netexec.go

/*
Copyright 2014 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package netexec

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"net/url"
	"os"
	"os/exec"
	"os/signal"
	"strconv"
	"strings"
	"sync/atomic"
	"syscall"
	"time"

	"github.com/ishidawataru/sctp"
	"github.com/spf13/cobra"

	utilnet "k8s.io/apimachinery/pkg/util/net"
	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
	"k8s.io/apimachinery/pkg/util/sets"
	netutils "k8s.io/utils/net"
)

var (
	httpPort           = 8080
	udpPort            = 8081
	sctpPort           = -1
	shellPath          = "/bin/sh"
	serverReady        = &atomicBool{0}
	certFile           = ""
	privKeyFile        = ""
	httpOverride       = ""
	udpListenAddresses = ""
	delayShutdown      = 0
)

const bindToAny = ""

// CmdNetexec is used by agnhost Cobra.
var CmdNetexec = &cobra.Command{
	Use:   "netexec",
	Short: "Creates HTTP(S), UDP, and (optionally) SCTP servers with various endpoints",
	Long: `Starts a HTTP(S) server on given port with the following endpoints:

- /: Returns the request's timestamp.
- /clientip: Returns the request's IP address.
- /header: Returns the request's header value corresponding to the key provided or the entire 
  header marshalled as json, if no form value (key) is provided.
  ("/header?key=X-Forwarded-For" or /header)
- /dial: Creates a given number of requests to the given host and port using the given protocol,
  and returns a JSON with the fields "responses" (successful request responses) and "errors" (
  failed request responses). Returns "200 OK" status code if the last request succeeded,
  "417 Expectation Failed" if it did not, or "400 Bad Request" if any of the endpoint's parameters
  is invalid. The endpoint's parameters are:
  - "host": The host that will be dialed.
  - "port": The port that will be dialed.
  - "request": The HTTP endpoint or data to be sent through UDP. If not specified, it will result
    in a "400 Bad Request" status code being returned.
  - "protocol": The protocol which will be used when making the request. Default value: "http".
    Acceptable values: "http", "udp", "sctp".
  - "tries": The number of times the request will be performed. Default value: "1".
- "/echo": Returns the given "msg" ("/echo?msg=echoed_msg"), with the optional status "code".
- "/exit": Closes the server with the given code and graceful shutdown. The endpoint's parameters
	are:
	- "code": The exit code for the process. Default value: 0. Allows an integer [0-127].
	- "timeout": The amount of time to wait for connections to close before shutting down.
		Acceptable values are golang durations. If 0 the process will exit immediately without
		shutdown.
	- "wait": The amount of time to wait before starting shutdown. Acceptable values are
	  golang durations. If 0 the process will start shutdown immediately.
- "/healthz": Returns "200 OK" if the server is healthy, "412 Status Precondition Failed"
  otherwise. The server is considered not ready if the UDP server did not start yet or
  it exited.
- "/readyz": Returns "200 OK" if the server is ready to receive traffic, "412 Status Precondition Failed", if the
  server is not yet ready to receive traffic, but may be ready later, and "503" if the server is shutting down.
  When a sig-term is observed, the /readyz will report 503, but healthz will report 200 to indicate that the
  server is healthy (don't kill it), but the it should not be sent traffic (remove from endpoints).
- "/hostname": Returns the server's hostname.
- "/hostName": Returns the server's hostname.
- "/redirect": Returns a redirect response to the given "location", with the optional status "code"
  ("/redirect?location=/echo%3Fmsg=foobar&code=307").
- "/shell": Executes the given "shellCommand" or "cmd" ("/shell?cmd=some-command") and
  returns a JSON containing the fields "output" (command's output) and "error" (command's
  error message). Returns "200 OK" if the command succeeded, "417 Expectation Failed" if not.
- "/shutdown": Closes the server with the exit code 0.
- "/upload": Accepts a file to be uploaded, writing it in the "/uploads" folder on the host.
  Returns a JSON with the fields "output" (containing the file's name on the server) and
  "error" containing any potential server side errors.

