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The reliable way to monitor SSL certificate expiry is to check every certificate actually presented by your public and internal endpoints on a schedule, alert before the validity window closes, and separately watch for unexpected certificate issuance. For a small estate, a hosted certificate-monitoring service is usually fastest. For controlled environments that already run Prometheus, the Prometheus SSL Exporter provides a self-hosted route.
“SSL” remains the term people use, but modern certificates protect TLS connections. An expiry monitor helps you avoid outages; it does not automatically renew a certificate unless the selected product explicitly documents renewal.
What an SSL certificate expiry monitor should do
A useful monitor connects to each target in the same way a client does, reads the presented certificate and evaluates its validity dates. Good implementations also verify hostname matching, trust-chain or configuration problems, and whether the endpoint is reachable from an appropriate network location.
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- Configuration checks: detect a certificate that is expired, not yet valid, mismatched to the hostname or otherwise misconfigured.
- Coverage of real endpoints: include every public hostname, load-balancer listener, reverse proxy, SMTP service and internal name that clients depend on.
- Issuance awareness: certificate-transparency or similar monitoring can help identify unexpected certificates. Let’s Encrypt says monitoring TLS certificate status is helpful for many subscribers and lists expiry and issuance monitoring as useful capabilities.
- Actionable delivery: alerts should reach the team that can replace or renew the certificate, not only a shared dashboard.
Monitoring is not the same as inventory. Keep an authoritative list of domains, owners, environments and renewal methods, then use monitoring to validate what is live.
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Hosted certificate-monitoring services
A hosted service removes most scheduler, storage and alert-routing work. Let’s Encrypt’s “Monitoring Service Options” page (last updated July 13, 2026) lists Red Sift Certificates, UptimeRobot, Datadog SSL Monitoring, TrackSSL, Host-Tracker, HeyOnCall self-hosted scripts, CertKit, CertObserver and Chill SSL. Let’s Encrypt states that these services are unaffiliated with ISRG and that ISRG does not endorse or guarantee their safety, reliability or effectiveness.
Let’s Encrypt specifically attributes a free allowance of up to 250 certificates to Red Sift Certificates Lite (formerly Hardenize)—a figure attributed to Let’s Encrypt in 2026, not a general limit for other providers. Check each vendor’s current documentation for limits, pricing, alert channels and supported certificate sources before committing.
When hosted monitoring fits
- You need coverage running quickly without operating another service.
- Your team wants email, chat or incident integrations managed by the provider.
- Endpoints are distributed across clouds, regions or networks that a hosted probe can reach.
- You need a central inventory for many domains and owners.
Questions to verify before choosing one
- Does it test only public HTTPS, or can it reach private hosts and non-HTTP services such as SMTP?
- Can it check every certificate in a chain, or only the leaf certificate?
- Which probe locations and network paths are available?
- Can you set several thresholds, maintenance windows and per-endpoint contacts?
- Does it watch certificate issuance as well as expiry?
- What happens when a probe cannot connect: is that an alert, a retry, or an unreported gap?
- How are credentials, private endpoints and certificate data protected?
Datadog SSL monitoring
Datadog documents SSL API tests that monitor public or internal hosts from multiple locations and detect certificates nearing expiry or misconfiguration: Datadog SSL Monitoring. This is a strong fit when your organization already operates Datadog and wants certificate checks alongside uptime, logs and incident workflows. Verify current plan requirements and pricing in Datadog’s documentation; the cited material does not establish a universal price or alert policy.
Datadog also provides an installed Agent TLS check, documented in its TLS Integration. The Agent check supports TCP and verifies only leaf or end-user certificates. It does not verify intermediate or root certificates, so it cannot replace a full chain-validation strategy where intermediates matter.
API test or Agent check?
| Choice | Best fit | Important boundary |
|---|---|---|
| SSL API test | Public or internal hosts checked from multiple locations | Confirm network access, plan requirements, locations and current alert options |
| Agent TLS check | Services reachable from a host running the Datadog Agent | TCP-based and leaf-certificate verification only; intermediates and roots are not checked |
Self-hosted monitoring with Prometheus SSL Exporter
The SSL Exporter for Prometheus scrapes configured HTTPS and SMTP targets, reads the presented certificate and exposes validity dates for Prometheus alerting. You operate the exporter, Prometheus, Alertmanager and their storage, but retain control over network placement and data.
Basic workflow
- Run the exporter where it can reach the target networks.
- Configure HTTPS or SMTP targets and the exporter’s scrape endpoint.
- Add the exporter to Prometheus as a scrape target.
- Create alerts for certificate expiry and scrape failures.
- Route alerts through Alertmanager to the responsible team and test the complete path.
Illustrative Prometheus configuration
The exact metric names and configuration keys depend on the exporter version you deploy; use the project’s current README as the authority. A typical Prometheus job uses HTTP parameters to pass a target through the exporter:
scrape_configs:
- job_name: ssl_exporter
metrics_path: /probe
params:
target: [https://example.com:443]
static_configs:
- targets: [ssl-exporter:9219]After confirming the exporter’s exposed metrics, create an alert using its certificate-valid-until metric and a seconds-to-expiry expression appropriate to your version. Do not copy an unverified metric name into production; inspect /metrics, then test an alert with a deliberately short threshold in a non-production target.
