feat(dgw): route KDC traffic through agent tunnel (DGW-384)

When an agent advertises the KDC's subnet or DNS domain, route Kerberos
traffic through the QUIC tunnel just like every other proxy path. This
closes the last gap left after the transparent routing PR (#1741):

- `/jet/KdcProxy` HTTP endpoint — `send_krb_message` now consults the
  routing pipeline before falling back to direct TCP. The HTTP handler
  has no parent association, so it mints a fresh session_id purely for
  agent-side log correlation.

- RDP CredSSP/NLA — `rdp_proxy.rs::send_network_request` previously
  hard-coded `None` for the agent handle. Plumb `agent_tunnel_handle`
  and `session_id` from `RdpProxy` down through `perform_credssp_with_*`
  → `resolve_*_generator` → `send_network_request`. The same change
  reaches the credential-injection clean path (`rd_clean_path.rs`).
  `session_id` here is `session_info.id` / `claims.jet_aid` so the
  agent log ties KDC sub-traffic to its parent RDP session.

Stack: based on #1741. Picks up `agent_tunnel::routing::try_route`.

`send_krb_message` signature gains `(agent_tunnel_handle, session_id)`
in that order — required `Uuid`, no `Option<>` — so the call site is
honest about which UUID it's logging. The UDP scheme guard (KDC over
UDP keeps going direct because the agent protocol only carries TCP)
and the 64 KiB `MAX_KDC_REPLY_MESSAGE_LEN` DoS cap (and the matching
generic `read_kdc_reply_message`) come along since they live in the
same file and serve the same end.

Author: irvingoujAtDevolution

crates/agent-tunnel/src/routing.rs

@@ -1,7 +1,9 @@
 //! Shared routing pipeline for agent tunnel.
 //!
 //! Consumed by the upstream connection paths (forwarding, RDP clean path,
-//! generic client) to ensure consistent routing behavior and error messages.
+//! generic client) and by the KDC proxy (HTTP endpoint plus the CredSSP/NLA
+//! sub-flow inside `rdp_proxy.rs`) to ensure consistent routing behavior and
+//! error messages.
 
 use std::net::IpAddr;
 use std::sync::Arc;

devolutions-gateway/src/api/kdc_proxy.rs

@@ -9,6 +9,7 @@ use kdc::handle_kdc_proxy_message;
 use picky_krb::messages::KdcProxyMessage;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::{TcpStream, UdpSocket};
+use uuid::Uuid;
 
 use crate::DgwState;
 use crate::http::{HttpError, HttpErrorBuilder};
@@ -25,6 +26,7 @@ async fn kdc_proxy(
         token_cache,
         jrl,
         recordings,
+        agent_tunnel_handle,
         ..
     }): State<DgwState>,
     extract::Path(token): extract::Path<String>,
@@ -105,7 +107,19 @@ async fn kdc_proxy(
         &claims.krb_kdc
     };
 
-    let kdc_reply_message = send_krb_message(kdc_addr, &kdc_proxy_message.kerb_message.0.0).await?;
+    // The HTTP /jet/KdcProxy endpoint stands on its own — its token does not carry
+    // a parent association — so we mint a fresh session_id purely for log/agent
+    // correlation. The RDP CredSSP/NLA caller (rdp_proxy.rs::send_network_request)
+    // passes `claims.jet_aid` instead so KDC sub-traffic correlates with its RDP session.
+    let session_id = Uuid::new_v4();
+
+    let kdc_reply_message = send_krb_message(
+        kdc_addr,
+        &kdc_proxy_message.kerb_message.0.0,
+        agent_tunnel_handle.as_deref(),
+        session_id,
+    )
+    .await?;
 
     let kdc_reply_message = KdcProxyMessage::from_raw_kerb_message(&kdc_reply_message)
         .map_err(HttpError::internal().with_msg("couldn't create KDC proxy reply").err())?;
@@ -115,11 +129,33 @@ async fn kdc_proxy(
     kdc_reply_message.to_vec().map_err(HttpError::internal().err())
 }
 
