SSL_ERROR_RX_UNKNOWN_RECORD_TYPE

Enterno.io Editorial Team

By Anatoly Oshmanovsky · Updated Apr 18, 2026

Key idea:

SSL_ERROR_RX_UNKNOWN_RECORD_TYPE — Firefox received a TLS record with unknown content type (not 20, 21, 22, 23, 24). Frequent: MITM proxy inserting garbage, HTTP error page served on an HTTPS port (80 vs 443 mix-up), captive portal interception, or broken load balancer. Fix: confirm the server actually speaks TLS on 443.

Below: causes, fixes, FAQ.

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Common Causes

HTTP server listens on 443 (nginx misconfig)
Captive portal (hotel Wi-Fi) intercepts and returns HTML
MITM proxy with broken TLS handling
Broken load balancer forwards non-TLS
nginx stream module routes wrong (TCP pass-through broken)

Step-by-Step Fix

openssl s_client -connect host:443 — should show TLS handshake
curl -vI https://host — HTTP/2 200 ok?
Check nginx config: listen 443 ssl — did you forget ssl?
Test from another network — captive portal issue?
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Related SSL Errors

CertificateExpiry, issuer, domains (SAN)

ChainIntermediate and root CA validation

TLS ProtocolTLS version and cipher suite

VulnerabilitiesHeartbleed, POODLE, weak ciphers

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Common Mistakes

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Expired certificateBrowsers block sites with expired SSL. Set up auto-renewal or monitoring.

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Incomplete certificate chainWithout intermediate CA, some browsers and bots cannot verify the certificate.

❌

Mixed content on HTTPS siteHTTP resources on an HTTPS page — the browser lock icon disappears, reducing trust.

❌

Using TLS 1.0/1.1Legacy TLS versions have known vulnerabilities. Use TLS 1.2+ or 1.3.

❌

Domain mismatch in certificateThe certificate must cover all site domains, including www and subdomains.

Best Practices

✓

Set up auto-renewalLet's Encrypt + certbot with cron — certificate renews automatically every 60-90 days.

✓

Enable HSTSStrict-Transport-Security header forces browsers to always use HTTPS.

✓

Use TLS 1.3TLS 1.3 is faster (1-RTT handshake) and safer — legacy ciphers removed.

✓

Monitor expiration datesCreate a monitor on Enterno.io — get notified well before expiration.

✓

Verify chain after renewalAfter certificate renewal, confirm that intermediate certificates are installed.

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Research

TLS 1.3 Adoption 2026: Growth Pace

Errors

Frequently Asked Questions

Captive portal — how to tell?

From hotel/cafe Wi-Fi — yes. Try mobile hotspot. Error gone → captive portal. Usually a 302 redirect over HTTP.

nginx "listen 443" without ssl?

Fatal config error. nginx serves plain HTTP on 443, client sends TLS, server answers plain HTTP → garbage bytes.

MITM detection?

Cert pinning / HPKP (deprecated) / DANE. Or compare cert fingerprint: <code>openssl s_client | openssl x509 -fingerprint</code>.

TLS record types?

ChangeCipherSpec(20), Alert(21), Handshake(22), ApplicationData(23), Heartbeat(24). Anything else is malformed.