# enterno.io

> enterno.io — 48+ free web tools for HTTP, DNS, SSL, security, SEO, and network analysis. Plus uptime monitoring with 8 alert channels, API (v2/v4), team support, and white-label reports. Trusted by developers, sysadmins, and webmasters.

## Tools
- [HTTP Header Checker](https://enterno.io/en/): Inspect HTTP response headers, redirects, cookies, cache-control, and security headers for any URL
- [DNS Lookup](https://enterno.io/en/dns): Query DNS records — A, AAAA, MX, NS, TXT, CNAME, SOA, PTR
- [SSL/TLS Checker](https://enterno.io/en/ssl): Check SSL certificate validity, expiry, chain, issuer, TLS version support
- [Ping / Response Time](https://enterno.io/en/ping): Measure server response time and HTTP availability
- [IP Geolocation](https://enterno.io/en/ip): Resolve IP or hostname to country, ISP, ASN, coordinates
- [PageSpeed Analysis](https://enterno.io/en/speed): Analyse page performance: Core Web Vitals, LCP, CLS, FCP, TTFB
- [Health Score](https://enterno.io/en/health): Aggregated health score: SSL, DNS, security headers, redirects, performance
- [WHOIS Lookup](https://enterno.io/en/whois): Query domain registration data
- [Traceroute](https://enterno.io/en/traceroute): Trace network route, show each hop latency and IP
- [AI Readiness Check](https://enterno.io/en/ai-check): Audit site for AI-agent compatibility: llms.txt, MCP, agent cards, content-signals
- [Security Scanner](https://enterno.io/en/security): HTTP headers, HTTPS, cookies, information disclosure audit
- [Port Checker](https://enterno.io/en/ports): Scan common ports + 70+ port glossary pages with security notes
- [Uptime Monitoring](https://enterno.io/en/monitors): Continuous HTTP/SSL/Ping/DNS monitoring, 8 alert channels, SLA reports
- [RUM — Web Vitals](https://enterno.io/en/rum): Real User Monitoring: LCP, INP, CLS from actual visitors
- [API v2 / v4](https://enterno.io/en/api-docs): REST API for programmatic access to all tools
- [MCP Server](https://enterno.io/en/mcp-server): JSON-RPC 2.0 MCP server at /mcp — install enterno.io into Claude Desktop, Cursor, Zed

## Knowledge base
- [Glossary](https://enterno.io/en/glossary): 136 pages
- [How-to guides](https://enterno.io/en/how-to): 95 pages
- [Network ports](https://enterno.io/en/ports): 79 pages
- [Alternatives](https://enterno.io/en/alternatives): 82 pages
- [SSL/TLS errors](https://enterno.io/en/ssl-errors): 68 pages
- [Research reports](https://enterno.io/en/research): 47 pages

