TechnitiumSoftware/DnsServer · error · InvalidDomainNameException

Invalid domain name [{domain}]: invalid character [{labelCha

Error message

Invalid domain name [{domain}]: invalid character [{labelChar}] was found.

What it means

Thrown by DomainTree.ConvertToByteKey when a character in a label has a code point value greater than or equal to the size of the internal _keyMap lookup table. The library's character map only covers a limited ASCII-ish range, so any character whose int value falls outside that range (e.g. high Unicode codepoints or characters beyond the table) cannot be mapped to a key byte. Only thrown when throwException is true.

Source

Thrown at DnsServerCore/Dns/Trees/DomainTree.cs:166

                    if (throwException)
                        throw new InvalidDomainNameException("Invalid domain name [" + domain + "]: label length cannot exceed 63 bytes.");

                    return null;
                }

                if ((labelLength == 1) && (domain[labelStart + 1] == '*')) //[*]
                {
                    key[keyOffset++] = 1;
                }
                else
                {
                    for (i = labelStart + 1; i <= labelEnd; i++)
                    {
                        labelChar = domain[i];
                        if (labelChar >= _keyMap.Length)
                        {
                            if (throwException)
                                throw new InvalidDomainNameException("Invalid domain name [" + domain + "]: invalid character [" + labelChar + "] was found.");

                            return null;
                        }

                        labelKeyCode = _keyMap[labelChar];
                        if (labelKeyCode == 0xff)
                        {
                            if (throwException)
                                throw new InvalidDomainNameException("Invalid domain name [" + domain + "]: invalid character [" + labelChar + "] was found.");

                            return null;
                        }

                        key[keyOffset++] = labelKeyCode;
                    }
                }

                key[keyOffset++] = 0; //[.]

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Encode internationalized domain names with IdnMapping.GetAscii (Punycode) before passing them to the tree.
2. Sanitize names to the allowed DNS character set [a-zA-Z0-9-._*] at ingestion.
3. Probe with throwException=false to obtain null for unmappable characters.

Example fix

// before
_tree.GetOrAdd("café.example.com", value);

// after
var idn = new System.Globalization.IdnMapping();
string ascii = idn.GetAscii(domain);
_tree.GetOrAdd(ascii, value);

string ToAsciiDomain(string d)
{
    var idn = new System.Globalization.IdnMapping();
    try { return idn.GetAscii(d); } catch { return null; }
}

static bool IsAsciiDomain(string d) =>
    d.Length > 0 && d.All(c => (int)c < KEYMAP_SIZE);

When it happens

Trigger: Passing a domain name containing a character whose numeric value is >= _keyMap.Length to a DomainTree-derived tree operation.

Common situations: Non-ASCII / Unicode hostnames not converted to Punycode (IDNA) before being passed to the tree; a stray emoji or accented character in a record owner name; binary garbage in a name.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Invalid domain name [{domain}]: length cannot exceed 255 byt
• Invalid domain name [{domain}]: label length cannot be 0 byt
• Invalid domain name [{domain}]: label length cannot exceed 6
• Zone does not contain SOA record.
• Failed to load DNS zone file: the zone file does not contain

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/a29f6b9621275ca1. Report an issue: GitHub.