start_tunnel · Mcp Untun MCP: Risk & Policy

WHAT IT DOES

What start_tunnel does on Mcp Untun

AI agents invoke start_tunnel to trigger actions in Mcp Untun. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why start_tunnel needs a policy

This tool executes a tunnel creation operation that initiates network connectivity changes and external processes. While not destructive (tunnels can be stopped), it goes beyond read/write operations by actively triggering and managing network services.

From the tool's definition Tool description states it "Creates a secure tunnel from a public internet address to your local server" and "Start an untun tunnel process connecting to your specified local URL".

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

start_tunnel Rate-limited

start_tunnel stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "start_tunnel": {
      "limits": [
        {
          "counter": "start_tunnel_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Mcp Untun, so you can run agents against it safely.

FAQ

Questions about start_tunnel

What does the start_tunnel tool do? +

Creates a secure tunnel from a public internet address to your local server. This tool will: - Start an untun tunnel process connecting to your specified local URL - Return a public URL that can be used to access your local server - Allow you to name your tunnel for easier management After starting a tunnel, wait a few seconds and use. It is categorised as a Execute tool in the Mcp Untun MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on start_tunnel? +

Register the Mcp Untun MCP server in PolicyLayer and add a rule for start_tunnel: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Mcp Untun. Nothing to install.

What risk level is start_tunnel? +

start_tunnel is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit start_tunnel? +

Yes. Add a rate_limit block to the start_tunnel rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block start_tunnel completely? +

Set action: deny in the PolicyLayer policy for start_tunnel. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides start_tunnel? +

start_tunnel is provided by the Mcp Untun MCP server (minte-app/untun-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.