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- Strength: private endpoints can be checked from inside the network, and alerting can use your existing Prometheus policies.
- Cost: you maintain upgrades, target configuration, persistence, high availability and secret handling.
- Risk: a failed exporter or Prometheus server can create a monitoring blind spot. Monitor the monitor with a separate health check.
- Coverage: include SMTP and every listener, not just the website’s main HTTPS URL.
DIY checks for diagnosis and scheduled jobs
Command-line checks are useful for troubleshooting and for a small internal script, but a one-off command is not a monitoring system until it has scheduling, state, alert delivery and failure handling.
Inspect a live certificate with OpenSSL
echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | openssl x509 -noout -subject -issuer -dates -ext subjectAltNameThe -servername option matters on shared hosting: without SNI, you may inspect the wrong certificate. Compare the hostname with the certificate’s SAN entries and check the complete chain separately when diagnosing trust failures.
Simple expiry calculation
END_DATE=$(echo | openssl s_client -connect example.com:443 -servername example.com 2>/dev/null | openssl x509 -noout -enddate | cut -d= -f2)
END_EPOCH=$(date -d "$END_DATE" +%s)
NOW_EPOCH=$(date +%s)
DAYS_LEFT=$(( (END_EPOCH - NOW_EPOCH) / 86400 ))
printf 'example.com: %s days remainingn' "$DAYS_LEFT"GNU date -d is not portable to every operating system. In production, use a language library that parses certificate dates, define the timezone behavior, and return a non-zero exit code when the threshold is breached. Always add connection timeouts and retry limits so a hung endpoint cannot block the whole job.
Scheduling and ownership
- Run checks at least daily; use a shorter interval for high-risk or frequently changing services.
- Alert at more than one threshold, for example 30, 14 and 7 days, with a separate expired state.
- Attach an owner, renewal runbook and replacement method to every target.
- Suppress alerts during a documented maintenance window, not by deleting the target.
- Test with a staging certificate or a temporary low-threshold rule so notification failures are found early.
How to choose an approach
| Requirement | Hosted service | Datadog | Prometheus SSL Exporter |
|---|---|---|---|
| Fastest deployment | Usually strongest | Fast if Datadog is already deployed | Requires operating exporter and Prometheus |
| Private-network checks | Verify provider connectivity or agent options | API tests support internal hosts; Agent checks run from your network | Place exporter inside the required network |
| Multiple probe locations | Verify current vendor plan | Documented for SSL API tests | You operate the locations |
| Leaf versus full chain | Verify per service | Agent check is leaf-only | Verify exporter behavior and validate chain separately where required |
| Operations burden | Lower platform maintenance | Low if already standardized | Higher: upgrades, storage, alerting and availability |
| Current price comparison | Not established uniformly; check vendor pages | Check Datadog plan requirements | Software is self-hosted; infrastructure and operations are your cost |
For a mixed estate, a practical design is hosted or external checks for public availability plus an internal exporter or Agent for private listeners. Avoid assuming that one successful HTTPS check proves every certificate path is healthy.
Troubleshooting common failures
The monitor reports an unexpected certificate
Check DNS resolution, load-balancer routing and SNI. Probe the exact hostname and port, then compare certificates from each backend or region. A shared IP can legitimately serve different certificates for different names.
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The certificate is valid in a browser but not in the monitor
The monitor may use a different network path, IPv4/IPv6 address, proxy or trust store. Test from the monitor’s location and inspect the served chain. Internal names may require an agent or an exporter inside the network.
Only some clients fail
Look for an incomplete intermediate chain, protocol or cipher incompatibility, or an old client trust store. Datadog’s Agent TLS check does not validate intermediate or root certificates, so use a chain-aware diagnostic as well.
Alerts arrive after the outage
Increase the warning window, confirm the monitor’s schedule and timezone, and verify that alert routing is not muted. Include renewal lead time: replacement may require DNS changes, approval or a deployment window.
Checks time out or create noise
Set bounded connection and read timeouts, retry transient failures, and alert separately on “target unreachable” and “certificate near expiry.” Do not classify every network failure as an expired certificate.
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Does monitoring renew an expiring certificate?
Not by itself. Monitoring reports status and alerts; use a documented renewal system or a provider that explicitly includes renewal.
Should internal certificates be monitored?
Yes, if an internal service can cause an outage. Run the check from a network that can reach the service and protect any private certificate or endpoint data.
How far ahead should an alert fire?
Choose thresholds that match your replacement process. Multiple warnings are safer than relying on a single final-day notification.
Frequently Asked Questions
Can I monitor SMTP certificates as well as HTTPS?
Yes. The Prometheus SSL Exporter project documents scraping HTTPS and SMTP targets; verify the target and protocol configuration for your deployed version.
Recommended Free Tools
Is a certificate-transparency alert a replacement for expiry monitoring?
No. Issuance monitoring helps identify unexpected certificates, while endpoint checks tell you whether the certificate currently served is valid and near expiry.
Quick Recap
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