摘要：假设你对kube-proxy的工作原理有一定的了解，本文基于kubernetes v1.5代码对kube-proxy的源码目录结构进行了分析，并以iptables mode为例进行了完整流程的源码分析，给出了其内部实现的模块逻辑图，希望对你深入理解kube-proxy有所帮助。

kube-proxy介绍

请参考我的另一篇博文：kube-proxy工作原理

源码目录结构分析

cmd/kube-proxy      //负责kube-proxy的创建，启动的入口
.
├── app
│   ├── conntrack.go    //linux kernel的nf_conntrack-sysctl的interface定义，更多关于conntracker的定义请看https://www.kernel.org/doc/Documentation/networking/nf_conntrack-sysctl.txt
│   ├── options
│   │   └── options.go    //kube-proxy的参数定义ProxyServerConfig及相关方法
│   ├── server.go    //ProxyServer结构定义及其创建(NewProxyServerDefault)和运行(Run)的方法。
│   └── server_test.go
└── proxy.go    //kube-proxy的main方法


pkg/proxy
.
├── OWNERS
├── config
│   ├── api.go    //给proxy配置Service和Endpoint的Reflectors和Cache.Store
│   ├── api_test.go
│   ├── config.go    //定义ServiceUpdate，EndpointUpdate结构体以及ServiceConfigHandler，EndpointConfigHandler来处理Service和Endpoint的Update
│   ├── config_test.go
│   └── doc.go
├── doc.go
├── healthcheck    //负责service listener和endpoint的health check，add/delete请求。
│   ├── api.go
│   ├── doc.go
│   ├── healthcheck.go
│   ├── healthcheck_test.go
│   ├── http.go
│   ├── listener.go
│   └── worker.go
├── iptables    //proxy mode为iptables的实现
│   ├── proxier.go
│   └── proxier_test.go
├── types.go
├── userspace    //proxy mode为userspace的实现
│   ├── loadbalancer.go
│   ├── port_allocator.go
│   ├── port_allocator_test.go
│   ├── proxier.go
│   ├── proxier_test.go
│   ├── proxysocket.go
│   ├── rlimit.go
│   ├── rlimit_windows.go
│   ├── roundrobin.go
│   ├── roundrobin_test.go
│   └── udp_server.go
└── winuserspace    //windows OS时，proxy mode为userspace的实现
    ├── loadbalancer.go
    ├── port_allocator.go
    ├── port_allocator_test.go
    ├── proxier.go
    ├── proxier_test.go
    ├── proxysocket.go
    ├── roundrobin.go
    ├── roundrobin_test.go
    └── udp_server.go

内部实现模块逻辑图

源码分析

main

kube-proxy的main入口在：cmd/kube-proxy/proxy.go:39

func main() {
    //创建kube-proxy的默认config对象
    config := options.NewProxyConfig()
    //用kube-proxy命令行的参数替换默认参数
    config.AddFlags(pflag.CommandLine)

    flag.InitFlags()
    logs.InitLogs()
    defer logs.FlushLogs()

    verflag.PrintAndExitIfRequested()

    //根据config创建ProxyServer
    s, err := app.NewProxyServerDefault(config)
    if err != nil {
        fmt.Fprintf(os.Stderr, "%v\n", err)
        os.Exit(1)
    }

    //执行Run方法让kube-proxy开始干活了
    if err = s.Run(); err != nil {
        fmt.Fprintf(os.Stderr, "%v\n", err)
        os.Exit(1)
    }
}

main方法中，我们重点关注app.NewProxyServerDefault(config)创建ProxyServer和Run方法。

创建ProxyServer

NewProxyServerDefault负责根据提供的config参数创建一个新的ProxyServer对象，其代码比较长，逻辑相对复杂，下面会挑重点说一下。

cmd/kube-proxy/app/server.go:131

func NewProxyServerDefault(config *options.ProxyServerConfig) (*ProxyServer, error) {
    ...

    // Create a iptables utils.
    execer := exec.New()

    if runtime.GOOS == "windows" {
        netshInterface = utilnetsh.New(execer)
    } else {
        dbus = utildbus.New()
        iptInterface = utiliptables.New(execer, dbus, protocol)
    }

    ...
    //设置OOM_SCORE_ADJ
    var oomAdjuster *oom.OOMAdjuster
    if config.OOMScoreAdj != nil {
        oomAdjuster = oom.NewOOMAdjuster()
        if err := oomAdjuster.ApplyOOMScoreAdj(0, int(*config.OOMScoreAdj)); err != nil {
            glog.V(2).Info(err)
        }
    }

    ...

    // Create a Kube Client
    ...

    // 创建event Broadcaster和event recorder
    hostname := nodeutil.GetHostname(config.HostnameOverride)
    eventBroadcaster := record.NewBroadcaster()
    recorder := eventBroadcaster.NewRecorder(v1.EventSource{Component: "kube-proxy", Host: hostname})

    //定义proxier和endpointsHandler，分别用于处理services和endpoints的update event。
    var proxier proxy.ProxyProvider
    var endpointsHandler proxyconfig.EndpointsConfigHandler

    //从config中获取proxy mode
    proxyMode := getProxyMode(string(config.Mode), client.Core().Nodes(), hostname, iptInterface, iptables.LinuxKernelCompatTester{})

    // proxy mode为iptables场景
    if proxyMode == proxyModeIPTables {
        glog.V(0).Info("Using iptables Proxier.")
        if config.IPTablesMasqueradeBit == nil {
            // IPTablesMasqueradeBit must be specified or defaulted.
            return nil, fmt.Errorf("Unable to read IPTablesMasqueradeBit from config")
        }

