Hi,

I’m currently fiddling with Docker Swarm on three sandbox servers running CentOS 7.

Unfortunately I couldn’t get even the most basic configuration running. After some experimenting and investigating, it turns out there’s a problem with FirewallD.

Here’s what I did first on every single node.

# firewall-cmd –permanent –add-service=docker-swarm
# firewall-cmd –reload

I launched a simple web service like this:

# docker service create –name web -p 80:80 nginx

My three nodes appear correctly with ‘docker node ls’.

From here, I should be able to open the Nginx default web page in a web browser using any node’s hostname.

But nothing happens. No page is loaded.

On a whim, I turned off FirewallD, and suddenly everything worked as expected.

Now here’s the list of ports that have to be open on every single node for Docker Swarm to work:

* 2376/tcp
* 2377/tcp
* 7946/tcp
* 7946/udp
* 4789/udp

And here’s what docker-swarm.xml looks like under CentOS 7:

# cat /usr/lib/firewalld/services/docker-swarm.xml


Docker integrated swarm mode
Natively managed cluster of Docker Engines (>=1.12.0), where you deploy services.

So it looks like TCP port 2376 was forgotten in that service definition.

I opened it up manually on every node:

# firewall-cmd –permanent –add-port=2376/tcp
# firewall-cmd –reload

And now Docker Swarm works correctly.

Cheers from the sunny South of France,

Niki Kovacs

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