CaptainDNS

DNS Lookup

Choose a DNS record type to search for.

Propagation & diagnostics

Compare resolvers worldwide and inspect returned answers.

Propagation testPublic resolvers, remaining TTLs, divergent responses.
Domain DNS auditParent/local NS, authority and response times.
API & integrations
Keep history, monitor your zones and automate recurring checks.

Email authentication

Tools to verify and validate your email authentication setup.

Syntax validation

Record inspectors

Generators

SPF

SPF syntax validator

SPF record inspector

SPF record generator

DKIM

DKIM syntax validator

DKIM record check

DKIM generator

DMARC

DMARC syntax validator

DMARC record inspector

DMARC record generator

BIMI

BIMI syntax validator

BIMI record inspector

BIMI record generator

MTA-STS

MTA-STS syntax validator

MTA-STS record inspector

MTA-STS generator

TLS-RPT

TLS-RPT syntax validator

TLS-RPT record inspector

TLS-RPT generator

Deliverability

Email deliverability audit

Audit MX, SPF, DKIM and DMARC to confirm a domain is ready to send.

Mail header analyzer

Decode participants, authentication results and routing from raw headers.

Email tester

Send a test email and get instant authentication and deliverability feedback.

IP Blacklist Checker

Check if your IP is on email blacklists

Domain Blacklist Checker

Check if your domain is on URI blacklists

SPF Flattener

Flatten SPF records to reduce DNS lookups below the 10-lookup limit.

DKIM Selector Finder

Auto-discover all DKIM selectors for a domain via DNS brute-force.

Text

Transform, encode and measure your content in seconds.

Text case converter

Convert any block of text to upper or lower case instantly.

Base64 encoder / decoder

Encode or decode any content in Base64 without leaving the browser.

Slug generator

Transform any sentence into an SEO-friendly slug in seconds.

Word & character counter

Measure the length of any text, with instant word and character counts.

Web & Certificates

BIMI logos, VMC/CSR certificates and web page diagnostics.

BIMI logo lookup

Inspect a BIMI logo URL - format, metadata and live rendering - before rollout.

CSR parser

Inspect a CSR, extract subject details, fingerprints and requested SANs.

VMC inspector

Inspect a Verified Mark Certificate - issuer, validity and SAN coverage - before you publish BIMI.

Page size checker

Check if your page exceeds Google's 2 MB crawl limit.

Look up addresses, owners, reverse records and ranges.

Reverse lookup

Resolve a PTR record and validate DNS consistency.

IP WhoIs

Identify the owner of an IP range and its contacts.

IPv4 netmask

Calculate network, broadcast and usable hosts for any IPv4 block.

My IP address

Detect your IPv4/IPv6 addresses and their geolocation.

CaptainDNS

CaptainDNS platform features, tutorials, and usage guides.

3 articles

December 2025

Illustration of the 3 CaptainDNS MCP widgets in ChatGPT

CaptainDNS · December 17, 2025

Connect CaptainDNS to ChatGPT via MCP: 3 widgets for DNS audits and deliverability

Add the CaptainDNS MCP server to ChatGPT and use 3 widgets to troubleshoot DMARC/DNS and analyze email headers without leaving the conversation.

November 2025

CaptainDNS · November 7, 2025

New in CaptainDNS: central history and continuous DNS monitoring

Diagnose, archive, share, then monitor: CaptainDNS now supervises the full lifecycle of a DNS query from the first lookup to automated watch jobs.

October 2025

CaptainDNS · October 17, 2025

Building Resilient DNS Monitoring

Ensure visibility of your domains by combining zone differences, active queries and propagation tests.