19.1 IP-Based Access Control Lists

For the SNMP adaptor, the Java Dynamic Management Kit (Java DMK) provides access control based on the IP address and community of the manager's host machine. Information on the access rights for communities and host machines is stored in access control lists (InetAddressAcl). The default implementation provided with the product uses an InetAddressAcl file, but you can provide your own implementation as described in 19.1.3 Custom Access Control.

The InetAddressAcl mechanism can also be used to define the communities and managers to which the agent will send traps. When you rely on the InetAddressAcl trap group, the agent will send traps to all hosts listed in the InetAddressAcl file. See 16.3.1 Specifying the Trap Destination for the different ways that an agent application can send traps.

The following code example gives the contents of the examplesDir/current/Snmp/Agent/jdmk.acl file used when running the SNMP example applications. When using it in the security examples, you should replace yourmanager with the complete IP address or hostname of the host running your SNMP manager application.

Example 19–1 jdmk.acl File

acl = {
 {
 communities = public
 access = read-only
 managers = yourmanager
 }
 {
 communities = private
 access = read-write
 managers = yourmanager
 } 
} 

trap = {
  {
  trap-community = public
  hosts = yourmanager
  }
}

19.1.1 InetAddressAcl File Format

An InetAddressAcl file contains an acl group defining community and manager access rights and a trap group defining the community and hosts for sending traps.

19.1.1.1 Format of the acl Group

The acl group contains one or more access configurations.

acl = {
   access1
   access2
     ...
   accessN
}

Each access configuration has the following format:

{
   communities = communityList
   access = accessRights
   managers = hostList
}

The communityList is a list of SNMP community names to which this access control applies. The community names in this list are separated by commas.

The accessRights specifies the rights to be granted to all managers connecting from the hosts specified in the hostList. There are two possible values: either read-write or read-only.

The hostList item gives the hosts of the managers to be granted the access rights. The hostList is a comma-separated list of hosts, each of which can be expressed as any one of the following:

• A host name

• An IP address

• A subnet mask, in the format A.B.C.D/24

The set of all access configurations defines the access policy of the SNMP agent. A manager whose host is specified in a hostList and that identifies itself in one of the communities of the same configuration will be granted the permissions defined by the corresponding accessRights. A manager's host can appear in several access configurations provided it is associated with a different community list. This will define different access communities with different rights from the same manager.

A manager whose host-community identification pair does not appear in any of the access configurations will be denied all access. This means that protocol data units (PDU) from this manager will be dropped without being processed.

19.1.1.2 Format of the Trap Group

The trap group specifies the hosts to which the agent will send traps if the InetAddressAcl mechanism is used. This group contains one or more trap community definitions.

trap = {
   community1
   community2
   ...
   communityN
}

Each community definition defines the association between a set of hosts and the SNMP community string in the traps to be sent to them. Each trap definition has the following format:

{
   trap-community = trapCommunityName
   hosts = trapHostList
}

The trapCommunityName item specifies a single SNMP community string. It will be included in the traps sent to the hosts specified in the hosts item. SNMPv3 does use the community string, so use IP addresses or the context name instead.

The trapHostList item specifies a comma-separated list of hosts. Each host must be identified by its name or complete IP address.

When the SNMP protocol adaptor is instructed to send a trap using the InetAddressAcl mechanism, it will send a trap to every host listed in the trap community definitions. If a host is present in more than one list, it will receive more than one trap, each one identified by its corresponding trap community.

19.1.1.3 Format of the Inform Group

The inform group specifies the hosts to which your agent or manager will send informs if the InetAddressAcl mechanism is used. This group contains one or more inform community definitions.

inform = {
   community1
   community2
   ...
   communityN
}

Each community definition defines the association between a set of hosts and the SNMP community string in the informs to be sent to them. Each inform definition has the following format:

{
   inform-community = informCommunityName
   hosts = informHostList
}

The informCommunityName item specifies a single SNMP community string. It will be included in the informs sent to the hosts specified in the hosts item. SNMPv3 does use the community string, so use IP addresses or the context name instead.

The informHostList item specifies a comma-separated list of hosts. Each host must be identified by its name or complete IP address.

When the SNMP protocol adaptor is instructed to send an inform using the InetAddressAcl mechanism, it will send an inform to every host listed in the inform community definitions. If a host is present in more than one list, it will receive more than one inform, each one identified by its corresponding inform community.