If "--tls-cert-file" is added (ideally in conjunction with "--tls-private-key-file", the HTTP server
will be upgraded to HTTPS. The image has default, "localhost"-based cert/privkey files at
"/localhost.crt" and "/localhost.key" (see: "porter" subcommand)

If "--http-override" is set, the HTTP(S) server will always serve the override path & options,
ignoring the request URL.

It will also start a UDP server on the indicated UDP port and addresses that responds to the following commands:

- "hostname": Returns the server's hostname
- "echo <msg>": Returns the given <msg>
- "clientip": Returns the request's IP address

The UDP server can be disabled by setting --udp-port to -1.

Additionally, if (and only if) --sctp-port is passed, it will start an SCTP server on that port,
responding to the same commands as the UDP server.
`,
	Args: cobra.MaximumNArgs(0),
	Run:  main,
}

func init() {
	CmdNetexec.Flags().IntVar(&httpPort, "http-port", 8080, "HTTP Listen Port")
	CmdNetexec.Flags().StringVar(&certFile, "tls-cert-file", "",
		"File containing an x509 certificate for HTTPS. (CA cert, if any, concatenated after server cert)")
	CmdNetexec.Flags().StringVar(&privKeyFile, "tls-private-key-file", "",
		"File containing an x509 private key matching --tls-cert-file")
	CmdNetexec.Flags().IntVar(&udpPort, "udp-port", 8081, "UDP Listen Port")
	CmdNetexec.Flags().IntVar(&sctpPort, "sctp-port", -1, "SCTP Listen Port")
	CmdNetexec.Flags().StringVar(&httpOverride, "http-override", "", "Override the HTTP handler to always respond as if it were a GET with this path & params")
	CmdNetexec.Flags().StringVar(&udpListenAddresses, "udp-listen-addresses", "", "A comma separated list of ip addresses the udp servers listen from")
	CmdNetexec.Flags().IntVar(&delayShutdown, "delay-shutdown", 0, "Number of seconds to delay shutdown when receiving SIGTERM.")
}

// atomicBool uses load/store operations on an int32 to simulate an atomic boolean.
type atomicBool struct {
	v int32
}

// set sets the int32 to the given boolean.
func (a *atomicBool) set(value bool) {
	if value {
		atomic.StoreInt32(&a.v, 1)
		return
	}
	atomic.StoreInt32(&a.v, 0)
}

// get returns true if the int32 == 1
func (a *atomicBool) get() bool {
	return atomic.LoadInt32(&a.v) == 1
}

func main(cmd *cobra.Command, args []string) {
	exitCh := make(chan shutdownRequest)

	sigTermReceived := make(chan struct{})
	go func() {
		termCh := make(chan os.Signal, 1)
		signal.Notify(termCh, syscall.SIGTERM)

		<-termCh
		close(sigTermReceived)
	}()

	go func() {
		<-sigTermReceived
		if delayShutdown > 0 {
			log.Printf("Sleeping %d seconds before terminating...", delayShutdown)
			time.Sleep(time.Duration(delayShutdown) * time.Second)
		}
		os.Exit(0)
	}()

	if httpOverride != "" {
		mux := http.NewServeMux()
		addRoutes(mux, sigTermReceived, exitCh)

		http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
			overrideReq, err := http.NewRequestWithContext(r.Context(), "GET", httpOverride, nil)
			if err != nil {
				http.Error(w, fmt.Sprintf("override request failed: %v", err), http.StatusInternalServerError)
				return
			}
			mux.ServeHTTP(w, overrideReq)
		})
	} else {
		addRoutes(http.DefaultServeMux, sigTermReceived, exitCh)
	}

	// UDP server
	if udpPort != -1 {
		udpBindTo, err := parseAddresses(udpListenAddresses)
		if err != nil {
			log.Fatal(err)
		}

		for _, address := range udpBindTo {
			go startUDPServer(address, udpPort)
		}
	}

