原始碼路徑： https://github.com/containernetworking/plugins

版本： v.0.10.0

flannel cni路徑： plugins/plugins/meta/flannel/flannel.go

subnet.env檔案

# cat /run/flannel/subnet.env 
FLANNEL_NETWORK=172.30.0.0/16
FLANNEL_SUBNET=172.30.45.1/24
FLANNEL_MTU=1500
FLANNEL_IPMASQ=false

cni 配置檔案

# cat /etc/cni/net.d/10-flannel.conf 
{"name":"cbr0","type":"flannel","delegate": {"bridge": "docker0", "isDefaultGateway": true, "ipMasq": false}}

NetConf結構體

    subnetFile檔案預設為：/run/flannel/subnet.env

    dataDIr預設為：/var/lib/cni/flannel

type NetConf struct {
	types.NetConf

	SubnetFile string                 `json:"subnetFile"`
	DataDir    string                 `json:"dataDir"`
	Delegate   map[string]interface{} `json:"delegate"`
}
 

1. cmdAdd函式

  1.1 loadFlannelNetConf函式

    將傳入的引數解析到NetConf結構體，這裡設定了預設的subnetFile，與dataDir

	n, err := loadFlannelNetConf(args.StdinData)
	if err != nil {
		return err
	}

  1.2 loadFlannelSubnetEnv函式

    從/run/flannel/subnet.env讀取配置，包括：

• FLANNEL_NETWORK=172.30.0.0/16
• FLANNEL_SUBNET=172.30.46.1/24
• FLANNEL_MTU=1500
• FLANNEL_IPMASQ=false

	fenv, err := loadFlannelSubnetEnv(n.SubnetFile)
	if err != nil {
		return err
	}

  1.3 subnet.env驗證引數合法性

	if n.Delegate == nil {
		n.Delegate = make(map[string]interface{})
	} else {
		if hasKey(n.Delegate, "type") && !isString(n.Delegate["type"]) {
			return fmt.Errorf("'delegate' dictionary, if present, must have (string) 'type' field")
		}
		if hasKey(n.Delegate, "name") {
			return fmt.Errorf("'delegate' dictionary must not have 'name' field, it'll be set by flannel")
		}
		if hasKey(n.Delegate, "ipam") {
			return fmt.Errorf("'delegate' dictionary must not have 'ipam' field, it'll be set by flannel")
		}
	}

  1.4 doCmdAdd函式

    對於未設定的引數初始化，如果未設定ipam則使用預設host-local

func doCmdAdd(args *skel.CmdArgs, n *NetConf, fenv *subnetEnv) error {
	n.Delegate["name"] = n.Name

	if !hasKey(n.Delegate, "type") {
		n.Delegate["type"] = "bridge"
	}

	if !hasKey(n.Delegate, "ipMasq") {
		// if flannel is not doing ipmasq, we should
		ipmasq := !*fenv.ipmasq
		n.Delegate["ipMasq"] = ipmasq
	}

	if !hasKey(n.Delegate, "mtu") {
		mtu := fenv.mtu
		n.Delegate["mtu"] = mtu
	}

	if n.Delegate["type"].(string) == "bridge" {
		if !hasKey(n.Delegate, "isGateway") {
			n.Delegate["isGateway"] = true
		}
	}
	if n.CNIVersion != "" {
		n.Delegate["cniVersion"] = n.CNIVersion
	}

	n.Delegate["ipam"] = map[string]interface{}{
		"type":   "host-local",
		"subnet": fenv.sn.String(),
		"routes": []types.Route{
			{
				Dst: *fenv.nw,
			},
		},
	}

	return delegateAdd(args.ContainerID, n.DataDir, n.Delegate)
}

2. delegateAdd函式

  saveScratchNetConf函式建立/var/lib/cni/flannel目錄

  呼叫DelegateAdd函式

func delegateAdd(cid, dataDir string, netconf map[string]interface{}) error {
	netconfBytes, err := json.Marshal(netconf)
	if err != nil {
		return fmt.Errorf("error serializing delegate netconf: %v", err)
	}

	// save the rendered netconf for cmdDel
	if err = saveScratchNetConf(cid, dataDir, netconfBytes); err != nil {
		return err
	}

	result, err := invoke.DelegateAdd(netconf["type"].(string), netconfBytes, nil)
	if err != nil {
		return err
	}

	return result.Print()
}

3. 根據設定的bridge，呼叫bridge cni

參照https://blog.csdn.net/zhonglinzhang/article/details/82733201

  3.1 setupBridge函式

• 通過netlink.LinkAdd(br)建立網橋，相當於ip link add br-test type bridge
•  然後通過 netlink.LinkSetUp(br)啟動網橋，相當於ip link set dev br-test up

  3.2 setupVeth函式

•  呼叫netlink.LinkAdd(veth)建立veth，這個是一個管道，Linux的網絡卡對，在容器對應的namespace下建立好虛擬網路介面，相當於ip link add test-veth0 type veth peer name test-veth1
•  呼叫netlink.LinkSetUp(contVeth)啟動容器端網絡卡，相當於ip link set dev test-veth0 up
• 呼叫netlink.LinkSetNsFd(hostVeth, int(hostNS.Fd()))將host端加入namespace中，相當於ip link set $link netns $ns
•  呼叫netlink.LinkSetMaster(hostVeth, br)綁到bridge，相當於ip link set dev test-veth0 master br-test
   

與calico不同點：

calico 綁到eth0

flannel 綁到docker0 bridge上，在呼叫bridage cni建立一大堆網路相關