SSL Certificate Exporter

The blackbox_exporter allows you to test the expiry date of a certificate as part of its HTTP(S) probe - which is great. It doesn't, however, tell you which certificate in the chain is nearing expiry or give you any other information that might be useful when sending alerts.

For instance, there's a definite value in knowing, upon first receiving an alert, if it's a certificate you manage directly or one further up the chain. It's also not always necessarily clear from the address you're polling what kind of certificate renewal you're looking at. Is it a Let's Encrypt, in which case it should be handled by automation? Or your organisation's wildcard? Maybe the domain is managed by a third-party and you need to submit a ticket to get it renewed.

Whatever it is, the SSL exporter gives you visibility over those dimensions at the point at which you receive an alert. It also allows you to produce more meaningful visualisations and consoles.

Building

go build
./ssl_exporter <flags>

Similarly to the blackbox_exporter, visiting http://localhost:9219/probe?target=https://example.com will return certificate metrics for example.com. The ssl_https_connect_success metric indicates if the probe has been successful.

Flags

./ssl_exporter --help

• --tls.insecure: Skip certificate verification (default false). This is insecure but does allow you to collect metrics in the case where a certificate has expired. That being said, I feel that it's more important to catch verification failures than it is to identify an expired certificate, especially as the former includes the latter.
• --web.listen-address: The port (default ":9219").
• --web.metrics-path: The path metrics are exposed under (default "/metrics")
• --web.probe-path: The path the probe endpoint is exposed under (default "/probe")

Metrics

Metrics are exported for each certificate in the chain individually. All of the metrics are labelled with the Issuer's Common Name and the Serial ID, which is pretty much a unique identifier.

I considered having a series for each ssl_cert_subject_alternative_* value but these labels aren't actually very cardinal, considering the most frequently they'll change is probably every three months, which is longer than most metric retention times anyway. Joining them within commas as I've done allows for easy parsing and relabelling.

Metric	Meaning	Labels
ssl_cert_not_after	The date after which the certificate expires. Expressed as a Unix Epoch Time.	issuer_cn, serial_no
ssl_cert_not_before	The date before which the certificate is not valid. Expressed as a Unix Epoch Time.	issuer_cn, serial_no
ssl_cert_subject_common_name	The common name of the certificate. Always has a value of 1	issuer_cn, serial_no, subject_cn
ssl_cert_subject_alternative_dnsnames	The subject alternative names (if any). Always has a value of 1	issuer_cn, serial_no, dnsnames
ssl_cert_subject_alternative_emails	The subject alternative email addresses (if any). Always has a value of 1	issuer_cn, serial_no, emails
ssl_cert_subject_alternative_ips	The subject alternative IP addresses (if any). Always has a value of 1	issuer_cn, serial_no, ips
ssl_https_connect_success	Was the HTTPS connection successful? Boolean.

Prometheus

Configuration

Just like with the blackbox_exporter, you should pass the targets to a single instance of the exporter in a scrape config with a clever bit of relabelling. This allows you to leverage service discovery and keeps configuration centralised to your Prometheus config.

scrape_configs:
  - job_name: 'ssl'
    metrics_path: /probe
    static_configs:
      - targets:
        - https://example.com
        - https://prometheus.io
    relabel_configs:
      - source_labels: [__address__]
        target_label: __param_target
      - source_labels: [__param_target]
        target_label: instance
      - target_label: __address__
        replacement: 127.0.0.1:9219  # SSL exporter.

Example Queries

Certificates that expire within 7 days, with Subject Common Name and Subject Alternative Names joined on:

((ssl_cert_not_after - time() < 86400 * 7) * on (instance,issuer_cn,serial_no) group_left (dnsnames) ssl_cert_subject_alternative_dnsnames) * on (instance,issuer_cn,serial_no) group_left (subject_cn) ssl_cert_subject_common_name

Only return wildcard certificates that are expiring:

((ssl_cert_not_after - time() < 86400 * 7) * on (instance,issuer_cn,serial_no) group_left (subject_cn) ssl_cert_subject_common_name{subject_cn=~"\\*.*"})

Number of certificates in the chain:

count(ssl_cert_subject_common_name) by (instance)

Identify instances that have failed to create a valid SSL connection:

ssl_https_connect_success == 0

Limitations

I've only exported a subset of the information you could extract from a certificate. It would be simple to add more, for instance organisational information, if there's a need.

Acknowledgements

The overall structure and implementation of this exporter is based on the consul_exporter. The probing functionality borrows from the blackbox_exporter.