 넘어서는 DR(Disaster Recovery) 시스템 구축을 진행 중입니다. 이 때 DR의 핵심 목적인 High Availability 및 High Reliability를 달성하고자 GSLB를 도입하게 되었습니다.

GSLB는 Global Server Load Balancing의 약자로써 말 그대로 여러 리전에 분산된 서버들에 대한 로드밸런서로써의 역할을 수행하게 됩니다. GSLB는 통상시에는 DNS와 유사하게 GSLB의 도메인으로 요청한 주소에 대한 IP 를 로드밸런싱 기능을 통해 제공합니다. 하지만 일반적인 DNS와는 달리 GSLB는 각 서버에 대한 헬스체크 기능을 수행하기 때문에 GSLB에 어떤 요청을 할 때 장애가 발생하거나 기타 이유로 가용하지 않는 서버의 IP는 제외하고 가용한 서버의 IP만을 제공한다는 차이점이 존재합니다.

이 때 GSLB의 특징에서 언급한 바와 같이 GSLB는 DNS 기반으로 동작하기 때문에 아무리 GSLB가 헬스체크를 통해 가용한 서버의 IP만을 제공한다고 할지라도 정작 해당 도메인에 요청을 보내고자 하는 JVM에 위치한 서버 또는 클라이언트 애플리케이션이 기존 도메인에 매핑된 가용하지 않는 서버 IP를 사용하게 된다면 정작 구축해놓은 GSLB 환경이 아무 소용이 없을 것입니다.

이에 따라 본 포스팅에서는 통상적으로 서버 애플리케이션 개발에 사용되는 Spring Boot 및 Java 환경에서 OS 또는 DNS에서 도메인에 해당하는 IP 매핑값이 반영되는 시간이 어느 정도일지 알아보기 위해 테스트를 수행하였으며, 추가적으로 실험되지 않은 환경에서는 어떻게 작동하는지 리서치한 결과를 공유드리고자 합니다.

2. 테스트 환경

테스트 시 사용한 환경은 다음과 같으며, JDK 설정은 추가 설정이 없는 기본값을 따랐습니다.

Macbook Pro(16형, 2021년 모델), Apple M1 Pro, 32GB RAM
Mac OS
OpenJDK 1.8, Hotspot (temurin-8.jdk)

3. 테스트 절차

테스트는 통상적인 Java 8 버전 JVM 및 Spring Boot 환경하에서의 수행을 확인하기 위해 Local 맥북 환경하에서 다음 과정에 따라 테스트를 수행하였습니다.

Hosts 파일에 특정 Domain Name에 대한 IP를 최초 127.0.0.1로 설정
테스트 프로그램 시작
테스트 프로그램상에서 최초 IP 또는 변경된 IP가 확인되면 hosts 파일 내 IP를 127.0.0.2 ~ 127.0.0.4 까지 점진적으로 증가시키며 JVM상에서 변경된 IP가 적용되는지 확인

4. 테스트 코드

테스트 시 사용된 코드는 다음과 같습니다.

DnsTestApplication.java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DnsTestApplication {
    public static void main(String[] args) {
        SpringApplication.run(DnsTestApplication.class, args);
    }
}

DnsTestListener.java

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.concurrent.Executors;
import javax.annotation.PreDestroy;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.context.event.ApplicationStartedEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.stereotype.Component;
import lombok.extern.slf4j.Slf4j;

@Slf4j
@Component
public class DnsTestListener implements ApplicationListener<ApplicationStartedEvent> {
    @Value("${url.dns.test}")
    private String dnsAddress;
    private final DnsTestRunner dnsTestRunner = new DnsTestRunner();
    private boolean isRunning = true;

    @Override
    public void onApplicationEvent(ApplicationStartedEvent event) {
        Executors.newSingleThreadExecutor().execute(dnsTestRunner);
    }

    @PreDestroy
    public void destroy() {
        isRunning = false;
    }

    public class DnsTestRunner implements Runnable {
        @Override
        public void run() {
            while(isRunning) {
                log.info("--------------------------------------------------------");
                log.info("DNS Test ADDR: {}", dnsAddress);
                try {
                    log.info("Result IP: {}", InetAddress.getByName(dnsAddress));
                } catch (UnknownHostException e) {
                    log.error("DNS \"{}\" is UnknownHost", dnsAddress);
                }

                // 3초 sleep
                try {
                    Thread.sleep(3000L);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.7.5</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.dns.test</groupId>
    <artifactId>dns-test</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>dns-test</name>
    <description>dns-test</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.24</version>
        </dependency>
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.12.0</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

application.properties

server.port=28080
url.dns.test=www.naverdnstesturlforgslb.com

5. 기본 JVM 설정에서 테스트 결과

테스트 결과 로그는 다음과 같습니다.

