Resolves#42185
## Summary
- Added `flatcar` and `coreos` to `HostLinuxOSs` in
`server/fleet/hosts.go`
- Added both to `HostNeitherDebNorRpmPackageOSs` (neither distro uses
deb or rpm)
- Added both to `HOST_LINUX_PLATFORMS` in
`frontend/interfaces/platform.ts`
- Added test cases in `server/fleet/hosts_test.go`
- Updated platform lists in
`docs/Contributing/product-groups/orchestration/understanding-host-vitals.md`
- Added changelog entry
## Problem
Flatcar Container Linux reports `platform=flatcar` and
`platform_like=coreos` via osquery's `os_version` table. Neither value
is in `HostLinuxOSs`, so `PlatformFromHost("flatcar")` returns `""` and
`RunsForPlatform` skips all Linux-platform-filtered detail queries.
**Symptoms:** Flatcar hosts enroll successfully, appear online, and
respond to live queries. But host details (private IP, disk space, etc.)
are never populated because the detail queries that collect this data
are never sent to the host.
## Context
Flatcar Container Linux is an immutable, container-optimized Linux
distribution (successor to CoreOS Container Linux). We deploy Fleet's
Orbit agent on Flatcar via systemd-sysext and have confirmed that all
osquery tables work correctly — the only gap was this platform string
not being recognized.
This follows the same pattern as prior platform additions: #19011
(tuxedo), #28977 (neon), #34357 (manjaro-arm).
## Changes file
- [x] Changes file added in `changes/`
## Checklist
- [x] Added/updated automated tests
- [x] Manual QA: Verified on Flatcar Container Linux 4459.2.4 with
osquery 5.21.0 and Orbit 1.53.0 — confirmed detail queries work after
patching `HostLinuxOSs` locally
- [x] No database migrations needed
- [x] No endpoint changes
- [x] No backward compatibility concerns (additive change only)
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