The Blacklist Plugin

The sample blacklisting plugin included in the Traffic Server SDK is blacklist_1.c. This plugin checks every incoming HTTP client request against a list of blacklisted web sites. If the client requests a blacklisted site, then the plugin returns an Access forbidden message to the client.

The flow of HTTP processing with the blacklist plugin is illustrated in the figure titled “Blacklist Plugin”. This example also contains a simple configuration management interface. It can read a list of blacklisted sites from a file (blacklist.txt) that can be updated by a Traffic Server administrator. When the configuration file is updated, Traffic Server sends an event to the plugin that wakes it up to do some work.

Creating the Parent Continuation

You create the static parent continuation in the mandatory INKPluginInit function. This parent continuation effectively is the plugin: the plugin does work when this continuation receives an event from Traffic Server. Traffic Server passes the event as an argument to the continuation’s handler function. When you create continuations, you must create and specify their handler functions.

You can specify an optional mutex lock when you create continuations. The mutex lock protects data shared by asynchronous processes. Because Traffic Server has a multi-threaded design, race conditions can occur if several threads try to access the same continuation’s data.

Here is how the static parent continuation is created in blacklist-1.c

void
INKPluginInit (int argc, const char *argv[])
{ ...
       INKCont contp;

       contp = INKContCreate (blacklist_plugin, NULL);
...
}

The handler function for the plugin is blacklist_plugin, and the mutex is null. The continuation handler function’s job is to handle the events that are sent to it; accordingly, the blacklist_plugin routine consists of a switch statement that covers each of the events that might be sent to it:

static int
blacklist_plugin (INKCont contp, INKEvent event, void *edata)
{
     INKHttpTxn txnp = (INKHttpTxn) edata;
     switch (event) {
     case INK_EVENT_HTTP_OS_DNS:
          handle_dns (txnp, contp);
          return 0;
     case INK_EVENT_HTTP_SEND_RESPONSE_HDR:
          handle_response (txnp);
          return 0;
     case INK_EVENT_MGMT_UPDATE:
     read_blacklist ();
     return 0;
     default:
          break;
     }
     return 0;
}

When you write handler functions, you have to anticipate any events that might be sent to the handler by hooks or by other functions. In the Blacklist plugin, INK_EVENT_OS_DNS is sent because of the global hook established in INKPluginInit, INK_EVENT_HTTP_SEND_RESPONSE_HDR is sent because the plugin contains a transaction hook (see Setting Up a Transaction Hook), and INK_EVENT_MGMT_UPDATE is sent by Traffic Manager whenever there is a configuration change (see Setting Up UI Update Callbacks). It is good practice to have a default case in your switch statements.

Traffic Server Software Developers Kit

The Blacklist Plugin

Creating the Parent Continuation