ArthurChiao's Blog

Cilium Code Walk Through: Node & Endpoint Health Probe

Published at 2020-12-31 | Last Update 2020-12-31

This post also provides a Chinese version.

This post belongs to the Cilium Code Walk Through Series.

This post gives a brief introduction to the Cilium health probe stuffs. Code based on 1.8.4.


Call diagram:

Call stack:

runDaemon                                        // daemon/cmd/daemon_main.go
  |-initHealth                                   // daemon/cmd/health.go
  |  |-health.Launch                             // cilium-health/launch/launcher.go
  |  |  |-ch.server = server.NewServer()
  |  |  |             |-newServer
  |  |  |                |-api := restapi.NewCiliumHealthAPI
  |  |  |                |-srv := healthApi.NewServer(api)
  |  |  |-ch.client = client.NewDefaultClient()
  |  |  |-go runServer()                         // cilium-health/launch/launcher.go
  |  |      |-go ch.server.Serve()
  |  |      |     |-for { go tcpServer.Serve() }
  |  |      |     |-go runActiveServices()
  |  |      |           |-s.FetchStatusResponse
  |  |      |           |   |-getAllNodes
  |  |      |           |   | |-s.Daemon.GetClusterNodes
  |  |      |           |   |    |-"GET /cluster/nodes" to cilium-agent API
  |  |      |           |   |-prober.Run
  |  |      |           |   |   |-p.Pinger.Run
  |  |      |           |   |   |-p.runHTTPProbe
  |  |      |           |   |        |-for node in nodes:
  |  |      |           |   |            for ip in [nodeIP, healthIP]:
  |  |      |           |   |              for port in ports:
  |  |      |           |   |                httpProbe(node, ip, port) // port=4240
  |  |      |           |   |-updateCluster(prober.Results)
  |  |      |           |-s.getNodes
  |  |      |           |-prober.OnIdle = func() {
  |  |      |           |   updateCluster(prober.Results)
  |  |      |           |   nodesAdded, nodesRemoved := getNodes()
  |  |      |           |     |-s.Daemon.GetClusterNodes
  |  |      |           |        |-"GET /cluster/nodes" to cilium-agent API
  |  |      |           |   prober.setNodes(nodesAdded, nodesRemoved)
  |  |      |           | }
  |  |      |           |-prober.RunLoop
  |  |      |           |-s.Server.Serve()  // api/v1/health/server/server.go
  |  |      |              |-s.Listen()     // listen at unix://xxx/health.sock
  |  |      |              |-if unix sock:
  |  |      |                  |-domainSocket.Serve(l)
  |  |      |-for {
  |  |          ch.client.Restapi.GetHealthz()
  |  |          ch.setStatus(status)
  |  |        }
  |  |
  |  |-pidfile.Remove("/var/run/cilium/state/")
  |  |-UpdateController("cilium-health-ep", func() {
  |      DoFunc: func() {
  |          LaunchAsEndpoint 
  |          RunInterval: 60 * time.Second,
  |      }
  |    })
  |-startAgentHealthHTTPService                 // daemon/cmd/agenthealth.go
      |- mux.Handle("/healthz")

1 Design

1.1 Full-mesh probe

By design, each Cilium node can actively probe all other nodes about their health statuses, and in this way each node always keeps a fresh view of the global health status of the cluster.

As each node will probe all other nodes, the probing paths turn out to be a full-mesh network, as shown below:

Fig. Full-mesh health probing of Cilium nodes

The probing behavior is controlled by two options in cilium-agent, both with default true:

  1. enable-health-checking: probe the status of other nodes.
  2. enable-endpoint-health-checking: probe the status of the cilium-health-ep on other nodes. We will detail on cilium-health-ep later.

1.2 Types of probes

Speaking from the viewpoint of network layers, there are two kinds of probes:

  1. L3 probe: ping (ICMP)
  2. L7 probe: GET <HTTP API>

And by combining the two options, there are 4 types of probes in total:

  1. enable-health-checking=true:

    1. ICMP probe (L3): ping <NodeIP>
    2. HTTP probe (L7): GET http://<NodeIP>:4240/hello
  2. enable-endpoint-health-checking=true:

    1. ICMP probe (L3): ping <HealthIP>
    2. HTTP probe (L7): GET http://<HealthIP>:4240/hello

1.3 Probe results

The agent stores the probe results to a local cache, and we can retrieve it via CLI:

