diff --git a/vhost-user-backend/tests/vhost-user-server.rs b/vhost-user-backend/tests/vhost-user-server.rs
index 70b3cc5..e371299 100644
--- a/vhost-user-backend/tests/vhost-user-server.rs
+++ b/vhost-user-backend/tests/vhost-user-server.rs
@@ -59,7 +59,8 @@ impl VhostUserBackendMut for MockVhostBackend {
     }
 
     fn protocol_features(&self) -> VhostUserProtocolFeatures {
-        VhostUserProtocolFeatures::all()
+        // Exclude REPLY_ACK to test that it is automatically added.
+        VhostUserProtocolFeatures::all() - VhostUserProtocolFeatures::REPLY_ACK
     }
 
     fn reset_device(&mut self) {
diff --git a/vhost/CHANGELOG.md b/vhost/CHANGELOG.md
index dbbb0c1..ead2f47 100644
--- a/vhost/CHANGELOG.md
+++ b/vhost/CHANGELOG.md
@@ -5,6 +5,10 @@
 - [[#268]](https://github.com/rust-vmm/vhost/pull/268) Add support for `VHOST_USER_GET_SHARED_OBJECT`
 
 ### Changed
+- [[#290]](https://github.com/rust-vmm/vhost/pull/290) Backends now
+  always support `VHOST_USER_PROTOCOL_F_REPLY_ACK`, without the
+  `VhostUserBackendReqHandler` implementation having to include it in
+  the features returned from `get_protocol_features`.
 
 ### Deprecated
 
diff --git a/vhost/src/vhost_user/backend_req_handler.rs b/vhost/src/vhost_user/backend_req_handler.rs
index 409073c..d74b045 100644
--- a/vhost/src/vhost_user/backend_req_handler.rs
+++ b/vhost/src/vhost_user/backend_req_handler.rs
@@ -327,7 +327,6 @@ pub struct BackendReqHandler<S: VhostUserBackendReqHandler> {
 
     virtio_features: u64,
     acked_virtio_features: u64,
-    protocol_features: VhostUserProtocolFeatures,
     acked_protocol_features: u64,
 
     // sending ack for messages without payload
@@ -347,7 +346,6 @@ impl<S: VhostUserBackendReqHandler> BackendReqHandler<S> {
             backend,
             virtio_features: 0,
             acked_virtio_features: 0,
-            protocol_features: VhostUserProtocolFeatures::empty(),
             acked_protocol_features: 0,
             reply_ack_enabled: false,
             error: None,
@@ -512,7 +510,9 @@ impl<S: VhostUserBackendReqHandler> BackendReqHandler<S> {
             }
             Ok(FrontendReq::GET_PROTOCOL_FEATURES) => {
                 self.check_request_size(&hdr, size, 0)?;
-                let features = self.backend.get_protocol_features()?;
+                // REPLY_ACK is implemented entirely by this library, so it's always supported.
+                let features =
+                    self.backend.get_protocol_features()? | VhostUserProtocolFeatures::REPLY_ACK;
 
                 // Enable the `XEN_MMAP` protocol feature for backends if xen feature is enabled.
                 #[cfg(feature = "xen")]
@@ -520,7 +520,6 @@ impl<S: VhostUserBackendReqHandler> BackendReqHandler<S> {
 
                 let msg = VhostUserU64::new(features.bits());
                 self.send_reply_message(&hdr, &msg)?;
-                self.protocol_features = features;
                 self.update_reply_ack_flag();
             }
             Ok(FrontendReq::SET_PROTOCOL_FEATURES) => {
@@ -952,7 +951,6 @@ impl<S: VhostUserBackendReqHandler> BackendReqHandler<S> {
         let pflag = VhostUserProtocolFeatures::REPLY_ACK;
 
         self.reply_ack_enabled = (self.virtio_features & vflag) != 0
-            && self.protocol_features.contains(pflag)
             && (self.acked_protocol_features & pflag.bits()) != 0;
     }