Docker image jenkins/jenkins:lts-alpine DNS issue - docker

Question

I have a docker image based on jenkins/jenkins:lts-alpine. For some reason, it is not able to resolve DNS names, as shown below:
[root#inf-jenkins02-prd ~]# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
716ceece2017 myjenkins/jenkins:latest "/sbin/tini -- /us..." 12 minutes ago Up 12 minutes 8080/tcp, 50000/tcp jenkins_main.1.heaupued6g8eygrgrk6c8hlvy
[root#inf-jenkins02-prd ~]# docker exec -ti 716ceece2017 nslookup www.google.com
nslookup: can't resolve '(null)': Name does not resolve
nslookup: can't resolve 'www.google.com': Try again
[root#inf-jenkins02-prd ~]#
However, running the following works as expected:
[root#inf-jenkins02-prd ~]# docker run myjenkins/jenkins:latest nslookup google.com
nslookup: can't resolve '(null)': Name does not resolve
Name: google.com
Address 1: 216.58.204.142 par21s05-in-f142.1e100.net
Here is the Dockerfile use to build the image:
FROM jenkins/jenkins:lts-alpine
ENV JAVA_OPTS="-Djenkins.install.runSetupWizard=false -Dhudson.DNSMultiCast.disabled=true -Dhudson.udp=-1"
USER jenkins
RUN ssh-keygen -q -t rsa -N '' -f /var/jenkins_home/.ssh/id_rsa
COPY security.groovy /usr/share/jenkins/ref/init.groovy.d/security.groovy
COPY xlocation.groovy /usr/share/jenkins/ref/init.groovy.d/xlocation.groovy
COPY znodes.groovy /usr/share/jenkins/ref/init.groovy.d/znodes.groovy
COPY plugins.txt /usr/share/jenkins/ref/plugins.txt
RUN /usr/local/bin/install-plugins.sh < /usr/share/jenkins/ref/plugins.txt
And this is the docker-compose file:
version: "3.1"
services:
main:
container_name: jenkins-mine
image: myjenkins/jenkins
deploy:
placement:
constraints:
- node.role == manager
ports:
- 8080:8080
- 50000:50000
volumes:
- /home/jenkins:/var/jenkins_home/
- /var/run/docker.sock:/var/run/docker.sock
Then, I'm just doing:
docker stack deploy -c /.../docker-compose.yml jenkins
Everything seems to be okay:
[root#inf-jenkins02-prd ~]# docker service ls
ID NAME MODE REPLICAS IMAGE
71h4jrygqp7m jenkins_main replicated 1/1 myjenkins/jenkins
[root#inf-jenkins02-prd ~]# docker service ps 71h4jrygqp7m
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
heaupued6g8e jenkins_main.1 myjenkins/jenkins inf-jenkins02-prd Running Running 23 minutes ago
[root#inf-jenkins02-prd ~]#
but google.com can't resolve:
[root#inf-jenkins02-prd ~]# docker exec -ti 716ceece2017 nslookup www.google.conf
nslookup: can't resolve '(null)': Name does not resolve
nslookup: can't resolve 'www.google.conf': Try again
[root#inf-jenkins02-prd ~]# docker exec -ti 716ceece2017 cat /etc/resolv.conf
search ...
nameserver 127.0.0.11
options ndots:0
[root#inf-jenkins02-prd ~]#
Any idea of what's happening here ? Many thanks in advance.
Kind regards,
Nicolas

Answer 1

If you ever see the message nslookup: can't resolve '(null)': Name does not resolve, it's because nslookup is looking up the name of the server you're going to send the DNS query to. Since you didn't provide one, it's null. This is a known issue in nslookup (see https://github.com/gliderlabs/docker-alpine/issues/476). See also this SO question for more details..
One thing to check is that you're looking up the right domain.
Here, you looked up www.google.conf, not www.google.com. www.google.conf does not exist, so the output you got is expected.
# docker exec -ti 716ceece2017 nslookup www.google.conf
nslookup: can't resolve '(null)': Name does not resolve
nslookup: can't resolve 'www.google.conf': Try again
Another thing to check is to try nslookup with a different DNS resolver.
For example:
docker exec -ti 716ceece2017 nslookup www.google.conf 8.8.8.8
Then you can see if the issue is the Internet, or the DNS server.