TruncateFCS: new element to truncate the FCS

Removes FCS from ethernet frames.

In practice, just removes 4 bytes...

elements/ethernet/truncatefcs.cc

@@ -0,0 +1,57 @@
+// -*- mode: c++; c-basic-offset: 4 -*-
+/*
+ * truncate.{cc,hh} -- limits packet length
+ * Eddie Kohler
+ *
+ * Copyright (c) 2004 Regents of the University of California
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, subject to the conditions
+ * listed in the Click LICENSE file. These conditions include: you must
+ * preserve this copyright notice, and you cannot mention the copyright
+ * holders in advertising related to the Software without their permission.
+ * The Software is provided WITHOUT ANY WARRANTY, EXPRESS OR IMPLIED. This
+ * notice is a summary of the Click LICENSE file; the license in that file is
+ * legally binding.
+ */
+
+#include <click/config.h>
+#include "truncatefcs.hh"
+#include <click/args.hh>
+#include <click/error.hh>
+#include <click/glue.hh>
+#include <click/packet_anno.hh>
+CLICK_DECLS
+
+TruncateFCS::TruncateFCS()
+{
+}
+
+int
+TruncateFCS::configure(Vector<String> &conf, ErrorHandler *errh)
+{
+    _extra_anno = true;
+    return Args(conf, this, errh)
+	.read("EXTRA_LENGTH", _extra_anno)
+	.complete();
+}
+
+Packet *
+TruncateFCS::simple_action(Packet *p)
+{
+	if (_extra_anno && EXTRA_LENGTH_ANNO(p) > 4)
+	    SET_EXTRA_LENGTH_ANNO(p, EXTRA_LENGTH_ANNO(p) - 4);
+    else
+        p->take(4);
+    return p;
+}
+
+void
+TruncateFCS::add_handlers()
+{
+}
+
+CLICK_ENDDECLS
+EXPORT_ELEMENT(TruncateFCS)
+ELEMENT_MT_SAFE(TruncateFCS)

elements/ethernet/truncatefcs.hh

@@ -0,0 +1,41 @@
+// -*- mode: c++; c-basic-offset: 4 -*-
+#ifndef CLICK_TRUNCATECS_HH
+#define CLICK_TRUNCATEFCS_HH
+#include <click/element.hh>
+CLICK_DECLS
+
+/*
+ * =c
+ * TruncateFCS()
+ * =s ethernet
+ * remove the FCS from packet
+ * =d
+ * Does not check if the FCS is present, therefore the packets is always shortened by 4 bytes.
+ *
+ * The EXTRA_LENGTH keyword argument determines whether packets' extra length
+ * annotations are updated to account for any dropped bytes.  Default is true.
+ * =a Strip, Pad, Truncate
+ */
+
+class TruncateFCS : public Element { public:
+
+    TruncateFCS() CLICK_COLD;
+
+    const char *class_name() const override		{ return "TruncateFCS"; }
+    const char *port_count() const override		{ return PORTS_1_1; }
+
+    int configure(Vector<String> &, ErrorHandler *) CLICK_COLD;
+    bool can_live_reconfigure() const		{ return true; }
+
+    Packet *simple_action(Packet *);
+
+    void add_handlers() CLICK_COLD;
+
+  private:
+
+    bool _extra_anno;
+
+};
+
+CLICK_ENDDECLS
+#endif