Java 无法解析主机名

Question

我正在尝试在 AWS ECS 容器中启动 Java 应用程序，但 Java 无法查找主机名。

ECS 设置主机名并正确设置 /etc/hosts 和 /etc/resolv.conf。如果我这样做 nslookup `hostname` 我得到的分辨率就好了。我还可以针对 /etc/resolv.conf 中设置的名称服务器解决这个问题

但是，如果我启动尝试解析主机名的 java 应用程序，我会收到一个异常，指出无法解析主机名。

java -version 输出：

bash# java -version
java version "1.8.0_45"
Java(TM) SE Runtime Environment (build 1.8.0_45-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

于是我写了一个小测试程序：

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.io.PrintWriter;
import java.io.StringWriter;

public class DomainResolutionTest {

    public static void main(String[] args) {

        try {
            InetAddress ip = InetAddress.getLocalHost();
            System.out.println(ip.toString());
        }catch (UnknownHostException uhx) {
            System.out.println("ERROR: " + uhx.getMessage() + "\n" + getStackTrace(uhx));
            Throwable cause = uhx.getCause();
            if (cause != null) System.out.println("CAUSE: " + cause.getMessage());
        }

    }

    public static String getStackTrace(Throwable t)
    {
        StringWriter sw = new StringWriter();
        PrintWriter pw = new PrintWriter(sw, true);
        t.printStackTrace(pw);
        pw.flush();
        sw.flush();
        return sw.toString();
    }

}

当主机名是'f17a0bdca449'时会抛出以下错误：

ERROR: f17a0bdca449: f17a0bdca449: unknown error
java.net.UnknownHostException: f17a0bdca449: f17a0bdca449: unknown error
    at java.net.InetAddress.getLocalHost(InetAddress.java:1484)
    at DomainResolutionTest.main(DomainResolutionTest.java:11)
Caused by: java.net.UnknownHostException: f17a0bdca449: unknown error
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
    at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:907)
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302)
    at java.net.InetAddress.getLocalHost(InetAddress.java:1479)
    ... 1 more

CAUSE: f17a0bdca449: unknown error

如果我使用 -Djava.net.preferIPv4Stack=true 切换 java 以使用 ipv4 堆栈，我会得到：

$> java -Djava.net.preferIPv4Stack=true DomainResolutionTest
ERROR: f17a0bdca449: f17a0bdca449: unknown error
java.net.UnknownHostException: f17a0bdca449: f17a0bdca449: unknown error
    at java.net.InetAddress.getLocalHost(InetAddress.java:1484)
    at DomainResolutionTest.main(DomainResolutionTest.java:11)
Caused by: java.net.UnknownHostException: f17a0bdca449: unknown error
    at java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method)
    at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:907)
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302)
    at java.net.InetAddress.getLocalHost(InetAddress.java:1479)
    ... 1 more

CAUSE: f17a0bdca449: unknown error

如果我强制它使用 sun 实现并指定 NAMESERVER 和 SEARCH_DOMAIN，我仍然会得到相同的结果：

$> export NAMESERVER=$(cat /etc/resolv.conf | grep nameserver | awk '{print $2}')
$> export SEARCH_DOMAIN=$(cat /etc/resolv.conf | grep search | awk '{print $2}')
$> java -Dsun.net.spi.nameservice.provider.1=dns,sun -Dsun.net.spi.nameservice.nameservers=${NAMESERVER} -Dsun.net.spi.nameservice.domain=${SEARCH_DOMAIN} -Djava.net.preferIPv4Stack=true DomainResolutionTest
ERROR: f17a0bdca449: DNS name not found [response code 3]
java.net.UnknownHostException: f17a0bdca449: DNS name not found [response code 3]
    at java.net.InetAddress.getLocalHost(InetAddress.java:1484)
    at DomainResolutionTest.main(DomainResolutionTest.java:11)
Caused by: java.net.UnknownHostException: DNS name not found [response code 3]
    at sun.net.spi.nameservice.dns.DNSNameService.resolve(DNSNameService.java:180)
    at sun.net.spi.nameservice.dns.DNSNameService.lookupAllHostAddr(DNSNameService.java:351)
    at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302)
    at java.net.InetAddress.getLocalHost(InetAddress.java:1479)
    ... 1 more

CAUSE: DNS name not found [response code 3]

我束手无策。有什么建议吗？

Answer 1

您获得的主机信息是 docker 容器的主机名。 这个问题不是 ecs 独有的，任何在 docker 容器中运行的 java 应用程序都是如此。

使用 ECS，您无法选择将主机名作为运行时配置注入。

在 AWS 环境中运行时更好的解决方案是使用 AWS 元数据服务来获取主机名。

对 AWS 元数据服务的 http GET 请求应该会为您提供主机的详细信息。

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html

如果您的应用程序中包含 java aws sdk，您可以利用以下类 http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/util/EC2MetadataUtils.InstanceInfo.html