etcdmain: advertise-client-urls must be set if listen-client-urls is set

Before this PR, people can set listen-client-urls without setting
advertise-client-urls, and leaves advertise-client-urls as default
localhost value. The client libraries which sync the cluster info
fetch wrong advertise-client-urls and cannot connect to the cluster.
This PR avoids this case and provides better UX.

On the other hand, this change is safe because people always want to set
advertise-client-urls if listen-client-urls is set. The default localhost
advertise url cannot be accessed from the outside, and should always be
set except that etcd is bootstrapped with no flag.

etcdmain/config.go

@@ -61,6 +61,7 @@ var (
 
 	ErrConflictBootstrapFlags = fmt.Errorf("multiple discovery or bootstrap flags are set" +
 		"Choose one of \"initial-cluster\", \"discovery\" or \"discovery-srv\"")
+	errUnsetAdvertiseClientURLsFlag = fmt.Errorf("-advertise-client-urls is required when -listen-client-urls is set explicitly")
 )
 
 type config struct {
@@ -264,6 +265,9 @@ func (cfg *config) Parse(arguments []string) error {
 	if err != nil {
 		return err
 	}
+	if flags.IsSet(cfg.FlagSet, "listen-client-urls") && !flags.IsSet(cfg.FlagSet, "advertise-client-urls") {
+		return errUnsetAdvertiseClientURLsFlag
+	}
 
 	if 5*cfg.TickMs > cfg.ElectionMs {
 		return fmt.Errorf("-election-timeout[%vms] should be at least as 5 times as -heartbeat-interval[%vms]", cfg.ElectionMs, cfg.TickMs)

etcdmain/config_test.go

@@ -29,6 +29,8 @@ func TestConfigParsingMemberFlags(t *testing.T) {
 		"-snapshot-count=10",
 		"-listen-peer-urls=http://localhost:8000,https://localhost:8001",
 		"-listen-client-urls=http://localhost:7000,https://localhost:7001",
+		// it should be set if -listen-client-urls is set
+		"-advertise-client-urls=http://localhost:7000,https://localhost:7001",
 	}
 	wcfg := &config{
 		dir:          "testdir",

etcdmain/etcd.go

@@ -60,7 +60,12 @@ func Main() {
 	cfg := NewConfig()
 	err := cfg.Parse(os.Args[1:])
 	if err != nil {
-		log.Fatalf("error verifying flags, %v. See 'etcd -help'.", err)
+		log.Printf("error verifying flags, %v. See 'etcd -help'.", err)
+		switch err {
+		case errUnsetAdvertiseClientURLsFlag:
+			log.Printf("When listening on specific address(es), this etcd process must advertise accessible url(s) to each connected client.")
+		}
+		os.Exit(1)
 	}
 	setupLogging(cfg)
 

etcdmain/help.go

@@ -56,7 +56,8 @@ clustering flags:
 	--initial-cluster-token 'etcd-cluster'
 		initial cluster token for the etcd cluster during bootstrap.
 	--advertise-client-urls 'http://localhost:2379,http://localhost:4001'
-		list of this member's client URLs to advertise to the rest of the cluster.
+		list of this member's client URLs to advertise to the public.
+		The client URLs advertised should be accessible to machines that talk to etcd cluster. etcd client libraries parse these URLs to connect to the cluster.
 	--discovery ''
 		discovery URL used to bootstrap the cluster.
 	--discovery-fallback 'proxy'