TechnitiumSoftware/DnsServer · error · InvalidDomainNameException

Invalid domain name [{domain}]: label length cannot exceed 6

Error message

Invalid domain name [{domain}]: label length cannot exceed 63 bytes.

What it means

Thrown by DomainTree.ConvertToByteKey when a single label within the domain name exceeds 63 bytes, the RFC 1035 per-label limit (the length must fit in a 6-bit/2-byte length field on the wire). Only thrown when throwException is true.

Source

Thrown at DnsServerCore/Dns/Trees/DomainTree.cs:149

            {
                if (labelEnd < 0)
                    labelEnd = 0;

                labelStart = domain.LastIndexOf('.', labelEnd);
                labelLength = labelEnd - labelStart;

                if (labelLength == 0)
                {
                    if (throwException)
                        throw new InvalidDomainNameException("Invalid domain name [" + domain + "]: label length cannot be 0 byte.");

                    return null;
                }

                if (labelLength > 63)
                {
                    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.");

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Validate each label's length is <= 63 before inserting/querying.
2. Split overly long single tokens across multiple labels or store the data in record RDATA instead of the name.
3. Probe with throwException=false to silently skip invalid names.

Example fix

// before
_tree.GetOrAdd(longLabelDomain, value);

// after
foreach (var label in domain.Split('.'))
    if (label.Length > 63) { /* reject */ return; }
_tree.GetOrAdd(domain, value);

bool LabelsWithinLimit(string d) => d.Split('.').All(l => l.Length <= 63);

static bool IsValidDnsName(string d) =>
    d.Length <= 255 && d.Split('.').All(l => l.Length >= 1 && l.Length <= 63);

When it happens

Trigger: Passing a domain name with at least one dot-delimited label whose character count exceeds 63 to a DomainTree-derived tree operation.

Common situations: A TXT/record owner name built from a long token, a base64/blob string used as a label, DKIM/long selector names, or accidentally concatenating labels without splitting.

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}]: invalid character [{labelCha
• 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/11ccbe3e870dd1fb. Report an issue: GitHub.