This adds a --usermap and a --groupmap option.

TODO:  make this work when --numeric-ids was specified.

--- old/flist.c
+++ new/flist.c
@@ -61,6 +61,8 @@ extern int copy_links;
 extern int copy_unsafe_links;
 extern int protocol_version;
 extern int sanitize_paths;
+extern char *usermap;
+extern char *groupmap;
 extern struct stats stats;
 
 extern char curr_dir[MAXPATHLEN];
@@ -1882,8 +1884,13 @@ struct file_list *recv_file_list(int f)
 	int dstart, flags;
 	int64 start_read;
 
-	if (!first_flist)
+	if (!first_flist) {
 		rprintf(FLOG, "receiving file list\n");
+		if (usermap)
+			parse_name_map(usermap, 1);
+		if (groupmap)
+			parse_name_map(groupmap, 0);
+	}
 	if (show_filelist_p())
 		start_filelist_progress("receiving file list");
 	else if (inc_recurse && verbose && !am_server && !first_flist)
--- old/options.c
+++ new/options.c
@@ -156,6 +156,8 @@ char *rsync_path = RSYNC_PATH;
 char *backup_dir = NULL;
 char backup_dir_buf[MAXPATHLEN];
 char *sockopts = NULL;
+char *usermap = NULL;
+char *groupmap = NULL;
 int rsync_port = 0;
 int compare_dest = 0;
 int copy_dest = 0;
@@ -367,6 +369,8 @@ void usage(enum logcode F)
   rprintf(F,"     --delay-updates         put all updated files into place at transfer's end\n");
   rprintf(F," -m, --prune-empty-dirs      prune empty directory chains from the file-list\n");
   rprintf(F,"     --numeric-ids           don't map uid/gid values by user/group name\n");
+  rprintf(F,"     --usermap=STRING        custom username mapping\n");
+  rprintf(F,"     --groupmap=STRING       custom groupname mapping\n");
   rprintf(F,"     --timeout=TIME          set I/O timeout in seconds\n");
   rprintf(F," -I, --ignore-times          don't skip files that match in size and mod-time\n");
   rprintf(F,"     --size-only             skip files that match in size\n");
@@ -568,6 +572,8 @@ static struct poptOption long_options[] 
   {"files-from",       0,  POPT_ARG_STRING, &files_from, 0, 0, 0 },
   {"from0",           '0', POPT_ARG_NONE,   &eol_nulls, 0, 0, 0},
   {"numeric-ids",      0,  POPT_ARG_NONE,   &numeric_ids, 0, 0, 0 },
+  {"usermap",          0,  POPT_ARG_STRING, &usermap, 0, 0, 0 },
+  {"groupmap",         0,  POPT_ARG_STRING, &groupmap, 0, 0, 0 },
   {"timeout",          0,  POPT_ARG_INT,    &io_timeout, 0, 0, 0 },
   {"rsh",             'e', POPT_ARG_STRING, &shell_cmd, 0, 0, 0 },
   {"rsync-path",       0,  POPT_ARG_STRING, &rsync_path, 0, 0, 0 },
@@ -1857,6 +1863,22 @@ void server_options(char **args,int *arg
 		args[ac++] = "--numeric-ids";
 
 	if (am_sender) {
+		if (usermap) {
+			if (strchr(usermap, '\'') != NULL)
+				usermap = "INVALID";
+			if (asprintf(&arg, "--usermap='%s'", usermap) < 0)
+				goto oom;
+			args[ac++] = arg;
+		}
+
+		if (groupmap) {
+			if (strchr(groupmap, '\'') != NULL)
+				groupmap = "INVALID";
+			if (asprintf(&arg, "--groupmap='%s'", groupmap) < 0)
+				goto oom;
+			args[ac++] = arg;
+		}
+
 		if (ignore_existing)
 			args[ac++] = "--ignore-existing";
 
