Denylist Plugin¶

The sample denylisting plugin included in the Traffic Server SDK is denylist_1.cc. This plugin checks every incoming HTTP client request against a list of listed web sites. If the client requests a listed site, then the plugin returns an Access forbidden message to the client.

The flow of HTTP processing with the denylist plugin is illustrated in the figure titled Denylist Plugin. This example also contains a simple configuration management interface. It can read a list of sites from a file (denylist.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 TSPluginInit function. This parent continuation effectively is the plugin: the plugin executes only 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 denylist_1.cc:

void
TSPluginInit (int argc, const char *argv[])
{
   // ...
   TSCont contp;

   contp = TSContCreate (denylist_plugin, nullptr);
   // ...
}

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

namespace
{
DbgCtl dbg_ctl{"denylist_plugin"};
}

static int
denylist_plugin (TSCont contp, TSEvent event, void *edata)
{
   TSHttpTxn txnp = static_cast<TSHttpTxn>(edata);
   switch (event) {
      case TS_EVENT_HTTP_OS_DNS:
         handle_dns (txnp, contp);
         return 0;
      case TS_EVENT_HTTP_SEND_RESPONSE_HDR:
         handle_response (txnp);
         return 0;
      default:
         Dbg (dbg_ctl, "This event was unexpected: %d", int(event) );
         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 Denylist plugin, TS_EVENT_HTTP_OS_DNS is sent because of the global hook established in TSPluginInit, TS_EVENT_HTTP_SEND_RESPONSE_HDR is sent because the plugin contains a transaction hook (see Setting Up a Transaction Hook). It is good practice to have a default case in your switch statements.