	// SCTP server
	if sctpPort != -1 {
		go startSCTPServer(sctpPort)
	}

	server := &http.Server{Addr: fmt.Sprintf(":%d", httpPort)}
	if len(certFile) > 0 {
		startServer(server, exitCh, func() error { return server.ListenAndServeTLS(certFile, privKeyFile) })
	} else {
		startServer(server, exitCh, server.ListenAndServe)
	}
}

func addRoutes(mux *http.ServeMux, sigTermReceived chan struct{}, exitCh chan shutdownRequest) {
	mux.HandleFunc("/", rootHandler)
	mux.HandleFunc("/clientip", clientIPHandler)
	mux.HandleFunc("/header", headerHandler)
	mux.HandleFunc("/dial", dialHandler)
	mux.HandleFunc("/echo", echoHandler)
	mux.HandleFunc("/exit", func(w http.ResponseWriter, req *http.Request) { exitHandler(w, req, exitCh) })
	mux.HandleFunc("/healthz", healthzHandler)
	mux.HandleFunc("/readyz", readyzHandler(sigTermReceived))
	mux.HandleFunc("/hostname", hostnameHandler)
	mux.HandleFunc("/redirect", redirectHandler)
	mux.HandleFunc("/shell", shellHandler)
	mux.HandleFunc("/upload", uploadHandler)
	// older handlers
	mux.HandleFunc("/hostName", hostNameHandler)
	mux.HandleFunc("/shutdown", shutdownHandler)
}

func startServer(server *http.Server, exitCh chan shutdownRequest, fn func() error) {
	log.Printf("Started HTTP server on port %d", httpPort)
	go func() {
		re := <-exitCh
		ctx, cancelFn := context.WithTimeout(context.Background(), re.timeout)
		defer cancelFn()
		err := server.Shutdown(ctx)
		log.Printf("Graceful shutdown completed with: %v", err)
		os.Exit(re.code)
	}()

	if err := fn(); err != nil {
		if err == http.ErrServerClosed {
			// wait until the goroutine calls os.Exit()
			select {}
		}
		log.Fatal(err)
	}
}

func rootHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /")
	fmt.Fprintf(w, "NOW: %v", time.Now())
}

func echoHandler(w http.ResponseWriter, r *http.Request) {
	msg := r.FormValue("msg")
	codeString := r.FormValue("code")
	log.Printf("GET /echo?msg=%s&code=%s", msg, codeString)
	if codeString != "" {
		code, err := strconv.Atoi(codeString)
		if err != nil && codeString != "" {
			fmt.Fprintf(w, "argument 'code' must be an integer or empty, got %q\n", codeString)
			return
		}
		w.WriteHeader(code)
	}
	fmt.Fprintf(w, "%s", msg)
}

func clientIPHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /clientip")
	fmt.Fprintf(w, r.RemoteAddr)
}
func headerHandler(w http.ResponseWriter, r *http.Request) {
	key := r.FormValue("key")
	if key != "" {
		log.Printf("GET /header?key=%s", key)
		fmt.Fprintf(w, "%s", r.Header.Get(key))
	} else {
		log.Printf("GET /header")
		data, err := json.Marshal(r.Header)
		if err != nil {
			fmt.Fprintf(w, "error marshalling header, err: %v", err)
			return
		}
		fmt.Fprintf(w, "%s", string(data))
	}
}

