diff --git a/common/rdr/SubstitutingInStream.h b/common/rdr/SubstitutingInStream.h
new file mode 100644
index 0000000..325b01c
--- /dev/null
+++ b/common/rdr/SubstitutingInStream.h
@@ -0,0 +1,102 @@
+/* Copyright (C) 2002-2005 RealVNC Ltd.  All Rights Reserved.
+ * 
+ * This is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ * 
+ * This software is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this software; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307,
+ * USA.
+ */
+
+#ifndef __RDR_SUBSTITUTINGINSTREAM_H__
+#define __RDR_SUBSTITUTINGINSTREAM_H__
+
+#include <rdr/InStream.h>
+#include <rdr/Exception.h>
+
+namespace rdr {
+
+  class Substitutor {
+  public:
+    virtual char* substitute(const char* varName) = 0;
+  };
+
+  class SubstitutingInStream : public InStream {
+  public:
+    SubstitutingInStream(InStream* underlying_, Substitutor* s,
+                         int maxVarNameLen_)
+      : underlying(underlying_), dollar(0), substitutor(s), subst(0),
+        maxVarNameLen(maxVarNameLen_)
+    {
+      ptr = end = underlying->getptr();
+      varName = new char[maxVarNameLen+1];
+    }
+    ~SubstitutingInStream() {
+      delete underlying;
+      delete [] varName;
+      delete [] subst;
+    }
+
+    int pos() { return underlying->pos(); }
+
+    virtual int overrun(int itemSize, int nItems, bool wait=true) {
+      if (itemSize != 1)
+        throw new rdr::Exception("SubstitutingInStream: itemSize must be 1");
+
+      if (subst) {
+        delete [] subst;
+        subst = 0;
+      } else {
+        underlying->setptr(ptr);
+      }
+
+      underlying->check(1);
+      ptr = underlying->getptr();
+      end = underlying->getend();
+      dollar = (const U8*)memchr(ptr, '$', end-ptr);
+      if (dollar) {
+        if (dollar == ptr) {
+          try {
+            int i = 0;
+            while (i < maxVarNameLen) {
+              varName[i++] = underlying->readS8();
+              varName[i] = 0;
+              subst = substitutor->substitute(varName);
+              if (subst) {
+                ptr = (U8*)subst;
+                end = (U8*)subst + strlen(subst);
+                break;
+              }
+            }
+          } catch (EndOfStream&) {
+          }
+
+          if (!subst)
+            dollar = (const U8*)memchr(ptr+1, '$', end-ptr-1);
+        }
+        if (!subst && dollar) end = dollar;
+      }
+
+      if (itemSize * nItems > end - ptr)
+        nItems = (end - ptr) / itemSize;
+
+      return nItems;
+    }
+
+    InStream* underlying;
+    const U8* dollar;
+    Substitutor* substitutor;
+    char* varName;
+    char* subst;
+    int maxVarNameLen;
+  };
+}
+#endif