Browser Error Codes | ERR_SSL, 502, ERR_NAME_NOT_RESOLVED

Look up browser and HTTP error codes - ERR_SSL_VERSION_OR_CIPHER_MISMATCH, 502 Bad Gateway, ERR_NAME_NOT_RESOLVED, and more. See causes and step-by-step fixes. Designed specifically for developers and professionals, this free online networking runs entirely in your browser to process tasks quickly and securely. Enjoy unlimited usage with no data tracking and no registration required.

Also known as: browser error codes · err ssl version or cipher mismatch fix · 502 bad gateway · 502 bad gateway meaning · err name not resolved fix · dns probe finished nxdomain · http error codes list · err connection refused fix · http error codes

encryption tools

networking

developer utilities

ERR_SSL_VERSION_OR_CIPHER_MISMATCH

SSL

Rank ~7.2

SSL Version or Cipher Mismatch

The browser and server could not agree on a secure encryption protocol. Usually caused by an outdated TLS version or unsupported cipher suite on the server.

Common Causes

•Server only supports TLS 1.0 or 1.1 (disabled in modern browsers)
•Misconfigured or outdated cipher suites on the server
•Antivirus or corporate proxy intercepting the SSL connection
•Expired or self-signed certificate using weak algorithms

How to Fix

✓Update server to support TLS 1.2 or TLS 1.3
✓Reconfigure cipher suites to modern standards (AES-256-GCM, ChaCha20-Poly1305)
✓Temporarily disable antivirus SSL scanning to test
✓Renew the certificate and verify the full chain is installed

ERR_CERT_AUTHORITY_INVALID

SSL

Certificate Authority Invalid

The SSL certificate was issued by a certificate authority the browser does not trust.

Common Causes

•Self-signed certificate used in production
•Corporate proxy or security appliance replacing the certificate
•Intermediate certificate missing from the server chain
•Root CA not installed on the client device

How to Fix

✓Replace self-signed cert with one from a trusted CA (Let's Encrypt is free)
✓Install the corporate root CA certificate on the device
✓Ensure the full certificate chain including intermediates is served

ERR_CERT_COMMON_NAME_INVALID

SSL

Certificate Name Mismatch

The SSL certificate is valid but was not issued for the domain being visited.

Common Causes

•Cert issued for www.example.com but visiting example.com (or vice versa)
•Wildcard certificate does not cover the specific subdomain
•Wrong certificate installed on the server or CDN edge

How to Fix

✓Install a certificate that exactly matches the domain
✓Use a SAN (Subject Alternative Names) certificate covering all variants
✓Add a redirect from the uncovered variant to the covered one

ERR_CERT_DATE_INVALID

SSL

Certificate Expired

The SSL certificate's validity period has ended. Browsers block access to prevent MITM attacks using stale certificates.

Common Causes

•Certificate was not renewed before its expiry date
•System clock on the server or client is set incorrectly
•Auto-renewal cron job (certbot) failed silently

How to Fix

✓Renew the SSL certificate immediately using your CA or certbot
✓Verify and correct the system clock (correct date/time/timezone)
✓Set up certbot --deploy-hook to reload the web server after renewal

NET::ERR_CERT_REVOKED

SSL

Certificate Revoked

The SSL certificate has been explicitly revoked by the issuing Certificate Authority.

Common Causes

•Private key was compromised and CA revoked the cert
•Domain ownership was transferred
•Certificate was mistakenly revoked

How to Fix

✓Issue and install a brand new SSL certificate
✓Contact your CA to understand why revocation occurred
✓Do not reuse the old private key - generate a fresh key pair

ERR_SSL_PROTOCOL_ERROR

SSL

SSL Protocol Error

A low-level SSL/TLS protocol error occurred. Often a handshake failure due to incompatible protocol versions.

