INCIDENT REPLAY · 2018-01-24

NHS Wales national data-centre outage (routine firewall change)

A routine firewall change cut access to both NWIS data centres on the morning of 24 Jan 2018. Twenty-six nationally hosted clinical systems (appointments, referrals, test results, GP test requesting) and NHS Wales email were unavailable; at one point at least half of GP practices were affected. Systems were restored by 16:15 and normal operations by 20:00.

A reconstruction, not the operator's network. Hammerhead's authors rebuilt the change from the public record as a small network of their own. No configuration here is the operator's. What follows is what the tool reported on that reconstruction; it is not a claim that the tool would have prevented the outage.

What happened

Who: NHS Wales Informatics Service (NWIS)

The summary at the top is paraphrased from the sources below. Hammerhead's catalog rates its confidence in this account high.

Sources

What the public record says

A routine firewall change cut access to both NWIS data centres on the morning of 24 Jan 2018. Twenty-six nationally hosted clinical systems (appointments, referrals, test results, GP test requesting) and NHS Wales email were unavailable; at one point at least half of GP practices were affected. Systems were restored by 16:15 and normal operations by 20:00.
Who: NHS Wales Informatics Service (NWIS)

What the reconstruction assumed

Decided by Hammerhead's authors to build a network that runs. None of it describes the operator's network.

Everything below is ours.

The family template (tools/replays/gen_family_policy.py): CLINICAL-1 (clinical systems at the hospitals) reaches SHARED-SVC-1 (the service both data centres depend on) through DC-FW-1 (the data-centre firewall the routine change was made on), which filters inbound on its outside interface. The intent file names the flow: clinical systems reach the shared service on tcp/443.
A routine firewall change; the vendor said the rule change itself was not at fault and both data centres shared the dependency. It is modelled as the HTTPS permit being dropped by a change meant to remove SSH; the shared dependency is one service behind one firewall.
Cisco IOS syntax and stand-in address space throughout. A reconstruction, not any operator's configuration.

What Hammerhead said

Each run below is pinned by Hammerhead's replay tests at commit 0ee9732: the verdict and the quoted text are checked against the tool's output, and a run that disagrees fails the test. Reason lines are copied verbatim; a pinned line can be part of a longer reason. Every preflight run uses --max-blast-radius 100, so a change is never counted as caught merely because it touched many devices of a small network.

the change as intended

now: pass

On the reconstruction, hammerhead preflight on a before and an after snapshot with --policy @nhs_wales_2018/policy.yaml --allow-flow-changes returned pass.

First run, before the fixes this batch prompted

first: fail

policy gate: high/critical failure(s) on the 'after' network: clinical systems reach the shared service

Pinned in tests/gauntlet/replays-policy.yaml.

the change as pushed

now: fail

On the reconstruction, hammerhead preflight on a before and an after snapshot with --policy @nhs_wales_2018/policy.yaml --allow-flow-changes returned fail, for the reasons below.

Reason text pinned (some reason contains each line)

policy gate: high/critical failure(s) on the 'after' network: clinical systems reach the shared service
NOT reachable (expected reachable)

First run, before the fixes this batch prompted

first: fail

policy gate: high/critical failure(s) on the 'after' network: clinical systems reach the shared service

Pinned in tests/gauntlet/replays-policy.yaml.

What changed in Hammerhead because of it

From the “What was fixed” table in Hammerhead's replay notes, where each fix landed after a failing test. “Found by” is copied as written; a family name means the batch this incident belongs to.

A flow rule names its protocol and port

Every reachability rule probed TCP/80, so an intent about SIP could not tell the push from the intended change: 7 of 11 intended changes failed on main and the one that dropped only the named port passed. protocol: and dst_port: on the rule; the policy gate's reason now carries the violation, not only the policy's name.

Found by: Required-flow family

Limits

The configurations are a reconstruction. Where the record is silent, the assumptions above fill the gap; if one is wrong, the replay is wrong in that respect.
A verdict describes the reconstructed network's converged state. Timing, traffic load, and anything a device does that no configuration states, such as software reacting to a health check or a firmware bug, are outside it.
A pass is reported as a pass. A fail says a gate fired on the reconstruction, nothing more.
Why the catalog counts this outage as partly in scope: A firewall change was the trigger and both data centres shared the dependency, which a model can show; but the vendor says the rule change itself was benign and the device misbehaved, which no config model predicts.

PUT THE IDEA TO WORK

Green links. Broken checkout.

Chicago payments network. Four routed metro nodes form a ring; the service gateway has a separate handoff. Route checks and ICMP succeed. The primary service transport fails even though an allow rule appears in the service access list. Investigate the two service networks from R2: 10.201.130.0/24 to 10.201.136.0/24 (TCP/443), and 10.201.131.0/24 to 10.201.137.0/24 (TCP/8443).

Physical links · Service health not shown

Link addressing (5)

R1 R210.201.144.0/30
R2 R310.201.145.0/30
R3 R410.201.146.0/30
R4 R110.201.147.0/30
R4 R510.201.148.0/30

Free to practice. No account required.

Imported from docs/replays/nhs-wales-2018/FACTS.md and the replay manifests in the Hammerhead repository at commit 0ee9732.