type shutdownRequest struct {
	code    int
	timeout time.Duration
}

func exitHandler(w http.ResponseWriter, r *http.Request, exitCh chan<- shutdownRequest) {
	waitString := r.FormValue("wait")
	timeoutString := r.FormValue("timeout")
	codeString := r.FormValue("code")
	log.Printf("GET /exit?code=%s&timeout=%s&wait=%s", codeString, timeoutString, waitString)
	timeout, err := time.ParseDuration(timeoutString)
	if err != nil && timeoutString != "" {
		fmt.Fprintf(w, "argument 'timeout' must be a valid golang duration or empty, got %q\n", timeoutString)
		return
	}
	wait, err := time.ParseDuration(waitString)
	if err != nil && waitString != "" {
		fmt.Fprintf(w, "argument 'wait' must be a valid golang duration or empty, got %q\n", waitString)
		return
	}
	code, err := strconv.Atoi(codeString)
	if err != nil && codeString != "" {
		fmt.Fprintf(w, "argument 'code' must be an integer [0-127] or empty, got %q\n", codeString)
		return
	}
	log.Printf("Will begin shutdown in %s, allowing %s for connections to close, then will exit with %d", wait, timeout, code)
	time.Sleep(wait)
	if timeout == 0 {
		os.Exit(code)
	}
	exitCh <- shutdownRequest{code: code, timeout: timeout}
}

func hostnameHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /hostname")
	fmt.Fprint(w, getHostName())
}

// healthHandler response with a 200 if the UDP server is ready. It also serves
// as a health check of the HTTP server by virtue of being a HTTP handler.
func healthzHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /healthz")
	if serverReady.get() {
		w.WriteHeader(200)
		return
	}
	w.WriteHeader(http.StatusPreconditionFailed)
}

// readyzHandler response with a 200 if the UDP server is ready. It serves as a readyz that will return a 503
// once a sig-term has been received.   This allows for graceful removal from endpoints during a pod delete flow.
func readyzHandler(sigTermReceived chan struct{}) func(w http.ResponseWriter, r *http.Request) {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Printf("GET /readyz")

		select {
		case <-sigTermReceived:
			w.WriteHeader(http.StatusServiceUnavailable)
			if _, err := w.Write([]byte("shutting down")); err != nil {
				utilruntime.HandleError(err)
			}
			return

		default:
			if serverReady.get() {
				if _, err := w.Write([]byte("ok")); err != nil {
					utilruntime.HandleError(err)
				}
				return
			}
			w.WriteHeader(http.StatusPreconditionFailed)
		}
	}
}

func shutdownHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /shutdown")
	os.Exit(0)
}

func dialHandler(w http.ResponseWriter, r *http.Request) {
	values, err := url.Parse(r.URL.RequestURI())
	if err != nil {
		http.Error(w, fmt.Sprintf("%v", err), http.StatusBadRequest)
		return
	}

	host := values.Query().Get("host")
	port := values.Query().Get("port")
	request := values.Query().Get("request") // hostName
	protocol := values.Query().Get("protocol")
	tryParam := values.Query().Get("tries")
	log.Printf("GET /dial?host=%s&protocol=%s&port=%s&request=%s&tries=%s", host, protocol, port, request, tryParam)
	tries := 1
	if len(tryParam) > 0 {
		tries, err = strconv.Atoi(tryParam)
	}
	if err != nil {
		http.Error(w, fmt.Sprintf("tries parameter is invalid. %v", err), http.StatusBadRequest)
		return
	}
	if len(request) == 0 {
		http.Error(w, fmt.Sprintf("request parameter not specified. %v", err), http.StatusBadRequest)
		return
	}

	hostPort := net.JoinHostPort(host, port)
	var addr net.Addr
	var dialer func(string, net.Addr) (string, error)
	switch strings.ToLower(protocol) {
	case "", "http":
		dialer = dialHTTP
		addr, err = net.ResolveTCPAddr("tcp", hostPort)
	case "udp":
		dialer = dialUDP
		addr, err = net.ResolveUDPAddr("udp", hostPort)
	case "sctp":
		dialer = dialSCTP
		addr, err = sctp.ResolveSCTPAddr("sctp", hostPort)
	default:
		http.Error(w, fmt.Sprintf("unsupported protocol. %s", protocol), http.StatusBadRequest)
		return
	}
	if err != nil {
		http.Error(w, fmt.Sprintf("host and/or port param are invalid. %v", err), http.StatusBadRequest)
		return
	}

