CAUTION:  This patch compiles, but is otherwise totally untested!

This patch also implements --times-only.

Implementation details for the --source-filter and -dest-filter options:

 - These options open a *HUGE* security hole in daemon mode unless they
   are refused in your rsyncd.conf!

 - Filtering disables rsync alogrithm. (This should be fixed.)

 - Source filter makes temporary files in /tmp. (Should be overridable.)

 - If source filter fails, data is send unfiltered. (Should be changed
   to abort.)

 - Failure of destination filter, causes data loss!!! (Should be changed
   to abort.)

 - If filter changes size of file, you should use --times-only option to
   prevent repeated transfers of unchanged files.

 - If the COMMAND contains single quotes, option-passing breaks.  (Needs
   to be fixed.)

To use this patch, run these commands for a successful build:

    patch -p1 <patches/source-filter_dest-filter.diff
    ./prepare-source
    ./configure                                (optional if already run)
    make

based-on: 0ac7ebceef70417355f25daf9e2fd94e84c49749
diff --git a/generator.c b/generator.c
--- a/generator.c
+++ b/generator.c
@@ -67,6 +67,7 @@ extern int append_mode;
 extern int make_backups;
 extern int csum_length;
 extern int ignore_times;
+extern int times_only;
 extern int size_only;
 extern OFF_T max_size;
 extern OFF_T min_size;
@@ -618,7 +619,7 @@ int quick_check_ok(enum filetype ftype, const char *fn, struct file_struct *file
 {
 	switch (ftype) {
 	  case FT_REG:
-		if (st->st_size != F_LENGTH(file))
+		if (!times_only && st->st_size != F_LENGTH(file))
 			return 0;
 
 		/* If always_checksum is set then we use the checksum instead
diff --git a/main.c b/main.c
--- a/main.c
+++ b/main.c
@@ -189,7 +189,7 @@ int shell_exec(const char *cmd)
 }
 
 /* Wait for a process to exit, calling io_flush while waiting. */
-static void wait_process_with_flush(pid_t pid, int *exit_code_ptr)
+void wait_process_with_flush(pid_t pid, int *exit_code_ptr)
 {
 	pid_t waited_pid;
 	int status;
diff --git a/options.c b/options.c
--- a/options.c
+++ b/options.c
@@ -116,6 +116,7 @@ int safe_symlinks = 0;
 int copy_unsafe_links = 0;
 int munge_symlinks = 0;
 int size_only = 0;
+int times_only = 0;
 int daemon_bwlimit = 0;
 int bwlimit = 0;
 int fuzzy_basis = 0;
@@ -176,6 +177,8 @@ char *logfile_name = NULL;
 char *logfile_format = NULL;
 char *stdout_format = NULL;
 char *password_file = NULL;
+char *source_filter = NULL;
+char *dest_filter = NULL;
 char *early_input_file = NULL;
 char *rsync_path = RSYNC_PATH;
 char *backup_dir = NULL;
@@ -685,6 +688,7 @@ static struct poptOption long_options[] = {
   {"chmod",            0,  POPT_ARG_STRING, 0, OPT_CHMOD, 0, 0 },
   {"ignore-times",    'I', POPT_ARG_NONE,   &ignore_times, 0, 0, 0 },
   {"size-only",        0,  POPT_ARG_NONE,   &size_only, 0, 0, 0 },
+  {"times-only",       0,  POPT_ARG_NONE,   &times_only , 0, 0, 0 },
   {"one-file-system", 'x', POPT_ARG_NONE,   0, 'x', 0, 0 },
   {"no-one-file-system",0, POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
   {"no-x",             0,  POPT_ARG_VAL,    &one_file_system, 0, 0, 0 },
@@ -822,6 +826,8 @@ static struct poptOption long_options[] = {
   {"early-input",      0,  POPT_ARG_STRING, &early_input_file, 0, 0, 0 },
   {"blocking-io",      0,  POPT_ARG_VAL,    &blocking_io, 1, 0, 0 },
   {"no-blocking-io",   0,  POPT_ARG_VAL,    &blocking_io, 0, 0, 0 },
+  {"source-filter",    0,  POPT_ARG_STRING, &source_filter, 0, 0, 0 },
+  {"dest-filter",      0,  POPT_ARG_STRING, &dest_filter, 0, 0, 0 },
   {"outbuf",           0,  POPT_ARG_STRING, &outbuf_mode, 0, 0, 0 },
   {"remote-option",   'M', POPT_ARG_STRING, 0, 'M', 0, 0 },
   {"protocol",         0,  POPT_ARG_INT,    &protocol_version, 0, 0, 0 },
@@ -2419,6 +2425,16 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 		}
 	}
 