--- old/rsync.yo
+++ new/rsync.yo
@@ -361,6 +361,8 @@ to the detailed description below for a 
      --delay-updates         put all updated files into place at end
  -m, --prune-empty-dirs      prune empty directory chains from file-list
      --numeric-ids           don't map uid/gid values by user/group name
+     --usermap=STRING        custom username mapping
+     --groupmap=STRING       custom groupname mapping
      --timeout=TIME          set I/O timeout in seconds
  -I, --ignore-times          don't skip files that match size and time
      --size-only             skip files that match in size
@@ -1445,6 +1447,25 @@ from the source system is used instead. 
 the chroot setting affects rsync's ability to look up the names of the
 users and groups and what you can do about it.
 
+dit(bf(--usermap=STRING, --groupmap=STRING)) These options allow you to
+specify user/group names and IDs that should be mapped to other values by
+the receiving side.  The bf(STRING) is one or more FROM:TO pairs of values
+separated by commas.  Any matching FROM value from the sender is replaced
+with a TO value from the receiver.  You may specify usernames or user IDs
+for the FROM and TO values, and the FROM value may also be a wild-card
+string, which will be matched against the sender's names (it will not match
+IDs).  For example:
+
+  --usermap=0:foo,bar:baz,*:nobody --groupmap=root:1,1:root
+
+The first match in the list is the one that is used.
+
+For the bf(--usermap) option to be effective you will need to have specified
+the bf(-o) (bf(--owner)) option and the receiver will need to be running as
+root (see also the bf(--fake-root) option).  For the bf(--groupmap) option
+to be effective you will need to have specified the bf(-g) (bf(--groups))
+option, and the receiver will need to have permissions to set that group.
+
 dit(bf(--timeout=TIMEOUT)) This option allows you to set a maximum I/O
 timeout in seconds. If no data is transferred for the specified time
 then rsync will exit. The default is 0, which means no timeout.
--- old/uidlist.c
+++ new/uidlist.c
@@ -38,6 +38,7 @@ extern int preserve_uid;
 extern int preserve_gid;
 extern int preserve_acls;
 extern int numeric_ids;
+extern int protocol_version;
 
 struct idlist {
 	struct idlist *next;
@@ -45,8 +46,8 @@ struct idlist {
 	char *name;
 };
 
-static struct idlist *uidlist;
-static struct idlist *gidlist;
+static struct idlist *uidlist, *uidmap;
+static struct idlist *gidlist, *gidmap;
 
 static struct idlist *add_to_list(struct idlist **root, int id, char *name,
 				  int id2)
@@ -158,8 +159,33 @@ static int is_in_group(gid_t gid)
 /* Add a uid to the list of uids.  Only called on receiving side. */
 static uid_t recv_add_uid(uid_t id, char *name)
 {
-	uid_t id2 = name ? map_uid(id, name) : id;
 	struct idlist *node;
+	uid_t id2;
+
+	if (name) {
+		struct idlist *list;
+		for (list = uidmap; list; list = list->next) {
+			switch (list->id) {
+			case -2:
+				if (!wildmatch(list->name, name))
+					continue;
+				break;
+			case -1:
+				if (strcmp(list->name, name) != 0)
+					continue;
+				break;
+			default:
+				if (list->id != (int)id)
+					continue;
+				break;
+			}
+			id2 = list->id2;
+			break;
+		}
+		if (!list)
+			id2 = id ? map_uid(id, name) : 0; /* don't map root */
+	} else
+		 id2 = id;
 
 	node = add_to_list(&uidlist, (int)id, name, (int)id2);
 