        //调用pkg/proxy/iptables/proxier.go:222中的iptables.NewProxier来创建proxier，赋值给前面定义的proxier和endpointsHandler，表示由该proxier同时负责service和endpoint的event处理。
        proxierIPTables, err := iptables.NewProxier(iptInterface, utilsysctl.New(), execer, config.IPTablesSyncPeriod.Duration, config.IPTablesMinSyncPeriod.Duration, config.MasqueradeAll, int(*config.IPTablesMasqueradeBit), config.ClusterCIDR, hostname, getNodeIP(client, hostname))
        if err != nil {
            glog.Fatalf("Unable to create proxier: %v", err)
        }
        proxier = proxierIPTables
        endpointsHandler = proxierIPTables
        // No turning back. Remove artifacts that might still exist from the userspace Proxier.
        glog.V(0).Info("Tearing down userspace rules.")
        userspace.CleanupLeftovers(iptInterface)
    } 
    // proxy mode为userspace场景
    else {
        glog.V(0).Info("Using userspace Proxier.")
        // This is a proxy.LoadBalancer which NewProxier needs but has methods we don‘t need for
        // our config.EndpointsConfigHandler.
        loadBalancer := userspace.NewLoadBalancerRR()
        // set EndpointsConfigHandler to our loadBalancer
        endpointsHandler = loadBalancer

        var proxierUserspace proxy.ProxyProvider

        // windows OS场景下，调用pkg/proxy/winuserspace/proxier.go:146的winuserspace.NewProxier来创建proxier。
        if runtime.GOOS == "windows" {
            proxierUserspace, err = winuserspace.NewProxier(
                loadBalancer,
                net.ParseIP(config.BindAddress),
                netshInterface,
                *utilnet.ParsePortRangeOrDie(config.PortRange),
                // TODO @pires replace below with default values, if applicable
                config.IPTablesSyncPeriod.Duration,
                config.UDPIdleTimeout.Duration,
            )
        } 

        // linux OS场景下，调用pkg/proxy/userspace/proxier.go:143的userspace.NewProxier来创建proxier。
        else {
            proxierUserspace, err = userspace.NewProxier(
                loadBalancer,
                net.ParseIP(config.BindAddress),
                iptInterface,
                *utilnet.ParsePortRangeOrDie(config.PortRange),
                config.IPTablesSyncPeriod.Duration,
                config.IPTablesMinSyncPeriod.Duration,
                config.UDPIdleTimeout.Duration,
            )
        }
        if err != nil {
            glog.Fatalf("Unable to create proxier: %v", err)
        }
        proxier = proxierUserspace
        // Remove artifacts from the pure-iptables Proxier, if not on Windows.
        if runtime.GOOS != "windows" {
            glog.V(0).Info("Tearing down pure-iptables proxy rules.")
            iptables.CleanupLeftovers(iptInterface)
        }
    }

    // Add iptables reload function, if not on Windows.
    if runtime.GOOS != "windows" {
        iptInterface.AddReloadFunc(proxier.Sync)
    }

    // Create configs (i.e. Watches for Services and Endpoints)
    // 创建serviceConfig负责service的watchforUpdates
    serviceConfig := proxyconfig.NewServiceConfig()

    //给serviceConfig注册proxier，既添加对应的listener用来处理service update时逻辑。
    serviceConfig.RegisterHandler(proxier)

    // 创建endpointsConfig负责endpoint的watchforUpdates
    endpointsConfig := proxyconfig.NewEndpointsConfig()

    //给endpointsConfig注册endpointsHandler，既添加对应的listener用来处理endpoint update时的逻辑。
    endpointsConfig.RegisterHandler(endpointsHandler)

    //NewSourceAPI creates config source that watches for changes to the services and endpoints.
    //NewSourceAPI通过ListWatch apiserver的Service和endpoint，并周期性的维护serviceStore和endpointStore的更新
    proxyconfig.NewSourceAPI(
        client.Core().RESTClient(),
        config.ConfigSyncPeriod,
        serviceConfig.Channel("api"), //Service Update Channel
        endpointsConfig.Channel("api"),  //endpoint update channel
    )

    ...

    //把前面创建的对象作为参数，构造出ProxyServer对象。
    return NewProxyServer(client, config, iptInterface, proxier, eventBroadcaster, recorder, conntracker, proxyMode)
}

NewProxyServerDefault中的核心逻辑我都已经在上述代码中添加了注释，其中有几个地方需要我们再深入跟进去看看：proxyconfig.NewServiceConfig，proxyconfig.NewEndpointsConfig，serviceConfig.RegisterHandler，endpointsConfig.RegisterHandler，proxyconfig.NewSourceAPI。

proxyconfig.NewServiceConfig

我们对ServiceConfig的代码分析一遍，EndpointsConfig的代码则类似。

pkg/proxy/config/config.go:192
func NewServiceConfig() *ServiceConfig {
    // 创建updates channel
    updates := make(chan struct{}, 1)

    // 构建serviceStore对象
    store := &serviceStore{updates: updates, services: make(map[string]map[types.NamespacedName]api.Service)}
    mux := config.NewMux(store)

    // 新建Broadcaster，在后续的serviceConfig.RegisterHandler会注册该Broadcaster的listener。
    bcaster := config.NewBroadcaster()

