doc/sdk/header-based-plugin-examples/blacklist-plugin/working-with-http-header-functions.en.rst

Working with HTTP Header Functions
**********************************

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   distributed with this work for additional information
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   to you under the Apache License, Version 2.0 (the
   "License"); you may not use this file except in compliance
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The Blacklist plugin examines the host header in every client
transaction. This is done in the ``handle_dns`` routine, using
``TSHttpTxnClientReqGet``, ``TSHttpHdrUrlGet``, and ``TSUrlHostGet``.

.. code-block:: c

   static void
   handle_dns (TSHttpTxn txnp, TSCont contp)
   {
   TSMBuffer bufp;
   TSMLoc hdr_loc;
   TSMLoc url_loc;
   const char *host;
   int i;
   int host_length;
 
   if (TSHttpTxnClientReqGet(txnp, &bufp, &hdr_loc) != TS_SUCCESS) {
      TSError("couldn't retrieve client request header\n");
      goto done;
   }
 
   if (TSHttpHdrUrlGet(bufp, hdr_loc, &url_loc) != TS_SUCCESS) {
      TSError("couldn't retrieve request url\n");
      TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
      goto done;
   }
 
   host = TSUrlHostGet(bufp, url_loc, &host_length);
   if (!host) {
      TSError("couldn't retrieve request hostname\n");
      TSHandleMLocRelease(bufp, hdr_loc, url_loc);
      TSHandleMLocRelease(bufp, TS_NULL_MLOC, hdr_loc);
      goto done;
   }

To access the host header, the plugin must first get the client request,
retrieve the URL portion, and then obtain the host header. See `HTTP
Headers <../../http-headers>`__ for more information about these calls.
See `Release Marshal Buffer
Handles <../../http-headers/guide-to-trafficserver-http-header-system/release-marshal-buffer-handles>`__
for guidelines on using ``TSHandleMLocRelease``.