This patch is a back-port of the --link-dest option contained in rsync-2.5.6
to rsync-2.5.5.

diff -ru rsync-2.5.5/generator.c rsync-2.5.5L/generator.c
--- rsync-2.5.5/generator.c	Sun Mar 24 22:54:31 2002
+++ rsync-2.5.5L/generator.c	Thu Jan 16 11:32:43 2003
@@ -41,6 +41,7 @@
 extern int always_checksum;
 extern int modify_window;
 extern char *compare_dest;
+extern int link_dest;
 
 
 /* choose whether to skip a particular file */
@@ -51,6 +52,15 @@
 		return 0;
 	}
 	
+        if (link_dest) {
+                if((st->st_mode & ~_S_IFMT) !=  (file->mode & ~_S_IFMT)) {
+                        return 0;
+                }
+                if (st->st_uid != file->uid || st->st_gid != file->gid) {
+                        return 0;
+                }
+        }
+ 
 	/* if always checksum is set then we use the checksum instead 
 	   of the file time to determine whether to sync */
 	if (always_checksum && S_ISREG(st->st_mode)) {
@@ -382,6 +392,18 @@
 			statret = -1;
 		if (statret == -1)
 			errno = saveerrno;
+#if HAVE_LINK
+                else if (link_dest && !dry_run) {
+                        if (do_link(fnamecmpbuf, fname) != 0) {
+                                if (verbose > 0)
+                                        rprintf(FINFO,"link %s => %s : %s\n",
+                                                fnamecmpbuf,
+                                                fname,
+                                                strerror(errno));
+                        }
+                        fnamecmp = fnamecmpbuf;
+                }
+#endif
 		else
 			fnamecmp = fnamecmpbuf;
 	}
diff -ru rsync-2.5.5/options.c rsync-2.5.5L/options.c
--- rsync-2.5.5/options.c	Tue Mar 19 13:16:42 2002
+++ rsync-2.5.5L/options.c	Thu Jan 16 11:34:14 2003
@@ -113,6 +113,7 @@
 char *rsync_path = RSYNC_PATH;
 char *backup_dir = NULL;
 int rsync_port = RSYNC_PORT;
+int link_dest = 0;
 
 int verbose = 0;
 int quiet = 0;
@@ -282,7 +283,7 @@
       OPT_EXCLUDE_FROM, OPT_DELETE, OPT_DELETE_EXCLUDED, OPT_NUMERIC_IDS,
       OPT_RSYNC_PATH, OPT_FORCE, OPT_TIMEOUT, OPT_DAEMON, OPT_CONFIG, OPT_PORT,
       OPT_INCLUDE, OPT_INCLUDE_FROM, OPT_STATS, OPT_PARTIAL, OPT_PROGRESS,
-      OPT_COPY_UNSAFE_LINKS, OPT_SAFE_LINKS, OPT_COMPARE_DEST,
+      OPT_COPY_UNSAFE_LINKS, OPT_SAFE_LINKS, OPT_COMPARE_DEST, OPT_LINK_DEST,
       OPT_LOG_FORMAT, OPT_PASSWORD_FILE, OPT_SIZE_ONLY, OPT_ADDRESS,
       OPT_DELETE_AFTER, OPT_EXISTING, OPT_MAX_DELETE, OPT_BACKUP_DIR, 
       OPT_IGNORE_ERRORS, OPT_BWLIMIT, OPT_BLOCKING_IO,
@@ -341,6 +342,7 @@
   {"timeout",          0,  POPT_ARG_INT,    &io_timeout , 0, 0, 0 },
   {"temp-dir",        'T', POPT_ARG_STRING, &tmpdir , 0, 0, 0 },
   {"compare-dest",     0,  POPT_ARG_STRING, &compare_dest , 0, 0, 0 },
+  {"link-dest",        0,  POPT_ARG_STRING, 0,               OPT_LINK_DEST, 0, 0 },
   /* TODO: Should this take an optional int giving the compression level? */
   {"compress",        'z', POPT_ARG_NONE,   &do_compression , 0, 0, 0 },
   {"daemon",           0,  POPT_ARG_NONE,   &am_daemon , 0, 0, 0 },
@@ -563,6 +565,19 @@
 			read_batch = 1;
 			break;
 
+                case OPT_LINK_DEST:
+#if HAVE_LINK
+                        compare_dest = poptGetOptArg(pc);
+                        link_dest = 1;
+                        break;
+#else
+                        snprintf(err_buf,sizeof(err_buf),
+                                  "hard links are not supported on this %s\n",
+                                 am_server ? "server" : "client");
+                        rprintf(FERROR,"ERROR: hard links not supported on this platform\n");
+                        return 0;
+#endif
+ 
 		default:
                         /* FIXME: If --daemon is specified, then errors for later
                          * parameters seem to disappear. */
@@ -785,7 +800,7 @@
 		 *   and it may be an older version that doesn't know this
 		 *   option, so don't send it if client is the sender.
 		 */
-		args[ac++] = "--compare-dest";
+                args[ac++] = link_dest ? "--link-dest" : "--compare-dest";
 		args[ac++] = compare_dest;
 	}
 
diff -ru rsync-2.5.5/rsync.yo rsync-2.5.5L/rsync.yo
--- rsync-2.5.5/rsync.yo	Wed Feb  6 14:20:49 2002
+++ rsync-2.5.5L/rsync.yo	Thu Jan 16 11:29:05 2003
@@ -261,6 +261,7 @@
      --modify-window=NUM     Timestamp window (seconds) for file match (default=0)
  -T  --temp-dir=DIR          create temporary files in directory DIR
      --compare-dest=DIR      also compare destination files relative to DIR
+     --link-dest=DIR         create hardlinks to DIR for unchanged files
  -P                          equivalent to --partial --progress
  -z, --compress              compress file data
      --exclude=PATTERN       exclude files matching PATTERN
@@ -596,6 +597,11 @@
 temporary destination until they have a chance to be completed.  If DIR is
 a relative path, it is relative to the destination directory.
 
+dit(bf(--link-dest=DIR)) This option behaves like \fB--compare-dest\fP but
+also will create hard links from \fIDIR\fP to the destination directory for
+unchanged files.  Files with changed ownership or permissions will not be
+linked.
+
 dit(bf(-z, --compress)) With this option, rsync compresses any data from
 the files that it sends to the destination machine.  This
 option is useful on slow links.  The compression method used is the