Setting a Global Hook
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Global hooks are always added in ``TSPluginInit`` using
``TSHttpHookAdd``. The two arguments of ``TSHttpHookAdd`` are the hook
ID and the continuation to call when processing the event corresponding
to the hook. In ``blacklist-1.c``, the global hook is added as follows:

.. code-block:: c

   TSHttpHookAdd (TS_HTTP_OS_DNS_HOOK, contp);

Above, ``TS_HTTP_OS_DNS_HOOK`` is the ID for the origin server DNS
lookup hook and ``contp`` is the parent continuation created earlier.

This means that the Blacklist plugin is called at every origin server
DNS lookup. When it is called, the handler functio ``blacklist_plugin``
receives ``TS_EVENT_HTTP_OS_DNS`` and calls ``handle_dns`` to see if the
request is forbidden.
