TechnitiumSoftware/DnsServer · error · ArgumentException

Username length cannot exceed 255 characters.

Error message

Username length cannot exceed 255 characters.

What it means

Thrown by User.IsUsernameValid(username, throwException: true) when the username is longer than 255 characters (after the null/empty check passes). It is an ArgumentException (parameter Username) enforcing the configured storage width for usernames.

Source

Thrown at DnsServerCore/Auth/User.cs:179

            user._ssoIdentifier = ssoIdentifier;

            return user;
        }

        public static bool IsUsernameValid(string username, bool throwException = false)
        {
            if (string.IsNullOrWhiteSpace(username))
            {
                if (throwException)
                    throw new ArgumentException("Username cannot be null or empty.", nameof(Username));

                return false;
            }

            if (username.Length > 255)
            {
                if (throwException)
                    throw new ArgumentException("Username length cannot exceed 255 characters.", nameof(Username));

                return false;
            }

            foreach (char c in username)
            {
                if ((c >= 97) && (c <= 122)) //[a-z]
                    continue;

                if ((c >= 65) && (c <= 90)) //[A-Z]
                    continue;

                if ((c >= 48) && (c <= 57)) //[0-9]
                    continue;

                if (c == '@')
                    continue;

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Shorten the username to <= 255 characters.
2. Enforce client-side maxlength and server-side validation on the username field.
3. If using email-as-username, truncate or map long addresses to a shorter local identifier.

Example fix

// before
User.IsUsernameValid(username, throwException: true);

// after
if (username?.Length > 255)
    return BadRequest("Username length cannot exceed 255 characters.");

if (username is null || username.Length > 255)
    return BadRequest("Username length cannot exceed 255 characters.");

try { User.IsUsernameValid(username, throwException: true); }
catch (ArgumentException ex) when (ex.ParamName == "Username" && ex.Message.Contains("255"))
{ return BadRequest(ex.Message); }

When it happens

Trigger: Calling IsUsernameValid(longUsername, true), or a create/rename path that delegates to it, with a username over 255 chars.

Common situations: A UI without a maxlength on the username field; an automated provisioner that builds usernames from long email/UPN values; a migration that did not trim source identifiers.

Related errors

• Group name length cannot exceed 255 characters.
• Username cannot be null or empty.
• Username can contain only alpha numeric, '@', '-', '_', or '
• Display name length cannot exceed 255 characters.
• Token name length cannot exceed 255 characters.

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/27e46d48fd696a4f. Report an issue: GitHub.