    //启动协程，马上开始watch updates channel
    go watchForUpdates(bcaster, store, updates)

    return &ServiceConfig{mux, bcaster, store}
}

下面我们再跟进watchForUpdates去看看。

pkg/proxy/config/config.go:292
func watchForUpdates(bcaster *config.Broadcaster, accessor config.Accessor, updates <-chan struct{}) {
    for true {
        <-updates
        bcaster.Notify(accessor.MergedState())
    }
}

watchForUpdates就是一直在watch updates channel，如果有数据，则立刻由该Broadcaster Notify到注册的listeners。
Notify的代码如下，可见，它负责将数据通知给所有的listener，并调用各个listener的OnUpdate方法。

pkg/util/config/config.go:133
// Notify notifies all listeners.
func (b *Broadcaster) Notify(instance interface{}) {
    b.listenerLock.RLock()
    listeners := b.listeners
    b.listenerLock.RUnlock()
    for _, listener := range listeners {
        listener.OnUpdate(instance)
    }
}

func (f ListenerFunc) OnUpdate(instance interface{}) {
    f(instance)
}

serviceConfig.RegisterHandler

上面分析的proxyconfig.NewServiceConfig负责创建ServiceConfig，开始watch updates channel了，当从channel中取到值的时候，Broadcaster就会通知listener进行处理。serviceConfig.RegisterHandler正是负责给Broadcaster注册listener的，其代码如下。

pkg/proxy/config/config.go:205

func (c *ServiceConfig) RegisterHandler(handler ServiceConfigHandler) {
    //给ServiceConfig的Broadcaster注册listener。
    c.bcaster.Add(config.ListenerFunc(func(instance interface{}) {
        glog.V(3).Infof("Calling handler.OnServiceUpdate()")
        handler.OnServiceUpdate(instance.([]api.Service))
    }))
}

上面分析proxyconfig.NewServiceConfig时可知，当从updates channel中取到值的时候，最终会调用对应的ListenerFunc(instance)进行处理，在这里，也就是调用:

        glog.V(3).Infof("Calling handler.OnServiceUpdate()")
        handler.OnServiceUpdate(instance.([]api.Service))
    }

即调用到handler.OnServiceUpdate。每种proxymode对应的proxier都有对应的handler.OnServiceUpdate接口实现，我们以iptables mode为例，看看handler.OnServiceUpdate的实现：

pkg/proxy/iptables/proxier.go:428
func (proxier *Proxier) OnServiceUpdate(allServices []api.Service) {
    ...

    proxier.syncProxyRules()
    proxier.deleteServiceConnections(staleUDPServices.List())

}

因此，最终关键的逻辑都转向了proxier.syncProxyRules()，从我们上面给出的内部模块交互图也能看得出来。对于proxier.syncProxyRules()，我们放到后面来详细讨论，现在你只要知道proxier.syncProxyRules()负责将proxy中缓存的service/endpoint同步更新到iptables中生成对应Chain和NAT Rules。

proxyconfig.NewEndpointsConfig

endpointsConfig的逻辑和serviceConfig的类似，在这里只给出对应代码，不再跟进分析。

pkg/proxy/config/config.go:84

func NewEndpointsConfig() *EndpointsConfig {
    // The updates channel is used to send interrupts to the Endpoints handler.
    // It‘s buffered because we never want to block for as long as there is a
    // pending interrupt, but don‘t want to drop them if the handler is doing
    // work.
    updates := make(chan struct{}, 1)
    store := &endpointsStore{updates: updates, endpoints: make(map[string]map[types.NamespacedName]api.Endpoints)}
    mux := config.NewMux(store)
    bcaster := config.NewBroadcaster()
    go watchForUpdates(bcaster, store, updates)
    return &EndpointsConfig{mux, bcaster, store}
}

endpointsConfig.RegisterHandler

pkg/proxy/config/config.go:97

func (c *EndpointsConfig) RegisterHandler(handler EndpointsConfigHandler) {
    c.bcaster.Add(config.ListenerFunc(func(instance interface{}) {
        glog.V(3).Infof("Calling handler.OnEndpointsUpdate()")
        handler.OnEndpointsUpdate(instance.([]api.Endpoints))
    }))
}

proxyconfig.NewSourceAPI

proxyconfig.NewSourceAPI是很关键的，它负责给service updates channel和endpoint updates channel配置数据源，它是通过周期性的List和Watch kube-apiserver中的all service and endpoint来提供数据的，发给对应的channel。默认的List周期是15min，可通过--config-sync-period修改。下面来看其具体代码：

func NewSourceAPI(c cache.Getter, period time.Duration, servicesChan chan<- ServiceUpdate, endpointsChan chan<- EndpointsUpdate) {
    servicesLW := cache.NewListWatchFromClient(c, "services", api.NamespaceAll, fields.Everything())
    cache.NewReflector(servicesLW, &api.Service{}, NewServiceStore(nil, servicesChan), period).Run()

    endpointsLW := cache.NewListWatchFromClient(c, "endpoints", api.NamespaceAll, fields.Everything())
    cache.NewReflector(endpointsLW, &api.Endpoints{}, NewEndpointsStore(nil, endpointsChan), period).Run()
}

// NewServiceStore creates an undelta store that expands updates to the store into
// ServiceUpdate events on the channel. If no store is passed, a default store will
// be initialized. Allows reuse of a cache store across multiple components.
func NewServiceStore(store cache.Store, ch chan<- ServiceUpdate) cache.Store {
    fn := func(objs []interface{}) {
        var services []api.Service
        for _, o := range objs {
            services = append(services, *(o.(*api.Service)))
        }
        ch <- ServiceUpdate{Op: SET, Services: services}
    }
    if store == nil {
        store = cache.NewStore(cache.MetaNamespaceKeyFunc)
    }
    return &cache.UndeltaStore{
        Store:    store,
        PushFunc: fn,
    }
}