테스트 결과의 로그시간과 IP를 확인해본 결과 약 30초 간격으로 변경된 DNS가 JVM에 반영되는 것을 확인하였습니다.

2022-10-24 18:11:03.151  INFO 54568 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 28080 (http)
2022-10-24 18:11:03.156  INFO 54568 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2022-10-24 18:11:03.156  INFO 54568 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.68]
2022-10-24 18:11:03.249  INFO 54568 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2022-10-24 18:11:03.249  INFO 54568 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1148 ms
2022-10-24 18:11:03.628  INFO 54568 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 28080 (http) with context path ''
2022-10-24 18:11:03.642  INFO 54568 --- [           main] c.d.t.dnstest.DnsTestApplication     : Started DnsTestApplication in 2.033 seconds (JVM running for 2.677)
2022-10-24 18:11:03.643  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:03.643  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:03.689  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:06.695  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:06.695  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:06.695  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:09.699  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:09.699  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:09.699  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:12.703  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:12.704  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:12.704  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:15.711  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:15.722  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:15.722  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:18.723  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:18.724  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:18.724  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:21.728  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:21.728  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:21.728  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:24.733  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:24.733  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:24.733  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:27.738  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:27.740  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:27.740  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:30.742  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:30.744  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:30.744  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-24 18:11:33.748  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:33.752  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:33.767  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:36.769  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:36.771  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:36.772  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:39.774  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:39.776  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:39.777  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:42.782  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:42.783  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:42.783  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:45.786  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:45.786  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:45.787  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:48.793  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:48.799  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:48.809  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:51.815  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:51.818  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:51.818  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:54.821  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:54.822  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:54.823  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:11:57.827  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:11:57.828  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:11:57.828  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:12:00.833  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:00.833  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:00.833  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-24 18:12:03.839  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:03.840  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:03.859  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:06.860  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:06.861  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:06.861  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:09.862  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:09.863  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:09.863  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:12.865  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:12.868  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:12.868  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:15.874  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:15.875  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:15.876  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:18.881  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:18.881  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:18.881  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:21.884  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:21.885  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:21.885  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:24.891  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:24.899  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:24.902  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:27.904  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:27.905  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:27.905  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:30.908  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:30.908  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:30.908  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.3
2022-10-24 18:12:33.914  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:33.919  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:33.972  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.4
2022-10-24 18:12:36.973  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:36.974  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:36.974  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.4
2022-10-24 18:12:39.979  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-24 18:12:39.979  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-24 18:12:39.979  INFO 54568 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.4
Process finished with exit code 130 (interrupted by signal 2: SIGINT)

이에 따라 앞선 테스트의 결과를 정리하자면 다음과 같았습니다.

별도의 설정을 하지 않은 OpenJDK 8의 JVM 환경에서는 약 30초마다 JVM의 DNS 캐시를 변경하는 것으로 확인되었습니다.
다만 AWS 내 JVM TTL 설정 관련 문서 에서는 JVM에 대한 추가 설정 및 Security Manager 의 설정 여부에 따라 DNS 갱신시간(TTL)이 달라질 수 있거나 심지어 갱신 없이 무제한이 될 수도 있기에 명시적으로 설정하는 것을 권장하였습니다.
기타 여러 레퍼런스를 확인하여 보니 JVM의 버전에 따라 10초~30초로 버전 및 운영체제 종류 등의 환경에 따라서도 변화하는 것으로 보였습니다.

6. 추가 실험

앞선 실험 결과에서 언급하였듯이 여러 레퍼런스에서는 JVM TTL을 명시적으로 설정하는 방법에 대해 설명된 부분이 많았습니다. 이에 따라 추가적으로 JVM TTL을 직접 조정하였을 때의 결과는 어떻게 될것인지 추가 테스트를 기획하였습니다.

이에 따라 DNS 캐시의 TTL 시간을 10초 이내로 조정하기 위해 networkaddress.cache.ttl 및 networkaddress.cache.negative.ttl 프로퍼티를 수정하여 JVM 상의 Positive Cache TTL 및 Negative Cache TTL을 1초로 변경 후 테스트를 수행하였지만, 실제 Cache내 만료 및 갱신시간은 그대로 30초로 변화가 없었습니다.