19.1.2 Enabling InetAddressAcl

The default InetAddressAcl mechanism provided with the Java DMK relies on an InetAddressAcl file to define the access rights and trap recipients. To enable access control with this mechanism, you must first write an InetAddressAcl file to reflect the access and trap policy of your SNMP agent. Then, there are two ways to enable file-based access control, one way to modify the file in use and one way to disable access control.

The simplest way of enabling access control and traps is to ensure that an InetAddressAcl file exists when the SNMP protocol adaptor MBean is instantiated. To be automatically detected, the InetAddressAcl file must be named jdmk.acl and must be located in the configuration directory of the Java DMK installation. On UNIX systems with a standard installation of the product, the configuration directory is owned by root and requires superuser privileges to write or modify the InetAddressAcl file.

The other way of enabling file-based access control is to specify a different file through the jdmk.acl.file system property. The filename associated with the property will override any InetAddressAcl file in the configuration directory. This property can be set programmatically, but it is usually done on the command line when starting your agent. For example, if the full pathname of your InetAddressAcl file is MyAclFile, use this command to start the agent with SNMP access control enabled:

$ java -classpath classpath -Djdmk.acl.file=MyAclFile MyAgent

If an InetAddressAcl file exists, the access rights it defines apply to all management applications that access the agent through its SNMP adaptor. This includes managers on the agent's local host: the InetAddressAcl groups must explicitly give permissions to localhost or the host's name or IP address for such managers. If the InetAddressAcl file does not exist when the SNMP adaptor is instantiated, either in the configuration directory or defined as a property, all SNMP requests will be processed, and traps will be sent only to the local host.

The InetAddressAcl file-based mechanism relies on the JdmkAcl class to provide the access control functionality. This is the class that is initialized with the contents of the InetAddressAcl file. This class provides the rereadTheFile method to reset the access control and trap lists with the contents of the InetAddressAcl file. This method will reload the same file that was used originally, regardless of any new property definitions. After you have updated the InetAddressAcl file, call the following methods to update the access control lists:

// assuming mySnmpAdaptor is declared as an SnmpAdaptorServer object
JdmkAcl theAcl = (JdmkAcl)(mySnmpAdaptor.getInetAddressAcl());
theAcl.rereadTheFile();

The JdmkAcl class that is used by default might not be suitable for all environments. For example, it relies on the java.security.acl package that is not available in the PersonalJava^TM runtime environment. Therefore, one constructor of the SnmpAdaptorServer class lets you override this default, forcing the adaptor not to use access control, regardless of any existing InetAddressAcl file. If you specify false for the useAcl parameter of this constructor, the SNMP adaptor will not even search for an InetAddressAcl file. In this case, no access control is performed, as if there were no InetAddressAcl file: all SNMP requests will be processed, and traps will be sent only to the local host. For security considerations, the use of access control cannot be toggled once the SNMP adaptor has been instantiated.

19.1.3 Custom Access Control

The JdmkAcl class that relies on an ACL file is the default access control mechanism in the SNMP adaptor. For greater adaptability, the SnmpAdaptorServer class has constructors that let you specify your own access control mechanism. For example, if your agent runs on a device with no file system, you could implement a mechanism that does not rely on the jdmk.acl file.

To instantiate an SNMP adaptor with your own access control, use one of the constructors that takes an acl parameter of the type InetAddressAcl. Note that if this parameter's value is null, or if you use a constructor that does not specify an acl parameter, the SNMP adaptor will use the JdmkAcl class by default. If you want to instantiate an SNMP adaptor without access control, call the constructor with the useAcl parameter set to false.

Your access control mechanism must be a class that implements the InetAddressAcl interface. This interface specifies the methods that the SNMP adaptor uses to check permissions when processing a request. If you instantiate the SNMP adaptor with your access control class, the adaptor will call your implementation of the access control methods. Again, for security reasons, the InetAddressAcl implementation in use cannot be changed once the SNMP adaptor has been instantiated.

The JdmkAcl class implements the default access mechanism that uses the jdmk.acl file. It is also an implementation of the InetAddressAcl interface, and it provides a few other methods, such as rereadTheFile, to control the InetAddressAcl mechanism.