+	if (source_filter || dest_filter) {
+		if (whole_file == 0) {
+			snprintf(err_buf, sizeof err_buf,
+				 "--no-whole-file cannot be used with --%s-filter\n",
+				 source_filter ? "source" : "dest");
+			return 0;
+		}
+		whole_file = 1;
+	}
+
 	if (files_from) {
 		char *h, *p;
 		int q;
@@ -2807,6 +2823,25 @@ void server_options(char **args, int *argc_p)
 	else if (missing_args == 1 && !am_sender)
 		args[ac++] = "--ignore-missing-args";
 
+	if (times_only && am_sender)
+		args[ac++] = "--times-only";
+
+	if (source_filter && !am_sender) {
+		/* Need to single quote the arg to keep the remote shell
+		 * from splitting it.  FIXME: breaks if command has single quotes. */
+	        if (asprintf(&arg, "--source-filter='%s'", source_filter) < 0)
+			goto oom;
+		args[ac++] = arg;
+	}
+
+	if (dest_filter && am_sender) {
+		/* Need to single quote the arg to keep the remote shell
+		 * from splitting it.  FIXME: breaks if command has single quotes. */
+	        if (asprintf(&arg, "--dest-filter='%s'", dest_filter) < 0)
+			goto oom;
+		args[ac++] = arg;
+	}
+
 	if (modify_window_set && am_sender) {
 		char *fmt = modify_window < 0 ? "-@%d" : "--modify-window=%d";
 		if (asprintf(&arg, fmt, modify_window) < 0)
diff --git a/pipe.c b/pipe.c
--- a/pipe.c
+++ b/pipe.c
@@ -27,6 +27,7 @@ extern int am_server;
 extern int blocking_io;
 extern int filesfrom_fd;
 extern int munge_symlinks;
+extern mode_t orig_umask;
 extern char *logfile_name;
 extern int remote_option_cnt;
 extern const char **remote_options;
@@ -176,3 +177,77 @@ pid_t local_child(int argc, char **argv, int *f_in, int *f_out,
 
