TechnitiumSoftware/DnsServer · error · ArgumentException

Boot file name cannot exceed 127 bytes.

Error message

Boot file name cannot exceed 127 bytes.

What it means

Constructor argument guard in DhcpMessage: the boot file name (file) must be at most 127 ASCII bytes. RFC 2131 fixes the 'file' field at 128 bytes (127 usable + a null terminator); the constructor ASCII-encodes file and rejects a result of 128+ bytes. The check uses >= 128, so up to 127 bytes are allowed.

Source

Thrown at DnsServerCore/Dhcp/DhcpMessage.cs:159

            if (sname != null)
            {
                _serverHostName = sname;

                byte[] buffer = Encoding.ASCII.GetBytes(sname);
                if (buffer.Length >= 64)
                    throw new ArgumentException("Server host name cannot exceed 63 bytes.", nameof(sname));

                Buffer.BlockCopy(buffer, 0, _sname, 0, buffer.Length);
            }

            _file = new byte[128];
            if (file != null)
            {
                _bootFileName = file;

                byte[] buffer = Encoding.ASCII.GetBytes(file);
                if (buffer.Length >= 128)
                    throw new ArgumentException("Boot file name cannot exceed 127 bytes.", nameof(file));

                Buffer.BlockCopy(buffer, 0, _file, 0, buffer.Length);
            }

            _options = options;

            foreach (DhcpOption option in _options)
            {
                if (option.Code == DhcpOptionCode.ServerIdentifier)
                {
                    _serverIdentifier = option as ServerIdentifierOption;
                    break;
                }
            }
        }

        public DhcpMessage(Stream s)
        {

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Use a boot file path whose ASCII encoding is <= 127 bytes.
2. Store only the path (not a full URL or embedded script) in the file field; move long payloads to DHCP options (e.g. option 66/67 or vendor options).
3. Validate Encoding.ASCII.GetByteCount(file) < 128 before constructing.
4. Shorten directory structure / use a shorter filename on the TFTP server.

Example fix

// before
string file = "https://tftp.example.corp.local/boot/very/deeply/nested/path/to/this/image/bootstrap-image-filename.bin"; // > 127 bytes
var msg = new DhcpMessage(op, htype, xid, secs, flags, ..., sname, file, opts); // throws [219]

// after: bare path within 127 bytes
string file = "/boot/pxelinux.0";
if (Encoding.ASCII.GetByteCount(file) >= 128) throw new InvalidOperationException("file too long");
var msg = new DhcpMessage(op, htype, xid, secs, flags, ..., sname, file, opts);

if (file is not null && Encoding.ASCII.GetByteCount(file) >= 128)
    throw new ArgumentException("Boot file name (file) must be <= 127 ASCII bytes.", nameof(file));

var msg = new DhcpMessage(op, htype, xid, secs, flags, ciaddr, yiaddr, siaddr, giaddr, chaddr, sname, file, opts);

static bool IsValidBootFile(string file) =>
    file is null || Encoding.ASCII.GetByteCount(file) < 128;

When it happens

Trigger: Calling the DhcpMessage constructor with a non-null file whose ASCII encoding is 128 or more bytes.

Common situations: A long boot image path with many directory segments; a full URL placed in the file field instead of a path; iPXE scripts/embedded data; a deeply nested TFTP path.

Related errors

• Client hardware address cannot exceed 16 bytes.
• Transaction ID must be 4 bytes.
• Seconds elapsed must be 2 bytes.
• Server host name cannot exceed 63 bytes.
• Address family not supported.

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/93704fca73a21241. Report an issue: GitHub.