// NewEndpointsStore creates an undelta store that expands updates to the store into
// EndpointsUpdate events on the channel. If no store is passed, a default store will
// be initialized. Allows reuse of a cache store across multiple components.
func NewEndpointsStore(store cache.Store, ch chan<- EndpointsUpdate) cache.Store {
    fn := func(objs []interface{}) {
        var endpoints []api.Endpoints
        for _, o := range objs {
            endpoints = append(endpoints, *(o.(*api.Endpoints)))
        }
        ch <- EndpointsUpdate{Op: SET, Endpoints: endpoints}
    }
    if store == nil {
        store = cache.NewStore(cache.MetaNamespaceKeyFunc)
    }
    return &cache.UndeltaStore{
        Store:    store,
        PushFunc: fn,
    }
}

代码很简单，不需要过多解释。

执行Run开始工作

创建完ProxyServer后，就执行Run方法开始工作了,它主要负责周期性(default 30s)的同步proxy中的services/endpionts到iptables中生成对应Chain and NAT Rules。

cmd/kube-proxy/app/server.go:308
func (s *ProxyServer) Run() error {
    ...

    // Start up a webserver if requested
    if s.Config.HealthzPort > 0 {
        http.HandleFunc("/proxyMode", func(w http.ResponseWriter, r *http.Request) {
            fmt.Fprintf(w, "%s", s.ProxyMode)
        })
        configz.InstallHandler(http.DefaultServeMux)
        go wait.Until(func() {
            err := http.ListenAndServe(s.Config.HealthzBindAddress+":"+strconv.Itoa(int(s.Config.HealthzPort)), nil)
            if err != nil {
                glog.Errorf("Starting health server failed: %v", err)
            }
        }, 5*time.Second, wait.NeverStop)
    }

    ...

    // Just loop forever for now...
    s.Proxier.SyncLoop()
    return nil
}

Run方法关键代码很简单，就是执行对应proxier的SyncLoop()。我们还是以iptables mode为例，看看它是如何实现SyncLoop()的：

pkg/proxy/iptables/proxier.go:416
// SyncLoop runs periodic work.  This is expected to run as a goroutine or as the main loop of the app.  It does not return.
func (proxier *Proxier) SyncLoop() {
    t := time.NewTicker(proxier.syncPeriod)
    defer t.Stop()
    for {
        <-t.C
        glog.V(6).Infof("Periodic sync")
        proxier.Sync()
    }
}

SyncLoop中，通过设置定时器，默认每30s会执行一次proxier.Sync()，可以通过--iptables-sync-period修改默认时间。那我们继续跟进Sync()的代码：

pkg/proxy/iptables/proxier.go:409
// Sync is called to immediately synchronize the proxier state to iptables
func (proxier *Proxier) Sync() {
    proxier.mu.Lock()
    defer proxier.mu.Unlock()
    proxier.syncProxyRules()
}

可见，最终还是调用proxier.syncProxyRules()。前一节中创建ProxyServer的分析也是一样，最终watch到service/endpoint有更新时，都会调用到proxier.syncProxyRules()。那下面我们就来看看proxier.syncProxyRules()的代码。

proxier.syncProxyRules

下面的proxier.syncProxyRules代码是iptables mode对应的实现。userspace mode的代码我就不贴了。

pkg/proxy/iptables/proxier.go:791
// This is where all of the iptables-save/restore calls happen.
// The only other iptables rules are those that are setup in iptablesInit()
// assumes proxier.mu is held
func (proxier *Proxier) syncProxyRules() {
    if proxier.throttle != nil {
        proxier.throttle.Accept()
    }
    start := time.Now()
    defer func() {
        glog.V(4).Infof("syncProxyRules took %v", time.Since(start))
    }()
    // don‘t sync rules till we‘ve received services and endpoints
    if !proxier.haveReceivedEndpointsUpdate || !proxier.haveReceivedServiceUpdate {
        glog.V(2).Info("Not syncing iptables until Services and Endpoints have been received from master")
        return
    }
    glog.V(3).Infof("Syncing iptables rules")

    // Create and link the kube services chain.
    {
        tablesNeedServicesChain := []utiliptables.Table{utiliptables.TableFilter, utiliptables.TableNAT}
        for _, table := range tablesNeedServicesChain {
            if _, err := proxier.iptables.EnsureChain(table, kubeServicesChain); err != nil {
                glog.Errorf("Failed to ensure that %s chain %s exists: %v", table, kubeServicesChain, err)
                return
            }
        }

        tableChainsNeedJumpServices := []struct {
            table utiliptables.Table
            chain utiliptables.Chain
        }{
            {utiliptables.TableFilter, utiliptables.ChainOutput},
            {utiliptables.TableNAT, utiliptables.ChainOutput},
            {utiliptables.TableNAT, utiliptables.ChainPrerouting},
        }
        comment := "kubernetes service portals"
        args := []string{"-m", "comment", "--comment", comment, "-j", string(kubeServicesChain)}
        for _, tc := range tableChainsNeedJumpServices {
            if _, err := proxier.iptables.EnsureRule(utiliptables.Prepend, tc.table, tc.chain, args...); err != nil {
                glog.Errorf("Failed to ensure that %s chain %s jumps to %s: %v", tc.table, tc.chain, kubeServicesChain, err)
                return
            }
        }
    }

    // Create and link the kube postrouting chain.
    {
        if _, err := proxier.iptables.EnsureChain(utiliptables.TableNAT, kubePostroutingChain); err != nil {
            glog.Errorf("Failed to ensure that %s chain %s exists: %v", utiliptables.TableNAT, kubePostroutingChain, err)
            return
        }

        comment := "kubernetes postrouting rules"
        args := []string{"-m", "comment", "--comment", comment, "-j", string(kubePostroutingChain)}
        if _, err := proxier.iptables.EnsureRule(utiliptables.Prepend, utiliptables.TableNAT, utiliptables.ChainPostrouting, args...); err != nil {
            glog.Errorf("Failed to ensure that %s chain %s jumps to %s: %v", utiliptables.TableNAT, utiliptables.ChainPostrouting, kubePostroutingChain, err)
            return
        }
    }