@@ -174,8 +200,33 @@ static uid_t recv_add_uid(uid_t id, char
 /* Add a gid to the list of gids.  Only called on receiving side. */
 static gid_t recv_add_gid(gid_t id, char *name)
 {
-	gid_t id2 = name ? map_gid(id, name) : id;
 	struct idlist *node;
+	gid_t id2;
+
+	if (name) {
+		struct idlist *list;
+		for (list = gidmap; list; list = list->next) {
+			switch (list->id) {
+			case -2:
+				if (!wildmatch(list->name, name))
+					continue;
+				break;
+			case -1:
+				if (strcmp(list->name, name) != 0)
+					continue;
+				break;
+			default:
+				if (list->id != (int)id)
+					continue;
+				break;
+			}
+			id2 = list->id2;
+			break;
+		}
+		if (!list)
+			id2 = id ? map_gid(id, name) : 0; /* don't map root */
+	} else
+		 id2 = id;
 
 	if (!am_root && !is_in_group(id2))
 		id2 = GID_NONE;
@@ -195,9 +246,6 @@ uid_t match_uid(uid_t uid)
 	static uid_t last_in, last_out;
 	struct idlist *list;
 
-	if (uid == 0)
-		return 0;
-
 	if (uid == last_in)
 		return last_out;
 
@@ -238,7 +286,7 @@ char *add_uid(uid_t uid)
 	struct idlist *list;
 	struct idlist *node;
 
-	if (uid == 0)	/* don't map root */
+	if (uid == 0 && protocol_version < 30)
 		return NULL;
 
 	for (list = uidlist; list; list = list->next) {
@@ -256,7 +304,7 @@ char *add_gid(gid_t gid)
 	struct idlist *list;
 	struct idlist *node;
 
-	if (gid == 0)	/* don't map root */
+	if (gid == 0 && protocol_version < 30)
 		return NULL;
 
 	for (list = gidlist; list; list = list->next) {
@@ -356,3 +404,70 @@ void recv_uid_list(int f, struct file_li
 			F_GROUP(flist->files[i]) = match_gid(F_GID(flist->files[i]));
 	}
 }
+
+void parse_name_map(char *map, int usernames)
+{
+	char *colon, *end, *cp = map + strlen(map);
+	int id1, id2;
+
+	while (1) {
+		end = cp;
+		while (cp > map && cp[-1] != ',') cp--;
+		if (!(colon = strchr(cp, ':'))) {
+			rprintf(FERROR, "No colon found in --%smap: %s\n",
+				usernames ? "user" : "group", cp);
+			exit_cleanup(RERR_SYNTAX);
+		}
+		*colon = '\0';
+
+		if (isDigit(cp)) {
+			if (strspn(cp, "0123456789") != (size_t)(colon - cp)) {
+			  bad_number:
+				rprintf(FERROR, "Invalid number in --%smap: %s\n",
+					usernames ? "user" : "group", cp);
+				exit_cleanup(RERR_SYNTAX);
+			}
+			id1 = atoi(cp);
+		} else if (strpbrk(cp, "*[?"))
+			id1 = -2;
+		else
+			id1 = -1;
+
+		if (isDigit(colon+1)) {
+			if (strspn(colon+1, "0123456789") != (size_t)(end - colon - 1)) {
+				cp = colon+1;
+				goto bad_number;
+			}
+			id2 = atoi(colon+1);
+		} else {
+			if (usernames) {
+				uid_t uid;
+				if (name_to_uid(colon+1, &uid))
+					id2 = (int)uid;
+				else
+					id2 = -1;
+			} else {
+				gid_t gid;
+				if (name_to_gid(colon+1, &gid))
+					id2 = (int)gid;
+				else
+					id2 = -1;
+			}
+			if (id2 < 0) {
+				rprintf(FERROR, "Invalid name in --%smap: %s\n",
+					usernames ? "user" : "group", colon+1);
+				exit_cleanup(RERR_SYNTAX);
+			}
+		}
+
+		if (usernames)
+			add_to_list(&uidmap, id1, id1 < 0 ? cp : NULL, id2);
+		else
+			add_to_list(&gidmap, id1, id1 < 0 ? cp : NULL, id2);
+
+		if (cp == map)
+			break;
+
+		*--cp = '\0'; /* replace comma */
+	}
+}