# Agent not reporting: the host vanished from Infrastructure
What you see: the host was there, now it is gone or stale. `datadog.agent.up` is missing or not OK.
## Triage order
1. **Is the host alive?** SSH in. If the box is down, this is not a Datadog problem.
2. **Is the agent running?** `systemctl status datadog-agent` (or the platform equivalent). If dead, check why: OOM killer, disk full (the agent buffers to disk), a bad config push that fails validation on restart.
3. **Did the config change?** A recent `datadog.yaml` edit with bad indentation prevents startup. The agent logs the parse error; read them before reinstalling.
4. **Network path to Datadog.** From the host, reach the intake endpoints for your site. Proxy changes and egress firewall rules are the usual silent killers.
5. **Clock skew.** TLS to the intake fails with a wildly wrong system clock. NTP issues are in the troubleshooting index for a reason.
## The flapping variant
Host appears and disappears every few minutes: usually **duplicate hostnames** (two machines reporting as one, each overwriting the other) or the agent restarting in a loop (config error plus a supervisor restarting it). Check the agent uptime in status: if it is always a few minutes old, it is crash-looping.
## Fix and confirm
Fix the underlying cause, restart, then `datadog-agent status` should show the forwarder accepting payloads and the host reappears in Infrastructure within a couple minutes. Set a monitor on `datadog.agent.up` per host so the next one pages you instead of being discovered in a dashboard.