Allow the server listening address to be changed

It may be desired to change the listening address of the http server to
set it for a specific address or interface.

Allow this to be set with a "host" config or environment variable
CLOUDPROBER_HOST and listen on that hostname/address if given.

If not available, default to the existing setting of listening on all
addresses.

PiperOrigin-RevId: 239244493

cloudprober.go

@@ -50,9 +50,11 @@ const (
 	sysvarsModuleName = "sysvars"
 )
 
-// Constants defining the default server port.
+// Constants defining the default server host and port.
 const (
+	DefaultServerHost = ""
 	DefaultServerPort = 9313
+	ServerHostEnvVar  = "CLOUDPROBER_HOST"
 	ServerPortEnvVar  = "CLOUDPROBER_PORT"
 )
 
@@ -74,6 +76,16 @@ type Prober struct {
 }
 
 func (pr *Prober) initDefaultServer() error {
+	serverHost := pr.c.GetHost()
+	if serverHost == "" {
+		serverHost = DefaultServerHost
+		// If ServerHostEnvVar is defined, it will override the default
+		// server host.
+		if host := os.Getenv(ServerHostEnvVar); host != "" {
+			serverHost = host
+		}
+	}
+
 	serverPort := int(pr.c.GetPort())
 	if serverPort == 0 {
 		serverPort = DefaultServerPort
@@ -89,7 +101,7 @@ func (pr *Prober) initDefaultServer() error {
 		}
 	}
 
-	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", serverPort))
+	ln, err := net.Listen("tcp", fmt.Sprintf("%s:%d", serverHost, serverPort))
 	if err != nil {
 		return fmt.Errorf("error while creating listener for default HTTP server. Err: %v", err)
 	}

config/proto/config.proto

@@ -1,14 +1,14 @@
 syntax = "proto2";
 
+package cloudprober;
+
 import "github.com/google/cloudprober/probes/proto/config.proto";
 import "github.com/google/cloudprober/servers/proto/config.proto";
-import "github.com/google/cloudprober/targets/proto/targets.proto";
 import "github.com/google/cloudprober/surfacers/proto/config.proto";
+import "github.com/google/cloudprober/targets/proto/targets.proto";
 import "github.com/google/cloudprober/targets/rds/server/proto/config.proto";
 import "github.com/google/cloudprober/targets/rtc/rtcreporter/proto/rtcreporter.proto";
 
-package cloudprober;
-
 message ProberConfig {
   // Probes to run.
   repeated probes.ProbeDef probe = 1;
@@ -28,6 +28,9 @@ message ProberConfig {
   // other probes.
   repeated servers.ServerDef server = 3;
 
+  // Common services related options.
+  // Next tag: 102
+
   // Resource discovery server
   optional targets.rds.ServerConf rds_server = 95;
 
@@ -37,6 +40,11 @@ message ProberConfig {
   // CLOUDPROBER_PORT.
   optional int32 port = 96;
 
+  // Host for the default HTTP server. Default listens on all addresses. If not
+  // specified in the config, default port can be overridden by the environment
+  // variable CLOUDPROBER_HOST.
+  optional string host = 101;
+
   // How often to export system variables. To learn more about system variables:
   // http://godoc.org/github.com/google/cloudprober/sysvars.
   optional int32 sysvars_interval_msec = 97 [default = 10000];