Java无法解析主机名

Question

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

ECS设置了主机名并正确设置了/etc/hosts和/etc/resolv.conf。如果我执行nslookup `hostname`，解析就没问题。我还可以将其解析为/etc/resolv.conf中设置的名称服务器。

但是，如果我启动试图解析主机名的Java应用程序，我会收到一个异常，指示无法解析主机名。

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)

所以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();
    }

}

当主机名为'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使用IPv4堆栈，我会得到以下结果：

$> 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

如果我强制使用Sun实现并指定NAMESERVER和SEARCH_DOMAIN，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]

我感到十分困惑，有什么建议吗？

- Nick Palmer

---

getent hosts f17a0bdca449 会给你什么结果？ - delephin

正确的地址。我认为问题实际上是我正在运行的Linux版本。在不同的平台上重新构建。 - Nick Palmer

Answer 1

您获取的主机信息是Docker容器的主机名。 这个问题不仅适用于ECS，对于在Docker容器中运行的任何Java应用程序都是如此。 在ECS中，您无法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。 向AWS元数据服务发送HTTP GET请求，应该可以获取到主机的详细信息。

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，则可以利用以下类http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/util/EC2MetadataUtils.InstanceInfo.html。

- Shibashis

---

对于“在ECS中，您无法将主机名作为运行时配置注入”的说法，我认为部分正确。 在ECS中的“EC2”任务类型中可以配置主机名。 - aksinghdce