René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Guide: Resolving \"421 Invalid Host Header\" (DNS Rebinding Protection)","articleBody":"### **Context**\n\nA recent update (Commit: `86bb54c`, PR: #861) 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).\n\n### **Affected Components**\n\n* **File:** `src/mcp/server/fastmcp/server.py:178`\n\n---\n\n### **Solutions**\n\nDepending on your security requirements, you can resolve this in two ways:\n\n#### **Option 1: Explicitly Allow Specific Hosts (Recommended)**\n\nUse this approach if you are running in production or through a gateway. You can wildcard the ports using `*`.\n\n```python\nfrom mcp.server.fastmcp import FastMCP\nfrom mcp.server.transport_security import TransportSecuritySettings\n\nmcp = FastMCP(\n    \"MyServer\",\n    transport_security=TransportSecuritySettings(\n        enable_dns_rebinding_protection=True,\n        # Add your specific gateway or domain here\n        allowed_hosts=[\"localhost:*\", \"127.0.0.1:*\", \"your-gateway-host:*\"],\n        allowed_origins=[\"http://localhost:*\", \"http://your-gateway-host:*\"],\n    )\n)\n\n```\n\n#### **Option 2: Disable DNS Rebinding Protection**\n\nUse this approach for local development or if you are managing security at a different layer of your infrastructure.\n\n```python\nfrom mcp.server.fastmcp import FastMCP\nfrom mcp.server.transport_security import TransportSecuritySettings\n\nmcp = FastMCP(\n    \"MyServer\",\n    transport_security=TransportSecuritySettings(\n        enable_dns_rebinding_protection=False,\n    )\n)\n\n```\n\n---\n\n### **References**\n\n* **Original Reported by:** @leoswing in #861\n* **Then Reported by:** @longz30 in #1797\n\n\u003e **Note:** If you are using a reverse proxy (like Nginx or Caddy), ensure your proxy is passing the correct `Host` header to the MCP server.\n\n\n### Additional Context\n\nIf your problem is solved, remember to give a thumbs up 👍!","author":{"url":"https://github.com/Junyi-99","@type":"Person","name":"Junyi-99"},"datePublished":"2025-12-17T18:17:58.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":12},"url":"https://github.com/1798/python-sdk/issues/1798"}