title: Nextjs How to send Etag thorugh vercel?
body: <p>I want to send Etag.</p>
  <pre><code>export const getServerSideProps: GetServerSideProps = async ({ req, res }) =&gt; {
  
   const targetIndex =
    req.rawHeaders.findIndex((val) =&gt; val === &quot;If-None-Match&quot;) + 1;
    const EtagVal = req.rawHeaders[targetIndex];
  
    res.setHeader(&quot;Cache-Control&quot;, &quot;public, max-age=0, must-revalidate&quot;);
    res.setHeader(&quot;ETag&quot;, &quot;aaaa&quot;);
    return {
      props: {},
    };
  };
  </code></pre>
  <p>This Works locally. <code>build &amp; next start.</code>.<br />
  It does not work after <code>vercel</code> deployment.</p>
  <p>I tried configure <code>next.config.js</code> like this</p>
  <pre><code>const nextConfig = {
    reactStrictMode: true,
    async headers() {
      return [
        {
          source: '/:path*',
          &quot;headers&quot; : [
            {
              &quot;key&quot; : &quot;ETag&quot;,
            },
            {
              &quot;key&quot; : &quot;If-None-Match&quot;,
            }
          ]
        }
      ]
    }
  }
  </code></pre>
  <p>Unlike the document that any value is allowed if value is not defined,<br />
  An error occurs during the build phase.</p>