FeministWiki:LDAP Schema

    Da FeministWiki.
    Versione del 2 apr 2020 alle 21:35 di Technician (discussione | contributi) (→‎Password policy)
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    The member database of the FeministWiki is stored via LDAP. This page explains some details about the setup.

    Structure

    The basic structure looks like this:

    dc=feministwiki,dc=org
      ou=members
        - cn=username
          objectClass: fwMember
          cn: username
          uid: username
          sn: Display name
          userPassword: {SSHA}saltedhash
          mail: username@feministwiki.org
          fwRecoveryMail: user@example.org
        - cn=username2
          objectClass: fwMembe
          cn: username2
          uid: username2
          sn: Display name
          userPassword: {SSHA}saltedhash2
          mail: username2@feministwiki.org
          manager: cn=username,ou=members,dc=feministwiki,dc=org
        - ...
      ou=groups
        cn=members
        objectClass: groupOfNames
        cn: members
        member: username
        member: username2
        member: ...
    

    Notes:

    • The cn (common name) and uid (user ID) fields both contain the username. This is because some software is preconfigured to look at uid, while most look at cn.
    • The sn (surname) field is used to hold a display name that may be different from the username. The field is filled with the username by default.
    • The mail field holds the primary mail address for communication with the member. It's filled with the FeministWiki mail address of the member by default, but can be changed freely.
    • The fwRecoveryMail field may hold a mail address that will be used for password reset requests. It's different from the primary mail address because that one may be the member's FeministWiki address, which they can't access if they've lost their password.
    • The manager contains the DN (distinguished name) of the member who added the member. It may be empty for special member accounts like "Administrator" or the "Deleted" pseudo-account.

    Read-only user

    For security purposes, it's a good idea to have a "read-only" user for LDAP read operations, instead of using the admin for everything.

    # Add read-only user
    ldapadd -xy ~/pwd/ldap <<EOF
    dn: cn=readonly,dc=feministwiki,dc=org
    objectClass: simpleSecurityObject
    objectClass: organizationalRole
    cn: readonly
    description: Read-only user
    EOF
    

    No fiddling with access control is needed, since read-only access is the default.

    Custom objectClass

    The following command creates the fwMember object class.

    ldapadd -Y external -H ldapi:// <<EOF
    dn: cn=feministwiki,cn=schema,cn=config
    objectClass: olcSchemaConfig
    cn: feministwiki
    olcAttributeTypes: {0}( 1.3.6.1.4.1.42.2.27.99.1.1
       NAME 'fwRecoveryMail'
       DESC 'FeministWiki password recovery mail'
       EQUALITY caseIgnoreMatch
       SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 )
    olcObjectClasses: {1}( 1.3.6.1.4.1.42.2.27.99.2.1
       NAME 'fwMember'
       DESC 'FeministWiki member'
       SUP inetOrgPerson
       STRUCTURAL
       MAY ( fwRecoveryMail ) )
    EOF
    

    Attribute permissions

    We want to make the following changes to the default LDAP permissions:

    • Members should be able to change their own display name (sn) and e-mail address (mail).
    • The read-only user should be able to find users via the combination of their username and recovery mail address (fwRecoveryMail), but not actually see their recovery mail addresses. (The password reset mechanism uses this.)
    • Members should not be able to see who a member was added by (the manager field).

    The following command makes the necessary access control changes:

    ldapmodify -Y external -H ldapi:// <<EOF
    dn: olcDatabase={1}mdb,cn=config
    changetype: modify
    add: olcAccess
    olcAccess: {2}to attrs=sn,mail by self write
    olcAccess: {3}to attrs=fwRecoveryMail by self write by dn.exact="cn=readonly,dc=feministwiki,dc=org" search
    olcAccess: {4}to attrs=manager by self read
    EOF
    

    Note that olcAccess entries are evaluated in order, and the first match takes effect. This can affect performance. In the statement above, we start inserting entries from index 2, because indexes 0 and 1 already have some meaningful default entries.

    Password policy

    To make sure passwords are stored with the {SSHA} scheme rather than plain text, the ppolicy "password policy overlay" is used. ZYTRAX has a very nice book about LDAP which documents how to enable this: http://www.zytrax.com/books/ldap/ch6/ppolicy.html

    In short, the steps go as follows (these commands should work verbatim):

    # Add the ppolicy schema
    ldapadd -Y external -H ldapi:// < /etc/ldap/schema/ppolicy.ldif
    
    # Enable the ppolicy dynamic module
    ldapmodify -Y external -H ldapi:// <<EOF
    dn: cn=module{0},cn=config
    changetype: modify
    add: olcModuleLoad
    olcModuleLoad: ppolicy
    EOF
    
    # Add the ppolicy overlay with olcPPolicyHashCleartext set to TRUE
    ldapadd -Y external -H ldapi:// <<EOF
    dn: olcOverlay=ppolicy,olcDatabase={1}mdb,cn=config
    objectClass: olcPPolicyConfig
    olcOverlay: ppolicy
    olcPPolicyHashCleartext: TRUE
    EOF
    

    Further, ppolicy is used to enable brute-force protection. For this, we need to add an entry of the object class pwdPolicy to the directory, add attributes related to brute-force protection, and then set it as the default password policy:

    # Add an OU for password policies
    ldapadd -xy ~/pwd/ldap <<EOF
    dn: ou=pwdPolicies,dc=feministwiki,dc=org
    objectClass: organizationalUnit
    ou: pwdPolicies
    EOF
    
    # Add the pwdPolicy object
    ldapadd -xy ~/pwd/ldap <<EOF
    dn: cn=default,ou=pwdPolicies,dc=feministwiki,dc=org
    objectClass: applicationProcess
    objectClass: pwdPolicy
    cn: default
    pwdAttribute: userPassword
    pwdLockout: TRUE
    pwdLockoutDuration: 3600
    pwdMaxFailure: 10
    pwdFailureCountInterval: 3600
    EOF
    
    # Set it as the default password policy
    ldapmodify -Y external -H ldapi:// <<EOF
    dn: olcOverlay={0}ppolicy,olcDatabase={1}mdb,cn=config
    changetype: modify
    add: olcPPolicyDefault
    olcPPolicyDefault: cn=default,ou=pwdPolicies,dc=feministwiki,dc=org
    EOF
    

    With these settings, ten consecutive authentication failures with a username will lock the account for an hour. Login failures are also cleared after an hour. This means it's possible to try ten passwords per hour during a brute-force attack, which won't get the attacker very far.

    Time of last login

    The lastbind module of OpenLDAP keeps track of when a user last logged in.

    # Load the module
    ldapmodify -Y external -H ldapi:// <<EOF
    dn: cn=module{0},cn=config
    changetype: modify
    add: olcModuleLoad
    olcModuleLoad: lastbind
    EOF
    
    # Enable the overlay
    ldapadd -Y external -H ldapi:// <<EOF
    dn: olcOverlay=lastbind,olcDatabase={1}mdb,cn=config
    objectClass: olcLastBindConfig
    olcOverlay: lastbind
    olcLastBindPrecision: 60
    EOF