Fix issue #28: SNMPv2c does not work anymore (ErrNoPermission)

1. Introduction of SnmpV3Only Configuration:

A new `SecurityConfig` configuration option, `SnmpV3Only`, has been added.

This setting instructs the server to accept SNMP version 3 (SNMPv3) requests
exclusively. This addition is a response to the request made in issue #23,
aiming to provide enhanced security by leveraging SNMPv3's advanced features.

@see #23 (comment)

2. Code Improvements and Bug Fix for Issue #28:

The contributions from @adenis78 and @ConsoleTVs have been refined and
incorporated to resolve the issue where SNMPv2c was not functioning correctly.

3. Defensive Rechecking of Configuration and Community Handling:

Enhanced validation checks have been implemented to ensure robust error
handling and response mechanisms. In scenarios where the specified community
is not found within the subagent configuration, and there's an absence of a
SecurityConfig.NoSecurity setting, the server will now refrain from responding.

4. Utilization of NoSecurity for Compatibility and Testing:

The SecurityConfig.NoSecurity configuration has been introduced
to facilitate bypassing certain security checks.

agentcontrol.go

@@ -31,6 +31,9 @@ type MasterAgent struct {
 type SecurityConfig struct {
 	NoSecurity bool
 
+	// SnmpV3Only is used for mark only snmpv3 is supported
+	SnmpV3Only bool
+
 	// AuthoritativeEngineID is SNMPV3 AuthoritativeEngineID
 	AuthoritativeEngineID SNMPEngineID
 	// AuthoritativeEngineBoots is SNMPV3 AuthoritativeEngineBoots
@@ -130,6 +133,10 @@ func (t *MasterAgent) ResponseForBuffer(i []byte) ([]byte, error) {
 
 	switch request.Version {
 	case gosnmp.Version1, gosnmp.Version2c:
+		if t.SecurityConfig.SnmpV3Only {
+			return nil, errors.WithMessagef(ErrUnsupportedProtoVersion, "Server sets snmpV3 Only")
+		}
+
 		return t.marshalPkt(t.ResponseForPkt(request))
 		//
 	case gosnmp.Version3:
@@ -152,6 +159,30 @@ func (t *MasterAgent) ResponseForBuffer(i []byte) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
+
+		if !t.SecurityConfig.NoSecurity{
+			// https://pkg.go.dev/github.com/gosnmp/gosnmp#SnmpV3MsgFlags
+			userAuthMode := gosnmp.NoAuthNoPriv
+
+
+			if usm.AuthenticationProtocol > gosnmp.NoAuth {
+					userAuthMode = gosnmp.AuthNoPriv
+			}
+
+
+			if usm.PrivacyProtocol > gosnmp.NoPriv {
+					userAuthMode = gosnmp.AuthPriv
+			}
+
+			requestAuthMode := request.MsgFlags&gosnmp.AuthPriv /*3*/ 
+
+			if requestAuthMode != gosnmp.SnmpV3MsgFlags(userAuthMode) {
+				return nil, 
+					errors.WithMessagef(ErrNoPermission, 
+						"user %v required %v, got %v", username, userAuthMode.String(), request.MsgFlags.String())
+			}
+		}
+
 		if decodeError != nil {
 			t.Logger.Debugf("v3 decode [will fail with non password] meet %v", err)
 			vhandle.SecurityParameters = &gosnmp.UsmSecurityParameters{
@@ -288,8 +319,11 @@ func (t *MasterAgent) findForSubAgent(community string) *SubAgent {
 	if val, ok := t.priv.communityToSubAgent[community]; ok {
 		return val
 	} else {
-		return t.priv.defaultSubAgent
-	}
+		if t.SecurityConfig.NoSecurity {
+			return t.priv.defaultSubAgent
+		}
+		return nil
+	} 
 }
 
 func DefaultAuthoritativeEngineID() SNMPEngineID {

agentcontrol_test.go

@@ -22,6 +22,7 @@ func (suite *ResponseForBufferTestSuite) SetupTest() {
 		Logger: suite.Logger,
 		SecurityConfig: SecurityConfig{
 			AuthoritativeEngineBoots: 123,
+			NoSecurity: true,
 			Users: []gosnmp.UsmSecurityParameters{
 				{
 					UserName:                 "pippo",

cmd/gosnmpserver/main.go

@@ -26,6 +26,7 @@ func makeApp() *cli.App {
 					&cli.StringFlag{Name: "v3Username", Value: "testuser"},
 					&cli.StringFlag{Name: "v3AuthenticationPassphrase", Value: "testauth"},
 					&cli.StringFlag{Name: "v3PrivacyPassphrase", Value: "testpriv"},
+					&cli.BoolFlag{Name: "v3Only", Value: false},
 				},
 				Action: runServer,
 			},
@@ -58,6 +59,7 @@ func runServer(c *cli.Context) error {
 		Logger: logger,
 		SecurityConfig: GoSNMPServer.SecurityConfig{
 			AuthoritativeEngineBoots: 1,
+			SnmpV3Only:                   c.Bool("v3Only"),
 			Users: []gosnmp.UsmSecurityParameters{
 				{
 					UserName:                 c.String("v3Username"),
@@ -75,6 +77,9 @@ func runServer(c *cli.Context) error {
 			},
 		},
 	}
+
+
+
 	logger.Infof("V3 Users:")
 	for _, val := range master.SecurityConfig.Users {
 		logger.Infof(