Network MTU specific path failure response

When this happens

Use this when only one page, endpoint, static asset, or api route fails. Do not stop at the screen message; validate the route, rewrite rule, API path, host header, and upstream mapping first.

Symptom checklist

• MTU appears repeatedly in the Network UI or logs.
• DNS, TCP handshake, proxy, VPN, packet loss differs between successful and failed requests.
• The issue appears only after separating server outage from network path issue.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• MTU specifically changes the investigation surface for Network: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The client can resolve the host but cannot complete connection or response transfer.
• Firewall, security group, VPN, or proxy rules differ by source network.
• The service is listening on a different interface or port than the route expects.
• Large packets or long-running responses fail only on specific paths.
• Retries hide the original refusal, reset, or timeout point.
• For the specific path failure case, the first useful clue is the route, rewrite rule, API path, host header, and upstream mapping.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare DNS, TCP handshake, proxy, VPN, packet loss for success and failure in the same window.
3. Test the hypothesis: network failure caused by blocked port, refused listener, packet loss, VPN/proxy path, MTU, VPC rule, or long-running request.
4. Classify this as specific path failure: the route, rewrite rule, API path, host header, and upstream mapping.
5. Capture current values before changing configuration.

First evidence

Treat MTU as a specific path failure case. First collect evidence for the route, rewrite rule, API path, host header, and upstream mapping.

Output examples

Normal output

Working and failing paths use the intended host, status, and upstream.

Failing output

Only one route changes status, redirect target, content type, or upstream.

Output-to-action branches

• Only one page, endpoint, static asset, or API route fails.
  Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
• The working and failing outputs differ.
  Act on the differing layer first: For MTU, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not clear every cache before proving the route mapping.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "MTU" ./logs

dig example.com

curl -Iv https://example.com

traceroute example.com

nc -vz example.com 443

nc -vz example.com 443

curl --connect-timeout 5 -Iv https://example.com

traceroute example.com || tracert example.com

mtr -rw example.com

curl -I https://example.com/failing-path && curl -I https://example.com/working-path

Fix order

1. Record the full MTU message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for network failure caused by blocked port, refused listener, packet loss, VPN/proxy path, MTU, VPC rule, or long-running request.
3. Compare the failing case with a successful case before editing settings.
4. If this is the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For MTU, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Test DNS, TCP connection, TLS, and HTTP separately.
• Compare office/VPN and external network results.
• Open only the required port and source range.
• Move long work out of synchronous requests.
• Save traceroute and direct connection output beside the incident.
• For the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.

Evidence links

• curl everything official
• Cloudflare network troubleshooting official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for Network MTU.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change server outage from network path issue results.
• Do not trust only the Network UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for DNS, TCP handshake, proxy, VPN, packet loss.
• Add DNS cache, proxy bypass, MTU, firewall ports to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Network timeout first triage response
• Network connection refused first triage response
• Network connection reset first triage response
• Network TLS handshake first triage response
• Network proxy first triage response
• Network VPN first triage response
• Network MTU first triage response
• Network DNS cache first triage response