The initial version of this patch was originally posted David Woodhouse, and
dman gets the credit for first integrating it with SA-Exim.

I have since then maintained it by first making a few minor changes, and
later switching it to a major/minor number scheme to support upgrades in
the exim API that don't affect backward compatibility (you can rely on
a feature denoted by the minor number and be compatible with future versions
of exim until Philip has to break the API and increase the major number)

Marc MERLIN <marc_soft@merlins.org>

diff -urN exim-4.14-0/src/EDITME exim-4.14-1/src/EDITME
--- exim-4.14-0/src/EDITME	Tue Mar 11 04:20:18 2003
+++ exim-4.14-1/src/EDITME	Sun Mar 23 15:34:15 2003
@@ -388,6 +388,20 @@
 
 
 #------------------------------------------------------------------------------
+# On systems which support dynamic loading of shared libraries, Exim can
+# load a local_scan function specified in its config file instead of having
+# to be recompiled with the desired local_scan function. For a full 
+# description of the API to this function, see the Exim specification.
+
+DLOPEN_LOCAL_SCAN=yes
+
+# If you set DLOPEN_LOCAL_SCAN, then you need to include -rdynamic in the
+# linker flags.  Without it, the loaded .so won't be able to access any
+# functions from exim.
+
+LFLAGS=-rdynamic -ldl
+
+#------------------------------------------------------------------------------
 # The default distribution of Exim contains only the plain text form of the
 # documentation. Other forms are available separately. If you want to install
 # the documentation in "info" format, first fetch the Texinfo documentation
diff -urNad 50_localscan_dlopen.tmp/src/config.h.defaults 50_localscan_dlopen/src/config.h.defaults
--- 50_localscan_dlopen.tmp/src/config.h.defaults      Sun Dec 29 11:55:42 2002
+++ 50_localscan_dlopen/src/config.h.defaults  Sun Dec 29 11:56:44 2002
@@ -17,6 +17,8 @@
 #define AUTH_PLAINTEXT
 #define AUTH_SPA
 
+#define DLOPEN_LOCAL_SCAN
+
 #define BIN_DIRECTORY
 
 #define CONFIGURE_FILE
diff -urN exim-4.14-0/src/globals.c exim-4.14-1/src/globals.c
--- exim-4.14-0/src/globals.c	Tue Mar 11 04:20:20 2003
+++ exim-4.14-1/src/globals.c	Sun Mar 23 15:34:15 2003
@@ -103,6 +103,9 @@
 uschar *tls_verify_hosts       = NULL;
 #endif
 
+#ifdef DLOPEN_LOCAL_SCAN
+uschar *local_scan_path        = NULL;
+#endif
 
 /* Input-reading functions for messages, so we can use special ones for
 incoming TCP/IP. The defaults use stdin. We never need these for any
diff -urN exim-4.14-0/src/globals.h exim-4.14-1/src/globals.h
--- exim-4.14-0/src/globals.h	Tue Mar 11 04:20:20 2003
+++ exim-4.14-1/src/globals.h	Sun Mar 23 15:34:15 2003
@@ -67,6 +67,9 @@
 extern uschar *tls_verify_hosts;       /* Mandatory client verification */
 #endif
 
+#ifdef DLOPEN_LOCAL_SCAN
+extern uschar *local_scan_path;        /* Path to local_scan() library */
+#endif
 
 /* Input-reading functions for messages, so we can use special ones for
 incoming TCP/IP. */
diff -urN exim-4.14-0/src/local_scan.c exim-4.14-1/src/local_scan.c
--- exim-4.14-0/src/local_scan.c	Tue Mar 11 04:20:20 2003
+++ exim-4.14-1/src/local_scan.c	Sun Mar 23 15:34:15 2003
@@ -5,60 +5,131 @@
 /* Copyright (c) University of Cambridge 1995 - 2003 */
 /* See the file NOTICE for conditions of use and distribution. */
 
+#include "exim.h"
 
