通過修改etcd來設置或修改節點flannel子網信息

在首次啟動flannel服務的時候可以手動指定subnet.env文件，配置所在節點的flannel子網網段，如果不指定配置文件，flannel將自動分配一個子網網段並生成配置文件 /var/run/flannel/subnet.env，配置信息寫入etcd集群。
如果節點失效超時，etcd將清除該節點的子網信息。在不移除節點的情況下，修改配置文件並重啟flannel服務，flannel會將etcd中的信息寫回subnet.env，即subnet.env文件會被重置。因此必須通過修改etcd來修改節點flannel子網信息。

[root@master1 ~]# etcdctl ls /coreos.com/network/subnets/
/coreos.com/network/subnets/172.7.16.0-24
/coreos.com/network/subnets/172.7.17.0-24
/coreos.com/network/subnets/172.7.28.0-24

[root@master1 ~]# etcdctl set /coreos.com/network/subnets/172.7.18.0-24 '{"PublicIP":"172.25.25.18","BackendType":"host-gw"}'
{"PublicIP":"172.25.25.18","BackendType":"host-gw"}

[root@master1 ~]# etcdctl ls /coreos.com/network/subnets/
/coreos.com/network/subnets/172.7.16.0-24
/coreos.com/network/subnets/172.7.17.0-24
/coreos.com/network/subnets/172.7.28.0-24
/coreos.com/network/subnets/172.7.18.0-24

[root@master1 ~]# etcdctl get /coreos.com/network/subnets/172.7.28.0-24
{"PublicIP":"172.25.25.18","BackendType":"host-gw"}
[root@master1 ~]# etcdctl get /coreos.com/network/subnets/172.7.18.0-24
{"PublicIP":"172.25.25.18","BackendType":"host-gw"}

[root@master1 ~]# etcdctl rm /coreos.com/network/subnets/172.7.28.0-24
PrevNode.Value: {"PublicIP":"172.25.25.18","BackendType":"host-gw"}

[root@master1 ~]# etcdctl ls /coreos.com/network/subnets/
/coreos.com/network/subnets/172.7.16.0-24
/coreos.com/network/subnets/172.7.17.0-24
/coreos.com/network/subnets/172.7.18.0-24

[root@master1 ~]# etcdctl get /coreos.com/network/subnets/172.7.18.0-24
{"PublicIP":"172.25.25.18","BackendType":"host-gw"}
[root@master1 ~]# etcdctl get /coreos.com/network/subnets/172.7.17.0-24
{"PublicIP":"172.25.25.17","BackendType":"host-gw"}
[root@master1 ~]# etcdctl get /coreos.com/network/subnets/172.7.16.0-24
{"PublicIP":"172.25.25.16","BackendType":"host-gw"}