Common Causes

•Server only offers SSLv3 or TLS 1.0 (very old and insecure)
•Server's SSL configuration has syntax errors
•Firewall or load balancer breaking the TLS handshake

How to Fix

✓Upgrade server SSL config to TLS 1.2+ only
✓Test SSL config with SSL Labs (ssllabs.com/ssltest)
✓Check for firewall rules that inspect and modify HTTPS traffic

ERR_NAME_NOT_RESOLVED

DNS

Rank ~3

DNS Name Not Resolved

The browser could not translate the domain name into an IP address. The domain may not exist, or DNS is unreachable.

Common Causes

•Domain name is misspelled in the URL
•Domain does not exist or has expired
•DNS server is unreachable (no internet connection)
•Local DNS cache contains stale or corrupt data

How to Fix

✓Double-check the spelling of the URL
✓Flush DNS cache: ipconfig /flushdns (Windows) or sudo dscacheutil -flushcache (macOS)
✓Try a different DNS server: Google (8.8.8.8) or Cloudflare (1.1.1.1)
✓Check internet connection and router

DNS_PROBE_FINISHED_NXDOMAIN

DNS

Domain Does Not Exist (NXDOMAIN)

DNS lookup completed but the domain was not found. NXDOMAIN = Non-Existent Domain.

Common Causes

•Domain is not registered or has expired
•Typo in the URL
•DNS propagation still in progress after recent changes
•Recently deleted domain

How to Fix

✓Verify the domain name spelling carefully
✓Check domain registration status (use WHOIS)
✓Wait 24-48 hours for DNS propagation after changes
✓Use a DNS lookup tool to verify records exist

DNS_PROBE_FINISHED_BAD_CONFIG

DNS

DNS Bad Configuration

Your local DNS resolver is misconfigured and cannot process DNS queries.

Common Causes

•VPN software modified DNS settings without restoring them
•Network adapter set to use a non-functioning DNS server
•ISP DNS outage

How to Fix

✓Set DNS to 8.8.8.8 (Google) or 1.1.1.1 (Cloudflare) in network settings
✓Disconnect and reconnect your VPN
✓Restart router and modem

DNS_PROBE_FINISHED_NO_INTERNET

DNS

No Internet Connection

DNS probe completed and confirmed there is no internet connectivity at all.

Common Causes

•Wi-Fi or ethernet cable is disconnected
•Router or modem has lost its connection
•ISP outage in your area

How to Fix

✓Check physical cable and Wi-Fi connection
✓Restart your router and modem (unplug for 30 seconds)
✓Contact your ISP if the issue persists

DNS_PROBE_POSSIBLE

DNS

DNS Probe Possible

Chrome suspects a DNS issue. The domain may have inconsistent DNS records or the resolver returned an unusual response.

Common Causes

•DNS records not fully propagated
•Inconsistent responses from different DNS servers
•Captive portal (hotel/airport Wi-Fi) intercepting requests

How to Fix

✓Try using Google DNS (8.8.8.8) or Cloudflare DNS (1.1.1.1)
✓If on public Wi-Fi, open a plain HTTP site to trigger the login portal
✓Wait for DNS propagation to complete

ERR_CONNECTION_REFUSED

CONNECTION

Connection Refused

The server actively rejected the connection. The host is reachable but nothing is listening on the requested port.

Common Causes

•Web server is not running or has crashed
•Wrong port number in the URL
•Firewall blocking the specific port
•Service failed to start (check server logs)

How to Fix

✓Start or restart the web server
✓Verify the correct port is being used
✓Check firewall rules for that port
✓For localhost: run npm run dev, python manage.py runserver, etc.

ERR_CONNECTION_TIMED_OUT

CONNECTION

Connection Timed Out

The connection was attempted but no response arrived within the browser's time limit. Packets may be silently dropped.

Common Causes

•Server is overloaded or unreachable
•Firewall silently dropping packets (not refusing them)
•Network routing issue between client and server
•Server behind NAT without proper port forwarding

How to Fix

✓Check if the server is overloaded and restart if needed
✓Review firewall rules for the destination IP and port
✓Try from a different network to isolate the problem
✓Verify port forwarding rules if behind a NAT router

ERR_NETWORK_CHANGED

CONNECTION

Network Changed

Your network configuration changed while a request was in flight.

