Batched Ssh Enumeration

Batched SSH Enumeration

A loop that SSHes to a list of hosts — a fleet probe, a deploy fan-out, a shepherd that visits N repos/nodes — has two failure modes that quietly wedge the whole run. Both are avoidable with a few flags and a heartbeat.

Failure 1: one host drops to a password prompt and blocks forever

If key auth fails for any host (key rotated, host re-imaged, account removed), OpenSSH falls back to an interactive password prompt. In an unattended loop there is no human to type, so that one ssh call blocks indefinitely, and every host after it never gets visited. One bad host = total stall.

Force non-interactive, bounded SSH on every call:

ssh -o BatchMode=yes \        # never prompt; fail fast if key auth can't work
    -o ConnectTimeout=10 \    # cap the TCP/handshake wait
    -o StrictHostKeyChecking=accept-new \
    "$host" "$remote_cmd"

• BatchMode=yes turns any prompt (password, passphrase) into an immediate non-zero exit instead of a hang.
• ConnectTimeout bounds connection setup.
• Wrap the whole call in an overall timeout too, because a connection that succeeds but then hangs mid-command is not covered by ConnectTimeout:

timeout 60 ssh -o BatchMode=yes -o ConnectTimeout=10 "$host" "$remote_cmd" \
  || log "host $host unreachable/timed out — skipping"

Treat a failed host as skip-and-continue with a logged warning, never as a reason to block. Isolate each host so one failure cannot take down the sweep.

Failure 2: the watchdog trusts liveness, not progress

A watchdog that only checks "is the process / tmux session / pod still up?" is fooled by a live-but-stuck worker: the process is running, so liveness is green, but it has been blocked on one hung SSH call for hours making zero progress. Liveness ≠ progress.

Watch a heartbeat, not just a pulse. The worker writes a monotonic heartbeat — a timestamp file, a counter, a last_progress_at field — every time it actually advances (per host visited, per item done). The watchdog reads that heartbeat and declares the worker WEDGED when it is stale beyond a threshold, then restarts/alerts:

age=$(( $(date +%s) - $(stat -c %Y "$HEARTBEAT_FILE") ))
if (( age > STALE_THRESHOLD )); then
  alert "worker WEDGED — heartbeat stale ${age}s"; restart_worker
fi

Real incident: a global PR shepherd ran wedged for ~37 hours. The process was alive (liveness checks passed) but stuck on a single un-authable host's interactive prompt; no heartbeat check existed to notice it had stopped making progress.

Checklist

• Every ssh uses BatchMode=yes + ConnectTimeout.
• Every remote call is wrapped in an overall timeout.
• A failed host is skipped-and-logged, never blocking.
• The worker emits a heartbeat on real progress.
• The watchdog flags stale heartbeat as wedged, not just dead process.