TechnitiumSoftware/DnsServer · error · DnsWebServiceException

User already exists: {username}

Error message

User already exists: {username}

What it means

Thrown as DnsWebServiceException from CreateUser when _users.TryAdd(username, user) returns false, meaning a user with that username already exists. Usernames are lowercased before storage, so the uniqueness check is effectively case-insensitive. Returned over the API as HTTP 200 with status 'error'.

Source

Thrown at DnsServerCore/Auth/AuthManager.cs:941

                throw new DnsWebServiceException("Cannot create more than 255 users.");

            username = username.ToLowerInvariant();

            User user = User.CreateLocalUser(displayName, username, password, iterations);

            if (_users.TryAdd(username, user))
            {
                if (_users.Count > byte.MaxValue)
                {
                    _users.TryRemove(username, out _); //undo
                    throw new DnsWebServiceException("Cannot create more than 255 users.");
                }

                user.AddToGroup(GetGroup(Group.EVERYONE));
                return user;
            }

            throw new DnsWebServiceException("User already exists: " + username);
        }

        public User CreateSsoUser(string displayName, string username, string ssoIdentifier)
        {
            if (_users.Count >= byte.MaxValue)
                throw new DnsWebServiceException("Cannot create more than 255 users.");

            username = username.ToLowerInvariant();

            User user = User.CreateSsoUser(displayName, username, ssoIdentifier);

            if (_users.TryAdd(username, user))
            {
                if (_users.Count > byte.MaxValue)
                {
                    _users.TryRemove(username, out _); //undo
                    throw new DnsWebServiceException("Cannot create more than 255 users.");
                }

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Check whether the username exists (case-insensitively) before creating, and skip or update instead.
2. Pick a unique username.
3. Delete the existing user first if a replacement is intended.

Example fix

// before
_dnsWebService._authManager.CreateUser(name, username, password);
// after
if (GetUser(username) is null)
    _dnsWebService._authManager.CreateUser(name, username, password);

if (await GetUserAsync(username) is not null)
    throw new InvalidOperationException($"User '{username}' already exists.");

try { await client.CreateUserAsync(name, username, password); }
catch (HttpApiClientException ex) when (ex.Message.StartsWith("User already exists"))
{
    // skip or update instead
}

When it happens

Trigger: Calling CreateUser with a username (lowercased) that is already a key in _users; e.g., creating 'Admin' when 'admin' exists.

Common situations: Re-running a provisioning script; an admin/User already created the same name; case-only differences (the code stores lowercased keys).

Related errors

• User already exists: {newUsername}
• User account is disabled. Please contact your administrator.
• Cannot create more than 255 users.
• Group already exists: {name}
• Group already exists: {newGroupName}

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/edd15803a2efab82. Report an issue: GitHub.