doq: configure QUIC keep-alive and retry on idle timeout

Pass a quic.Config with KeepAlivePeriod (15s) to DoQ dial calls instead
of nil, so pooled connections send periodic QUIC PINGs to stay alive and
detect dead paths proactively.

Also add IdleTimeoutError to the DoQ retry conditions alongside io.EOF,
so stale pooled connections trigger a transparent retry instead of
propagating as a query failure.

doq.go

@@ -51,11 +51,12 @@ const doqPoolSize = 16
 
 // doqConnPool manages a pool of QUIC connections for DoQ queries using a buffered channel.
 type doqConnPool struct {
-	uc        *UpstreamConfig
-	addrs     []string
-	port      string
-	tlsConfig *tls.Config
-	conns     chan *doqConn
+	uc         *UpstreamConfig
+	addrs      []string
+	port       string
+	tlsConfig  *tls.Config
+	quicConfig *quic.Config
+	conns      chan *doqConn
 }
 
 type doqConn struct {
@@ -74,12 +75,17 @@ func newDOQConnPool(_ context.Context, uc *UpstreamConfig, addrs []string) *doqC
 		ServerName: uc.Domain,
 	}
 
+	quicConfig := &quic.Config{
+		KeepAlivePeriod: 15 * time.Second,
+	}
+
 	pool := &doqConnPool{
-		uc:        uc,
-		addrs:     addrs,
-		port:      port,
-		tlsConfig: tlsConfig,
-		conns:     make(chan *doqConn, doqPoolSize),
+		uc:         uc,
+		addrs:      addrs,
+		port:       port,
+		tlsConfig:  tlsConfig,
+		quicConfig: quicConfig,
+		conns:      make(chan *doqConn, doqPoolSize),
 	}
 
 	// Use SetFinalizer here because we need to call a method on the pool itself.
@@ -94,12 +100,17 @@ func newDOQConnPool(_ context.Context, uc *UpstreamConfig, addrs []string) *doqC
 
 // Resolve performs a DNS query using a pooled QUIC connection.
 func (p *doqConnPool) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
-	// Retry logic for io.EOF errors (as per original implementation)
+	// Retry logic for transient errors: io.EOF (connection reset) and
+	// IdleTimeoutError (stale pooled connection timed out).
 	for range 5 {
 		answer, err := p.doResolve(ctx, msg)
 		if err == io.EOF {
 			continue
 		}
+		var idleErr *quic.IdleTimeoutError
+		if errors.As(err, &idleErr) {
+			continue
+		}
 		if err != nil {
 			return nil, wrapCertificateVerificationError(err)
 		}
@@ -235,7 +246,7 @@ func (p *doqConnPool) dialConn(ctx context.Context) (string, *quic.Conn, error)
 			udpConn.Close()
 			return "", nil, err
 		}
-		conn, err := quic.DialEarly(ctx, udpConn, remoteAddr, p.tlsConfig, nil)
+		conn, err := quic.DialEarly(ctx, udpConn, remoteAddr, p.tlsConfig, p.quicConfig)
 		if err != nil {
 			udpConn.Close()
 			return "", nil, err
@@ -250,7 +261,7 @@ func (p *doqConnPool) dialConn(ctx context.Context) (string, *quic.Conn, error)
 	}
 
 	pd := &quicParallelDialer{}
-	conn, err := pd.Dial(ctx, dialAddrs, p.tlsConfig, nil)
+	conn, err := pd.Dial(ctx, dialAddrs, p.tlsConfig, p.quicConfig)
 	if err != nil {
 		return "", nil, err
 	}