Docker Cookbook

6.4 Using fleet to Start Containers on a CoreOS Cluster | 179

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Jan 12 22:06:21 core-02 ...[cluster.service ] [Wendigo] new_master [Wendigo]...
Jan 12 22:06:21 core-02 ...[http ] [Wendigo] bound_address ...
Jan 12 22:06:21 core-02 ...[node ] [Wendigo] started
Jan 12 22:06:21 core-02 ...[gateway ] [Wendigo] recovered [0] ...
See Also
• Launching containers with 
fleet
6.5 Deploying a flannel Overlay Between CoreOS Instances
Contributed by Eugene Yakubovich
Problem
You have a CoreOS cluster and would like Docker containers to communicate using
overlay networking instead of port forwarding.
Solution
Set up 
flannel
on all of the CoreOS instances. Include the following snippet in your
cloud-config
as part of CoreOS provisioning:
#cloud-config
coreos:
units:
- name: flanneld.service
drop-ins:
- name: 50-network-config.conf
content: |
[Service]
ExecStartPre=/usr/bin/etcdctl set \
/coreos.com/network/config \
'{ "Network": "10.1.0.0/16" }'
command: start
Make sure to pick an IP address range that is unused by your orga‐
nization. 
flannel
uses 
etcd
for coordination. Be sure that you also
follow the recipe to set up an 
etcd
cluster.
Make sure your security policy allows traffic on UDP port 8285. Start three CoreOS
instances and wait for 
flannel
to initialize. You can use the 
ifconfig
utility to check
that the 
flannel0
interface is up:
180 | Chapter 6: Optimized Operating System Distributions for Docker

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$ ifconfig
flannel0: flags=81 mtu 1472
inet 10.1.77.0 netmask 255.255.0.0 destination 10.1.77.0
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen ...
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
Next, run a container to print out its IP address and listen on TCP port 8000:
$ docker run -it --rm busybox /bin/sh -c \
"ifconfig eth0 && nc -l -p 8000"
eth0 Link encap:Ethernet HWaddr 02:42:0A:01:4D:03
inet addr:10.1.77.3 Bcast:0.0.0.0 Mask:255.255.255.0
UP BROADCAST MTU:1472 Metric:1
RX packets:3 errors:0 dropped:0 overruns:0 frame:0
TX packets:1 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:234 (234.0 B) TX bytes:90 (90.0 B)
Take note of the IP address reported by 
ifconfig
. Other containers that are part of
the 
flannel
network can use this IP to reach this container. On a different host, run a
container to send a string to the listener:
$ docker run -it --rm busybox /bin/sh -c \
"echo Hello, container | nc 10.1.77.3 8000"
The first container will print out “Hello, container” and exit.
When you add more items to the 
units
section of 
cloud-config
, be
sure that any services that start Docker containers are listed after
flanneld.service
. Since units are processed in order, this will
ensure that 
flannel
is ready prior to containers starting.