TechnitiumSoftware/DnsServer · error · InvalidDataException

DnsServer block list zone file format is invalid.

Error message

DnsServer block list zone file format is invalid.

What it means

Thrown when loading a block-list zone config file whose first 2 bytes are not the ASCII magic 'BL'. ReadConfigFrom reads a fixed 2-byte format header before the version byte; any other content (truncated file, wrong file, text/corruption) is treated as an invalid block-list format.

Source

Thrown at DnsServerCore/Dns/ZoneManagers/BlockListZoneManager.cs:235

            _dnsServer.LogManager.Write("DNS Server block list config file was saved: " + blockListConfigFile);
        }

        public void SaveConfigFile()
        {
            lock (_saveLock)
            {
                if (_pendingSave)
                    return;

                _pendingSave = true;
                _saveTimer.Change(SAVE_TIMER_INITIAL_INTERVAL, Timeout.Infinite);
            }
        }

        private void ReadConfigFrom(Stream s, bool isConfigTransfer)
        {
            if (Encoding.ASCII.GetString(s.ReadExactly(2)) != "BL") //format
                throw new InvalidDataException("DnsServer block list zone file format is invalid.");

            BinaryReader bR = new BinaryReader(s);

            byte version = bR.ReadByte();
            switch (version)
            {
                case 1:
                    int count = bR.ReadByte();
                    string[] blockListUrls = new string[count];

                    for (int i = 0; i < count; i++)
                        blockListUrls[i] = s.ReadShortString();

                    _blockListUpdateIntervalHours = bR.ReadInt32();

                    DateTime blockListLastUpdatedOn = s.ReadDateTime();
                    if (!isConfigTransfer)
                        _blockListLastUpdatedOn = blockListLastUpdatedOn;

View on GitHub (pinned to d0484b6c1e)

Solutions

  1. Move the offending block-list config file aside and let the server regenerate it (it recreates a valid 'BL' file on next save).
  2. Restore the file from a known-good backup if you need the previously configured block list URLs.
  3. Confirm only the block-list file (not a different feature's .bin) lives in the expected path.

Example fix

// before: config/blockListZone.bin has wrong header -> startup of block list fails
// after: remove so it regenerates
//   mv config/blockListZone.bin config/blockListZone.bin.bak   (then restart server)
Defensive patterns

Strategy: validation

Validate before calling

static bool IsValidBlockListFile(string path)
{
    using var fs = File.OpenRead(path);
    Span<byte> hdr = stackalloc byte[2];
    return fs.Read(hdr) == 2 && hdr[0] == (byte)'B' && hdr[1] == (byte)'L';
}

Try / catch

try { blockListZoneManager.Load(); }
catch (InvalidDataException ex) when (ex.Message.Contains("block list zone file format"))
{ File.Move(blockListPath, blockListPath + ".bad"); blockListZoneManager.Load(); }

Prevention

When it happens

Trigger: Pointing BlockListZoneManager at a non-blocklist file; a truncated or zero-length file; a file from a different feature (e.g. a 'BZ' blocked-zone file); manual edits that clobber the binary header.

Common situations: Backup restore put the wrong .bin into the config folder; disk corruption or partial write during a crash; upgrading across a format change where the old file is mislabeled.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/3fe395d618dea09b. Report an issue: GitHub.