SSL Certificate Checker
Analyze SSL/TLS certificate validity, dates, and chain details.
What this tool does
This tool checks the SSL/TLS certificate presented by a host and reports validity, issuer details, chain information, and SAN coverage. It helps you identify expiration risks and hostname mismatches before they affect visitors or compliance checks.
Inputs explained
- Hostname: The domain name to test, such as example.com.
- Port: The TLS port to connect to, typically 443.
How it works / Method
The tool opens a TLS connection to the host and retrieves the certificate chain. It checks validity dates, issuer fields, and SAN entries, then calculates days remaining. Results reflect the certificate presented at the time of the request and do not change server configuration.
SANs (Alternative Names)
Example
Input: Hostname: example.com, Port: 443. Expected output: A valid certificate status with issuer information, not-before and not-after dates, days remaining, and a list of SANs that includes the hostname.
Use cases
- Monitor certificate expiration dates for production sites.
- Verify SAN coverage for multiple subdomains.
- Detect chain issues after server or CDN changes.
- Support compliance checks for HTTPS security.
- Validate non-standard TLS ports for internal services.
Limitations & notes
- Only the certificate presented by the server is evaluated at query time.
- This tool does not enumerate supported TLS versions or ciphers.
- Network middleboxes or CDNs can alter the certificate you see.
- Private trust stores may treat certificates differently from public browsers.
Accuracy & Disclaimer
Certificate details reflect the live server response at query time. For compliance or production decisions, confirm results from your hosting provider or certificate authority.
Frequently Asked Questions
What makes an SSL/TLS certificate valid?
A certificate is considered valid when it is within its issued date range, matches the hostname being accessed, and chains to a trusted root through valid intermediate certificates. The server must also present the correct certificate for the requested hostname, typically via SNI. If any part of the chain is missing or untrusted, or if the certificate is expired, most clients will treat it as invalid.
Why might a certificate show as invalid or expired?
Common causes include an expired certificate, a hostname mismatch, or an incomplete chain missing intermediate certificates. Other causes include a revoked certificate, a misconfigured server presenting the wrong certificate, or client trust store issues. This tool checks what the server presents at the time of the request, so a recently updated certificate might still show as invalid if the server cache has not refreshed.
What is a SAN and why is it important?
SAN stands for Subject Alternative Name. It lists the hostnames that the certificate is valid for, such as example.com and www.example.com. Modern clients use SANs for hostname validation rather than the legacy Common Name field. If the hostname you are testing is not included in the SAN list, browsers will report a mismatch even if the certificate is otherwise valid.
What is the certificate chain?
The certificate chain links the server certificate to a trusted root certificate authority through one or more intermediate certificates. Clients validate each link to ensure the server certificate was issued by a trusted CA. If the chain is incomplete or misordered, clients may fail validation. This tool lists chain details so you can confirm the issuer hierarchy and troubleshoot missing intermediates.
Does this tool test TLS versions or cipher suites?
No. This checker focuses on certificate validity and chain details. It does not enumerate supported TLS versions, cipher suites, or protocol hardening settings. For full TLS configuration testing, use a dedicated TLS scanner. You can still use this tool to confirm expiration dates, issuer information, and SAN coverage as part of routine certificate monitoring. Pair it with periodic TLS scans for a complete view.
How often should certificates be renewed?
Renew certificates before their expiration date to avoid service disruption. Public certificate lifetimes are limited by CA and browser policies, so renewal is typically required at least annually. Many teams automate issuance and renewal through ACME or provider dashboards. This tool helps you track remaining days so you can schedule renewals well in advance. Consider alerting 30 to 60 days before expiry.
Sources & references
- RFC 5280: Internet X.509 Public Key Infrastructure - Defines certificate fields and validation concepts.
- RFC 8446: TLS 1.3 - Describes modern TLS protocol behavior.
- CA/Browser Forum Baseline Requirements - Industry requirements for public TLS certificates.
- RFC 9525: Service Identity in TLS - Hostname validation guidance for certificates.