-/******************************************************************************
-This file contains a template local_scan() function that just returns ACCEPT.
-If you want to implement your own version, you should copy this file to, say
-Local/local_scan.c, and edit the copy. To use your version instead of the
-default, you must set
-
-LOCAL_SCAN_SOURCE=Local/local_scan.c
-
-in your Local/Makefile. This makes it easy to copy your version for use with
-subsequent Exim releases.
-
-For a full description of the API to this function, see the Exim specification.
-******************************************************************************/
-
-
-/* This is the only Exim header that you should include. The effect of
-including any other Exim header is not defined, and may change from release to
-release. Use only the documented interface! */
-
-#include "local_scan.h"
-
-
-/* This is a "do-nothing" version of a local_scan() function. The arguments
-are:
-
-  fd             The file descriptor of the open -D file, which contains the
-                   body of the message. The file is open for reading and
-                   writing, but modifying it is dangerous and not recommended.
-
-  return_text    A pointer to an unsigned char* variable which you can set in
-                   order to return a text string. It is initialized to NULL.
-
-The return values of this function are:
-
-  LOCAL_SCAN_ACCEPT
-                 The message is to be accepted. The return_text argument is
-                   saved in $local_scan_data.
-
-  LOCAL_SCAN_REJECT
-                 The message is to be rejected. The returned text is used
-                   in the rejection message.
-
-  LOCAL_SCAN_TEMPREJECT
-                 This specifies a temporary rejection. The returned text
-                   is used in the rejection message.
-*/
+#ifdef DLOPEN_LOCAL_SCAN
+#include <dlfcn.h>
+static int (*local_scan_fn)(int fd, uschar **return_text) = NULL;
+static int load_local_scan_library(void);
+#endif
 
 int
 local_scan(int fd, uschar **return_text)
 {
 fd = fd;                      /* Keep picky compilers happy */
 return_text = return_text;
-return LOCAL_SCAN_ACCEPT;
+#ifdef DLOPEN_LOCAL_SCAN
+/* local_scan_path is defined AND not the empty string */
+if (local_scan_path && *local_scan_path)
+  {
+  if (!local_scan_fn)
+    {
+    if (!load_local_scan_library())
+      {
+        char *base_msg , *error_msg , *final_msg ;
+        int final_length = -1 ;
+
+        base_msg=US"Local configuration error - local_scan() library failure\n";
+        error_msg = dlerror() ;
+
+        final_length = strlen(base_msg) + strlen(error_msg) + 1 ;
+        final_msg = (char*)malloc( final_length*sizeof(char) ) ;
+        *final_msg = '\0' ;
+
+        strcat( final_msg , base_msg ) ;
+        strcat( final_msg , error_msg ) ;
+
+        *return_text = final_msg ;
+      return LOCAL_SCAN_TEMPREJECT;
+      }
+    }
+    return local_scan_fn(fd, return_text);
+  }
+else
+#endif
+  return LOCAL_SCAN_ACCEPT;
+}
+
+#ifdef DLOPEN_LOCAL_SCAN
+
+static int load_local_scan_library(void)
+{
+/* No point in keeping local_scan_lib since we'll never dlclose() anyway */
+void *local_scan_lib = NULL;
+int (*local_scan_version_fn)(void);
+int vers_maj;
+int vers_min;
+
+local_scan_lib = dlopen(local_scan_path, RTLD_NOW);
+if (!local_scan_lib)
+  {
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() library open failed - "
+    "message temporarily rejected");
+  return FALSE;
+  }
+
+local_scan_version_fn = dlsym(local_scan_lib, "local_scan_version_major");
+if (!local_scan_version_fn)
+  {
+  dlclose(local_scan_lib);
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() library doesn't contain "
+    "local_scan_version_major() function - message temporarily rejected");
+  return FALSE;
+  }
+
+/* The major number is increased when the ABI is changed in a non
+   backward compatible way. */
+vers_maj = local_scan_version_fn();
+
+local_scan_version_fn = dlsym(local_scan_lib, "local_scan_version_minor");
+if (!local_scan_version_fn)
+  {
+  dlclose(local_scan_lib);
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() library doesn't contain "
+    "local_scan_version_minor() function - message temporarily rejected");
+  return FALSE;
+  }
+
+/* The minor number is increased each time a new feature is added (in a
+   way that doesn't break backward compatibility) -- Marc */
+vers_min = local_scan_version_fn();
+
+
+if (vers_maj != LOCAL_SCAN_ABI_VERSION_MAJOR)
+  {
+  dlclose(local_scan_lib);
+  local_scan_lib = NULL;
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() has an incompatible major"
+    "version number, you need to recompile your module for this version"
+    "of exim (The module was compiled for version %d.%d and this exim provides"
+    "ABI version %d.%d)", vers_maj, vers_min, LOCAL_SCAN_ABI_VERSION_MAJOR,
+    LOCAL_SCAN_ABI_VERSION_MINOR);
+  return FALSE;
+  }
+else if (vers_min > LOCAL_SCAN_ABI_VERSION_MINOR)
+  {
+  dlclose(local_scan_lib);
+  local_scan_lib = NULL;
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() has an incompatible minor"
+    "version number, you need to recompile your module for this version"
+    "of exim (The module was compiled for version %d.%d and this exim provides"
+    "ABI version %d.%d)", vers_maj, vers_min, LOCAL_SCAN_ABI_VERSION_MAJOR,
+    LOCAL_SCAN_ABI_VERSION_MINOR);
+  return FALSE;
+  }
+
+local_scan_fn = dlsym(local_scan_lib, "local_scan");
+if (!local_scan_fn)
+  {
+  dlclose(local_scan_lib);
+  log_write(0, LOG_MAIN|LOG_REJECT, "local_scan() library doesn't contain "
+    "local_scan() function - message temporarily rejected");      
+  return FALSE;
+  }
+
+return TRUE;
 }
+
+#endif /* DLOPEN_LOCAL_SCAN */
 
