本文主要是介绍flannel 实战与源码分析（三），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
之前一直介绍flannel的使用，现在正是进入代码讲解，看看flannel是如何实现的，先看启动
func main() {flag.Set("logtostderr", "true")// 解析参数flag.Parse()if flag.NArg() > 0 || opts.help {fmt.Fprintf(os.Stderr, "Usage: %s [OPTION]...\n", os.Args[0])flag.PrintDefaults()os.Exit(0)}if opts.version {fmt.Fprintln(os.Stderr, version.Version)os.Exit(0)}flagutil.SetFlagsFromEnv(flag.CommandLine, "FLANNELD")// 确定使用的端口extIface, err := LookupExtIface(opts.iface)if err != nil {log.Error("Failed to find interface to use: ", err)os.Exit(1)}sm, err := newSubnetManager()if err != nil {log.Error("Failed to create SubnetManager: ", err)os.Exit(1)}// 注册 SIGINT and SIGTERM信号监听log.Info("Installing signal handlers")sigs := make(chan os.Signal, 1)signal.Notify(sigs, os.Interrupt, syscall.SIGTERM)ctx, cancel := context.WithCancel(context.Background())go shutdown(sigs, cancel)// 获取网络配置如backend等config, err := getConfig(ctx, sm)if err == errCanceled {exit()}// 创建backend manager bm := backend.NewManager(ctx, sm, extIface)be, err := bm.GetBackend(config.BackendType)if err != nil {log.Errorf("Error fetching backend: %s", err)exit()}bn, err := be.RegisterNetwork(ctx, config)if err != nil {log.Errorf("Error registering network: %s", err)exit()}// 启动ipmasqif opts.ipMasq {err = network.SetupIPMasq(config.Network)if err != nil {log.Errorf("Failed to set up IP Masquerade: %v", err)}defer func() {if err := network.TeardownIPMasq(config.Network); err != nil {log.Errorf("Failed to tear down IP Masquerade: %v", err)}}()}if err := WriteSubnetFile(opts.subnetFile, config.Network, opts.ipMasq, bn); err != nil {log.Warningf("Failed to write subnet file: %s", err)} else {log.Infof("Wrote subnet file to %s", opts.subnetFile)}// 启动 backend network阻塞直到服务启动go bn.Run(ctx)log.Infof("Finished starting backend.")daemon.SdNotify(false, "READY=1")// 重新租约_ = MonitorLease(ctx, sm, bn)exit()
}上面的代码是服务的入口，先解析参数，在之前的应用的时候就有使用参数
flag.StringVar(&opts.etcdEndpoints, "etcd-endpoints", "http://127.0.0.1:4001,http://127.0.0.1:2379", "a comma-delimited list of etcd endpoints")flag.StringVar(&opts.etcdPrefix, "etcd-prefix", "/coreos.com/network", "etcd prefix")flag.StringVar(&opts.iface, "iface", "", "interface to use (IP or name) for inter-host communication")flag.StringVar(&opts.subnetFile, "subnet-file", "/run/flannel/subnet.env", "filename where env variables (subnet, MTU, ... ) will be written to")flag.IntVar(&opts.subnetLeaseRenewMargin, "subnet-lease-renew-margin", 60, "Subnet lease renewal margin, in minutes.")flag.BoolVar(&opts.ipMasq, "ip-masq", false, "setup IP masquerade rule for traffic destined outside of overlay network")这些截取的参数有事比较重要，大概说一下
etcd-endpoints：连接etcd地址
etcd-prefix ：在etcd里面路径前缀
iface：主机间流量互通的网卡
subnet-file：生成docker网络信息的路径
subnet-lease-renew-margin：这个是自网段的租约时间
ip-masq：是否启动ipmasq，就是SANT
