How to fix MCP server '421 Misdirected Request: Invalid Host header' behind nginx, a domain, Docker or a tunnel (Python and Typ

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--- RECIPE v1: Fix MCP server '421 Misdirected Request: Invalid Host header' behind nginx, a domain, Docker or a tunnel (Python and Typ ---
> Status: tested once (2026-10-25, ~08:10 Warsaw) on Linux (Debian), Python 3.13.5. Servers: Python `mcp` 1.30.0 (`mcp.server.fastmcp.FastMCP`) and `mcp` 2.3.0 (`mcp.server.MCPServer`), both with uvicorn 0.54.0 / Starlette 1.10.0, plus the TypeScript SDK `@modelcontextprotocol/sdk` 1.32.2 (`createMcpExpressApp`, Express 5.3.0, Node 20.19.2). Client: Python `mcp` 1.30.0 `streamablehttp_client` (initialize, tools/list, tools/call) and curl. The "domain" was a local hosts-file name (`mcp.example.test`) in front of nginx 1.26.3, with three proxy configs. Docker, ngrok/Cloudflare Tunnel, Caddy/Traefik, real HTTPS and the standalone `fastmcp` package were NOT tested. Their `Host` values were simulated with curl `-H 'Host: ...'`. Every error string below was copied from a real run.

Your MCP server works on `http://localhost:8000/mcp`. As soon as you reach it through a domain, a reverse proxy, a LAN IP or `host.docker.internal`, every request fails before it reaches your code. The cause is the SDK's **DNS-rebinding protection**: it is switched on automatically when the server is configured for host `127.0.0.1`/`localhost` (the default), and then only accepts `Host: localhost:*` / `127.0.0.1:*` / `[::1]:*`. Binding uvicorn to `0.0.0.0` does not change that.

## Which error do you have?

| What you see | Where | Meaning |
|---|---|---|
| `421 Misdirected Request`, body `Invalid Host header`. Python client: `HTTPStatusError: Client error '421 Misdirected Request' for url 'https://your-domain/mcp'` | Python SDK (`FastMCP` 1.x, `MCPServer` 2.x), Streamable HTTP and SSE (`GET /sse` too) | `Host` header not in `allowed_hosts`. |
| Server log: `WARNING ... Invalid Host header: mcp.example.test` (or `...:8000`, `host.docker.internal:8000`, `192.168.1.20:8000`) | Python SDK | Same. The logged value is exactly what you must allow. |
| `403 Forbidden`, body `{"jsonrpc":"2.0","error":{"code":-32000,"message":"Invalid Host: mcp.example.test"},"id":null}` | TS SDK `createMcpExpressApp()` | Same check in the TypeScript SDK (403, not 421). |
| `403`, body `Invalid Origin header` | Python SDK | `Host` passed, but the request carried an `Origin` (browser) not in `allowed_origins`. For the TS SDK and CORS errors see the [CORS recipe](/recipe/fix-cors-errors-for-browser-mcp-clients-failed-to-fetch). |

Quick check, from the machine running the server (replace port/domain):
```bash
curl -s -o /dev/null -w '%{http_code}\n' -X POST http://127.0.0.1:8000/mcp \
  -H 'Host: your-domain.example' -H 'Content-Type: application/json' -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"curl","version":"1"}}}'
```
`421` (Python) or `403` (TS) = this recipe. `200` with `Host: localhost:8000` but `421` with your domain confirms it.

## What we measured (Python `mcp` 1.30.0 and 2.3.0 behaved identically)

| Server setup | `Host: localhost:8000` | `Host: mcp.example.test` / `:8000` | `host.docker.internal:8000`, `192.168.1.20:8000` |
|---|---|---|---|
| `FastMCP("demo")` / `MCPServer("demo")`, app served with `uvicorn.run(app, host="0.0.0.0")` | 200 | **421** | **421** |
| Same + env `FASTMCP_HOST=0.0.0.0` (1.30.0) | 200 | **421** (`settings.host` stayed `127.0.0.1`, protection still on) | **421** |
| `FastMCP("demo", host="0.0.0.0")` + `mcp.run(transport="streamable-http")` / `mcp.streamable_http_app(host="0.0.0.0")` (2.3.0) | 200 | 200 | 200 (no protection at all) |
| Explicit `allowed_hosts` with your domain (step 2) | 200 | 200 | 421 (not listed, as intended) |
| `allowed_hosts=["mcp.example.test"]` only, no `:*` entry | **421** | 200 without port, **421** with `:8000` | 421 |
| `enable_dns_rebinding_protection=False` | 200 | 200 | 200 |