 	return pid;
 }
+
+pid_t run_filter(char *command[], int out, int *pipe_to_filter)
+{
+	pid_t pid;
+	int pipefds[2];
+
+	if (DEBUG_GTE(CMD, 1))
+		print_child_argv("opening connection using:", command);
+
+	if (pipe(pipefds) < 0) {
+		rsyserr(FERROR, errno, "pipe");
+		exit_cleanup(RERR_IPC);
+	}
+
+	pid = do_fork();
+	if (pid == -1) {
+		rsyserr(FERROR, errno, "fork");
+		exit_cleanup(RERR_IPC);
+	}
+
+	if (pid == 0) {
+		if (dup2(pipefds[0], STDIN_FILENO) < 0
+		 || close(pipefds[1]) < 0
+		 || dup2(out, STDOUT_FILENO) < 0) {
+			rsyserr(FERROR, errno, "Failed dup/close");
+			exit_cleanup(RERR_IPC);
+		}
+		umask(orig_umask);
+		set_blocking(STDIN_FILENO);
+		if (blocking_io)
+			set_blocking(STDOUT_FILENO);
+		execvp(command[0], command);
+		rsyserr(FERROR, errno, "Failed to exec %s", command[0]);
+		exit_cleanup(RERR_IPC);
+	}
+
+	if (close(pipefds[0]) < 0) {
+		rsyserr(FERROR, errno, "Failed to close");
+		exit_cleanup(RERR_IPC);
+	}
+
+	*pipe_to_filter = pipefds[1];
+
+	return pid;
+}
+
+pid_t run_filter_on_file(char *command[], int out, int in)
+{
+	pid_t pid;
+
+	if (DEBUG_GTE(CMD, 1))
+		print_child_argv("opening connection using:", command);
+
+	pid = do_fork();
+	if (pid == -1) {
+		rsyserr(FERROR, errno, "fork");
+		exit_cleanup(RERR_IPC);
+	}
+
+	if (pid == 0) {
+		if (dup2(in, STDIN_FILENO) < 0
+		 || dup2(out, STDOUT_FILENO) < 0) {
+			rsyserr(FERROR, errno, "Failed to dup2");
+			exit_cleanup(RERR_IPC);
+		}
+		if (blocking_io)
+			set_blocking(STDOUT_FILENO);
+		execvp(command[0], command);
+		rsyserr(FERROR, errno, "Failed to exec %s", command[0]);
+		exit_cleanup(RERR_IPC);
+	}
+
+	return pid;
+}
diff --git a/receiver.c b/receiver.c
--- a/receiver.c
+++ b/receiver.c
@@ -61,6 +61,7 @@ extern BOOL want_progress_now;
 extern mode_t orig_umask;
 extern struct stats stats;
 extern char *tmpdir;
+extern char *dest_filter;
 extern char *partial_dir;
 extern char *basis_dir[MAX_BASIS_DIRS+1];
 extern char sender_file_sum[MAX_DIGEST_LEN];
@@ -528,6 +529,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 	char *fnametmp, fnametmpbuf[MAXPATHLEN];
 	char *fnamecmp, *partialptr;
 	char fnamecmpbuf[MAXPATHLEN];
+	char *filter_argv[MAX_FILTER_ARGS + 1];
 	uchar fnamecmp_type;
 	struct file_struct *file;
 	int itemizing = am_server ? logfile_format_has_i : stdout_format_has_i;
@@ -538,6 +540,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 	const char *parent_dirname = "";
 #endif
 	int ndx, recv_ok, one_inplace;
+	pid_t pid = 0;
 
 	if (DEBUG_GTE(RECV, 1))
 		rprintf(FINFO, "recv_files(%d) starting\n", cur_flist->used);
@@ -548,6 +551,23 @@ int recv_files(int f_in, int f_out, char *local_name)
 	if (whole_file < 0)
 		whole_file = 0;
 
+	if (dest_filter) {
+		char *p;
+		char *sep = " \t";
+		int i;
+		for (p = strtok(dest_filter, sep), i = 0;
+		     p && i < MAX_FILTER_ARGS;
+		     p = strtok(0, sep))
+			filter_argv[i++] = p;
+		filter_argv[i] = NULL;
+		if (p) {
+			rprintf(FERROR,
+				"Too many arguments to dest-filter (> %d)\n",
+				MAX_FILTER_ARGS);
+			exit_cleanup(RERR_SYNTAX);
+		}
+	}
+
 	progress_init();
 
 	while (1) {
@@ -870,6 +890,9 @@ int recv_files(int f_in, int f_out, char *local_name)
 		else if (!am_server && INFO_GTE(NAME, 1) && INFO_EQ(PROGRESS, 1))
 			rprintf(FINFO, "%s\n", fname);
 
+		if (dest_filter)
+			pid = run_filter(filter_argv, fd2, &fd2);
+
 		/* recv file data */
 		recv_ok = receive_data(f_in, fnamecmp, fd1, st.st_size, fname, fd2, file, inplace || one_inplace);
 
@@ -885,6 +908,16 @@ int recv_files(int f_in, int f_out, char *local_name)
 			exit_cleanup(RERR_FILEIO);
 		}
 