    // Get iptables-save output so we can check for existing chains and rules.
    // This will be a map of chain name to chain with rules as stored in iptables-save/iptables-restore
    existingFilterChains := make(map[utiliptables.Chain]string)
    iptablesSaveRaw, err := proxier.iptables.Save(utiliptables.TableFilter)
    if err != nil { // if we failed to get any rules
        glog.Errorf("Failed to execute iptables-save, syncing all rules: %v", err)
    } else { // otherwise parse the output
        existingFilterChains = utiliptables.GetChainLines(utiliptables.TableFilter, iptablesSaveRaw)
    }

    existingNATChains := make(map[utiliptables.Chain]string)
    iptablesSaveRaw, err = proxier.iptables.Save(utiliptables.TableNAT)
    if err != nil { // if we failed to get any rules
        glog.Errorf("Failed to execute iptables-save, syncing all rules: %v", err)
    } else { // otherwise parse the output
        existingNATChains = utiliptables.GetChainLines(utiliptables.TableNAT, iptablesSaveRaw)
    }

    filterChains := bytes.NewBuffer(nil)
    filterRules := bytes.NewBuffer(nil)
    natChains := bytes.NewBuffer(nil)
    natRules := bytes.NewBuffer(nil)

    // Write table headers.
    writeLine(filterChains, "*filter")
    writeLine(natChains, "*nat")

    // Make sure we keep stats for the top-level chains, if they existed
    // (which most should have because we created them above).
    if chain, ok := existingFilterChains[kubeServicesChain]; ok {
        writeLine(filterChains, chain)
    } else {
        writeLine(filterChains, utiliptables.MakeChainLine(kubeServicesChain))
    }
    if chain, ok := existingNATChains[kubeServicesChain]; ok {
        writeLine(natChains, chain)
    } else {
        writeLine(natChains, utiliptables.MakeChainLine(kubeServicesChain))
    }
    if chain, ok := existingNATChains[kubeNodePortsChain]; ok {
        writeLine(natChains, chain)
    } else {
        writeLine(natChains, utiliptables.MakeChainLine(kubeNodePortsChain))
    }
    if chain, ok := existingNATChains[kubePostroutingChain]; ok {
        writeLine(natChains, chain)
    } else {
        writeLine(natChains, utiliptables.MakeChainLine(kubePostroutingChain))
    }
    if chain, ok := existingNATChains[KubeMarkMasqChain]; ok {
        writeLine(natChains, chain)
    } else {
        writeLine(natChains, utiliptables.MakeChainLine(KubeMarkMasqChain))
    }

    // Install the kubernetes-specific postrouting rules. We use a whole chain for
    // this so that it is easier to flush and change, for example if the mark
    // value should ever change.
    writeLine(natRules, []string{
        "-A", string(kubePostroutingChain),
        "-m", "comment", "--comment", `"kubernetes service traffic requiring SNAT"`,
        "-m", "mark", "--mark", proxier.masqueradeMark,
        "-j", "MASQUERADE",
    }...)

    // Install the kubernetes-specific masquerade mark rule. We use a whole chain for
    // this so that it is easier to flush and change, for example if the mark
    // value should ever change.
    writeLine(natRules, []string{
        "-A", string(KubeMarkMasqChain),
        "-j", "MARK", "--set-xmark", proxier.masqueradeMark,
    }...)

    // Accumulate NAT chains to keep.
    activeNATChains := map[utiliptables.Chain]bool{} // use a map as a set

    // Accumulate the set of local ports that we will be holding open once this update is complete
    replacementPortsMap := map[localPort]closeable{}

    // Build rules for each service.
    for svcName, svcInfo := range proxier.serviceMap {
        protocol := strings.ToLower(string(svcInfo.protocol))

        // Create the per-service chain, retaining counters if possible.
        svcChain := servicePortChainName(svcName, protocol)
        if chain, ok := existingNATChains[svcChain]; ok {
            writeLine(natChains, chain)
        } else {
            writeLine(natChains, utiliptables.MakeChainLine(svcChain))
        }
        activeNATChains[svcChain] = true

        svcXlbChain := serviceLBChainName(svcName, protocol)
        if svcInfo.onlyNodeLocalEndpoints {
            // Only for services with the externalTraffic annotation set to OnlyLocal
            // create the per-service LB chain, retaining counters if possible.
            if lbChain, ok := existingNATChains[svcXlbChain]; ok {
                writeLine(natChains, lbChain)
            } else {
                writeLine(natChains, utiliptables.MakeChainLine(svcXlbChain))
            }
            activeNATChains[svcXlbChain] = true
        } else if activeNATChains[svcXlbChain] {
            // Cleanup the previously created XLB chain for this service
            delete(activeNATChains, svcXlbChain)
        }

        // Capture the clusterIP.
        args := []string{
            "-A", string(kubeServicesChain),
            "-m", "comment", "--comment", fmt.Sprintf(`"%s cluster IP"`, svcName.String()),
            "-m", protocol, "-p", protocol,
            "-d", fmt.Sprintf("%s/32", svcInfo.clusterIP.String()),
            "--dport", fmt.Sprintf("%d", svcInfo.port),
        }
        if proxier.masqueradeAll {
            writeLine(natRules, append(args, "-j", string(KubeMarkMasqChain))...)
        }
        if len(proxier.clusterCIDR) > 0 {
            writeLine(natRules, append(args, "! -s", proxier.clusterCIDR, "-j", string(KubeMarkMasqChain))...)
        }
        writeLine(natRules, append(args, "-j", string(svcChain))...)

