feat: implement M3 — UDP/TCP DNS listener

- Add DnsListener that binds UDP and TCP on configurable port
- Add [dns] config section with listen address (default 0.0.0.0:5353)
- DNS over TCP uses 2-byte length prefix (RFC 1035 §4.2.2)
- Forward queries through existing DoH pipeline
- Start DNS listener as background task alongside DoH server

src/config.rs

@@ -8,10 +8,13 @@ use crate::upstream::doh_client::Strategy;
 pub struct AppConfig {
     /// Server configuration.
     pub server: ServerConfig,
+    /// DNS listener configuration.
+    #[serde(default)]
+    pub dns: DnsListenerConfig,
     /// Upstream resolver configuration.
     pub upstream: UpstreamConfig,
     /// Cache configuration.
-    #[allow(dead_code)] // Used once M3 caching is implemented
+    #[allow(dead_code)] // Used once M4 caching is implemented
     pub cache: CacheConfig,
 }
 
@@ -22,6 +25,21 @@ pub struct ServerConfig {
     pub listen: String,
 }
 
+/// DNS listener configuration for UDP/TCP.
+#[derive(Debug, Clone, Deserialize)]
+pub struct DnsListenerConfig {
+    /// Address to listen on for DNS queries (e.g., "0.0.0.0:5353").
+    pub listen: String,
+}
+
+impl Default for DnsListenerConfig {
+    fn default() -> Self {
+        Self {
+            listen: "0.0.0.0:5353".to_string(),
+        }
+    }
+}
+
 /// Upstream DoH resolver settings.
 #[derive(Debug, Clone, Deserialize)]
 pub struct UpstreamConfig {

