TechnitiumSoftware/DnsServer · error · Exception

Please specify a valid connection string in 'connectionStrin

Error message

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

What it means

QueryLogsSqlServerApp needs a SQL Server connection string to operate. The 'connectionString' property is read with a null default; this error fires when it is missing or null, before any SqlConnection is constructed. The app cannot persist DNS query logs without server coordinates.

Source

Thrown at Apps/QueryLogsSqlServerApp/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.Contains("Initial Catalog", StringComparison.OrdinalIgnoreCase))
                    throw new Exception("The 'connectionString' parameter must not define 'Initial Catalog'. Configure the 'databaseName' parameter above instead.");

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

                async Task ApplyConfig()
                {
                    if (enableLogging)
                    {
                        await using (SqlConnection connection = new SqlConnection(_connectionString))
                        {
                            await connection.OpenAsync();

                            await using (SqlCommand command = connection.CreateCommand())
                            {
                                command.CommandText = @$"

View on GitHub (pinned to d0484b6c1e)

Solutions

Populate 'connectionString' in dnsApp.config with server/auth info, e.g. 'Server=localhost;User Id=sa;Password=pass;'.
Do NOT add 'Initial Catalog' (see error 44); use 'databaseName' for the schema.
Restart the DNS server/app so ApplyConfig reloads the corrected config.

Example fix

// before
"connectionString": null

// after
"connectionString": "Server=localhost;User Id=sa;Password=pass;TrustServerCertificate=true;"

Defensive patterns

Strategy: validation

Validate before calling

static void EnsureSqlServerConnectionString(string? cs)
{
    if (string.IsNullOrWhiteSpace(cs))
        throw new InvalidOperationException(
            "'connectionString' is missing. Set a SQL Server connection string.");
}

EnsureSqlServerConnectionString(connectionString);

Prevention

Mark connectionString required in config templates.
Validate config presence at startup with explicit messages.
Add a smoke-test that loads config and asserts required keys.

When it happens

Trigger: ApplyConfig is called with a JSON config lacking the 'connectionString' property or with it set to null. The null guard throws immediately.

Common situations: Newly added SqlServer query-log app whose config was never populated; a manual config edit that removed the line; or a templating/CI step that emitted an empty connectionString.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/9f4ed1bf4b879fd4. Report an issue: GitHub.