LemonLDAP::NG is a modular WebSSO (Single Sign On) written in Perl.

It is highly customizable, and can be deployed in many different environments and for many different uses. It manages both authentication and authorization and provides customizable logging for accounting. So you can have a full AAA protection for your applications.



Main components

  • Manager: used to manage LemonLDAP::NG configuration and to explore sessions. Dedicated to administrators

  • Portal: used to authenticate users, display applications list and provides identity provider service (SAML, OpenID, CAS). Futhermore, Portal offers many other features (see portal for more)

  • Handler: used to protect applications which can read HTTP headers or environment variables to get user information



We use “database” as a generic term for backends that are not included with LemonLDAP::NG but that we need in order to provide the SSO service. They can be a LDAP directory, a SQL database server, or even a simple local file.

We split databases in two categories:

  • External databases: are not managed by LemonLDAP::NG. Data is externally provided and LemonLDAP::NG generally just needs to read it.

  • Internal databases: its data is controlled by LemonLDAP::NG

Main external databases are:

  • Authentication: how users are authentified (password, certificate, etc.)

  • User: where to find user attributes

  • Password: how user passwords are changed

  • Registration: where new users are created

  • Second factors: how second factors are validated

Main internal databases are:

  • Configuration: where configuration is stored. This does not include web server configuration which is not managed by LemonLDAP::NG

  • Sessions: where sessions are stored.

  • Notifications: messages displayed to connected users

  • Cache: cache for configuration and sessions


Handler protection


  1. User tries to access a protected application, his request is caught by Handler

  2. SSO cookie is not detected, so Handler redirects user to Portal

  3. User authenticates on Portal

  4. Portal checks authentication

  5. If authentication succeeds, Portal collects user data

  6. Portal creates a session to store user data

  7. Portal gets the session Id

  8. Portal creates SSO cookie with the session Id as value

  9. User is redirected on protected application, with a SSO cookie

  10. Handler reads session Id from the cookie and retrieves user session data

  11. Handler stores user data in its cache

  12. Handler checks access rules and sends headers to the protected application

  13. Protected application sends response to Handler

  14. Handler forwards the response to the user

Then handler will check SSO cookies for each HTTP request.


Default use case:

  1. User clicks on the logout link in Portal

  2. Portal destroys session and redirects user on itself with an empty SSO cookies

  3. User is redirected on portal and his SSO cookies is empty

LemonLDAP::NG is also able to catch logout request on protected applications, with different behavior:

  • SSO logout: the request is not forwarded to application, only the SSO session is closed

  • Application logout: the request is forwarded to application but SSO session is not closed

  • SSO and Application logout: the request is forwarded to application and SSO session is closed

After logout process, the user is redirected on portal, or on a configured URL.

Session expiration

The session expires after 20 hours by default. This duration can be set in the manager’s Configuration tab (General Parameters > Sessions > Sessions Timeout).


  • Handlers have a session cache, with a default lifetime of 10 minutes. So for Handlers embedded by different physical servers than the Portal, a user with an expired session can still be authorized until the cache expiration.

  • Sessions are deleted by a scheduled task. Do not forget to install cron files!

Cross Domain Authentication (CDA)


For security reason, a cookie provided for a domain cannot be sent to another domain. To extend SSO on several domains, a cross-domain mechanism is implemented in LemonLDAP::NG.

  1. User owns SSO cookies on the main domain (see Login kinematics)

  2. User tries to access a protected application in a different domain

  3. Handler does not see SSO cookies (because it is not in main domain) and redirects user on Portal

  4. Portal recognizes the user with its SSO cookies, and see he is coming from a different domain

  5. Portal redirects user on protected application with a token as URL parameter. The token is linked to a session which contains the real session ID

  6. Handler detects URL parameter, gets the real session ID, delete the token session and creates a SSO cookies on its domain, with session ID as value

Authentication, Authorization and Accounting (AAA) mechanisms


If a user is not authenticated and attempts to connect to an area protected by a LemonLDAP::NG compatible Handler, he is redirected to the portal.

Authentication process main steps are:

  • Control asked URL: prevent XSS attacks and bad redirections

  • Control existing session: detect SSO session, apply configured constraints (1 session per user, 1 session per IP, …)

  • Extract form info: get login/password, certificate, environment variable (depending on authentication module)

  • Get user info: contact user database to collect attributes

  • Ask for second factor if required: TOTP, Webauthn key, etc…

  • Set macros: compute configured macros

  • Set groups: request user database to find groups

  • Set local groups: compute configured groups

  • Authenticate: contact authentication database to check credentials

  • Grant session: check rights to open SSO session

  • Store: store user info in session database

  • Build cookie: build SSO cookies with session ID

  • Redirect: redirect user on protected application or on Portal (applications menu)

LemonLDAP::NG SSO cookies are generated by Apache::Session, they are as secure as a 128-bit random cookie. You may use the securedCookie options to avoid session hijacking. (since version 1.4.0 you can use SHA256 for generating safer cookies)


Authorization is controlled only by Handlers. An authorization is defined by:

  • An URL pattern (or default to match other URLs)

  • An access rule


Authorizations are defined inside a virtualhost and takes effect only on it. There are no global authorizations except the right to open a session in the portal.

Access rules values can be:

  • accept: all authenticated users can pass

  • deny: nobody is welcomed

  • skip: all is open!

  • unprotect: all is open, but authenticated users are seen as authenticated

  • logout_sso, logout_app, logout_app_sso: catch logout request

  • Perl expression: perl code snippet that returns 0 or 1

Some examples:

  • Accept all authenticated users:

    • URL pattern: default

    • Access rule: accept

  • Restrict /admin to administrators group

    • URL pattern: ^/admin/

    • Access rule: $groups =~ /\badministrators\b/


\b means start or end of a word in PCRE (Perl Compatible Regular Expressions)

See Writing rules and headers chapter.


Logging portal access

Portal produce a notice message in Web server logs or syslog when a user authenticates (or fails to authenticate) and logs out.

Logging application access

Handler informs Web server of connected user (parameter whatToTrace), so you can see user login in Web server access logs.

The real accounting has to be done by the application itself since SSO logs can not understand transactions.

LemonLDAP::NG can export HTTP headers either using a proxy or protecting directly the application.

An HTTP header is defined by:

  • A name

  • A value


Headers are defined inside a virtualhost and take effect only on it. There are no global headers.

The header value is a Perl expression, returning a string.

Some examples:

  • Send login in Auth-User:

    • Name: Auth-User

    • Value: $uid

  • Send “Lastname, firstname” in Auth-Name:

    • Name: Auth-Name

    • Value: $sn + ", " + $gn

See Writing rules and headers for more.