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To check an HTTPS certificate from the command line, connect to the exact host and port, send the hostname with SNI, and require certificate verification. With OpenSSL, run:
openssl s_client -connect example.com:443 -servername example.com -verify_return_error </dev/nullReplace example.com with the hostname you want to test. A successful TLS handshake alone does not prove that the certificate is trusted or matches the hostname; check the verification result and certificate details as well.
What an SSL check tests
“SSL check” is the familiar term, but modern HTTPS uses TLS. A useful check tests the live TLS endpoint—not just a certificate file—and separates several questions:
- Can the client reach the host and complete a TLS handshake?
- Does the server present a certificate that is within its validity dates and chains to a trusted root?
- Does the certificate cover the hostname requested?
- Which TLS protocol and cipher did the server negotiate?
OpenSSL describes s_client as a generic SSL/TLS client that connects to a remote host. It is a diagnostic tool, so make verification failures explicit when using it to assess whether a normal client should trust the endpoint.
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Run a basic HTTPS certificate check with OpenSSL
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Open a terminal with OpenSSL installed.
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Run the command, replacing the example hostname with the exact DNS name used in the URL:
openssl s_client -connect example.com:443 -servername example.com -verify_return_error </dev/null -
Read the verification result and the handshake output. Check the peer certificate, certificate chain, negotiated protocol, and cipher. With
-verify_return_error, verification errors are returned rather than silently allowing the diagnostic session to proceed as if validation succeeded.Rank #2
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-connect specifies the destination host and port. -servername sends the DNS hostname using Server Name Indication (SNI). This matters when multiple HTTPS sites share an IP address: without the intended SNI name, the server may present a certificate for a different virtual host.
Inspect the certificate fields and names
To print the subject, issuer, validity dates, and Subject Alternative Name (SAN) entries from the server’s presented certificate, use:
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openssl s_client -connect example.com:443 -servername example.com -showcerts </dev/null 2>/dev/null | openssl x509 -noout -subject -issuer -dates -ext subjectAltNameThe SAN list is the key place to check whether the requested DNS name is covered. Certificate verification also checks the chain against the client’s trusted roots and the intended server purpose. OpenSSL’s guidance treats certificate presentation, trusted-chain validation, and hostname matching as distinct checks. Its hostname-verification options describe matching a hostname against a DNS name in SAN or, where applicable, the Common Name.
For an IP-based destination serving a name-based site, connect to the IP but supply the intended DNS name with -servername; the certificate still needs to cover the DNS name the client is meant to use. For protocols such as SMTP or LDAP that begin without TLS, use the appropriate OpenSSL -starttls protocol option rather than treating the service as HTTPS.
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How to interpret common SSL check errors
| Result or symptom | What it usually indicates | What to check next |
|---|---|---|
| Certificate expired or not yet valid | The current time falls outside the certificate’s notBefore and notAfter range. |
Renew or correct the certificate served by the endpoint, and check the server’s clock. |
| Unable to get local issuer or incomplete chain | The client cannot build a trusted path, often because an intermediate certificate is missing or its trust store lacks the relevant root. | Confirm the server sends the required intermediate certificates and that the client uses the expected trust store. |
| Hostname mismatch | The requested name is not covered by the certificate’s SAN (or applicable Common Name). | Serve a certificate containing the requested DNS name, or correct the URL or DNS configuration. |
| Unexpected certificate on a shared IP | The server may have selected a different virtual host because the request used the wrong or no SNI name. | Repeat the check with the exact intended hostname in -servername and inspect the virtual-host configuration. |
| Protocol or cipher negotiation failure | The client and server may have no mutually supported TLS version or cipher under their current policies. | Compare the client’s and server’s TLS settings and supported versions. |
| Handshake completes but verification fails | Negotiation succeeded, but identity or trust validation did not. | Treat the check as failed for an ordinary public HTTPS client; a completed handshake is not proof of a valid identity. |
OpenSSL notes that versions before 1.1.0 did not perform hostname verification automatically. Legacy scripts should explicitly check the hostname; current automation should also make hostname verification and verification-error handling explicit.
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Choose the right kind of check
- Local command or hosted scanner: a local check is scriptable and keeps the target within your environment; a hosted scanner tests from an external vantage point.
- Certificate inspection or live handshake: parsing a certificate reveals its fields, while a handshake test also checks reachability, SNI behavior, and negotiated protocol and cipher.
- One-time diagnosis or monitoring: a one-time command shows the endpoint’s current state. Monitoring is needed to catch later expiry or configuration drift.
- Public trust or private CA: an internal certificate may be trusted by an organisation’s private CA while failing validation for clients that rely on public trust roots.
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