TechnitiumSoftware/DnsServer · error · FormatException

No 'healthCheck' was configured for: {appRecordName}

Error message

No 'healthCheck' was configured for: {appRecordName}

What it means

Thrown by FailoverApp Address.ProcessRequestAsync in the A/AAAA branch when the per-record JSON data has no 'healthCheck' property (or it is empty). FailoverApp needs a named health check to decide which backend address is up, so a record without one is considered malformed and throws FormatException. The name in the message identifies which app record is broken.

Source

Thrown at Apps/FailoverApp/Address.cs:177

        public Task<DnsDatagram?> ProcessRequestAsync(DnsDatagram request, IPEndPoint remoteEP, DnsTransportProtocol protocol, bool isRecursionAllowed, string zoneName, string appRecordName, uint appRecordTtl, string appRecordData)
        {
            DnsQuestionRecord question = request.Question[0];

            if (!question.Name.Equals(appRecordName, StringComparison.OrdinalIgnoreCase) && !appRecordName.StartsWith('*'))
                return Task.FromResult<DnsDatagram?>(null);

            switch (question.Type)
            {
                case DnsResourceRecordType.A:
                case DnsResourceRecordType.AAAA:
                    {
                        using JsonDocument jsonDocument = JsonDocument.Parse(appRecordData, _jsonParseOptions);
                        JsonElement jsonAppRecordData = jsonDocument.RootElement;

                        string? healthCheck = jsonAppRecordData.GetPropertyValue("healthCheck", null);
                        if (string.IsNullOrEmpty(healthCheck))
                            throw new FormatException("No 'healthCheck' was configured for: " + appRecordName);

                        Uri? healthCheckUrl = null;

                        if (_healthService!.HealthChecks.TryGetValue(healthCheck, out HealthCheck? hc) && ((hc.Type == HealthCheckType.Https) || (hc.Type == HealthCheckType.Http)) && (hc.Url is null))
                        {
                            //read health check url only for http/https type checks and only when app config does not have an url configured
                            if (jsonAppRecordData.TryGetProperty("healthCheckUrl", out JsonElement jsonHealthCheckUrl) && (jsonHealthCheckUrl.ValueKind == JsonValueKind.String))
                            {
                                healthCheckUrl = new Uri(jsonHealthCheckUrl.GetString()!);
                            }
                            else
                            {
                                if (hc.Type == HealthCheckType.Https)
                                    healthCheckUrl = new Uri("https://" + question.Name);
                                else
                                    healthCheckUrl = new Uri("http://" + question.Name);
                            }
                        }

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Open the FailoverApp record named in the message and add a 'healthCheck' key whose value matches a healthCheck defined in the app config.
2. Confirm the referenced health check exists in the FailoverApp 'healthChecks' section.
3. If you do not want failover behaviour, remove the A/AAAA record from the FailoverApp zone instead of leaving it half-configured.

Example fix

// before
{ "addresses": ["10.0.0.1","10.0.0.2"] }
// after
{ "healthCheck": "web", "addresses": ["10.0.0.1","10.0.0.2"] }

string? hc = jsonAppRecordData.GetPropertyValue("healthCheck", null);
if (string.IsNullOrEmpty(hc))
    throw new ConfigValidationException($"FailoverApp A/AAAA record '{appRecordName}' is missing the required 'healthCheck' field.");

When it happens

Trigger: An A/AAAA app record under FailoverApp whose 'data' JSON lacks the 'healthCheck' key, e.g. {"addresses":["1.2.3.4"]} with no healthCheck reference. This branch is only reached for A/AAAA question types that match the record name.

Common situations: Record created from a template that omitted healthCheck, a health check was deleted but the record still references nothing, or a manual JSON edit dropped the field.

Related errors

• No 'healthCheck' was configured for: {appRecordName}
• Health check type 'ping' is not supported over proxy.
• Failed to add header '{pair.Key}'.
• Local end point group map contains an invalid end point: {lo
• Network group map contains an invalid network address: {netw

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/4c6ee412c53d0b4b. Report an issue: GitHub.