ngrok ERR_NGROK_3004 — invalid host header: how to fix

What it means

ngrok routes traffic by Host header. A request reached the edge with a Host that has no matching tunnel — typo, expired tunnel, or stale DNS cache.

Common causes

• The tunnel name in the URL is misspelled.
• The tunnel has been killed since you copied the URL.
• Browser cached an older subdomain in DNS or HSTS.

How to fix it on ngrok

1. 1. Confirm the tunnel is alive

   $ ngrok api tunnels list

2. 2. Re-copy the URL

   ngrok prints the active subdomain on its UI dashboard at http://localhost:4040.

3. 3. Clear DNS / HSTS cache

   Particularly on Chrome — chrome://net-internals/#dns and #hsts.

More ngrok issues

• ngrok URL keeps changing — how to keep one stable URL on the free plan
• ngrok ERR_NGROK_3200 — tunnel endpoint offline: how to fix