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Troubleshooting

This guide helps you resolve common issues when using the MCP Python SDK.

421 Invalid Host Header (DNS Rebinding Protection)

A recent update introduced DNS rebinding protection to the MCP Python SDK. 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 (common when using proxies, gateways, or custom domains).

Solutions

Depending on your security requirements, you can resolve this in two ways:

Option 1: Explicitly Allow Specific Hosts (Recommended)

Use this approach if you are running in production or through a gateway. You can wildcard the ports using *.

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

Use this approach for local development or if you are managing security at a different layer of your infrastructure.

from mcp.server.fastmcp import FastMCP
from mcp.server.transport_security import TransportSecuritySettings

mcp = FastMCP(
    "MyServer",
    transport_security=TransportSecuritySettings(
        enable_dns_rebinding_protection=False,
    )
)

Reverse Proxy Configuration

If you are using a reverse proxy (like Nginx or Caddy), ensure your proxy is passing the correct Host header to the MCP server.

Nginx example:

location / {
    proxy_pass http://localhost:8000;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}

Caddy example:

reverse_proxy localhost:8000 {
    header_up Host {upstream_hostport}
}

See Also