Client class level options specified in the config backend have empty values on the wire

Suppose the option-data is specified for a class in the Config Backend and it csv-format setting is set to true. The option data is correctly fetched by the DHCP servers from the config backend and the config-get command returns valid option-data setting. However, when a DHCP client asks for this option the server returns it with empty payload (length of 0).

The issue is in the cb_ctl_dhcp4.cc (and cb_ctl_dhcp6.cc). The following code fetches updated client classes from the config backend and applies to the runtime server configuration:

    if (audit_entries.empty() || !updated_entries.empty()) {
        ClientClassDictionary client_classes = getMgr().getPool()->getAllClientClasses4(backend_selector,
                                                                                        server_selector);
        // Match expressions are not initialized for classes returned from the config backend.
        // We have to ensure to initialize them before they can be used by the server.
        client_classes.initMatchExpr(AF_INET);
        external_cfg->setClientClassDictionary(boost::make_shared<ClientClassDictionary>(client_classes));
    }

That call sets the new dictionary with many classes, each class having cfg_option_ instance. For each such instance we should call createOption() using appropriate option definitions to prepare on-wire data for these options. Note that we already do it for subnet-level, shared-network-level and other options. For example, see CfgSubnets4::merge:

        // Create the subnet's options based on the given definitions.
        (*other_subnet)->getCfgOption()->createOptions(cfg_def);
        for (auto pool : (*other_subnet)->getPoolsWritable(Lease::TYPE_V4)) {
            pool->getCfgOption()->createOptions(cfg_def);
        }

Therefore, we don't observe this issue for option-data other than defined at the client class level.

It is also worth mentioning that a workaround exists for this problem. The option-data at the client class level should be specified with the "csv-format": false setting and the "data" should be in the hex format.