TechnitiumSoftware/DnsServer · error · Exception

Please specify a valid connection string in…

Error message

Please specify a valid connection string in 'connectionString' parameter.

What it means

QueryLogsPostgreSqlApp requires a PostgreSQL connection string to reach the server. The 'connectionString' config property is read with a null default, and this error fires when it is absent or explicitly null. Without it the app cannot build the NpgsqlDataSource used to persist DNS query logs.

Solutions

Add a non-null 'connectionString' value to dnsApp.config, e.g. 'Host=localhost;Username=postgres;Password=pass;'.
Do NOT include 'Database=' in it (see error 42); the databaseName parameter handles that.
Restart/reload the DNS server app after saving the config so ApplyConfig re-runs.

Example fix

// before
"databaseName": "DnsQueryLogs",
"connectionString": null

// after
"databaseName": "DnsQueryLogs",
"connectionString": "Host=localhost;Username=postgres;Password=pass;"

Defensive patterns

Strategy: validation

Validate before calling

static void EnsurePostgresConnectionString(string? cs)
{
    if (string.IsNullOrWhiteSpace(cs))
        throw new InvalidOperationException(
            "'connectionString' is missing in config. Set a PostgreSQL connection string.");
}

EnsurePostgresConnectionString(connectionString);

Prevention

Provide a config schema/template with connectionString marked required.
Fail fast at app startup with a clear message instead of letting the null reach ApplyConfig.
Use a deployment validation step that checks required config keys.

When it happens

Trigger: ApplyConfig is invoked with a JSON config that omits the 'connectionString' property entirely, or sets it to null. The null-check throws before any database interaction occurs.

Common situations: Fresh install of the QueryLogsPostgreSqlApp where the config template was not filled in; a config edit that deleted the connectionString line; or a deployment script that wrote an empty value.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/3ad34fc165afa04a. Report an issue: GitHub.

Appendix: source

Thrown at Apps/QueryLogsPostgreSqlApp/App.cs:369

            try
            {
                _dnsServer = dnsServer;

                if (config is null)
                    throw new InvalidOperationException();

                using JsonDocument jsonDocument = JsonDocument.Parse(config, _jsonParseOptions);
                JsonElement jsonConfig = jsonDocument.RootElement;

                bool enableLogging = jsonConfig.GetPropertyValue("enableLogging", false);
                int maxQueueSize = jsonConfig.GetPropertyValue("maxQueueSize", 1000000);
                _maxLogDays = jsonConfig.GetPropertyValue("maxLogDays", 0);
                _maxLogRecords = jsonConfig.GetPropertyValue("maxLogRecords", 0);
                _databaseName = jsonConfig.GetPropertyValue("databaseName", "DnsQueryLogs");
                _connectionString = jsonConfig.GetPropertyValue("connectionString", null);

                if (_connectionString is null)
                    throw new Exception("Please specify a valid connection string in 'connectionString' parameter.");

                if (_connectionString.Replace(" ", "").Contains("Database=", StringComparison.OrdinalIgnoreCase))
                    throw new Exception("The 'connectionString' parameter must not define 'Database'. Configure the 'databaseName' parameter above instead.");

                if (!_connectionString.TrimEnd().EndsWith(';'))
                    _connectionString += ";";

                _dataSource = NpgsqlDataSource.Create(_connectionString + $" Database={_databaseName};");

                async Task ApplyConfig()
                {
                    if (enableLogging)
                    {
                        await using (NpgsqlConnection connection = await _dataSource.OpenConnectionAsync())
                        {
                            await using (NpgsqlCommand command = connection.CreateCommand())
                            {
                                command.CommandText = @$"

View on GitHub (pinned to d0484b6c1e)