src/dns/listener.rs

@@ -0,0 +1,206 @@
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Arc;
+
+use tokio::net::{TcpListener, UdpSocket};
+use tracing::{debug, error, info, warn};
+
+use crate::dns::query::parse_query;
+use crate::dns::response::build_servfail;
+use crate::upstream::doh_client::{DohClient, Strategy};
+
+/// Errors that can occur in the DNS listener.
+#[derive(Debug, thiserror::Error)]
+pub enum ListenerError {
+    /// Failed to bind the UDP socket.
+    #[error("failed to bind UDP socket: {0}")]
+    UdpBind(#[source] std::io::Error),
+    /// Failed to bind the TCP listener.
+    #[error("failed to bind TCP listener: {0}")]
+    TcpBind(#[source] std::io::Error),
+}
+
+/// A DNS listener that accepts UDP and TCP queries and forwards them
+/// through the DoH pipeline.
+#[derive(Clone)]
+pub struct DnsListener {
+    doh_client: DohClient,
+    strategy: Strategy,
+    rr_counter: Arc<AtomicUsize>,
+}
+
+impl DnsListener {
+    /// Create a new DNS listener with the given DoH client and strategy.
+    pub fn new(doh_client: DohClient, strategy: Strategy) -> Self {
+        Self {
+            doh_client,
+            strategy,
+            rr_counter: Arc::new(AtomicUsize::new(0)),
+        }
+    }
+
+    /// Start the UDP and TCP listeners on the given address.
+    pub async fn run(&self, addr: &str) -> Result<(), ListenerError> {
+        let udp_listener = self.clone();
+        let tcp_listener = self.clone();
+        let udp_addr = addr.to_string();
+        let tcp_addr = addr.to_string();
+
+        let udp_handle = tokio::spawn(async move {
+            if let Err(e) = udp_listener.run_udp(&udp_addr).await {
+                error!(error = %e, "UDP listener failed");
+            }
+        });
+
+        let tcp_handle = tokio::spawn(async move {
+            if let Err(e) = tcp_listener.run_tcp(&tcp_addr).await {
+                error!(error = %e, "TCP listener failed");
+            }
+        });
+
+        // Wait for either to complete (they shouldn't unless they error)
+        tokio::select! {
+            _ = udp_handle => {},
+            _ = tcp_handle => {},
+        }
+
+        Ok(())
+    }
+
+    /// Run the UDP listener loop.
+    async fn run_udp(&self, addr: &str) -> Result<(), ListenerError> {
+        let socket = UdpSocket::bind(addr)
+            .await
+            .map_err(ListenerError::UdpBind)?;
+        info!(addr = %addr, "UDP DNS listener started");
+
+        let mut buf = vec![0u8; 512]; // Standard DNS UDP packet size
+
+        loop {
+            let (len, src) = match socket.recv_from(&mut buf).await {
+                Ok(v) => v,
+                Err(e) => {
+                    warn!(error = %e, "UDP recv error");
+                    continue;
+                }
+            };
+
+            let query_bytes = &buf[..len];
+            debug!(from = %src, bytes = len, "received UDP DNS query");
+
+            let response = self.process_query(query_bytes).await;
+
+            if let Err(e) = socket.send_to(&response, src).await {
+                warn!(error = %e, "UDP send error");
+            }
+        }
+    }
+
+    /// Run the TCP listener loop.
+    async fn run_tcp(&self, addr: &str) -> Result<(), ListenerError> {
+        let listener = TcpListener::bind(addr)
+            .await
+            .map_err(ListenerError::TcpBind)?;
+        info!(addr = %addr, "TCP DNS listener started");
+
+        loop {
+            let (stream, src) = match listener.accept().await {
+                Ok(v) => v,
+                Err(e) => {
+                    warn!(error = %e, "TCP accept error");
+                    continue;
+                }
+            };
+
+            let this = self.clone();
+            tokio::spawn(async move {
+                if let Err(e) = this.handle_tcp(stream, src).await {
+                    debug!(error = %e, from = %src, "TCP connection error");
+                }
+            });
+        }
+    }
+
+    /// Handle a single TCP DNS connection.
+    ///
+    /// DNS over TCP uses a 2-byte length prefix (RFC 1035 §4.2.2).
+    async fn handle_tcp(
+        &self,
+        mut stream: tokio::net::TcpStream,
+        src: std::net::SocketAddr,
+    ) -> anyhow::Result<()> {
+        use tokio::io::{AsyncReadExt, AsyncWriteExt};
+
+        // Read 2-byte length prefix
+        let mut len_buf = [0u8; 2];
+        stream.read_exact(&mut len_buf).await?;
+        let query_len = u16::from_be_bytes(len_buf) as usize;
+
+        if query_len > 65535 {
+            return Err(anyhow::anyhow!("TCP query too large: {query_len} bytes"));
+        }
+
+        // Read the query
+        let mut query_buf = vec![0u8; query_len];
+        stream.read_exact(&mut query_buf).await?;
+
+        debug!(from = %src, bytes = query_len, "received TCP DNS query");
+
+        let response = self.process_query(&query_buf).await;
+
+        // Write 2-byte length prefix + response
+        let resp_len = (response.len() as u16).to_be_bytes();
+        stream.write_all(&resp_len).await?;
+        stream.write_all(&response).await?;
+
+        Ok(())
+    }
+
+    /// Process a DNS query: parse, forward through DoH, return wire-format response.
+    async fn process_query(&self, query_bytes: &[u8]) -> Vec<u8> {
+        let query = match parse_query(query_bytes) {
+            Ok(q) => q,
+            Err(e) => {
+                warn!(error = %e, "invalid DNS query");
+                // Return a minimal SERVFAIL for unparseable queries
+                return build_minimal_servfail();
+            }
+        };
+
+        let qname = query
+            .queries()
+            .first()
+            .map(|q| q.name().to_string())
+            .unwrap_or_default();
+        let qtype = query
+            .queries()
+            .first()
+            .map(|q| q.query_type().to_string())
+            .unwrap_or_default();
+
+        let idx = self.rr_counter.fetch_add(1, Ordering::Relaxed);
+
+        match self
+            .doh_client
+            .forward(query_bytes, self.strategy, idx)
+            .await
+        {
+            Ok(response) => {
+                info!(query = %qname, qtype = %qtype, bytes = response.len(), "DNS query forwarded");
+                response
+            }
+            Err(e) => {
+                error!(error = %e, query = %qname, "upstream forward failed");
+                build_servfail(&query).unwrap_or_else(|_| build_minimal_servfail())
+            }
+        }
+    }
+}
+
+/// Build a minimal SERVFAIL response (12-byte header, no questions).
+fn build_minimal_servfail() -> Vec<u8> {
+    let mut buf = vec![0u8; 12];
+    buf[2] = 0x81; // QR=1, Opcode=0, AA=0, TC=0, RD=1
+    buf[3] = 0x80; // RA=1, Z=0, RCODE=0 (no error in this byte, RCODE in lower 4 bits)
+    buf[3] |= 0x02; // RCODE = SERVFAIL (2)
+    buf
+}

src/dns/mod.rs

@@ -1,4 +1,5 @@
-//! DNS wire-format parsing and response building.
+//! DNS wire-format parsing, response building, and UDP/TCP listener.
 
+pub mod listener;
 pub mod query;
 pub mod response;

src/main.rs

@@ -9,6 +9,7 @@ use tracing::info;
 use tracing_subscriber::EnvFilter;
 
 use doh_forwarder::config::AppConfig;
+use doh_forwarder::dns::listener::DnsListener;
 use doh_forwarder::routes::{dns_query_get, dns_query_post, AppState};
 use doh_forwarder::upstream::doh_client::DohClient;
 
@@ -44,6 +45,16 @@ async fn main() -> anyhow::Result<()> {
     let strategy = config.upstream.strategy()?;
     let doh_client = DohClient::new(config.upstream.resolvers.clone());
 
+    // Start DNS listener (UDP/TCP) in background
+    let dns_listener = DnsListener::new(doh_client.clone(), strategy);
+    let dns_addr = config.dns.listen.clone();
+    tokio::spawn(async move {
+        if let Err(e) = dns_listener.run(&dns_addr).await {
+            tracing::error!(error = %e, "DNS listener failed");
+        }
+    });
+    info!(addr = %config.dns.listen, "DNS listener starting");
+
     let state = AppState {
         doh_client,
         rr_counter: Arc::new(std::sync::atomic::AtomicUsize::new(0)),