Common Causes

•Wi-Fi network switched mid-request
•VPN connected or disconnected
•IP address renewed by DHCP

How to Fix

✓Reload the page once the network is stable
✓Ensure a stable connection before loading heavy pages
✓Disconnect/reconnect VPN to stabilize routing

ERR_INTERNET_DISCONNECTED

CONNECTION

Internet Disconnected

The browser detected that the device has no active internet connection.

Common Causes

•Wi-Fi disconnected or ethernet unplugged
•Network adapter disabled
•Router or modem offline

How to Fix

✓Check Wi-Fi or ethernet status
✓Restart your router/modem
✓Re-enable the network adapter in system settings

ERR_EMPTY_RESPONSE

CONNECTION

Empty Response

The server accepted the connection but returned no data - the response was completely empty.

Common Causes

•Server-side application crashed mid-response
•Server closed the connection before sending headers
•Nginx or Apache misconfiguration returning empty body
•Application threw an uncaught exception and died silently

How to Fix

✓Check server error logs for crashes or exceptions
✓Restart the application server
✓Verify nginx upstream and proxy_pass configuration
✓Add proper error handling to the server application

ERR_TOO_MANY_REDIRECTS

CONNECTION

Too Many Redirects

The browser detected a redirect loop - two or more URLs keep redirecting to each other.

Common Causes

•HTTP to HTTPS redirect plus HTTPS to HTTP redirect creating a loop
•www to non-www and non-www to www simultaneously configured
•Load balancer and application both applying conflicting redirects
•Cookie required for redirect is being blocked

How to Fix

✓Clear browser cookies for the domain and retry
✓Review .htaccess or nginx rewrite rules for conflicting redirects
✓Check that HTTPS is not redirecting back to HTTP anywhere in the chain
✓Disable browser extensions temporarily (some inject redirects)

502 Bad Gateway

HTTP

Rank ~3.8

502 Bad Gateway

The reverse proxy (nginx, load balancer) received an invalid or empty response from the upstream application server.

Common Causes

•Application server (Node, Python, PHP) crashed or was restarted
•Upstream returned malformed HTTP response
•nginx/Apache proxy_pass pointing to wrong address or port
•Deployment in progress - old process stopped before new one started

How to Fix

✓Check and restart the application server process
✓Review nginx/Apache error logs for upstream errors
✓Verify proxy_pass URL and upstream address are correct
✓Ensure zero-downtime deployment (use blue-green or rolling restarts)

502 Bad Gateway Meaning

HTTP

Rank ~2.6

502 Bad Gateway - What It Means

A 502 error means your web server (acting as a gateway) got a bad reply from another server it depends on. You are seeing the proxy's error page, not the app's.

Common Causes

•Your nginx/Apache is fine but the Node/Python/PHP app behind it is down
•Upstream API or microservice is unreachable
•Database connection pool exhausted causing app crash

How to Fix

✓SSH into the server and check: sudo systemctl status your-app
✓Restart the app: sudo systemctl restart your-app
✓Check app logs: sudo journalctl -u your-app -n 50
✓Monitor database connection counts if the app is DB-heavy

503 Service Unavailable

HTTP

503 Service Unavailable

The server is temporarily unable to handle requests, usually due to overload or maintenance.

Common Causes

•Server is overloaded with too many concurrent requests
•Maintenance mode is active
•Resource exhaustion: CPU, RAM, or database connections
•Rate limiting triggered

How to Fix

✓Wait and retry - often transient
✓Check server resource usage (top, htop, df -h)
✓Disable maintenance mode if accidentally left on
✓Scale up server capacity or add caching (Redis, CDN)

504 Gateway Timeout

HTTP

504 Gateway Timeout

The reverse proxy did not receive a timely response from the upstream server. The upstream took too long.

Common Causes

•Upstream application processing request for too long
•Slow or blocking database query
•External API call timing out inside the application
•Network congestion between proxy and upstream

How to Fix

✓Optimize slow database queries and add missing indexes
✓Increase proxy_read_timeout in nginx config
✓Move long-running tasks to background workers (queues)
✓Add timeouts to all external API calls in your application

500 Internal Server Error

HTTP

500 Internal Server Error

Something went wrong on the server. The request was valid but the server encountered an unexpected condition.