상세 추가 테스트 결과 로그는 아래와 같습니다.

2022-10-25 14:36:58.871  INFO 19611 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 28080 (http)
2022-10-25 14:36:58.875  INFO 19611 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2022-10-25 14:36:58.876  INFO 19611 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.68]
2022-10-25 14:36:58.998  INFO 19611 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2022-10-25 14:36:58.999  INFO 19611 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1440 ms
2022-10-25 14:36:59.464  INFO 19611 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 28080 (http) with context path ''
2022-10-25 14:36:59.477  INFO 19611 --- [           main] c.d.t.dnstest.DnsTestApplication     : Started DnsTestApplication in 2.345 seconds (JVM running for 2.867)
2022-10-25 14:36:59.478  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:36:59.478  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:36:59.480  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:36:59.480  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:36:59.484  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:02.490  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:02.490  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:02.490  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:02.490  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:02.490  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:05.493  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:05.499  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:05.500  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:05.500  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:05.501  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:08.505  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:08.512  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:08.512  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:08.512  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:08.513  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:11.518  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:11.520  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:11.523  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:11.523  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:11.524  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:14.529  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:14.531  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:14.531  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:14.531  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:14.532  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:17.537  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:17.541  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:17.541  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:17.542  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:17.542  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:20.547  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:20.550  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:20.550  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:20.551  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:20.551  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:23.556  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:23.558  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:23.558  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:23.559  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:23.560  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:26.563  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:26.565  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:26.565  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:26.565  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:26.566  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.2
2022-10-25 14:37:29.570  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------
2022-10-25 14:37:29.582  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Cache TTL: 1 seconds
2022-10-25 14:37:29.582  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Negative Cache TTL: 1 seconds
2022-10-25 14:37:29.583  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : DNS Test ADDR: www.naverdnstesturlforgslb.com
2022-10-25 14:37:29.715  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : Result IP: www.naverdnstesturlforgslb.com/127.0.0.1
2022-10-25 14:37:32.720  INFO 19611 --- [pool-1-thread-1] c.d.t.dnstest.DnsTestListener  : --------------------------------------------------------

해당 결과가 나오게 된 원인을 확인하기 위해 자바의 InetAddressCachePolicy.java 소스 코드를 분석해본 결과 security manger가 비활성되어있는 경우 설정값과 관계없이 DEFAULT_POSITIVE값인 30초를 TTL 시간으로 사용한다는것을 확인할 수 있었습니다.

...
package sun.net;
import java.security.PrivilegedAction;
import java.security.Security;
public final class InetAddressCachePolicy {
...
    /* default value for positive lookups */
    public static final int DEFAULT_POSITIVE = 30;

    /* The Java-level namelookup cache policy for successful lookups:
     *
     * -1: caching forever
     * any positive value: the number of seconds to cache an address for
     *
     * default value is forever (FOREVER), as we let the platform do the
     * caching. For security reasons, this caching is made forever when
     * a security manager is set.
     */
    private static int cachePolicy = FOREVER;
... 
    static {
...
        if (tmp != null) {
            cachePolicy = tmp.intValue();
            if (cachePolicy < 0) {
                cachePolicy = FOREVER;
            }
            propertySet = true;
        } else {
            /* No properties defined for positive caching. If there is no
             * security manager then use the default positive cache value.
             */
            if (System.getSecurityManager() == null) {
                cachePolicy = DEFAULT_POSITIVE;
            }
        }
...
    }
...
}

이에 따라 JVM상에서 DNS 캐시의 TTL(time-to-live) 시간은 다음과 같이 정의할 수 있었습니다.

1. Security Manager를 설정하지 않은 경우
- 기본값: 30초 (변경불가, reflection 등을 쓰지 않는 한….)
2. Security Manager를 설정한 경우
- 기본값: 무한
- 설정값을 통해 변경 가능

7. 마치며

지금까지 JVM의 DNS Cache 설정에 따른 실제 갱신시간을 테스트를 통해 알아보았습니다. 서버가 구동되는 환경은 각각 다르고 모든 환경에서 테스트가 이루어진 것은 아니라 일반화할 수는 없지만, Java 또는 JVM 환경에서 작동하는 애플리케이션 개발 시 도메인에 할당된 IP가 중간에 변경되는 상황에 도움이 되시길 기대합니다.