## Articles
- [Fix ERR_ADDRESS_UNREACHABLE: Causes and Solutions](https://enterno.io/en/articles/err-address-unreachable-fix): ERR_ADDRESS_UNREACHABLE means the IP resolved but there is no route to it. Learn the causes — router, VPN, IPv6, private IPs — plus diagnostics and fixes.
- [Fix ERR_CERT_AUTHORITY_INVALID: Causes and Solutions](https://enterno.io/en/articles/err-cert-authority-invalid-fix): Why Chrome shows NET::ERR_CERT_AUTHORITY_INVALID: self-signed certificates, incomplete chains, untrusted CAs. Step-by-step fixes for site owners and visitors.
- [Fix DNS_PROBE_FINISHED_NO_INTERNET: Causes and Solutions](https://enterno.io/en/articles/dns-probe-finished-no-internet-fix): DNS_PROBE_FINISHED_NO_INTERNET means Chrome cannot reach a DNS resolver. Causes and step-by-step fixes: flush DNS, reset Winsock, change DNS servers.
- [How to Check Open Ports: Online, Windows, Linux](https://enterno.io/en/articles/how-to-check-open-ports): How to check open ports: netstat and Get-NetTCPConnection on Windows, ss and lsof on Linux, nc and telnet from outside, plus a free online port scanner.
- [Perplexity SEO: How to Get Cited in Answers](https://enterno.io/en/articles/perplexity-seo-optimization): How to optimize for Perplexity: how it picks sources, allowing PerplexityBot in robots.txt, citation factors, GEO vs classic SEO, and checking your mentions.
- [Bing Copilot Optimization: How to Get Cited by AI](https://enterno.io/en/articles/bing-copilot-optimization): How to get cited by Bing Copilot: the link to Bing ranking, the role of Bingbot and Bing Webmaster Tools, Schema, IndexNow for fast indexing, and Google differences.
- [GEO vs SEO: How AI Optimization Differs](https://enterno.io/en/articles/geo-vs-seo-comparison): GEO vs SEO explained: what they share (E-E-A-T, technical health), how they differ by goal, metrics, format and crawlers, whether to choose, plus a 2026 plan.
- [How to Make Content Extractable for AI](https://enterno.io/en/articles/ai-content-extractability-guide): A practical guide to content extractability for AI: the 40–60 word answer-block rule, query-shaped headings, tables, Schema markup and a checklist.
- [Yandex Neuro: How to Appear in Its AI Answers](https://enterno.io/en/articles/yandex-neuro-optimization): How Yandex Neuro works, which signals its neuro-search uses to pick sources, and what raises your citation odds. Factors, checklist and a free test.
- [Google AI Overviews: How to Rank and Turn It Off](https://enterno.io/en/articles/google-ai-overviews-guide): How to rank in Google AI Overviews: query fan-out, E-E-A-T, extractable passages, Schema. Impact on CTR and traffic, plus how to turn AI Overviews off.
- [How to Promote a Website in AI Neural Networks (GEO)](https://enterno.io/en/articles/promote-website-in-ai-neural-networks): Promote your website in AI answers (ChatGPT, Perplexity, Google AI Overviews, Gemini): the three GEO pillars, cited content formats, llms.txt, and a checklist.
- [How to Appear in AI Answers: ChatGPT, Perplexity, Yandex](https://enterno.io/en/articles/how-to-appear-in-ai-answers): How to appear in AI answers from ChatGPT, Perplexity, Yandex and AI Overviews: extractable passages, sourced facts, Schema, AI crawler access, checklist.
- [NS DNS Record: What It Is and How to Check It](https://enterno.io/en/articles/ns-record-explained): What an NS DNS record is and how zone delegation works: primary and secondary servers, glue records, checking NS with dig and WHOIS, changing NS servers and common mistakes.
- [TXT DNS Record: What It's For and How to Add It](https://enterno.io/en/articles/txt-record-explained): What a TXT DNS record is and why you need it (verification, SPF, DKIM, DMARC, BIMI), the 255-character limit, quoting syntax, and how to add and verify it.
- [SRV DNS Record: Syntax, Priority, Examples](https://enterno.io/en/articles/srv-record-explained): What an SRV DNS record is per RFC 2782: the _service._proto.name syntax, how priority and weight work, examples for SIP and XMPP, and common mistakes.
- [SOA DNS Record: Fields, Serial, and How to Read It](https://enterno.io/en/articles/soa-record-explained): Understand the SOA DNS record: all fields (MNAME, RNAME, serial, refresh, retry, expire, minimum), the YYYYMMDDnn serial format, zone transfers, and dig checks.
- [SMTP Errors 550 and 554: Causes and Fixes](https://enterno.io/en/articles/smtp-error-550-554-fix): SMTP errors 550 and 554 explained: what the codes mean, enhanced 5.x.x status codes, rejection causes (SPF, DKIM, DMARC, blacklist) and how to fix them fast.
- [Traceroute: How to Read the Output and Find Problems](https://enterno.io/en/articles/traceroute-how-to-read): How to read traceroute and tracert output: hop numbers, RTT, asterisks. Learn to pinpoint where the problem is — your network, your ISP, or the server.
- [Email Ports Explained: 25, 465, 587, 110, 143, 993, 995](https://enterno.io/en/articles/email-ports-25-465-587-explained): Understand email ports: SMTP (25, 587, 465), POP3 (110, 995), IMAP (143, 993), STARTTLS vs implicit TLS, why port 25 is blocked, and which port to use.
- [What Is MTU: Fragmentation and How to Fix Issues](https://enterno.io/en/articles/mtu-explained): What MTU and MSS are, typical values (1500, 1492, jumbo 9000), Path MTU Discovery and black holes. How to measure MTU with ping and set it on your OS.
- [CAA DNS Record: Control Which CAs Issue Your SSL](https://enterno.io/en/articles/caa-record-explained): What a CAA DNS record is, why it matters, the issue/issuewild/iodef syntax, and how to check it. Stop unauthorized SSL certificate issuance for your domain.