Common Causes

•Unhandled exception in the application code
•Missing environment variable or configuration
•Database connection failure
•File permission issue preventing the server from reading a file

How to Fix

✓Check application error logs immediately
✓Reproduce the error locally with DEBUG mode enabled
✓Verify environment variables and config files are present
✓Check file/directory ownership and permissions

404 Not Found

HTTP

404 Not Found

The server could not find the resource at the requested URL.

Common Causes

•URL is incorrect or has a typo
•Resource was deleted or moved without a redirect
•Case sensitivity mismatch on Linux servers
•Server-side routing not configured for that path

How to Fix

✓Check the URL for typos or trailing slashes
✓Add a 301 redirect if the resource was moved
✓Check server routing config (routes.js, .htaccess, nginx.conf)
✓Verify file exists and path casing matches

403 Forbidden

HTTP

403 Forbidden

The server understood the request but refuses to authorize it. You do not have permission.

Common Causes

•Missing authentication or session expired
•File/directory permissions are too restrictive
•IP address is blocked by the server or CDN
•No index file and directory listing is disabled

How to Fix

✓Log in or provide valid API credentials
✓Fix file permissions: chmod 644 file, chmod 755 directory
✓Whitelist your IP in server or CDN firewall settings
✓Add an index.html or configure DirectoryIndex

401 Unauthorized

HTTP

401 Unauthorized

Authentication is required and either not provided or invalid.

Common Causes

•Missing Authorization header in API request
•Expired JWT or session token
•Wrong API key or credentials
•CORS preflight request blocked before auth can be sent

How to Fix

✓Include a valid Authorization: Bearer <token> header
✓Refresh or re-issue the access token
✓Verify API key is correct and active in your dashboard
✓Handle token expiry in your app and refresh automatically

429 Too Many Requests

HTTP

429 Too Many Requests

You have sent too many requests in a given time period. The server is rate-limiting your client.

Common Causes

•API rate limit exceeded
•Bot or scraper hitting the server too fast
•Retry logic in code not implementing exponential backoff
•Shared IP address flagged due to other users on the same network

How to Fix

✓Implement exponential backoff and jitter in retry logic
✓Check Retry-After response header for wait time
✓Cache API responses to reduce request frequency
✓Upgrade API plan for higher rate limits

ERR_BLOCKED_BY_CLIENT

BROWSER

Blocked by Client (Ad Blocker)

A browser extension (usually an ad blocker) blocked the request before it was sent.

Common Causes

•Ad blocker or privacy extension filtering the URL
•URL matched a known ad/tracking domain blocklist
•uBlock Origin, AdBlock Plus, or Brave Shields active

How to Fix

✓Temporarily disable the ad blocker and retry
✓Add the site to the ad blocker's allowlist
✓Check browser console for which extension is blocking

ERR_CACHE_MISS

BROWSER

Confirm Form Resubmission

The page requires data from a previous form submission that is no longer cached. Refreshing would resubmit the form.

Common Causes

•User refreshed a page that was the result of a POST request
•Browser back/forward navigation to a non-cacheable POST response

How to Fix

✓Use the Post/Redirect/Get (PRG) pattern on the server side
✓After form submission, redirect to a GET page instead of rendering directly
✓Inform users to navigate via the UI rather than refreshing after submit

CORS Error (blocked by CORS policy)

BROWSER

CORS Policy Blocked Request

The browser blocked a cross-origin request because the server did not include the correct CORS headers.

Common Causes

•API server missing Access-Control-Allow-Origin header
•Requested origin not whitelisted in server CORS config
•Preflight OPTIONS request not handled by the server
•Credentials sent without Access-Control-Allow-Credentials: true

How to Fix

✓Add Access-Control-Allow-Origin: * (or specific origin) to server responses
✓Handle OPTIONS preflight requests on all API routes
✓Use a CORS middleware (cors in Node.js, django-cors-headers in Django)
✓For development: use a local proxy or browser extension like CORS Unblock

Mixed Content Error

BROWSER

Mixed Content Blocked

An HTTPS page tried to load a resource (image, script, API) over HTTP. Browsers block this to prevent downgrade attacks.

