Guides

How things break, and how to know first: plain guides to each thing hostnag watches, with the check that catches it.

• How to monitor SSL certificate expiry (and why 30 days is the wrong first alert)

  15 Sep 2026

  What an expired certificate breaks, why a 30-day alert is a renewal that already failed, what to check beyond the date, and how to watch every port.

• What happens when a domain expires: the timeline from the date to the drop

  15 Sep 2026

  Expiry day, the renewal grace, 30 days of redemption at a premium, pending delete and the drop: what breaks at each step, and why a calendar entry fails.

• SPF, DKIM and DMARC explained: how to read the records and the mistakes that get mail rejected

  15 Sep 2026

  What SPF, DKIM and DMARC each do, how to read a record term by term, the dig commands, and the mistakes that get legitimate mail rejected.

• How to monitor cron jobs with heartbeats (a ping URL, one curl line and a grace period)

  15 Sep 2026

  A cron job has no address to check, so it checks in instead: a ping URL, curl -fsS on the last line, expected-every and grace, and what a missed job looks like.

• Ping is not uptime monitoring: what ICMP proves, what a TCP connect proves, and why a failure needs a second witness

  15 Sep 2026

  What an ICMP reply proves and what it misses, why a TCP connect on the right port is the first real test, and how a failure is confirmed before anyone is paged.

• How to check a mail server against blocklists (and why "not checkable" is not "clear")

  15 Sep 2026

  What a DNS blocklist is, the four lists that get mail refused, how a listing happens, the dig command to check one, delisting, and why silence is not a pass.

• Certificate transparency explained: the public logs, crt.sh, and the certificate you did not order

  15 Sep 2026

  What the certificate transparency logs are, how crt.sh reads them, why a certificate you did not order matters, and how a daily watch on your domain catches it.

• How to monitor a vendor's status page: five products, one board, and the incident title in every alert

  15 Sep 2026

  Paste a Statuspage, Instatus, Better Stack, incident.io or status.io page: an outage becomes Down on your board, with the incident title and link in the alert.

• How to monitor an office internet link from both ends: a connection from outside, a ping from inside, five verdicts

  15 Sep 2026

  A TCP connect to the gateway from outside plus a ping from a device inside, so a filtered port, a dead line and a new public address each read differently.

• How to monitor private PKI, client, code-signing and S/MIME certificates that no scanner can reach

  15 Sep 2026

  Internal CA, client, code-signing and S/MIME certificates are served on no port: post or paste them to a report URL and get the same expiry ladder.