- [High Ping and Packet Loss: Causes and Fixes](https://enterno.io/en/articles/high-ping-packet-loss-fix): Why ping is high and packets drop, what values are normal, how to measure with ping and traceroute, and how to isolate the problem — your network, ISP, or the server.
- [Fix ERR_CONNECTION_CLOSED: Causes and Solutions](https://enterno.io/en/articles/err-connection-closed-fix): ERR_CONNECTION_CLOSED in Chrome means the server dropped the connection. Causes: TCP RST, antivirus, proxy, MTU, TLS. Fix it as a user and on the server.
- [Your Site Is Blocked in Russia: Owner's Guide](https://enterno.io/en/articles/site-blocked-russia-owner-guide): How a site owner can check a block in Russia, tell it from a hosting or DNS failure, find the grounds in the EAIS register, and file an unblocking appeal to Roskomnadzor.
- [Fix ERR_QUIC_PROTOCOL_ERROR: QUIC / HTTP3 Error Guide](https://enterno.io/en/articles/err-quic-protocol-error-fix): What ERR_QUIC_PROTOCOL_ERROR means in Chrome, why QUIC/HTTP3 fails, and how to fix it: disable the QUIC flag, check UDP/443, the Alt-Svc header and server bugs.
- [HTTP 451 Unavailable For Legal Reasons Explained](https://enterno.io/en/articles/http-451-unavailable-legal-reasons): What HTTP 451 means under RFC 7725, how it differs from 403, who returns it, its link to Roskomnadzor blocks, and how to serve 451 with the blocked-by header.
- [TSPU, DPI and Traffic Throttling: How It Works and How to Check](https://enterno.io/en/articles/tspu-dpi-throttling-explained): How TSPU and DPI work, how throttling differs from a full block and an outage, the signs that reveal it, and how to run external availability diagnostics.
- [Fix ERR_SSL_VERSION_OR_CIPHER_MISMATCH: Causes and Solutions](https://enterno.io/en/articles/err-ssl-version-or-cipher-mismatch-fix): ERR_SSL_VERSION_OR_CIPHER_MISMATCH explained and fixed: outdated TLS 1.0/1.1, weak ciphers, nginx and Apache config, and openssl and curl verification steps.
- [Fix ERR_EMPTY_RESPONSE: Server Returned No Data](https://enterno.io/en/articles/err-empty-response-fix): ERR_EMPTY_RESPONSE means the server closed the connection with no reply. Learn the causes — backend crash, proxy timeout, WAF — and fix it via nginx and php-fpm logs.
- [How Roskomnadzor Blocks Websites: EAIS, TSPU, DPI](https://enterno.io/en/articles/rkn-how-blocking-works): How Roskomnadzor blocks websites: the EAIS register, TSPU under Law 90-FZ, DPI filtering, and the difference between IP, DNS and DPI blocking.
- [HTTP 400 Bad Request: Causes and Fixes](https://enterno.io/en/articles/http-400-bad-request-fix): What HTTP 400 Bad Request means, why it happens (broken cookies, JSON, long URLs) and how to fix it as a user and as a developer, with clear examples.
- [HTTP 401 Unauthorized: Causes and Fixes](https://enterno.io/en/articles/http-401-unauthorized-fix): What HTTP 401 Unauthorized means, the role of WWW-Authenticate and Authorization headers, how it differs from 403, and how to fix expired tokens or sessions.
- [Russia's Internet Blocklist in Numbers: 131,000 Blocked Domains Analyzed (2026)](https://enterno.io/en/articles/rkn-blocklist-in-numbers-2026): Enterno.io study: Russia's RKN registry holds 131,361 domains, 70% are international (not .ru), and 19% of the world's top sites are blocked. Data + method.
- [State of Security Headers Across Russia's Top Sites: A 50-Site Study (2026)](https://enterno.io/en/articles/state-of-security-headers-runet-2026): Enterno.io scanned 50 of Russia's top sites: only 2% use CSP, 25% use HSTS, and 57% score an F on security headers — while SSL/TLS hygiene is excellent.
- [Why Emails Bounce: Codes and Causes](https://enterno.io/en/articles/why-emails-bounce-guide): Why emails bounce: SMTP 5xx and 4xx codes explained, the real causes — bad address, reputation block, SPF/DKIM/DMARC failure — and how to fix each one.
- [Transactional Email Deliverability](https://enterno.io/en/articles/transactional-email-deliverability): How to make transactional email land: a dedicated subdomain, SPF/DKIM/DMARC, stream separation, and MX/SMTP monitoring — practical, battle-tested rules.
- [Warming Up a Sending Domain](https://enterno.io/en/articles/warm-up-sending-domain): Warm up a sending domain and IP for email: a day-by-day volume schedule, gradual ramp, starting with your most engaged subscribers, and reputation monitoring.
- [Mail Server Monitoring: MX/SMTP/Blacklist](https://enterno.io/en/articles/mail-server-monitoring): Mail server monitoring: checking MX, SMTP ports 25/465/587, TLS via openssl, blacklist status and alerts. Set up continuous mail monitoring in enterno.io.
- [Emails Going to Spam: Causes and Fix](https://enterno.io/en/articles/emails-going-to-spam-fix): Why your emails land in spam and how to fix it: SPF, DKIM, DMARC, IP reputation, content and blacklists. Step-by-step domain diagnosis and DNS record checks.
- [How to Get Off an Email Blacklist](https://enterno.io/en/articles/remove-from-email-blacklist): How to check and get off an email blacklist: Spamhaus, Barracuda, SORBS. dig commands for DNSBL, listing causes and the step-by-step delisting process.

## API + Docs
- [API documentation](https://enterno.io/en/api-docs): Interactive REST API docs (v2 + v4)
- [OpenAPI spec](https://enterno.io/.well-known/openapi.yaml): Machine-readable API definition
- [MCP Server Card](https://enterno.io/.well-known/mcp/server-card.json): Anthropic MCP for AI agents
- [A2A Agent Card](https://enterno.io/.well-known/agent-card.json): Google A2A protocol
- [Agent Skills](https://enterno.io/.well-known/agent-skills/index.json): agentskills.io index