	errors := make([]string, 0)
	responses := make([]string, 0)
	var response string
	for i := 0; i < tries; i++ {
		response, err = dialer(request, addr)
		if err != nil {
			errors = append(errors, fmt.Sprintf("%v", err))
		} else {
			responses = append(responses, response)
		}
	}
	output := map[string][]string{}
	if len(response) > 0 {
		output["responses"] = responses
	}
	if len(errors) > 0 {
		output["errors"] = errors
	}
	bytes, err := json.Marshal(output)
	if err == nil {
		fmt.Fprint(w, string(bytes))
	} else {
		http.Error(w, fmt.Sprintf("response could not be serialized. %v", err), http.StatusExpectationFailed)
	}
}

func dialHTTP(request string, addr net.Addr) (string, error) {
	transport := utilnet.SetTransportDefaults(&http.Transport{})
	httpClient := createHTTPClient(transport)
	resp, err := httpClient.Get(fmt.Sprintf("http://%s/%s", addr.String(), request))
	defer transport.CloseIdleConnections()
	if err == nil {
		defer resp.Body.Close()
		body, err := io.ReadAll(resp.Body)
		if err == nil {
			return string(body), nil
		}
	}
	return "", err
}

func createHTTPClient(transport *http.Transport) *http.Client {
	client := &http.Client{
		Transport: transport,
		Timeout:   5 * time.Second,
	}
	return client
}

func dialUDP(request string, addr net.Addr) (string, error) {
	Conn, err := net.DialUDP("udp", nil, addr.(*net.UDPAddr))
	if err != nil {
		return "", fmt.Errorf("udp dial failed. err:%v", err)
	}

	defer Conn.Close()
	buf := []byte(request)
	_, err = Conn.Write(buf)
	if err != nil {
		return "", fmt.Errorf("udp connection write failed. err:%v", err)
	}
	udpResponse := make([]byte, 2048)
	Conn.SetReadDeadline(time.Now().Add(5 * time.Second))
	count, err := Conn.Read(udpResponse)
	if err != nil || count == 0 {
		return "", fmt.Errorf("reading from udp connection failed. err:'%v'", err)
	}
	return string(udpResponse[0:count]), nil
}

func dialSCTP(request string, addr net.Addr) (string, error) {
	Conn, err := sctp.DialSCTP("sctp", nil, addr.(*sctp.SCTPAddr))
	if err != nil {
		return "", fmt.Errorf("sctp dial failed. err:%v", err)
	}

	defer Conn.Close()
	buf := []byte(request)
	_, err = Conn.Write(buf)
	if err != nil {
		return "", fmt.Errorf("sctp connection write failed. err:%v", err)
	}
	sctpResponse := make([]byte, 1024)
	Conn.SetReadDeadline(time.Now().Add(5 * time.Second))
	count, err := Conn.Read(sctpResponse)
	if err != nil || count == 0 {
		return "", fmt.Errorf("reading from sctp connection failed. err:'%v'", err)
	}
	return string(sctpResponse[0:count]), nil
}

func shellHandler(w http.ResponseWriter, r *http.Request) {
	cmd := r.FormValue("shellCommand")
	if cmd == "" {
		cmd = r.FormValue("cmd")
	}
	log.Printf("GET /shell?cmd=%s", cmd)
	cmdOut, err := exec.Command(shellPath, "-c", cmd).CombinedOutput()
	output := map[string]string{}
	if len(cmdOut) > 0 {
		output["output"] = string(cmdOut)
	}
	if err != nil {
		output["error"] = fmt.Sprintf("%v", err)
	}
	log.Printf("Output: %s", output)
	bytes, err := json.Marshal(output)
	if err == nil {
		fmt.Fprint(w, string(bytes))
	} else {
		http.Error(w, fmt.Sprintf("response could not be serialized. %v", err), http.StatusExpectationFailed)
	}
}

func uploadHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /upload")
	result := map[string]string{}
	file, _, err := r.FormFile("file")
	if err != nil {
		result["error"] = "Unable to upload file."
		bytes, err := json.Marshal(result)
		if err == nil {
			fmt.Fprint(w, string(bytes))
		} else {
			http.Error(w, fmt.Sprintf("%s. Also unable to serialize output. %v", result["error"], err), http.StatusInternalServerError)
		}
		log.Printf("Unable to upload file: %s", err)
		return
	}
	defer file.Close()