        // Capture externalIPs.
        for _, externalIP := range svcInfo.externalIPs {
            // If the "external" IP happens to be an IP that is local to this
            // machine, hold the local port open so no other process can open it
            // (because the socket might open but it would never work).
            if local, err := isLocalIP(externalIP); err != nil {
                glog.Errorf("can‘t determine if IP is local, assuming not: %v", err)
            } else if local {
                lp := localPort{
                    desc:     "externalIP for " + svcName.String(),
                    ip:       externalIP,
                    port:     svcInfo.port,
                    protocol: protocol,
                }
                if proxier.portsMap[lp] != nil {
                    glog.V(4).Infof("Port %s was open before and is still needed", lp.String())
                    replacementPortsMap[lp] = proxier.portsMap[lp]
                } else {
                    socket, err := proxier.portMapper.OpenLocalPort(&lp)
                    if err != nil {
                        glog.Errorf("can‘t open %s, skipping this externalIP: %v", lp.String(), err)
                        continue
                    }
                    replacementPortsMap[lp] = socket
                }
            } // We‘re holding the port, so it‘s OK to install iptables rules.
            args := []string{
                "-A", string(kubeServicesChain),
                "-m", "comment", "--comment", fmt.Sprintf(`"%s external IP"`, svcName.String()),
                "-m", protocol, "-p", protocol,
                "-d", fmt.Sprintf("%s/32", externalIP),
                "--dport", fmt.Sprintf("%d", svcInfo.port),
            }
            // We have to SNAT packets to external IPs.
            writeLine(natRules, append(args, "-j", string(KubeMarkMasqChain))...)

            // Allow traffic for external IPs that does not come from a bridge (i.e. not from a container)
            // nor from a local process to be forwarded to the service.
            // This rule roughly translates to "all traffic from off-machine".
            // This is imperfect in the face of network plugins that might not use a bridge, but we can revisit that later.
            externalTrafficOnlyArgs := append(args,
                "-m", "physdev", "!", "--physdev-is-in",
                "-m", "addrtype", "!", "--src-type", "LOCAL")
            writeLine(natRules, append(externalTrafficOnlyArgs, "-j", string(svcChain))...)
            dstLocalOnlyArgs := append(args, "-m", "addrtype", "--dst-type", "LOCAL")
            // Allow traffic bound for external IPs that happen to be recognized as local IPs to stay local.
            // This covers cases like GCE load-balancers which get added to the local routing table.
            writeLine(natRules, append(dstLocalOnlyArgs, "-j", string(svcChain))...)
        }

        // Capture load-balancer ingress.
        for _, ingress := range svcInfo.loadBalancerStatus.Ingress {
            if ingress.IP != "" {
                // create service firewall chain
                fwChain := serviceFirewallChainName(svcName, protocol)
                if chain, ok := existingNATChains[fwChain]; ok {
                    writeLine(natChains, chain)
                } else {
                    writeLine(natChains, utiliptables.MakeChainLine(fwChain))
                }
                activeNATChains[fwChain] = true
                // The service firewall rules are created based on ServiceSpec.loadBalancerSourceRanges field.
                // This currently works for loadbalancers that preserves source ips.
                // For loadbalancers which direct traffic to service NodePort, the firewall rules will not apply.

                args := []string{
                    "-A", string(kubeServicesChain),
                    "-m", "comment", "--comment", fmt.Sprintf(`"%s loadbalancer IP"`, svcName.String()),
                    "-m", protocol, "-p", protocol,
                    "-d", fmt.Sprintf("%s/32", ingress.IP),
                    "--dport", fmt.Sprintf("%d", svcInfo.port),
                }
                // jump to service firewall chain
                writeLine(natRules, append(args, "-j", string(fwChain))...)

                args = []string{
                    "-A", string(fwChain),
                    "-m", "comment", "--comment", fmt.Sprintf(`"%s loadbalancer IP"`, svcName.String()),
                }

                // Each source match rule in the FW chain may jump to either the SVC or the XLB chain
                chosenChain := svcXlbChain
                // If we are proxying globally, we need to masquerade in case we cross nodes.
                // If we are proxying only locally, we can retain the source IP.
                if !svcInfo.onlyNodeLocalEndpoints {
                    writeLine(natRules, append(args, "-j", string(KubeMarkMasqChain))...)
                    chosenChain = svcChain
                }

                if len(svcInfo.loadBalancerSourceRanges) == 0 {
                    // allow all sources, so jump directly to the KUBE-SVC or KUBE-XLB chain
                    writeLine(natRules, append(args, "-j", string(chosenChain))...)
                } else {
                    // firewall filter based on each source range
                    allowFromNode := false
                    for _, src := range svcInfo.loadBalancerSourceRanges {
                        writeLine(natRules, append(args, "-s", src, "-j", string(chosenChain))...)
                        // ignore error because it has been validated
                        _, cidr, _ := net.ParseCIDR(src)
                        if cidr.Contains(proxier.nodeIP) {
                            allowFromNode = true
                        }
                    }
                    // generally, ip route rule was added to intercept request to loadbalancer vip from the
                    // loadbalancer‘s backend hosts. In this case, request will not hit the loadbalancer but loop back directly.
                    // Need to add the following rule to allow request on host.
                    if allowFromNode {
                        writeLine(natRules, append(args, "-s", fmt.Sprintf("%s/32", ingress.IP), "-j", string(chosenChain))...)
                    }
                }