当解析完参数后LookupExtIface检查网卡是否存在，并且返回外部接口信息
&backend.ExternalInterface{Iface: iface,IfaceAddr: ifaceAddr,ExtAddr: extAddr,}Iface是制定的外网端口，如果publicip没有制定，IfaceAddr等于ExtAddr。这个容器出去的外部端口就设定成功了。
然后通过newSubnetManager去创建子网管理器，如果没有设置kubernetes子网管理的话，默认使用的就是通过etcd做子网管理，
func newSubnetManager() (subnet.Manager, error) {if opts.kubeSubnetMgr {return kube.NewSubnetManager()}cfg := &etcdv2.EtcdConfig{Endpoints: strings.Split(opts.etcdEndpoints, ","),Keyfile: opts.etcdKeyfile,Certfile: opts.etcdCertfile,CAFile: opts.etcdCAFile,Prefix: opts.etcdPrefix,Username: opts.etcdUsername,Password: opts.etcdPassword,}return etcdv2.NewLocalManager(cfg)
}连接上etcd。创建网sm以后，注册SIGINT and SIGTERM这个主要是优雅的停止服务使用。getConfig里面调用的是getNetworkConfig
func (esr *etcdSubnetRegistry) getNetworkConfig(ctx context.Context) (string, error) {key := path.Join(esr.etcdCfg.Prefix, "config")resp, err := esr.client().Get(ctx, key, &etcd.GetOptions{Quorum: true})if err != nil {return "", err}return resp.Node.Value, nil
}这个也是就是第一篇的实战的时候设置的config。从config里面获取backend是udp还是vxlan或者是别的。通过be, err := bm.GetBackend(config.BackendType)创建相应的backend。
befunc, ok := constructors[betype]if !ok {return nil, fmt.Errorf("unknown backend type: %v", betype)}be, err := befunc(bm.sm, bm.extIface)这样就获取到制定的backend，下面就是开始注册网络RegisterNetwork了，
每个backend都有自己的RegisterNetwork
下面以vxlan为例
func (be *VXLANBackend) RegisterNetwork(ctx context.Context, config *subnet.Config) (backend.Network, error) {// Parse our configurationcfg := struct {VNI intPort intGBP bool}{VNI: defaultVNI,}if len(config.Backend) > 0 {if err := json.Unmarshal(config.Backend, &cfg); err != nil {return nil, fmt.Errorf("error decoding VXLAN backend config: %v", err)}}devAttrs := vxlanDeviceAttrs{vni: uint32(cfg.VNI),name: fmt.Sprintf("flannel.%v", cfg.VNI),vtepIndex: be.extIface.Iface.Index,vtepAddr: be.extIface.IfaceAddr,vtepPort: cfg.Port,gbp: cfg.GBP,}dev, err := newVXLANDevice(&devAttrs)if err != nil {return nil, err}subnetAttrs, err := newSubnetAttrs(be.extIface.ExtAddr, dev.MACAddr())if err != nil {return nil, err}lease, err := be.subnetMgr.AcquireLease(ctx, subnetAttrs)switch err {case nil:case context.Canceled, context.DeadlineExceeded:return nil, errdefault:return nil, fmt.Errorf("failed to acquire lease: %v", err)}// vxlan's subnet is that of the whole overlay network (e.g. /16)// and not that of the individual host (e.g. /24)vxlanNet := ip.IP4Net{IP: lease.Subnet.IP,PrefixLen: config.Network.PrefixLen,}if err = dev.Configure(vxlanNet); err != nil {return nil, err}return newNetwork(be.subnetMgr, be.extIface, dev, vxlanNet, lease)
}配置vxlan的vtep设备，AcquireLease分配人一个子网租约，就是分配一个网段。再回到main里面，如果设置ipmaq就是添加一个NAT的IPtable是规则，接着就是就是写docker的网络配置文件WriteSubnetFile，最后启动go bn.Run(ctx)，它会阻塞，在另一个协程中运行，MonitorLease就是默认每隔60分钟续约一下。main解析完成，服务启动。
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