	f, err := os.CreateTemp("/uploads", "upload")
	if err != nil {
		result["error"] = "Unable to open file for write"
		bytes, err := json.Marshal(result)
		if err == nil {
			fmt.Fprint(w, string(bytes))
		} else {
			http.Error(w, fmt.Sprintf("%s. Also unable to serialize output. %v", result["error"], err), http.StatusInternalServerError)
		}
		log.Printf("Unable to open file for write: %s", err)
		return
	}
	defer f.Close()
	if _, err = io.Copy(f, file); err != nil {
		result["error"] = "Unable to write file."
		bytes, err := json.Marshal(result)
		if err == nil {
			fmt.Fprint(w, string(bytes))
		} else {
			http.Error(w, fmt.Sprintf("%s. Also unable to serialize output. %v", result["error"], err), http.StatusInternalServerError)
		}
		log.Printf("Unable to write file: %s", err)
		return
	}

	UploadFile := f.Name()
	if err := os.Chmod(UploadFile, 0700); err != nil {
		result["error"] = "Unable to chmod file."
		bytes, err := json.Marshal(result)
		if err == nil {
			fmt.Fprint(w, string(bytes))
		} else {
			http.Error(w, fmt.Sprintf("%s. Also unable to serialize output. %v", result["error"], err), http.StatusInternalServerError)
		}
		log.Printf("Unable to chmod file: %s", err)
		return
	}
	log.Printf("Wrote upload to %s", UploadFile)
	result["output"] = UploadFile
	w.WriteHeader(http.StatusCreated)
	bytes, err := json.Marshal(result)
	if err != nil {
		http.Error(w, fmt.Sprintf("%s. Also unable to serialize output. %v", result["error"], err), http.StatusInternalServerError)
		return
	}
	fmt.Fprint(w, string(bytes))
}

func hostNameHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("GET /hostName")
	fmt.Fprint(w, getHostName())
}

func redirectHandler(w http.ResponseWriter, r *http.Request) {
	location := r.FormValue("location")
	codeString := r.FormValue("code")
	log.Printf("%s /redirect?msg=%s&code=%s", r.Method, location, codeString)
	code := http.StatusFound
	if codeString != "" {
		var err error
		code, err = strconv.Atoi(codeString)
		if err != nil && codeString != "" {
			fmt.Fprintf(w, "argument 'code' must be an integer or empty, got %q\n", codeString)
			return
		}
	}
	http.Redirect(w, r, location, code)
}

// udp server supports the hostName, echo and clientIP commands.
func startUDPServer(address string, udpPort int) {
	serverAddress, err := net.ResolveUDPAddr("udp", net.JoinHostPort(address, strconv.Itoa(udpPort)))
	assertNoError(err, fmt.Sprintf("failed to resolve UDP address for port %d", udpPort))
	serverConn, err := net.ListenUDP("udp", serverAddress)
	assertNoError(err, fmt.Sprintf("failed to create listener for UDP address %v", serverAddress))
	defer serverConn.Close()
	buf := make([]byte, 2048)