                // If the packet was able to reach the end of firewall chain, then it did not get DNATed.
                // It means the packet cannot go thru the firewall, then mark it for DROP
                writeLine(natRules, append(args, "-j", string(KubeMarkDropChain))...)
            }
        }

        // Capture nodeports.  If we had more than 2 rules it might be
        // worthwhile to make a new per-service chain for nodeport rules, but
        // with just 2 rules it ends up being a waste and a cognitive burden.
        if svcInfo.nodePort != 0 {
            // Hold the local port open so no other process can open it
            // (because the socket might open but it would never work).
            lp := localPort{
                desc:     "nodePort for " + svcName.String(),
                ip:       "",
                port:     svcInfo.nodePort,
                protocol: protocol,
            }
            if proxier.portsMap[lp] != nil {
                glog.V(4).Infof("Port %s was open before and is still needed", lp.String())
                replacementPortsMap[lp] = proxier.portsMap[lp]
            } else {
                socket, err := proxier.portMapper.OpenLocalPort(&lp)
                if err != nil {
                    glog.Errorf("can‘t open %s, skipping this nodePort: %v", lp.String(), err)
                    continue
                }
                if lp.protocol == "udp" {
                    proxier.clearUdpConntrackForPort(lp.port)
                }
                replacementPortsMap[lp] = socket
            } // We‘re holding the port, so it‘s OK to install iptables rules.

            args := []string{
                "-A", string(kubeNodePortsChain),
                "-m", "comment", "--comment", svcName.String(),
                "-m", protocol, "-p", protocol,
                "--dport", fmt.Sprintf("%d", svcInfo.nodePort),
            }
            if !svcInfo.onlyNodeLocalEndpoints {
                // Nodeports need SNAT, unless they‘re local.
                writeLine(natRules, append(args, "-j", string(KubeMarkMasqChain))...)
                // Jump to the service chain.
                writeLine(natRules, append(args, "-j", string(svcChain))...)
            } else {
                // TODO: Make all nodePorts jump to the firewall chain.
                // Currently we only create it for loadbalancers (#33586).
                writeLine(natRules, append(args, "-j", string(svcXlbChain))...)
            }
        }

        // If the service has no endpoints then reject packets.
        if len(proxier.endpointsMap[svcName]) == 0 {
            writeLine(filterRules,
                "-A", string(kubeServicesChain),
                "-m", "comment", "--comment", fmt.Sprintf(`"%s has no endpoints"`, svcName.String()),
                "-m", protocol, "-p", protocol,
                "-d", fmt.Sprintf("%s/32", svcInfo.clusterIP.String()),
                "--dport", fmt.Sprintf("%d", svcInfo.port),
                "-j", "REJECT",
            )
            continue
        }

        // Generate the per-endpoint chains.  We do this in multiple passes so we
        // can group rules together.
        // These two slices parallel each other - keep in sync
        endpoints := make([]*endpointsInfo, 0)
        endpointChains := make([]utiliptables.Chain, 0)
        for _, ep := range proxier.endpointsMap[svcName] {
            endpoints = append(endpoints, ep)
            endpointChain := servicePortEndpointChainName(svcName, protocol, ep.ip)
            endpointChains = append(endpointChains, endpointChain)

            // Create the endpoint chain, retaining counters if possible.
            if chain, ok := existingNATChains[utiliptables.Chain(endpointChain)]; ok {
                writeLine(natChains, chain)
            } else {
                writeLine(natChains, utiliptables.MakeChainLine(endpointChain))
            }
            activeNATChains[endpointChain] = true
        }

        // First write session affinity rules, if applicable.
        if svcInfo.sessionAffinityType == api.ServiceAffinityClientIP {
            for _, endpointChain := range endpointChains {
                writeLine(natRules,
                    "-A", string(svcChain),
                    "-m", "comment", "--comment", svcName.String(),
                    "-m", "recent", "--name", string(endpointChain),
                    "--rcheck", "--seconds", fmt.Sprintf("%d", svcInfo.stickyMaxAgeMinutes*60), "--reap",
                    "-j", string(endpointChain))
            }
        }

        // Now write loadbalancing & DNAT rules.
        n := len(endpointChains)
        for i, endpointChain := range endpointChains {
            // Balancing rules in the per-service chain.
            args := []string{
                "-A", string(svcChain),
                "-m", "comment", "--comment", svcName.String(),
            }
            if i < (n - 1) {
                // Each rule is a probabilistic match.
                args = append(args,
                    "-m", "statistic",
                    "--mode", "random",
                    "--probability", fmt.Sprintf("%0.5f", 1.0/float64(n-i)))
            }
            // The final (or only if n == 1) rule is a guaranteed match.
            args = append(args, "-j", string(endpointChain))
            writeLine(natRules, args...)

            // Rules in the per-endpoint chain.
            args = []string{
                "-A", string(endpointChain),
                "-m", "comment", "--comment", svcName.String(),
            }
            // Handle traffic that loops back to the originator with SNAT.
            writeLine(natRules, append(args,
                "-s", fmt.Sprintf("%s/32", strings.Split(endpoints[i].ip, ":")[0]),
                "-j", string(KubeMarkMasqChain))...)
            // Update client-affinity lists.
            if svcInfo.sessionAffinityType == api.ServiceAffinityClientIP {
                args = append(args, "-m", "recent", "--name", string(endpointChain), "--set")
            }
            // DNAT to final destination.
            args = append(args, "-m", protocol, "-p", protocol, "-j", "DNAT", "--to-destination", endpoints[i].ip)
            writeLine(natRules, args...)
        }

