fix(dot): validate connections before reuse to prevent io.EOF errors

Add connection health check in getConn to validate TLS connections
before reusing them from the pool. This prevents io.EOF errors when
reusing connections that were closed by the server (e.g., due to idle
timeout).

dot.go

@@ -57,7 +57,7 @@ type dotConnPool struct {
 }
 
 type dotConn struct {
-	conn     net.Conn
+	conn     *tls.Conn
 	lastUsed time.Time
 	refCount int
 	mu       sync.Mutex
@@ -114,13 +114,6 @@ func (p *dotConnPool) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, erro
 		return nil, wrapCertificateVerificationError(err)
 	}
 
-	// Set deadline
-	deadline, ok := ctx.Deadline()
-	if !ok {
-		deadline = time.Now().Add(5 * time.Second)
-	}
-	_ = conn.SetDeadline(deadline)
-
 	client := dns.Client{Net: "tcp-tls"}
 	answer, _, err := client.ExchangeWithConnContext(ctx, msg, &dns.Conn{Conn: conn})
 	isGood := err == nil
@@ -145,7 +138,7 @@ func (p *dotConnPool) getConn(ctx context.Context) (net.Conn, string, error) {
 	// Try to reuse an existing connection
 	for addr, dotConn := range p.conns {
 		dotConn.mu.Lock()
-		if dotConn.refCount == 0 && dotConn.conn != nil {
+		if dotConn.refCount == 0 && dotConn.conn != nil && isAlive(dotConn.conn) {
 			dotConn.refCount++
 			dotConn.lastUsed = time.Now()
 			conn := dotConn.conn
@@ -202,7 +195,7 @@ func (p *dotConnPool) putConn(addr string, conn net.Conn, isGood bool) {
 }
 
 // dialConn creates a new TCP/TLS connection.
-func (p *dotConnPool) dialConn(ctx context.Context) (string, net.Conn, error) {
+func (p *dotConnPool) dialConn(ctx context.Context) (string, *tls.Conn, error) {
 	logger := LoggerFromCtx(ctx)
 	var endpoint string
 
@@ -224,7 +217,7 @@ func (p *dotConnPool) dialConn(ctx context.Context) (string, net.Conn, error) {
 	// Try bootstrap IPs in parallel
 	if len(p.addrs) > 0 {
 		type result struct {
-			conn net.Conn
+			conn *tls.Conn
 			addr string
 			err  error
 		}
@@ -316,3 +309,28 @@ func (p *dotConnPool) CloseIdleConnections() {
 		delete(p.conns, addr)
 	}
 }
+
+func isAlive(c *tls.Conn) bool {
+	// Set a very short deadline for the read
+	c.SetReadDeadline(time.Now().Add(1 * time.Millisecond))
+
+	// Try to read 1 byte without consuming it (using a small buffer)
+	one := make([]byte, 1)
+	_, err := c.Read(one)
+
+	// Reset the deadline for future operations
+	c.SetReadDeadline(time.Time{})
+
+	if err == io.EOF {
+		return false // Connection is definitely closed
+	}
+
+	// If we get a timeout, it means no data is waiting,
+	// but the connection is likely still "up."
+	var netErr net.Error
+	if errors.As(err, &netErr) && netErr.Timeout() {
+		return true
+	}
+
+	return err == nil
+}