-async fn read_kdc_reply_message(connection: &mut TcpStream) -> io::Result<Vec<u8>> {
-    let len = connection.read_u32().await?;
-    let mut buf = vec![0; (len + 4).try_into().expect("u32-to-usize")];
-    buf[0..4].copy_from_slice(&(len.to_be_bytes()));
-    connection.read_exact(&mut buf[4..]).await?;
+/// Hard ceiling on the announced length of a TCP-framed KDC reply.
+///
+/// The KDC TCP transport prefixes its message with a 4-byte big-endian length.
+/// A misbehaving (or malicious) peer can claim up to `u32::MAX` bytes, which
+/// without a cap would have us pre-allocate ~4 GiB on a single reply. 64 KiB
+/// is well above any realistic Kerberos reply size while keeping the worst
+/// case bounded.
+const MAX_KDC_REPLY_MESSAGE_LEN: u32 = 64 * 1024;
+
+async fn read_kdc_reply_message<R: AsyncReadExt + Unpin>(reader: &mut R) -> io::Result<Vec<u8>> {
+    let len = reader.read_u32().await?;
+
+    if len > MAX_KDC_REPLY_MESSAGE_LEN {
+        return Err(io::Error::new(
+            io::ErrorKind::InvalidData,
+            format!("KDC reply too large: announced {len} bytes, maximum is {MAX_KDC_REPLY_MESSAGE_LEN}"),
+        ));
+    }
+
+    let total_len = len
+        .checked_add(4)
+        .and_then(|n| usize::try_from(n).ok())
+        .ok_or_else(|| io::Error::new(io::ErrorKind::InvalidData, "KDC reply length prefix overflowed"))?;
+
+    let mut buf = vec![0; total_len];
+    buf[0..4].copy_from_slice(&len.to_be_bytes());
+    reader.read_exact(&mut buf[4..]).await?;
     Ok(buf)
 }
 
@@ -148,7 +184,60 @@ fn unable_to_reach_kdc_server_err(error: io::Error) -> HttpError {
 }
 
 /// Sends the Kerberos message to the specified KDC address.
-pub async fn send_krb_message(kdc_addr: &TargetAddr, message: &[u8]) -> Result<Vec<u8>, HttpError> {
+///
+/// Uses the same routing pipeline as connection forwarding:
+/// if an agent claims the KDC's domain/subnet, traffic goes through the tunnel.
+/// Falls back to direct connect when no agent matches.
+///
+/// `session_id` is forwarded to the agent as the QUIC stream's session ID for
+/// log correlation. Callers that have a parent association (RDP CredSSP) should
+/// pass the parent's `jet_aid`; callers with no parent (the HTTP `/jet/KdcProxy`
+/// endpoint) should mint a fresh `Uuid::new_v4()`.
+pub async fn send_krb_message(
+    kdc_addr: &TargetAddr,
+    message: &[u8],
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
+) -> Result<Vec<u8>, HttpError> {
+    // Route through agent tunnel using the SAME pipeline as connection forwarding,
+    // but only for `tcp` KDC targets. The agent tunnel currently has a single
+    // `ConnectRequest::tcp` shape, so a `udp://` KDC routed this way would be
+    // delivered to the agent as a TCP target — wrong protocol semantics that can
+    // silently break UDP Kerberos deployments. Fall through to the direct path
+    // (which honors the scheme) until an explicit UDP tunnel hop exists.
+    //
+    // `as_addr()` returns `host:port` (with IPv6 brackets), which is what the agent
+    // tunnel target parser expects — unlike `to_string()` which includes the scheme.
+    let kdc_target = kdc_addr.as_addr();
+    let tunnel_handle = if kdc_addr.scheme().eq_ignore_ascii_case("tcp") {
+        agent_tunnel_handle
+    } else {
+        None
+    };
+
+    let route_target = match kdc_addr.host_ip() {
+        Some(ip) => agent_tunnel::routing::RouteTarget::ip(ip),
+        None => agent_tunnel::routing::RouteTarget::hostname(kdc_addr.host()),
+    };
+
+    if let Some((mut stream, _agent)) =
+        agent_tunnel::routing::try_route(tunnel_handle, None, &route_target, session_id, kdc_target)
+            .await
+            .map_err(|e| HttpError::bad_gateway().build(format!("KDC routing through agent tunnel failed: {e:#}")))?
+    {
+        stream.write_all(message).await.map_err(
+            HttpError::bad_gateway()
+                .with_msg("unable to send KDC message through agent tunnel")
+                .err(),
+        )?;
+
+        return read_kdc_reply_message(&mut stream).await.map_err(
+            HttpError::bad_gateway()
+                .with_msg("unable to read KDC reply through agent tunnel")
+                .err(),
+        );
+    }
+
     let protocol = kdc_addr.scheme();
 
