Flannel is an overlay based networking technique for networking Docker containers on CoreOS platforms. This tutorial explains the theory, setup instructions and limtations of the mechanism.

1. Docker Networking
Tutorial – CoreOS Flannel
Srini Seetharaman
srini@sdnhub.org
November, 2014

2. CoreOS Flannel

3. • Lightweight OS based on Gentoo Linux
• Has a distributed key-value store at the core
• Read-only rootfs. Writeable /etc
o All services are in containers
CoreOS

4. • One CIDR subnet per machine, like Kubernetes
o Host 1: 10.10.10.0/24
o Host 2: 10.10.11.0/24
• No Docker port-based mapping
• Containers reach each other through IP
• Peer network configs exchanged over etcd
• Packets encapsulated using UDP, and soon VxLAN
Flannel Basic
4

6. 1. Build flannel on each host
2. Set key in etcd for network config
Instructions to Run Flannel
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$ curl -L http://127.0.0.1:4001/v2/keys/coreos.com/network/config
-XPUT -d value='{
"Network": "10.0.0.0/8",
"SubnetLen": 20,
"SubnetMin": "10.10.0.0",
"SubnetMax": "10.99.0.0",
"Backend": {"Type": "udp",
"Port": 7890}}
$ git clone https://github.com/coreos/flannel.git
$ cd flannel
$ docker run -v `pwd`:/opt/flannel -i -t google/golang /bin/bash -c "cd
/opt/flannel && ./build"

7. 3. Start flannel.
o flanneld port created and route is set for the full flat IP range.
Instructions to Run Flannel (contd.)
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$ sudo ./bin/flanneld &
Output:
I1219 17:34:41.159822 00809 main.go:247] Installing signal handlers
I1219 17:34:41.160030 00809 main.go:118] Determining IP address of default interface
I1219 17:34:41.160579 00809 main.go:205] Using 192.168.111.14 as external interface
I1219 17:34:41.212157 00809 subnet.go:83] Subnet lease acquired: 10.12.224.0/20
I1219 17:34:41.217829 00809 main.go:215] UDP mode initialized
I1219 17:34:41.218953 00809 udp.go:239] Watching for new subnet leases
I1219 17:34:41.219349 00809 udp.go:264] Subnet added: 10.13.128.0/20
core@coreos-05 ~ $ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.111.1 0.0.0.0 UG 1024 0 0 eth0
10.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 flannel0
10.12.224.0 0.0.0.0 255.255.240.0 U 0 0 0 docker0
192.168.111.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0

8. 4. Restart docker daemon with appropriate bridge IP
Instructions to Run Flannel (contd.)
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$ source /run/flannel/subnet.env
$ sudo ifconfig docker0 ${FLANNEL_SUBNET}
$ sudo docker -d --bip=${FLANNEL_SUBNET} --mtu=${FLANNEL_MTU} &

9. • Ping between two bash containers on two different hosts
succeeds. The traffic on wire is encapsulated by flanneld
Testing Flannel Networking
9
192.168.111.14
Docker0
10.12.224.1
bash
192.168.111.13
Docker0
10.13.128.1
bash
$ docker run -i -t ubuntu /bin/bash
root@36484def3b03:/# ifconfig eth0
eth0 Link encap:Ethernet
HWaddr 02:42:0a:0c:e0:02
inet addr:10.12.224.2
Bcast:0.0.0.0
Mask:255.255.240.0
root@36484def3b03:/# ping 10.13.128.2
Success!
$ docker run -i -t ubuntu /bin/bash
root@e0b9dd20d146:/# ifconfig eth0
eth0 Link encap:Ethernet
HWaddr 02:42:0a:0d:80:02
inet addr:10.13.128.2
Bcast:0.0.0.0
Mask:255.255.240.0

10. Packet on the Wire
10
Original ICMP
packet between
the two containers
Flannel
introduced encap
UDP header

11. • IP address overlap not possible
o VxLAN not used to create container groups
• User-space encapsulation and forwarding
o Potential performance bottleneck
Limitations
11

12. Thank you.
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