Network Path Diagnose

Network path diagnosis

Measure reachability and path quality (packet loss, latency, jitter) from this host to a user-specified target.

Out of scope: cloud VPC/security-group debugging, whole-host lag checks (diagnose_system_lag), DNS-only failures (dns-diagnose), changing routes or firewall rules.

Rules

1. Need a target first: ask for IP/host/URL if missing; do not invent defaults.
2. Read-only probes: ping/mtr/curl/ss are fine; do not change NICs, routes, or firewall.
3. Stop when enough: stop once the symptom is explained; do not add unrelated probes.
4. Missing tools: if mtr is absent, fall back to ping + traceroute/tracepath and note that in Evidence.
5. Uncontrollable peers: for public DNS and similar hosts you cannot log into, run forward tests only and state that reverse MTR is impossible.

Workflow

Confirm target & symptom → quick reachability → path quality (mtr) → TCP/port follow-up if needed → answer

1. Confirm inputs

2. Quick reachability

getent ahosts <host> | head -5
ping -c 5 -W 2 <host>
curl -sS -o /dev/null -w '%{http_code} time=%{time_total}\n' --connect-timeout 5 <URL>
# or
nc -zv -w 3 <host> <port> 2>&1

• Resolve fails → use or hand off to dns-diagnose; continue only with a user-provided IP.
• ICMP all lost but TCP port works → ICMP likely filtered; switch to TCP path tests; do not call the path “dead”.

3. Path quality (prefer mtr)

command -v mtr
mtr -rwzc 50 <host>
# without mtr:
traceroute -n <host> 2>/dev/null || tracepath -n <host>
ping -c 20 -W 2 <host>

How to read results

4. Follow-up (only if needed)

mtr -rwzc 30 --tcp -P <port> <host>

If the host itself looks suspicious:

ip -br link; ip route | head -20
ss -s

Bidirectional: if the user can run commands on the peer, ask for an mtr from the peer back to this host’s reachable address; otherwise note forward-only.

Output format

## Conclusion
1–2 sentences: reachable or not; loss / latency / DNS / port.

## Evidence
- Target and resolution
- ping / mtr (or fallback) numbers; mark forward-only when applicable
- Missing-tool fallbacks

## Recommendations
- Next checks (local Wi-Fi/cable, another network, peer side, ICMP filtered)
- Do not change config automatically

Reply in the user's language.

Related skills

Example

User: Intermittent timeouts to x.x.x.x on port 443

1. ping + nc -zv x.x.x.x 443
2. mtr -rwzc 50; if mid-hop loss but last hop fine → TCP mtr -P 443
3. Conclude from last-hop and TCP results; suggest peer/egress follow-up