+		if (dest_filter) {
+			int status;
+			wait_process_with_flush(pid, &status);
+			if (status != 0) {
+				rprintf(FERROR, "filter %s exited code: %d\n",
+					dest_filter, status);
+				continue;
+			}
+		}
+
 		if ((recv_ok && (!delay_updates || !partialptr)) || inplace) {
 			if (partialptr == fname)
 				partialptr = NULL;
diff --git a/rsync.1.md b/rsync.1.md
--- a/rsync.1.md
+++ b/rsync.1.md
@@ -424,6 +424,7 @@ has its own detailed description later in this manpage.
 --contimeout=SECONDS     set daemon connection timeout in seconds
 --ignore-times, -I       don't skip files that match size and time
 --size-only              skip files that match in size
+--times-only             skip files that match in mod-time
 --modify-window=NUM, -@  set the accuracy for mod-time comparisons
 --temp-dir=DIR, -T       create temporary files in directory DIR
 --fuzzy, -y              find similar file for basis if no dest file
@@ -472,6 +473,8 @@ has its own detailed description later in this manpage.
 --write-batch=FILE       write a batched update to FILE
 --only-write-batch=FILE  like --write-batch but w/o updating dest
 --read-batch=FILE        read a batched update from FILE
+--source-filter=COMMAND  filter file through COMMAND at source
+--dest-filter=COMMAND    filter file through COMMAND at destination
 --protocol=NUM           force an older protocol version to be used
 --iconv=CONVERT_SPEC     request charset conversion of filenames
 --checksum-seed=NUM      set block/file checksum seed (advanced)
@@ -3525,6 +3528,36 @@ your home directory (remove the '=' for that).
     [`--write-batch`](#opt).  If _FILE_ is `-`, the batch data will be read
     from standard input. See the "BATCH MODE" section for details.
 
+0.  `--source-filter=COMMAND`
+
+    This option allows the user to specify a filter program that will be
+    applied to the contents of all transferred regular files before the data is
+    sent to destination.  COMMAND will receive the data on its standard input
+    and it should write the filtered data to standard output.  COMMAND should
+    exit non-zero if it cannot process the data or if it encounters an error
+    when writing the data to stdout.
+
+    Example: `--source-filter="gzip -9"` will cause remote files to be
+    compressed.  Use of `--source-filter` automatically enables
+    [`--whole-file`](#opt).  If your filter does not output the same number of
+    bytes that it received on input, you should use `--times-only` to
+    disable size and content checks on subsequent rsync runs.
+
+0.  `--dest-filter=COMMAND`
+
+    This option allows you to specify a filter program that will be applied to
+    the contents of all transferred regular files before the data is written to
+    disk.  COMMAND will receive the data on its standard input and it should
+    write the filtered data to standard output.  COMMAND should exit non-zero
+    if it cannot process the data or if it encounters an error when writing the
+    data to stdout.
+
+    Example: --dest-filter="gzip -9" will cause remote files to be compressed.
+    Use of --dest-filter automatically enables --whole-file.  If your filter
+    does not output the same number of bytes that it received on input, you
+    should use --times-only to disable size and content checks on subsequent
+    rsync runs.
+
 0.  `--protocol=NUM`
 
     Force an older protocol version to be used.  This is useful for creating a
diff --git a/rsync.h b/rsync.h
--- a/rsync.h
+++ b/rsync.h
@@ -169,6 +169,7 @@
 #define IOERR_DEL_LIMIT (1<<2)
 
 #define MAX_ARGS 1000
+#define MAX_FILTER_ARGS 100
 #define MAX_BASIS_DIRS 20
 #define MAX_SERVER_ARGS (MAX_BASIS_DIRS*2 + 100)
 
diff --git a/sender.c b/sender.c
--- a/sender.c
+++ b/sender.c
@@ -21,6 +21,7 @@
 
 #include "rsync.h"
 #include "inums.h"
+#include "ifuncs.h"
 
 extern int do_xfers;
 extern int am_server;
@@ -49,6 +50,7 @@ extern int batch_fd;
 extern int write_batch;
 extern int file_old_total;
 extern BOOL want_progress_now;
+extern char *source_filter;
 extern struct stats stats;
 extern struct file_list *cur_flist, *first_flist, *dir_flist;
 
@@ -202,6 +204,26 @@ void send_files(int f_in, int f_out)
 	int f_xfer = write_batch < 0 ? batch_fd : f_out;
 	int save_io_error = io_error;
 	int ndx, j;
+	char *filter_argv[MAX_FILTER_ARGS + 1];
+	char *tmp = 0;
+	int unlink_tmp = 0;
+
+	if (source_filter) {
+		char *p;
+		char *sep = " \t";
+		int i;
+		for (p = strtok(source_filter, sep), i = 0;
+		     p && i < MAX_FILTER_ARGS;
+		     p = strtok(0, sep))
+			filter_argv[i++] = p;
+		filter_argv[i] = NULL;
+		if (p) {
+			rprintf(FERROR,
+				"Too many arguments to source-filter (> %d)\n",
+				MAX_FILTER_ARGS);
+			exit_cleanup(RERR_SYNTAX);
+		}
+	}
 
 	if (DEBUG_GTE(SEND, 1))
 		rprintf(FINFO, "send_files starting\n");
@@ -339,6 +361,7 @@ void send_files(int f_in, int f_out)
 			exit_cleanup(RERR_PROTOCOL);
 		}
 
