java.net.UnknownHostException on Docker

问题:

I am trying to create docker containers for ZooKeeper and configure them in cluster mode (full code is here and here).

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).

I use the following commands to start the cluster:

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).

If I wait some time before starting the second and third containers, then the host names zk2 and zk3 are put in /etc/hosts too late (by docker), and Java is unable to find them: I get java.net.UnknownHostException in the logs of zk1 for both zk2 and 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 Dockerfile in order to update the java.security settings:

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 10 seconds.

The variable networkaddress.cache.negative.ttl is already set to its default value (10).

The behavior does not change. I get lots of java.net.UnknownHostException repeatedly.

What can be the cause of the problem?

回答1:

In my case the java application was failing with java.net.UnknownHostException when running in docker. The reason was that I used --network=none docker flag (getting ip/hostname via dhcp and pipework). In this case, docker does not add automatically to /etc/hosts entry like

127.0.0.1 15e326aecf84

And getCanonicalHostName() Java function threw this exception.

Possible solutions:

• add hostname entry to /etc/hosts file via docker run parameter --hostname=your-hostname.com
• switch to docker-managed network configuration

回答2:

I managed to get rid of the DNS issues by switching to Oracle JRE 8 and using the following hack in the Dockerfile:

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).