Docker 上的 java.net.UnknownHostException
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java.net.UnknownHostException on Docker
提问by Nicola Ferraro
I am trying to create docker containers for ZooKeeper and configure them in cluster mode (full code is hereand here).
我正在尝试为 ZooKeeper 创建 docker 容器并在集群模式下配置它们（完整代码在这里和这里）。
Containers are based on Alpine Linux (alpine:3.2 on Docker Hub), but the problem that I'm going to describe happens also with the official Java container (java:7).
容器基于 Alpine Linux（Docker Hub 上的 alpine:3.2），但我将要描述的问题也发生在官方 Java 容器（java:7）上。
I use the following commands to start the cluster:
我使用以下命令启动集群：
docker run -d -h zk1 --name zk1 dockmob/zookeeper -s zk1,zk2,zk3
# wait some time ...
docker run -d -h zk2 --name zk2 dockmob/zookeeper -s zk1,zk2,zk3
docker run -d -h zk3 --name zk3 dockmob/zookeeper -s zk1,zk2,zk3
(They are available on docker hub, you can try them).
（它们在 docker hub 上可用，您可以尝试它们）。
If I wait some time before starting the second and third containers, then the host names zk2and zk3are put in /etc/hoststoo late (by docker), and Java is unable to find them: I get java.net.UnknownHostExceptionin the logs of zk1for both zk2and zk3.
如果我等待一段时间开始的第二和第三个容器，那么主机名之前zk2并zk3放入/etc/hosts太晚（由码头工人）和Java是无法找到他们：我得到java.net.UnknownHostException在日志zk1两个zk2和zk3。
I found on the web that I need to disable JVM DNS cache in order to refresh the host names, so I introduced the following command in the Dockerfilein order to update the java.securitysettings:
我在网上发现我需要禁用JVM DNS缓存才能刷新主机名，因此我在其中引入了以下命令Dockerfile以更新java.security设置：
RUN grep '^networkaddress.cache.ttl=' /usr/lib/jvm/java-1.7-openjdk/jre/lib/security/java.security || echo 'networkaddress.cache.ttl=10' >> /usr/lib/jvm/java-1.7-openjdk/jre/lib/security/java.security
It sets the DNS TTL property (networkaddress.cache.ttl) to 10seconds.
它将 DNS TTL 属性 ( networkaddress.cache.ttl) 设置为10秒。
The variable networkaddress.cache.negative.ttlis already set to its default value (10).
该变量networkaddress.cache.negative.ttl已设置为其默认值 ( 10)。
The behavior does not change. I get lots of java.net.UnknownHostExceptionrepeatedly.
行为不会改变。我java.net.UnknownHostException反复得到很多。
What can be the cause of the problem?
问题的原因是什么？
采纳答案by Nicola Ferraro
I managed to get rid of the DNS issues by switching to Oracle JRE 8 and using the following hack in the Dockerfile:
通过切换到 Oracle JRE 8 并在 Dockerfile 中使用以下技巧，我设法摆脱了 DNS 问题：
RUN echo 'hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4' >> /etc/nsswitch.conf
I created a working Java 8 docker container container on Docker Hub(the code is on github).
我在Docker Hub上创建了一个可用的 Java 8 docker 容器容器（代码在github 上）。
回答by Dmitriusan
In my case the java application was failing with java.net.UnknownHostExceptionwhen running in docker. The reason was that I used --network=nonedocker flag (getting ip/hostname via dhcp and pipework). In this case, docker does not add automatically to /etc/hostsentry like
在我的情况下，java 应用程序java.net.UnknownHostException在 docker 中运行时失败。原因是我使用了--network=nonedocker 标志（通过 dhcp 和管道获取 ip/hostname）。在这种情况下，docker 不会/etc/hosts像这样自动添加到条目中
127.0.0.1 15e326aecf84
127.0.0.1 15e326aecf84
And getCanonicalHostName()Java function threw this exception.
而getCanonicalHostName()Java 函数抛出了这个异常。
Possible solutions:
可能的解决方案：
- add hostname entry to
/etc/hostsfile viadocker runparameter--hostname=your-hostname.com - switch to docker-managed network configuration
/etc/hosts通过docker run参数将主机名条目添加到文件--hostname=your-hostname.com- 切换到 docker 管理的网络配置