	log.Printf("Started UDP server on port %s %d", address, udpPort)
	// Start responding to readiness probes.
	serverReady.set(true)
	defer func() {
		log.Printf("UDP server exited")
		serverReady.set(false)
	}()
	for {
		n, clientAddress, err := serverConn.ReadFromUDP(buf)
		assertNoError(err, fmt.Sprintf("failed accepting UDP connections"))
		receivedText := strings.ToLower(strings.TrimSpace(string(buf[0:n])))
		if receivedText == "hostname" {
			log.Println("Sending udp hostName response")
			_, err = serverConn.WriteToUDP([]byte(getHostName()), clientAddress)
			assertNoError(err, fmt.Sprintf("failed to write hostname to UDP client %s", clientAddress))
		} else if strings.HasPrefix(receivedText, "echo ") {
			parts := strings.SplitN(receivedText, " ", 2)
			resp := ""
			if len(parts) == 2 {
				resp = parts[1]
			}
			log.Printf("Echoing %v to UDP client %s\n", resp, clientAddress)
			_, err = serverConn.WriteToUDP([]byte(resp), clientAddress)
			assertNoError(err, fmt.Sprintf("failed to echo to UDP client %s", clientAddress))
		} else if receivedText == "clientip" {
			log.Printf("Sending clientip back to UDP client %s\n", clientAddress)
			_, err = serverConn.WriteToUDP([]byte(clientAddress.String()), clientAddress)
			assertNoError(err, fmt.Sprintf("failed to write clientip to UDP client %s", clientAddress))
		} else if len(receivedText) > 0 {
			log.Printf("Unknown UDP command received from %s: %v\n", clientAddress, receivedText)
		}
	}
}

// sctp server supports the hostName, echo and clientIP commands.
func startSCTPServer(sctpPort int) {
	serverAddress, err := sctp.ResolveSCTPAddr("sctp", fmt.Sprintf(":%d", sctpPort))
	assertNoError(err, fmt.Sprintf("failed to resolve SCTP address for port %d", sctpPort))
	listener, err := sctp.ListenSCTP("sctp", serverAddress)
	assertNoError(err, fmt.Sprintf("failed to create listener for SCTP address %v", serverAddress))
	defer listener.Close()
	buf := make([]byte, 1024)

	log.Printf("Started SCTP server")
	// Start responding to readiness probes.
	serverReady.set(true)
	defer func() {
		log.Printf("SCTP server exited")
		serverReady.set(false)
	}()
	for {
		conn, err := listener.AcceptSCTP()
		assertNoError(err, fmt.Sprintf("failed accepting SCTP connections"))
		clientAddress := conn.RemoteAddr().String()
		n, err := conn.Read(buf)
		assertNoError(err, fmt.Sprintf("failed to read from SCTP client %s", clientAddress))
		receivedText := strings.ToLower(strings.TrimSpace(string(buf[0:n])))
		if receivedText == "hostname" {
			log.Println("Sending SCTP hostName response")
			_, err = conn.Write([]byte(getHostName()))
			assertNoError(err, fmt.Sprintf("failed to write hostname to SCTP client %s", clientAddress))
		} else if strings.HasPrefix(receivedText, "echo ") {
			parts := strings.SplitN(receivedText, " ", 2)
			resp := ""
			if len(parts) == 2 {
				resp = parts[1]
			}
			log.Printf("Echoing %v to SCTP client %s\n", resp, clientAddress)
			_, err = conn.Write([]byte(resp))
			assertNoError(err, fmt.Sprintf("failed to echo to SCTP client %s", clientAddress))
		} else if receivedText == "clientip" {
			log.Printf("Sending clientip back to SCTP client %s\n", clientAddress)
			_, err = conn.Write([]byte(clientAddress))
			assertNoError(err, fmt.Sprintf("failed to write clientip to SCTP client %s", clientAddress))
		} else if len(receivedText) > 0 {
			log.Printf("Unknown SCTP command received from %s: %v\n", clientAddress, receivedText)
		}
		conn.Close()
	}
}

func getHostName() string {
	hostName, err := os.Hostname()
	assertNoError(err, "failed to get hostname")
	return hostName
}

func assertNoError(err error, detail string) {
	if err != nil {
		log.Fatalf("Error occurred: %s:%v", detail, err)
	}
}

func parseAddresses(addresses string) ([]string, error) {
	if addresses == "" {
		return []string{bindToAny}, nil
	}
	// Using a set to remove duplicates
	res := make([]string, 0)
	split := strings.Split(addresses, ",")
	for _, address := range split {
		netAddr := netutils.ParseIPSloppy(address)
		if netAddr == nil {
			return nil, fmt.Errorf("parseAddress: invalid address %s", address)
		}
		res = append(res, address)
	}
	set := sets.NewString(res...)
	return set.List(), nil
}