        // The logic below this applies only if this service is marked as OnlyLocal
        if !svcInfo.onlyNodeLocalEndpoints {
            continue
        }

        // Now write ingress loadbalancing & DNAT rules only for services that have a localOnly annotation
        // TODO - This logic may be combinable with the block above that creates the svc balancer chain
        localEndpoints := make([]*endpointsInfo, 0)
        localEndpointChains := make([]utiliptables.Chain, 0)
        for i := range endpointChains {
            if endpoints[i].localEndpoint {
                // These slices parallel each other; must be kept in sync
                localEndpoints = append(localEndpoints, endpoints[i])
                localEndpointChains = append(localEndpointChains, endpointChains[i])
            }
        }
        // First rule in the chain redirects all pod -> external vip traffic to the
        // Service‘s ClusterIP instead. This happens whether or not we have local
        // endpoints; only if clusterCIDR is specified
        if len(proxier.clusterCIDR) > 0 {
            args = []string{
                "-A", string(svcXlbChain),
                "-m", "comment", "--comment",
                fmt.Sprintf(`"Redirect pods trying to reach external loadbalancer VIP to clusterIP"`),
                "-s", proxier.clusterCIDR,
                "-j", string(svcChain),
            }
            writeLine(natRules, args...)
        }

        numLocalEndpoints := len(localEndpointChains)
        if numLocalEndpoints == 0 {
            // Blackhole all traffic since there are no local endpoints
            args := []string{
                "-A", string(svcXlbChain),
                "-m", "comment", "--comment",
                fmt.Sprintf(`"%s has no local endpoints"`, svcName.String()),
                "-j",
                string(KubeMarkDropChain),
            }
            writeLine(natRules, args...)
        } else {
            // Setup probability filter rules only over local endpoints
            for i, endpointChain := range localEndpointChains {
                // Balancing rules in the per-service chain.
                args := []string{
                    "-A", string(svcXlbChain),
                    "-m", "comment", "--comment",
                    fmt.Sprintf(`"Balancing rule %d for %s"`, i, svcName.String()),
                }
                if i < (numLocalEndpoints - 1) {
                    // Each rule is a probabilistic match.
                    args = append(args,
                        "-m", "statistic",
                        "--mode", "random",
                        "--probability", fmt.Sprintf("%0.5f", 1.0/float64(numLocalEndpoints-i)))
                }
                // The final (or only if n == 1) rule is a guaranteed match.
                args = append(args, "-j", string(endpointChain))
                writeLine(natRules, args...)
            }
        }
    }

    // Delete chains no longer in use.
    for chain := range existingNATChains {
        if !activeNATChains[chain] {
            chainString := string(chain)
            if !strings.HasPrefix(chainString, "KUBE-SVC-") && !strings.HasPrefix(chainString, "KUBE-SEP-") && !strings.HasPrefix(chainString, "KUBE-FW-") && !strings.HasPrefix(chainString, "KUBE-XLB-") {
                // Ignore chains that aren‘t ours.
                continue
            }
            // We must (as per iptables) write a chain-line for it, which has
            // the nice effect of flushing the chain.  Then we can remove the
            // chain.
            writeLine(natChains, existingNATChains[chain])
            writeLine(natRules, "-X", chainString)
        }
    }

    // Finally, tail-call to the nodeports chain.  This needs to be after all
    // other service portal rules.
    writeLine(natRules,
        "-A", string(kubeServicesChain),
        "-m", "comment", "--comment", `"kubernetes service nodeports; NOTE: this must be the last rule in this chain"`,
        "-m", "addrtype", "--dst-type", "LOCAL",
        "-j", string(kubeNodePortsChain))

    // Write the end-of-table markers.
    writeLine(filterRules, "COMMIT")
    writeLine(natRules, "COMMIT")

    // Sync rules.
    // NOTE: NoFlushTables is used so we don‘t flush non-kubernetes chains in the table.
    filterLines := append(filterChains.Bytes(), filterRules.Bytes()...)
    natLines := append(natChains.Bytes(), natRules.Bytes()...)
    lines := append(filterLines, natLines...)

    glog.V(3).Infof("Restoring iptables rules: %s", lines)
    err = proxier.iptables.RestoreAll(lines, utiliptables.NoFlushTables, utiliptables.RestoreCounters)
    if err != nil {
        glog.Errorf("Failed to execute iptables-restore: %v\nRules:\n%s", err, lines)
        // Revert new local ports.
        revertPorts(replacementPortsMap, proxier.portsMap)
        return
    }

    // Close old local ports and save new ones.
    for k, v := range proxier.portsMap {
        if replacementPortsMap[k] == nil {
            v.Close()
        }
    }
    proxier.portsMap = replacementPortsMap
}

看到这么长的方法，本来想多写一点分析注释的，结果我看完已经肌无力了。
如果你自己又k8s的环境，找一台node，查看其iptables，对着下面的代码来看会好很多。
如果你没有环境，没关系，可以参考到我的上一篇博文kube-proxy工作原理查看对应的Example。

总结

• kube-proxy实现了两种linux下的proxy mode：userspace和iptables，实现了一种windows下的proxy mode：userspace。
• kube-proxy通过周期性的List and Watch kube-apiserver的all service and endpiont Resources，通过Channels传给对应的Broadcaster，由Broadcaster Notify给Proxier注册的Listener。List周期默认15min，可通过--config-sync-period配置。
• Listener实现OnServiceUpdate和OnEndpointsUpdate接口，最终调用proxier.syncProxyRules()更新iptables。
• 另外，Proxy Run方法负责周期性的调用proxier.syncProxyRules()更新iptables，默认30s一次，可通过--iptables-sync-period配置。