+		unlink_tmp = 0;
 		fd = do_open(fname, O_RDONLY, 0);
 		if (fd == -1) {
 			if (errno == ENOENT) {
@@ -358,6 +381,33 @@ void send_files(int f_in, int f_out)
 			continue;
 		}
 
+		if (source_filter) {
+			int fd2;
+			char *tmpl = "/tmp/rsync-filtered_sourceXXXXXX";
+
+			tmp = strdup(tmpl);
+			fd2 = mkstemp(tmp);
+			if (fd2 == -1) {
+				rprintf(FERROR, "mkstemp %s failed: %s\n",
+					tmp, strerror(errno));
+			} else {
+				int status;
+				pid_t pid = run_filter_on_file(filter_argv, fd2, fd);
+				close(fd);
+				close(fd2);
+				wait_process_with_flush(pid, &status);
+				if (status != 0) {
+					rprintf(FERROR,
+					    "bypassing source filter %s; exited with code: %d\n",
+					    source_filter, status);
+					fd = do_open(fname, O_RDONLY, 0);
+				} else {
+					fd = do_open(tmp, O_RDONLY, 0);
+					unlink_tmp = 1;
+				}
+			}
+		}
+
 		/* map the local file */
 		if (do_fstat(fd, &st) != 0) {
 			io_error |= IOERR_GENERAL;
@@ -428,6 +478,8 @@ void send_files(int f_in, int f_out)
 			}
 		}
 		close(fd);
+		if (unlink_tmp)
+			unlink(tmp);
 
 		free_sums(s);
 
diff -Nurp a/rsync.1 b/rsync.1
--- a/rsync.1
+++ b/rsync.1
@@ -501,6 +501,7 @@ has its own detailed description later i
 --contimeout=SECONDS     set daemon connection timeout in seconds
 --ignore-times, -I       don't skip files that match size and time
 --size-only              skip files that match in size
+--times-only             skip files that match in mod-time
 --modify-window=NUM, -@  set the accuracy for mod-time comparisons
 --temp-dir=DIR, -T       create temporary files in directory DIR
 --fuzzy, -y              find similar file for basis if no dest file
@@ -549,6 +550,8 @@ has its own detailed description later i
 --write-batch=FILE       write a batched update to FILE
 --only-write-batch=FILE  like --write-batch but w/o updating dest
 --read-batch=FILE        read a batched update from FILE
+--source-filter=COMMAND  filter file through COMMAND at source
+--dest-filter=COMMAND    filter file through COMMAND at destination
 --protocol=NUM           force an older protocol version to be used
 --iconv=CONVERT_SPEC     request charset conversion of filenames
 --checksum-seed=NUM      set block/file checksum seed (advanced)
@@ -3609,6 +3612,32 @@ into the batch file without having to fl
 Apply all of the changes stored in FILE, a file previously generated by
 \fB\-\-write-batch\fP.  If \fIFILE\fP is \fB\-\fP, the batch data will be read
 from standard input. See the "BATCH MODE" section for details.
+.IP "\fB\-\-source-filter=COMMAND\fP"
+This option allows the user to specify a filter program that will be
+applied to the contents of all transferred regular files before the data is
+sent to destination.  COMMAND will receive the data on its standard input
+and it should write the filtered data to standard output.  COMMAND should
+exit non-zero if it cannot process the data or if it encounters an error
+when writing the data to stdout.
+.IP
+Example: \fB\-\-source-filter="gzip\ \-9"\fP will cause remote files to be
+compressed.  Use of \fB\-\-source-filter\fP automatically enables
+\fB\-\-whole-file\fP.  If your filter does not output the same number of
+bytes that it received on input, you should use \fB\-\-times-only\fP to
+disable size and content checks on subsequent rsync runs.
+.IP "\fB\-\-dest-filter=COMMAND\fP"
+This option allows you to specify a filter program that will be applied to
+the contents of all transferred regular files before the data is written to
+disk.  COMMAND will receive the data on its standard input and it should
+write the filtered data to standard output.  COMMAND should exit non-zero
+if it cannot process the data or if it encounters an error when writing the
+data to stdout.
+.IP
+Example: \-\-dest-filter="gzip \-9" will cause remote files to be compressed.
+Use of \-\-dest-filter automatically enables \-\-whole-file.  If your filter
+does not output the same number of bytes that it received on input, you
+should use \-\-times-only to disable size and content checks on subsequent
+rsync runs.
 .IP "\fB\-\-protocol=NUM\fP"
 Force an older protocol version to be used.  This is useful for creating a
 batch file that is compatible with an older version of rsync.  For
diff -Nurp a/rsync.1.html b/rsync.1.html
--- a/rsync.1.html
+++ b/rsync.1.html
@@ -419,6 +419,7 @@ has its own detailed description later i
 --contimeout=SECONDS     set daemon connection timeout in seconds
 --ignore-times, -I       don't skip files that match size and time
 --size-only              skip files that match in size
+--times-only             skip files that match in mod-time
 --modify-window=NUM, -@  set the accuracy for mod-time comparisons
 --temp-dir=DIR, -T       create temporary files in directory DIR
 --fuzzy, -y              find similar file for basis if no dest file
@@ -467,6 +468,8 @@ has its own detailed description later i
 --write-batch=FILE       write a batched update to FILE
 --only-write-batch=FILE  like --write-batch but w/o updating dest
 --read-batch=FILE        read a batched update from FILE
+--source-filter=COMMAND  filter file through COMMAND at source
+--dest-filter=COMMAND    filter file through COMMAND at destination
 --protocol=NUM           force an older protocol version to be used
 --iconv=CONVERT_SPEC     request charset conversion of filenames
 --checksum-seed=NUM      set block/file checksum seed (advanced)
@@ -3308,6 +3311,34 @@ into the batch file without having to fl
 from standard input. See the &quot;BATCH MODE&quot; section for details.</p>
 </dd>
 