     debug!("Connecting to KDC server located at {kdc_addr} using protocol {protocol}...");

devolutions-gateway/src/generic_client.rs

@@ -170,6 +170,7 @@ where
                                 .client_stream_leftover_bytes(leftover_bytes)
                                 .server_dns_name(selected_target.host().to_owned())
                                 .disconnect_interest(disconnect_interest)
+                                .agent_tunnel_handle(agent_tunnel_handle)
                                 .build()
                                 .run()
                                 .await

devolutions-gateway/src/rd_clean_path.rs

@@ -469,6 +469,8 @@ async fn handle_with_credential_injection(
         client_security_protocol,
         &credential_mapping.proxy,
         krb_server_config,
+        agent_tunnel_handle.as_deref(),
+        claims.jet_aid,
     );
 
     let krb_client_config = if conf.debug.enable_unstable
@@ -492,6 +494,8 @@ async fn handle_with_credential_injection(
         server_security_protocol,
         &credential_mapping.target,
         krb_client_config,
+        agent_tunnel_handle.as_deref(),
+        claims.jet_aid,
     );
 
     let (client_credssp_res, server_credssp_res) = tokio::join!(client_credssp_fut, server_credssp_fut);

devolutions-gateway/src/rdp_proxy.rs

@@ -10,6 +10,7 @@ use ironrdp_pdu::{mcs, nego, x224};
 use secrecy::ExposeSecret as _;
 use tokio::io::{AsyncRead, AsyncWrite, AsyncWriteExt};
 use typed_builder::TypedBuilder;
+use uuid::Uuid;
 
 use crate::api::kdc_proxy::send_krb_message;
 use crate::config::Conf;
@@ -33,6 +34,8 @@ pub struct RdpProxy<C, S> {
     subscriber_tx: SubscriberSender,
     server_dns_name: String,
     disconnect_interest: Option<DisconnectInterest>,
+    #[builder(default)]
+    agent_tunnel_handle: Option<Arc<agent_tunnel::AgentTunnelHandle>>,
 }
 
 impl<A, B> RdpProxy<A, B>
@@ -64,8 +67,12 @@ where
         subscriber_tx,
         server_dns_name,
         disconnect_interest,
+        agent_tunnel_handle,
     } = proxy;
 
+    // session_id used for KDC-via-tunnel correlation (see send_krb_message).
+    let session_id = session_info.id;
+
     let tls_conf = conf.credssp_tls.get().context("CredSSP TLS configuration")?;
     let gateway_hostname = conf.hostname.clone();
 
@@ -163,6 +170,8 @@ where
         handshake_result.client_security_protocol,
         &credential_mapping.proxy,
         krb_server_config,
+        agent_tunnel_handle.as_deref(),
+        session_id,
     );
 
     let krb_client_config = if conf.debug.enable_unstable
@@ -186,6 +195,8 @@ where
         handshake_result.server_security_protocol,
         &credential_mapping.target,
         krb_client_config,
+        agent_tunnel_handle.as_deref(),
+        session_id,
     );
 
     let (client_credssp_res, server_credssp_res) = tokio::join!(client_credssp_fut, server_credssp_fut);
@@ -393,6 +404,7 @@ where
     handshake_result
 }
 
+#[expect(clippy::too_many_arguments)]
 #[instrument(name = "server_credssp", level = "debug", ret, skip_all)]
 pub(crate) async fn perform_credssp_with_server<S>(
     framed: &mut ironrdp_tokio::Framed<S>,
@@ -401,6 +413,8 @@ pub(crate) async fn perform_credssp_with_server<S>(
     security_protocol: nego::SecurityProtocol,
     credentials: &crate::credential::AppCredential,
     kerberos_config: Option<ironrdp_connector::credssp::KerberosConfig>,
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
 ) -> anyhow::Result<()>
 where
     S: ironrdp_tokio::FramedRead + ironrdp_tokio::FramedWrite,
@@ -433,7 +447,7 @@ where
     loop {
         let client_state = {
             let mut generator = sequence.process_ts_request(ts_request);
-            resolve_client_generator(&mut generator).await?
+            resolve_client_generator(&mut generator, agent_tunnel_handle, session_id).await?
         }; // drop generator
 
