diff --git a/var/mongoose/lib/filerename b/var/mongoose/lib/filerename
new file mode 100644
index 0000000..227d572
--- /dev/null
+++ b/var/mongoose/lib/filerename
@@ -0,0 +1,21 @@
+
+proc {file rename} {{force {}} source target} {
+	try {
+		if {$force ni {{} -force}} {
+			error "bad option \"$force\": should be -force"
+		}
+
+		if {![file exists $source]} {
+			error "source file does not exist."
+		}
+
+		if {$force eq "" && [file exists $target]} {
+			error "error copying \"$source\" to \"$target\": file already exists"
+		}
+		exec /mod/bin/busybox/mv $source $target
+	} on error {msg opts} {
+		incr opts(-level)
+		return {*}$opts $msg
+	}
+}
+
diff --git a/var/mongoose/lib/setup b/var/mongoose/lib/setup
index 11c43ca..b4112a0 100644
--- a/var/mongoose/lib/setup
+++ b/var/mongoose/lib/setup
@@ -45,5 +45,9 @@ if {![exists -proc require]} {
 	# largefile safe.
 
 	require filecopy
+
+	# and {file rename} with one that supports cross-filesystem moves.
+
+	require filerename
 }