(node1) $ cilium-health status
Probe time:   2020-12-29T15:17:02Z
  cluster1/node1 (localhost):
    Host connectivity to        # <-- NodeIP
      ICMP to stack:   OK, RTT=9.557967ms
      HTTP to agent:   OK, RTT=405.072µs
    Endpoint connectivity to   # <-- HealthIP
      ICMP to stack:   OK, RTT=9.951333ms
      HTTP to agent:   OK, RTT=468.645µs
    Host connectivity to        # <-- NodeIP
      ICMP to stack:   OK, RTT=10.164048ms
      HTTP to agent:   OK, RTT=694.196µs
    Endpoint connectivity to     # <-- HealthIP
      ICMP to stack:   OK, RTT=11.282117ms
      HTTP to agent:   OK, RTT=765.092µs

If clustermesh is enabled, cilium-agent will also probe the nodes that are in other K8s clusters inside the clustermesh, that’s why we can see nodes from different clusters in the above output.

1.4 A special endpoint: cilium-health-ep

In its simple way, cilium-agent creates an Endpoint for each Pod on the node. cilium-health-ep is a special Endpoint:

(node) $ cilium endpoint list
ENDPOINT   POLICY (ingress)   POLICY (egress)   IDENTITY   LABELS (source:key[=value])  IPv4         STATUS
2399       Disabled           Disabled          4          reserved:health       ready

It is not a Pod, but as we can see, it has its own

  1. Endpoint ID: randomly allocated by cilium-agent, unique within each node.
  2. Identity: fixed value 4, which belongs to the reserved identity set. And that means, cilium-health-ep on each node has the same identity value 4.
  3. IP address: randomly allocated by cilium-agent.
  4. Veth pair: lxc_health@<peer>

We could also check the IP address of cilium-health-ep with the following command:

$ cilium status --all-addresses | grep health (health)

1.5 Summary

From the above discussions we can conclude that the health probe mechanism in Cilium is simple and straight forward.

But when it comes to implementation, things become complicated.

2 Implementation

The entire flow diagram and calling stack has been given at the very beginning of this post. In the next, let’s walk through them steps by steps.

2.1 Initialization: high level overview

Starting from the initialization of cilium-agent daemon.

Major steps:

  1. Call initHealth(), most of the remaining work will be done here.

    1. Init prober, probe all other nodes sequentially.
    2. Launch cilium-health-ep. This step doesn’t wait for the above step’s finishing, they are independent steps.
  2. Register cilium-agent /healthz API and start serving this API.

    • This API reports cilium-agent health status.
    • cilium status --brief internally calls this API.

One important thing to note is that, initHealth() creates many goroutines to initalize the health subsystem (including the sequential probing to all nodes), and /healthz API doesn’t wait former’s finishing. So usually cilium-agent /healthz API quickly becomes ready and returns OK if you GET it, but the health probing to all nodes may take several seconds, minutes, or even hours.

As most of the heavy work is done in initHealth(), let dig inside.

2.2 initHealth() -> Launch() -> runServer() -> server.Serve()

Steps in initHealth():

  1. Call health.Launch(), the latter performs

    1. Init ch.server
    2. Init ch.client
    3. Call go ch.runServer(), most of the remaining work will be done there.
  2. Remove stale pid file (/var/run/cilium/state/
  3. Create cilium-health-ep controller (periodic task), the latter will perform

    1. Create cilium-health-ep Endpoint,
    2. Make ensure cilium-health-ep is alive, recreate it if needed

Then let’s see the heavy work in go ch.runServer().

2.3 runServer()

Code at cilium-health/launch/launcher.go.


  1. Wait cilium-agent starts successfully (repeatedly GET /healthz until OK)
  2. Remove stale sock file /var/run/cilium/health.sock. When you execute cilium-health command, it will send requests to this sock.
  3. Call go ch.server.Serve(): start cilium-health API server in a new goroutine,

    1. Spawn TCP servers goroutines
    2. Spawn a goroutine to run runActiveServices(). This will create the prober and unix/http/https servers, and the unix server will re-create the health.sock file at its Listen() method.
  4. Blocking until new health.sock file is ready, then set permission for this file.

  5. Periodically GET /healthz from ciliu-agent, and cache the result to daemon.


// pkg/health/server/server.go

// Serve spins up the following goroutines:
// * TCP API Server: Responders to the health API "/hello" message, one per path
// * Prober: Periodically run pings across the cluster, update server's connectivity status cache.
// * Unix API Server: Handle all health API requests over a unix socket.
func (s *Server) Serve() (err error) {
    for i := range s.tcpServers {
        srv := s.tcpServers[i]
        go func() {
            errors <- srv.Serve()

    go func() {
        errors <- s.runActiveServices()

    err = <-errors // Block for the first error, then return.
    return err

Most of the remaing work will be done in runActiveServices().

