diff --git a/src/tftp/server.rs b/src/tftp/server.rs
index 1586c09..3a5891b 100644
--- a/src/tftp/server.rs
+++ b/src/tftp/server.rs
@@ -28,11 +28,15 @@ pub enum Error {
     Timeout,
 }
 
-pub async fn handle_connection(
+pub async fn handle_connection<F, Fut>(
     src: SocketAddr,
     packet: Packet,
-    callback: impl (Fn(&str) -> Option<Vec<u8>>) + Send + Clone + 'static,
-) -> Result<(), Error> {
+    callback: F,
+) -> Result<(), Error>
+where
+    F: Fn(&str) -> Fut,
+    Fut: Future<Output = Option<Vec<u8>>>,
+{
     let socket = UdpSocket::bind("127.0.0.1:0").await?;
     println!(
         "{src}: Connecting from {}",
@@ -50,7 +54,7 @@ pub async fn handle_connection(
     let filename = request.filename();
     println!("{src}: {filename} [{:?}]", request.mode());
 
-    let Some(data) = callback(filename) else {
+    let Some(data) = callback(filename).await else {
         Packet::error(ErrorCode::FileNotFound, "Unknown file")
             .send_to(&socket, &src)
             .await?;
@@ -141,9 +145,11 @@ pub async fn handle_connection(
     Ok(())
 }
 
-pub async fn serve(
-    callback: impl (Fn(&str) -> Option<Vec<u8>>) + Send + Clone + 'static,
-) -> Result<(), Error> {
+pub async fn serve<F, Fut>(callback: F) -> Result<(), Error>
+where
+    F: Fn(&str) -> Fut + Clone + Send + 'static,
+    Fut: Future<Output = Option<Vec<u8>>> + Send + 'static,
+{
     let socket = UdpSocket::bind("127.0.0.1:69").await?;
 
     println!("Server started");