docs: add DNS rebinding protection guide

Adds comprehensive documentation for resolving "421 Invalid Host Header"
errors when using MCP behind proxies or gateways.

Includes:
- Two resolution options (allowlist vs disable)
- Common scenarios (Nginx, Docker)
- Security considerations

Fixes #1798

Author: goingforstudying-ctrl

docs/guides/dns-rebinding-protection.md

@@ -0,0 +1,89 @@
+# DNS Rebinding Protection
+
+The MCP Python SDK includes DNS rebinding protection to prevent DNS rebinding attacks. While this improves security, it may cause existing setups to fail with a **421 Misdirected Request / Invalid Host Header** error if the host header doesn't match the allowed list.
+
+This commonly occurs when using:
+- Reverse proxies (Nginx, Caddy, etc.)
+- API gateways
+- Custom domains
+- Docker/Kubernetes networking
+
+## Resolving the Error
+
+Depending on your security requirements, you can resolve this in two ways:
+
+### Option 1: Explicitly Allow Specific Hosts (Recommended for Production)
+
+Use this approach if you are running in production or through a gateway. You can wildcard the ports using `*`.
+
+```python
+from mcp.server.fastmcp import FastMCP
+from mcp.server.transport_security import TransportSecuritySettings
+
+mcp = FastMCP(
+    "MyServer",
+    transport_security=TransportSecuritySettings(
+        enable_dns_rebinding_protection=True,
+        # Add your specific gateway or domain here
+        allowed_hosts=["localhost:*", "127.0.0.1:*", "your-gateway-host:*"],
+        allowed_origins=["http://localhost:*", "http://your-gateway-host:*"],
+    )
+)
+```
+
+### Option 2: Disable DNS Rebinding Protection (Development Only)
+
+Use this approach for local development or if you are managing security at a different layer of your infrastructure.
+
+```python
+from mcp.server.fastmcp import FastMCP
+from mcp.server.transport_security import TransportSecuritySettings
+
+mcp = FastMCP(
+    "MyServer",
+    transport_security=TransportSecuritySettings(
+        enable_dns_rebinding_protection=False,
+    )
+)
+```
+
+## Common Scenarios
+
+### Using with Nginx
+
+If you're using Nginx as a reverse proxy, ensure it's passing the correct headers:
+
+```nginx
+location / {
+    proxy_pass http://localhost:8000;
+    proxy_set_header Host $host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $scheme;
+}
+```
+
+And configure your MCP server to allow the Nginx host:
+
+```python
+allowed_hosts=["localhost:*", "your-domain.com:*"]
+```
+
+### Using with Docker
+
+When running in Docker, you may need to allow the container hostname:
+
+```python
+allowed_hosts=["localhost:*", "127.0.0.1:*", "mcp-server:*"]
+```
+
+## Security Considerations
+
+- **Production**: Always use Option 1 with explicit host allowlisting
+- **Development**: Option 2 is acceptable for local testing
+- **Never** disable DNS rebinding protection in production environments exposed to the internet
+
+## Related Issues
+
+- Original implementation: [#861](https://github.com/modelcontextprotocol/python-sdk/pull/861)
+- Common errors: [#1797](https://github.com/modelcontextprotocol/python-sdk/issues/1797)