LINUX: OPENSSL Command To Check the Validity and Expiration Of Certificate (Certs)

Below command can be used.

sudo openssl x509 -in <cert> --text

Below is sample output of above command.

sudo openssl x509 -in apiserver.crt --text

Certificate:

    Data:

        Version: 3 (0x2)

        Serial Number: XXXXXXX (XXXXXXX)

        Signature Algorithm: sha256WithRSAEncryption

        Issuer: CN = kubernetes

        Validity

            Not Before: Jul 26 16:04:29 2024 GMT

            Not After : Jul 26 16:09:29 2025 GMT

        Subject: CN = kube-apiserver

        Subject Public Key Info:

            Public Key Algorithm: rsaEncryption

                RSA Public-Key: (2048 bit)

                Modulus:

                    XXXXXXX

                Exponent: 65537 (0x10001)

        X509v3 extensions:

            X509v3 Key Usage: critical

                Digital Signature, Key Encipherment

            X509v3 Extended Key Usage: 

                TLS Web Server Authentication

            X509v3 Basic Constraints: critical

                CA:FALSE

            X509v3 Authority Key Identifier: 

                XXXXXXX

            X509v3 Subject Alternative Name: 

                XXXXXXX

    Signature Algorithm: sha256WithRSAEncryption

        XXXXXXX

-----BEGIN CERTIFICATE-----

        XXXXXXX

-----END CERTIFICATE-----