 /* End of local_scan.c */
diff -urN exim-4.14-0/src/local_scan.h exim-4.14-1/src/local_scan.h
--- exim-4.14-0/src/local_scan.h	Tue Mar 11 04:20:20 2003
+++ exim-4.14-1/src/local_scan.h	Sun Mar 23 15:34:57 2003
@@ -71,6 +71,15 @@
 
 #define SPOOL_DATA_START_OFFSET (MESSAGE_ID_LENGTH+3)
 
+/* local_scan() ABI version number for dynamic libraries
+   The major number is increased when the ABI is changed in a non
+   backward compatible way.
+   The minor number is increased each time a new feature is added (in a
+   way that doesn't break backward compatibility) -- Marc */
+#define LOCAL_SCAN_ABI_VERSION_MAJOR 1
+#define LOCAL_SCAN_ABI_VERSION_MINOR 0
+#define LOCAL_SCAN_ABI_VERSION \
+  LOCAL_SCAN_ABI_VERSION_MAJOR.LOCAL_SCAN_ABI_VERSION_MINOR
 
 /* Structure definitions that are documented as visible in the function. */
 
diff -urN exim-4.14-0/src/readconf.c exim-4.14-1/src/readconf.c
--- exim-4.14-0/src/readconf.c	Tue Mar 11 04:20:22 2003
+++ exim-4.14-1/src/readconf.c	Sun Mar 23 15:34:15 2003
@@ -182,6 +182,9 @@
   { "local_from_prefix",        opt_stringptr,   &local_from_prefix },
   { "local_from_suffix",        opt_stringptr,   &local_from_suffix },
   { "local_interfaces",         opt_stringptr,   &local_interfaces },
+#ifdef DLOPEN_LOCAL_SCAN
+  { "local_scan_path",          opt_stringptr,   &local_scan_path },
+#endif
   { "local_scan_timeout",       opt_time,        &local_scan_timeout },
   { "local_sender_retain",      opt_bool,        &local_sender_retain },
   { "localhost_number",         opt_stringptr,   &host_number_string },