         buf.clear();
@@ -465,13 +479,15 @@ where
 
 async fn resolve_server_generator(
     generator: &mut CredsspServerProcessGenerator<'_>,
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
 ) -> Result<sspi::credssp::ServerState, sspi::credssp::ServerError> {
     let mut state = generator.start();
 
     loop {
         match state {
             GeneratorState::Suspended(request) => {
-                let response = send_network_request(&request)
+                let response = send_network_request(&request, agent_tunnel_handle, session_id)
                     .await
                     .map_err(|err| sspi::credssp::ServerError {
                         ts_request: None,
@@ -489,13 +505,15 @@ async fn resolve_server_generator(
 
 async fn resolve_client_generator(
     generator: &mut CredsspClientProcessGenerator<'_>,
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
 ) -> anyhow::Result<sspi::credssp::ClientState> {
     let mut state = generator.start();
 
     loop {
         match state {
             GeneratorState::Suspended(request) => {
-                let response = send_network_request(&request).await?;
+                let response = send_network_request(&request, agent_tunnel_handle, session_id).await?;
                 state = generator.resume(Ok(response));
             }
             GeneratorState::Completed(client_state) => {
@@ -507,6 +525,7 @@ async fn resolve_client_generator(
     }
 }
 
+#[expect(clippy::too_many_arguments)]
 #[instrument(name = "client_credssp", level = "debug", ret, skip_all)]
 pub(crate) async fn perform_credssp_with_client<S>(
     framed: &mut ironrdp_tokio::Framed<S>,
@@ -515,6 +534,8 @@ pub(crate) async fn perform_credssp_with_client<S>(
     security_protocol: nego::SecurityProtocol,
     credentials: &crate::credential::AppCredential,
     kerberos_server_config: Option<sspi::KerberosServerConfig>,
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
 ) -> anyhow::Result<()>
 where
     S: ironrdp_tokio::FramedRead + ironrdp_tokio::FramedWrite,
@@ -535,6 +556,8 @@ where
         gateway_public_key,
         credentials,
         kerberos_server_config,
+        agent_tunnel_handle,
+        session_id,
     )
     .await;
 
@@ -555,13 +578,16 @@ where
 
     return result;
 
+    #[expect(clippy::too_many_arguments)]
     async fn credssp_loop<S>(
         framed: &mut ironrdp_tokio::Framed<S>,
         buf: &mut ironrdp_pdu::WriteBuf,
         client_computer_name: ironrdp_connector::ServerName,
         public_key: Vec<u8>,
         credentials: &crate::credential::AppCredential,
         kerberos_server_config: Option<sspi::KerberosServerConfig>,
+        agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+        session_id: Uuid,
     ) -> anyhow::Result<()>
     where
         S: ironrdp_tokio::FramedRead + ironrdp_tokio::FramedWrite,
@@ -603,7 +629,7 @@ where
 
             let result = {
                 let mut generator = sequence.process_ts_request(ts_request);
-                resolve_server_generator(&mut generator).await
+                resolve_server_generator(&mut generator, agent_tunnel_handle, session_id).await
             }; // drop generator
 
             buf.clear();
@@ -634,14 +660,14 @@ where
     Ok(())
 }
 
-async fn send_network_request(request: &NetworkRequest) -> anyhow::Result<Vec<u8>> {
+async fn send_network_request(
+    request: &NetworkRequest,
+    agent_tunnel_handle: Option<&agent_tunnel::AgentTunnelHandle>,
+    session_id: Uuid,
+) -> anyhow::Result<Vec<u8>> {
     let target_addr = TargetAddr::parse(request.url.as_str(), Some(88))?;
 
-    // TODO(DGW-384): plumb `agent_tunnel_handle` through `RdpProxy` so
-    // CredSSP-originated Kerberos requests can traverse the agent tunnel.
-    // Currently these go direct from the gateway host, bypassing the
-    // routing pipeline used by every other proxy path.
-    send_krb_message(&target_addr, &request.data)
+    send_krb_message(&target_addr, &request.data, agent_tunnel_handle, session_id)
         .await
         .map_err(|err| anyhow::Error::msg("failed to send KDC message").context(err))
 }