+<dt id="opt--source-filter"><code>--source-filter=COMMAND</code><a href="#opt--source-filter" class="tgt"></a></dt><dd>
+<p>This option allows the user to specify a filter program that will be
+applied to the contents of all transferred regular files before the data is
+sent to destination.  COMMAND will receive the data on its standard input
+and it should write the filtered data to standard output.  COMMAND should
+exit non-zero if it cannot process the data or if it encounters an error
+when writing the data to stdout.</p>
+<p>Example: <code>--source-filter=&quot;gzip -9&quot;</code> will cause remote files to be
+compressed.  Use of <code>--source-filter</code> automatically enables
+<a href="#opt--whole-file"><code>--whole-file</code></a>.  If your filter does not output the same number of
+bytes that it received on input, you should use <code>--times-only</code> to
+disable size and content checks on subsequent rsync runs.</p>
+</dd>
+
+<dt id="opt--dest-filter"><code>--dest-filter=COMMAND</code><a href="#opt--dest-filter" class="tgt"></a></dt><dd>
+<p>This option allows you to specify a filter program that will be applied to
+the contents of all transferred regular files before the data is written to
+disk.  COMMAND will receive the data on its standard input and it should
+write the filtered data to standard output.  COMMAND should exit non-zero
+if it cannot process the data or if it encounters an error when writing the
+data to stdout.</p>
+<p>Example: -&#8288;-&#8288;dest-filter=&quot;gzip -&#8288;9&quot; will cause remote files to be compressed.
+Use of -&#8288;-&#8288;dest-filter automatically enables -&#8288;-&#8288;whole-file.  If your filter
+does not output the same number of bytes that it received on input, you
+should use -&#8288;-&#8288;times-only to disable size and content checks on subsequent
+rsync runs.</p>
+</dd>
+
 <dt id="opt--protocol"><code>--protocol=NUM</code><a href="#opt--protocol" class="tgt"></a></dt><dd>
 <p>Force an older protocol version to be used.  This is useful for creating a
 batch file that is compatible with an older version of rsync.  For
