What causes "DnsServer blocked zone file format is invalid." in TechnitiumSoftware/DnsServer?

Move the bad blocked-zone config aside so the server recreates a valid 'BZ' file on next save.

TechnitiumSoftware/DnsServer · error · InvalidDataException

DnsServer blocked zone file format is invalid.

Error message

DnsServer blocked zone file format is invalid.

What it means

Thrown when loading a blocked-zone config file whose first 2 bytes are not the ASCII magic 'BZ'. ReadConfigFrom checks the header before reading the version byte and the zone count; any other content (wrong file, truncation, corruption) is rejected as an invalid blocked-zone format.

Source

Thrown at DnsServerCore/Dns/ZoneManagers/BlockedZoneManager.cs:209

            _dnsServer.LogManager.Write("DNS Server blocked zone file was saved: " + blockedZoneFile);
        }

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

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

        private void ReadConfigFrom(Stream s)
        {
            if (Encoding.ASCII.GetString(s.ReadExactly(2)) != "BZ") //format
                throw new InvalidDataException("DnsServer blocked zone file format is invalid.");

            BinaryReader bR = new BinaryReader(s);

            byte version = bR.ReadByte();
            switch (version)
            {
                case 1:
                    int length = bR.ReadInt32();
                    int i = 0;

                    AuthZoneManager zoneManager = new AuthZoneManager(_dnsServer);

                    zoneManager.LoadSpecialPrimaryZones(delegate ()
                    {
                        if (i++ < length)
                            return s.ReadShortString();

                        return null;

View on GitHub (pinned to d0484b6c1e)

Solutions

Move the bad blocked-zone config aside so the server recreates a valid 'BZ' file on next save.
Restore the file from a known-good backup if the previously blocked zones matter.
Verify the correct feature's .bin is in the expected location.

Example fix

// before: blockedZone.bin header != 'BZ' -> blocked-zone load fails
// after: remove to regenerate
//   mv config/blockedZone.bin config/blockedZone.bin.bak   (then restart)

Defensive patterns

Strategy: validation

Validate before calling

static bool IsValidBlockedZoneFile(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)'Z';
}

Try / catch

try { blockedZoneManager.Load(); }
catch (InvalidDataException ex) when (ex.Message.Contains("blocked zone file format"))
{ File.Move(blockedPath, blockedPath + ".bad"); blockedZoneManager.Load(); }

Prevention

Verify the 'BZ' header before loading.
Keep blocked-zone and block-list files in clearly named paths to avoid swaps.
Restore backups feature-by-feature, not by dumping all .bin files together.
Shut the server down cleanly to prevent truncated writes.

When it happens

Trigger: A non-blocked-zone file placed at the blocked-zone config path; truncated/empty file from a crash during write; a 'BL' block-list file mistakenly used here.

Common situations: Restoring backups into the wrong path; partial write left by a power loss; file copied between features by mistake.

Related errors

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/bf79eb4122d724b8. Report an issue: GitHub.