diff --git a/core/dns-api/fs.go b/core/dns-api/fs.go
new file mode 100644
index 0000000..597a088
--- /dev/null
+++ b/core/dns-api/fs.go
@@ -0,0 +1,67 @@
+package main
+
+import (
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+type FS interface {
+	Exists(path string) (bool, error)
+	Reader(path string) (io.ReadCloser, error)
+	Writer(path string) (io.WriteCloser, error)
+	Read(path string) (string, error)
+	Write(path string, data string) error
+	AbsolutePath(path string) string
+}
+
+type osFS struct {
+	root string
+}
+
+func (f osFS) AbsolutePath(path string) string {
+	return filepath.Join(f.root, path)
+}
+
+func (f osFS) Exists(path string) (bool, error) {
+	_, err := os.Stat(f.AbsolutePath(path))
+	if err == nil {
+		return true, nil
+	}
+	if errors.Is(err, os.ErrNotExist) {
+		return false, nil
+	}
+	return false, nil // TODO(gio): return err
+}
+
+func (f osFS) Reader(path string) (io.ReadCloser, error) {
+	return os.Open(f.AbsolutePath(path))
+}
+
+func (f osFS) Writer(path string) (io.WriteCloser, error) {
+	return os.Create(f.AbsolutePath(path))
+}
+
+func (f osFS) Read(path string) (string, error) {
+	r, err := f.Reader(path)
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+	d, err := io.ReadAll(r)
+	if err != nil {
+		return "", err
+	}
+	return string(d), err
+}
+
+func (f osFS) Write(path string, data string) error {
+	w, err := f.Writer(path)
+	if err != nil {
+		return err
+	}
+	defer w.Close()
+	_, err = io.WriteString(w, data)
+	return err
+}