Flannel是Overlay网络的一种，也是将源数据包封装在另一种网络包里面进行路由转发和通信，目前已经支持UDP、VXLAN、AWS VPC和GCE路由等数据转发方式。
Flannel通过给每台宿主机分配一个子网的方式为容器提供虚拟网络，它基于Linux TUN/TAP，使用UDP封装IP包来创建overlay网络，并借助etcd维护网络的分配情况。
去官网下载相应二进制包：https://github.com/coreos/flannel/releases
解压之后得到两个文件：flanneld和mk-docker-opts.sh
将其复制到flanel的专属目录中。这里我们统一放在/opt/kubernetes/bin下面。
通过以下文件来配置flannel的配置文件
cat <<EOF >/opt/kubernetes/cfg/flanneld
FLANNEL_OPTIONS="--etcd-endpoints=${ETCD_ENDPOINTS}
-etcd-cafile=/opt/kubernetes/ssl/ca.pem
-etcd-certfile=/opt/kubernetes/ssl/server.pem
-etcd-keyfile=/opt/kubernetes/ssl/server-key.pem"
EOF
注意：${ETCD_ENDPOINTS} 是etcd各节点路径，比如：“https://192.168.10.63:2379,https://192.168.10.64:2379,https://192.168.10.65:2379"。这里可以手动填写上去。
确定无误之后再用以下文件来配置service启动文件。
cat <<EOF >/usr/lib/systemd/system/flanneld.service
[Unit]
Description=Flanneld overlay address etcd agent
After=network-online.target network.target
Before=docker.service
[Service]
Type=notify
EnvironmentFile=/opt/kubernetes/cfg/flanneld
ExecStart=/opt/kubernetes/bin/flanneld --ip-masq $FLANNEL_OPTIONS
ExecStartPost=/opt/kubernetes/bin/mk-docker-opts.sh -k DOCKER_NETWORK_OPTIONS -d /run/flannel/subnet.env
Restart=on-failure
[Install]
WantedBy=multi-user.target
EOF
确保证书完整情况下便可启动flannel了。
启动完flannel之后还要配置docker使用flannel网络。
[root@k8s-node01-10 ~]# cat /usr/lib/systemd/system/docker.service
[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
After=network-online.target firewalld.service
Wants=network-online.target
[Service]
Type=notify
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set required
# for containers run by docker
#ExecStart=/usr/bin/dockerd （这是原来的，将其注释或者删除）
EnvironmentFile=/run/flannel/subnet.env
ExecStart=/usr/bin/dockerd $DOCKER_NETWORK_OPTIONS （这两行是替换上面哪一行的，有了这两行之后docker便可以使用flannel网络来）
ExecReload=/bin/kill -s HUP $MAINPID
# Having non-zero Limit*s causes performance problems due to accounting overhead
# in the kernel. We recommend using cgroups to do container-local accounting.
LimitNOFILE=infinity
LimitNPROC=infinity
LimitCORE=infinity
# Uncomment TasksMax if your systemd version supports it.
# Only systemd 226 and above support this version.
#TasksMax=infinity
TimeoutStartSec=0
# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes
# kill only the docker process, not all processes in the cgroup
KillMode=process
# restart the docker process if it exits prematurely
Restart=on-failure
StartLimitBurst=3
StartLimitInterval=60s
[Install]
WantedBy=multi-user.target
重启一下docker即可使用flannel网络。