2.4 runActiveServices()


  1. Call FetchStatusResponse(), which will probe all nodes sequentially.
  2. Set up OnIdle() handler for the prober, then start prober.RunLoop(), which will periodically update current node set.
  3. Call s.Server.Seve(), start to serve Unix/HTTP/HTTPS requests.

Note the 1st step,

  1. The probe process is sequential over all other nodes,
  2. If a node is not reachable, L7 check will time out after 30s.

So, if there are lots of nodes that not reachable via L7 probe, step 3 would be delayed. Specifically, we could see the following errors when executing cilium-health command:

$ cilium-health status
Error: Cannot get status: Get "http://%2Fvar%2Frun%2Fcilium%2Fhealth.sock/v1beta/status": dial unix /var/run/cilium/health.sock: connect: no such file or directory

As this sock file will be created later, in step 3 s.Server.Serve() -> Listen().


// pkg/health/server/server.go

// Run services that are actively probing other hosts and endpoints over ICMP and HTTP,
// and hosting the health admin API on a local Unix socket.
func (s *Server) runActiveServices() error {

    nodesAdded, _, _ := s.getNodes()
    prober := newProber(s, nodesAdded)
    prober.OnIdle = func() {
        // Fetch results and update set of nodes to probe every ProbeInterval

        if nodesAdded, nodesRemoved, err := s.getNodes(); err != nil {
            log.WithError(err).Error("unable to get cluster nodes")
        } else {
            prober.setNodes(nodesAdded, nodesRemoved)

    return s.Server.Serve()

The last line s.Server.Serve() is implemented as:

// api/v1/server/server.go

// Serve the api
func (s *Server) Serve() (err error) {
    if !s.hasListeners
        s.Listen() // net.Listen(s.SocketPath) -> create sock file

    if s.handler == nil // set default handler, if none is set

    if s.hasScheme(schemeUnix) { // "Serving cilium at unix://%s", s.SocketPath
        go func(l net.Listener) {

    if s.hasScheme(schemeHTTP) {
    if s.hasScheme(schemeHTTPS) {

    return nil

2.5 Launch cilium-health-ep

As shown above, creating cilium-health-ep is independent of the steps in section 2.2~2.4 (the latter is asynchronously executed since go runServer()).

cilium-health-ep is a Cilium Endpoint, so it will also go through the following endpoint creating procedure:

  1. Allocate IP address,
  2. Create netns,
  3. Create veth pair (lxc_health),
  4. Create Endpoint
  5. Allocate identity: note that in Cilium, identity is allocated after endpoint creation.
  6. Regenerate BPF

Code: cilium-health/launch/endpoint.go.

3 CLI cheat sheet

Some health related CLIs.

3.1 Check cilium agent status

  • cilium status --brief: brief output, which internally performs "GET /healthz"
  • cilium status: normal output
  • cilium status --verbose: verbose output

3.2 Check connectivity results

$ cilium-health status

$ cilium status --verbose
Cluster health:                          265/268 reachable   (2020-12-31T09:00:03Z)
  Name                         IP                Node          Endpoints
  cluster1/node1 (localhost)         reachable     reachable
  cluster2/node2             reachable     reachable

3.3 Check health info in CT/NAT tables

ICMP records in Conntrack (CT) table and NAT table:

$ cilium bpf ct list global | grep ICMP |head -n4
ICMP IN -> expires=1987899 RxPackets=1 RxBytes=50 RxFlagsSeen=0x00 LastRxReport=1987839 TxPackets=1 TxBytes=50 TxFlagsSeen=0x00 LastTxReport=1987839 Flags=0x0000 [ ] RevNAT=0 SourceSecurityID=2 IfIndex=0
ICMP OUT -> expires=1987951 RxPackets=0 RxBytes=0 RxFlagsSeen=0x00 LastRxReport=0 TxPackets=1 TxBytes=50 TxFlagsSeen=0x00 LastTxReport=1987891 Flags=0x0000 [ ] RevNAT=0 SourceSecurityID=0 IfIndex=0

$ cilium bpf nat list | grep ICMP |head -n4
ICMP OUT -> XLATE_SRC Created=1987884sec HostLocal=1
ICMP IN -> XLATE_DST Created=1987884sec HostLocal=1