diff --git a/src/extract_wintun_dll.rs b/src/extract_wintun_dll.rs
index a633404..9becd99 100644
--- a/src/extract_wintun_dll.rs
+++ b/src/extract_wintun_dll.rs
@@ -25,9 +25,55 @@ fn extract_wintun_impl() -> io::Result<()> {
         .and_then(|p| p.parent().map(|d| d.join("wintun.dll")))
         .unwrap_or_else(|| Path::new("wintun.dll").to_path_buf());
 
-    if !path.exists() {
-        let mut file = fs::File::create(&path)?;
-        file.write_all(WINTUN_DLL)?;
-    }
+    ensure_dll(&path)?;
     Ok(())
 }
+
+/// 确保 path 处的 dll 与内嵌版本一致，不一致（不存在/损坏/旧版）时重写。
+/// 返回是否发生了写入。只按存在性判断会让损坏或旧版 dll 永久残留。
+fn ensure_dll(path: &Path) -> io::Result<bool> {
+    let up_to_date = fs::read(path)
+        .map(|content| content.as_slice() == WINTUN_DLL)
+        .unwrap_or(false);
+    if up_to_date {
+        return Ok(false);
+    }
+    let mut file = fs::File::create(path)?;
+    file.write_all(WINTUN_DLL)?;
+    Ok(true)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn temp_dll_path(tag: &str) -> std::path::PathBuf {
+        std::env::temp_dir().join(format!("vnt_wintun_test_{}_{}.dll", std::process::id(), tag))
+    }
+
+    #[test]
+    fn test_ensure_dll_writes_when_missing() {
+        let path = temp_dll_path("missing");
+        let _ = fs::remove_file(&path);
+        assert!(ensure_dll(&path).unwrap());
+        assert_eq!(fs::read(&path).unwrap(), WINTUN_DLL);
+        let _ = fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_ensure_dll_rewrites_corrupt() {
+        let path = temp_dll_path("corrupt");
+        fs::write(&path, b"corrupt").unwrap();
+        assert!(ensure_dll(&path).unwrap());
+        assert_eq!(fs::read(&path).unwrap(), WINTUN_DLL);
+        let _ = fs::remove_file(&path);
+    }
+
+    #[test]
+    fn test_ensure_dll_skips_when_up_to_date() {
+        let path = temp_dll_path("uptodate");
+        fs::write(&path, WINTUN_DLL).unwrap();
+        assert!(!ensure_dll(&path).unwrap());
+        let _ = fs::remove_file(&path);
+    }
+}