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armani
Asked: 2017-02-28 13:29:31 +0800 CST

"ip route" Commands in Dockerfile Generate Error

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I create a Docker container like this: 

$sudo docker build -t "logstash-collector" ~/logging/logstash/

$sudo docker run -d \
--user=root \
--name="logstash-collector-01" \
--net=logging \
--cap-add=NET_ADMIN \
logstash-collector

And from there I can change its default gateway to an nginx container (for transparent load balancing):

$sudo docker exec -it logstash-collector-01 ip route delete default
$sudo docker exec -it logstash-collector-01 ip route add default via 172.18.0.10

And it works, and everything's fine:

$sudo docker exec -it logstash-collector-01 ip route 
default via 172.18.0.10 dev eth0 
172.18.0.0/16 dev eth0  proto kernel  scope link  src 172.18.0.3

But when I try to automate this by putting those commands in my ~/logging/logstash/Dockerfile like:

FROM logstash
COPY logstash.conf /usr/local/share
CMD ["-f", "/usr/local/share/logstash.conf", "--config.reload.automatic"]
CMD ["ip", "route", "delete", "default"]
CMD ["ip", "route", "add", "default", "via", "172.18.0.10"]

I see them getting executed during build: Logstash container build

But the container fails to launch. sudo docker logs logstash-collector-01 tells me: RTNETLINK answers: File exists

That error happens if I issue the second ip route command without issuing the first one (Linux won't add a route if one is already in its active routing table). But how can this be? Are my Dockerfile's CMD commands processing out of order?

linux networking docker

1 Answers

  1. Best Answer

    This is because you can only specify one CMD per dockerfile.

    CMD is what the container will be running (which would normally be logstash, a long-running, daemon-like process), so what are you doing is only running the last CMD.

    See https://stackoverflow.com/questions/23692470/why-cant-i-use-docker-cmd-multiple-times-to-run-multiple-services#23693804

    You could consider a lo-fi solution like using a single container's network to share between nginx and the logstash agents, but you then need to keep track of ports.

    Higher up the chain are solutions like kubernetes that make this kind of thing easier.

    In between, you might look into consul(/SkyDNS) to help with DNS discovery.

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