Through nginx (`proxy_pass http://127.0.0.1:8000;`, client URL `http://mcp.example.test:8080/mcp`), default protected server:

| nginx config | Host the server sees | Result |
|---|---|---|
| `proxy_set_header Host $host;` (most tutorials) | `mcp.example.test` | **421** |
| `proxy_set_header Host $http_host;` | `mcp.example.test:8082` | **421** |
| no `proxy_set_header Host` | `127.0.0.1:8000` (nginx default) | 200 (works by accident) |

TS SDK 1.32.2: `createMcpExpressApp()` (default host `127.0.0.1`) → `403 Invalid Host: mcp.example.test` for domain/`host.docker.internal`, 200 for localhost. `createMcpExpressApp({ host: '0.0.0.0' })` → all 200, and it prints `Warning: Server is binding to 0.0.0.0 without DNS rebinding protection.` `createMcpExpressApp({ host: '0.0.0.0', allowedHosts: [...] })` → listed names 200 (port-agnostic), others 403.

## Fix

1. **Find the Host value** the server rejects: it is in the server log line `Invalid Host header: ...` (Python) or the 403 body `Invalid Host: ...` (TS).
2. **Python (recommended): allow your real hostnames, keep the protection on.** Tested with 1.30.0:
   ```python
   from mcp.server.fastmcp import FastMCP
   from mcp.server.transport_security import TransportSecuritySettings

   mcp = FastMCP("demo", transport_security=TransportSecuritySettings(
       enable_dns_rebinding_protection=True,
       allowed_hosts=["127.0.0.1:*", "localhost:*", "mcp.example.com", "mcp.example.com:*"],
       allowed_origins=["http://127.0.0.1:*", "http://localhost:*", "https://mcp.example.com"],
   ))
   ```
   With 2.3.0 (`from mcp.server import MCPServer`) pass the same object to the app: `app = mcp.streamable_http_app(transport_security=TransportSecuritySettings(...))` (tested). Rules we confirmed: an entry matches exactly, so `"mcp.example.com"` does **not** match `Host: mcp.example.com:8443`. Add `"mcp.example.com:*"` for any port. Explicit settings replace the localhost defaults, so keep `localhost:*` / `127.0.0.1:*` if you still connect locally (without them `localhost:8000` got 421). Only browser requests send `Origin`, so `allowed_origins` only matters for web clients.
3. **TypeScript:** `const app = createMcpExpressApp({ host: '0.0.0.0', allowedHosts: ['localhost', '127.0.0.1', 'mcp.example.com'] });` Hostnames only, no ports (tested).
4. **Alternative: declare a non-local host.** `FastMCP("demo", host="0.0.0.0")` in 1.x, passed to the constructor (env `FASTMCP_HOST=0.0.0.0` did not help in our test; changing `mcp.settings.host` after construction was not tested), `mcp.streamable_http_app(host="0.0.0.0")` in 2.x, `createMcpExpressApp({ host: '0.0.0.0' })` in TS. This turns the Host check **off** completely. Only do it if the server is protected some other way (auth token, not reachable from a browser on the same machine).
5. **Behind nginx,** keep `proxy_set_header Host $host;` and add that domain to `allowed_hosts` as in step 2. Dropping the `Host` header also "fixed" it in our test, but only because nginx then sends `127.0.0.1:8000`. That works by accident.
6. **Verify:** rerun the curl check with your domain as `Host` → `200`, then connect your client through the real URL. Our Python client then completed initialize → tools/list → tools/call (`add(2,3)` = 5) via nginx.

## Rollback
Remove the `transport_security` / `allowedHosts` argument. The server returns to localhost-only mode and the domain gets 421/403 again. Nothing is persisted.

## Security note
This check exists so a malicious web page can't use DNS rebinding to talk to an MCP server on your laptop. On a local dev server, keep it on and list the hosts you need. Don't just set `enable_dns_rebinding_protection=False`. On a public server, you also need real authentication; the Host check is not auth.

## Not tested
Real Docker containers (we simulated `host.docker.internal:8000` with curl; a server started with the default `mcp.run(transport="streamable-http")` binds to 127.0.0.1 inside the container and is unreachable through `-p`, which is a different failure we did not test), ngrok / Cloudflare Tunnel / Tailscale Funnel (they forward a public `Host`, so expect the same 421; not run), Caddy/Traefik, HTTPS, uvicorn `--proxy-headers` / `X-Forwarded-Host` (the SDK checks `Host` itself), the standalone `fastmcp` 2.x/3.x package, and `mcp` versions older than 1.30.0. Related: [CORS / Invalid Origin header](/recipe/fix-cors-errors-for-browser-mcp-clients-failed-to-fetch), [Streamable HTTP 406 / session errors](/recipe/fix-mcp-streamable-http-errors-406-not-acceptable-server-not), [SSE 405/404](/recipe/fix-mcp-sse-error-non-200-status-code-405-404-and-other-sse), [-32000 Connection closed](/recipe/fix-mcp-error-32000-connection-closed-stdio-server-exits).


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--- END ---

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