Common Causes

•Hardcoded http:// URLs in HTML, CSS, or JavaScript
•Third-party embed or widget loading over HTTP
•API endpoint using HTTP instead of HTTPS
•Legacy CMS or plugin with absolute HTTP URLs

How to Fix

✓Change all resource URLs from http:// to https:// or use protocol-relative //
✓Enable HSTS to force all sub-resources to HTTPS
✓Scan with a tool like Why No Padlock to find all mixed content
✓Update CMS site URL settings to https://

About Browser Error Codes

A searchable reference for browser error codes and HTTP errors. Look up ERR_SSL_VERSION_OR_CIPHER_MISMATCH, 502 Bad Gateway, ERR_NAME_NOT_RESOLVED, CORS errors, and 25+ other common errors - with causes and step-by-step fixes.

Key Features

25+ browser and HTTP error codes with full explanations
Filter by SSL/TLS, DNS, Connection, HTTP, and Browser categories
Search across error codes, descriptions, and causes
Search ranking shown for high-traffic error codes
Step-by-step fix instructions with real commands

Use Cases

Diagnose ERR_SSL_VERSION_OR_CIPHER_MISMATCH on a production server
Understand what 502 Bad Gateway means and how to resolve it
Debug ERR_NAME_NOT_RESOLVED after a DNS migration
Fix CORS errors in a React or Vue frontend hitting an API
Quick reference for developers troubleshooting production incidents

Related Keywords

browser error codes

err_ssl_version_or_cipher_mismatch

502 bad gateway

502 bad gateway meaning

err_name_not_resolved

dns_probe_finished_nxdomain

http error codes

chrome error codes

err_connection_refused

cors error

Privacy & Security

All processing happens locally in your browser. Your data never leaves your computer, ensuring complete privacy and security.

Liked this tool?

DevToolLab is free to use. If this networking saved you some time today, please consider supporting our work.

Buy us a coffee ☕

Frequently asked questions

What is Browser Error Codes used for?

Look up browser and HTTP error codes - ERR_SSL_VERSION_OR_CIPHER_MISMATCH, 502 Bad Gateway, ERR_NAME_NOT_RESOLVED, and more. See causes and step-by-step fixes.

Is Browser Error Codes free to use?

Yes. Browser Error Codes is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use Browser Error Codes?

No. Browser Error Codes works immediately in your browser with no signup, login, or email address required.

Related Tools

HTTP Status Checker | URL Response Checker

Free URL status and HTTP response checker. See the status code, response headers, every 301/302 redirect hop and the response time for any link. No signup.

api and http tools

url and link tools

Webhook Receiver & Inspector | Test HTTP Webhooks

Generate a unique URL to easily test, receive and inspect HTTP webhooks in real-time. View request headers, query parameters, JSON payloads, and more.

json tools

api and http tools

Subnet Calculator | IPv4 CIDR Network Calculator Online

Subnet calculator computes IPv4 network masks, broadcast addresses, and host ranges from any IP and CIDR prefix - fast, free, and works entirely in your browser.

network tools

networking

More Tools You Might Like

Pastebin Online | Share Code & Text Snippets Free

Popular

Share code snippets, logs, and text with a shareable link. Syntax highlighting for 30+ languages, expiration controls, and password protection for Pro users.

text tools

password and key generators

math calculators

utilities

Use Tool

URL Tracker & Link Shortener | Track Clicks

Popular

Create short, trackable URLs with detailed click analytics. Track visitor location, devices, referrers, and more. Password protection & QR codes included.

text tools

password and key generators

url and link tools

networking

Use Tool

Webhook Receiver & Inspector | Test HTTP Webhooks

Popular

Generate a unique URL to easily test, receive and inspect HTTP webhooks in real-time. View request headers, query parameters, JSON payloads, and more.