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by ai.intodns
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Check forward-confirmed reverse DNS

check_fcrdns
Read-onlyIdempotent

Read-only FCrDNS (Forward-Confirmed Reverse DNS) audit for every IP that backs the domain's MX records. For each IP: looks up PTR record, then resolves that PTR's hostname back to A/AAAA records to confirm the round-trip. Returns per-IP PTR value, forward-resolution result, match verdict, and warnings (missing PTR, mismatched forward, generic ISP reverse). Use for mail deliverability audits, SpamExperts-style cluster checks, and any 'why is our mail being rejected' debugging; pair with check_blacklist for reputation signals. No auth.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "domain": "<domain>"
    },
    "name": "check_fcrdns"
  }
}

Input Schema

NameRequiredDescriptionDefault
domainYesDomain name only, e.g. example.com (no URL, path, or port)

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.5/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false, and openWorldHint, so 'Read-only' in the text mostly restates structured data. The description still adds real value by disclosing the per-IP resolution sequence, the four return fields, the specific warning categories (missing PTR, mismatched forward, generic ISP reverse), and the 'No auth' note. It stops short of mentioning rate limits or timeout behavior for the live DNS lookups.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The operation is front-loaded in the first clause, followed by the mechanism, then return shape, then usage. Each sentence carries distinct information with no filler, despite the description being fairly dense.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description compensates by enumerating what the result contains (PTR value, forward-resolution result, match verdict, warnings) and by specifying the resolution procedure. An agent has everything needed to decide to call it and to interpret the response shape.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, and the single 'domain' parameter is fully documented in the schema with format constraints (no URL, path, or port). The description adds no further parameter-level detail beyond implying MX records are derived from the domain, so the baseline of 3 for schema-covered parameters is right.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific operation (FCrDNS round-trip check) on a specific resource (IPs behind the domain's MX records), and names the exact mechanism: PTR lookup followed by forward A/AAAA resolution. This is clearly separable from siblings such as check_blacklist, check_spf, or lookup_dns, which do different jobs.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives concrete usage contexts (mail deliverability audits, cluster checks, 'why is our mail being rejected' debugging) and names a companion tool (check_blacklist) for reputation signals, which routes the agent appropriately. The only minor omission is an explicit 'do not use for X', but the positive guidance is strong enough to select correctly.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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