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LDAP and Kerberos: An Overview

LDAP and Kerberos: An Overview. Leveraging services provided by Active Directory for Unix/Linux authentication, authorization and name services. Jason Testart Computer Science Computing Facility University of Waterloo. March 2007. Summary.

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LDAP and Kerberos: An Overview

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  1. LDAP and Kerberos:An Overview Leveraging services provided by Active Directory for Unix/Linux authentication, authorization and name services Jason Testart Computer Science Computing Facility University of Waterloo March 2007

  2. Summary • Kerberos is for authentication only and provides Single Sign-on (SSO) • LDAP can be used for authentication, authorization, and name services (no SSO) • Active Directory is a kerberized directory service with an LDAP interface • Use Kerberos for authentication, LDAP for authorization and name services

  3. What do these technologies give us? • Eliminate password synchronization • Speed-up system deployment • Reduce development time when a new platform is introduced Improve the end-user experience

  4. LDAP Name services using the Lightweight Directory Access Protocol

  5. What is LDAP? • A protocol for accessing a directory service • What’s a directory service? Think DNS. • Database backend – do we care? • Schema: attributes with OIDs (à la SNMP) • Objects organized in a tree structure (DIT) • Operations: bind, search, modify • LDIF: text file format for describing directory contents

  6. Schema • Attribute – like a variable in a programming language, it holds a value • ObjectClass – a special attribute that all directory entries must have, as it acts as a template for the data (enforces a kind of internal consistency) • AD Schema:http://msdn.microsoft.com/library/en-us/ad/ad/active_directory_schema.asp • Unix Schema: http://www.ietf.org/rfc/rfc2307.txt

  7. Types of Object Classes • Structural – only one per entry! • Auxiliary – supplements structural • Abstract – can’t be used directly; only as an ancestor of another class (eg. “top”)

  8. Schema Examples Snippets of OpenLDAP’s RFC 2307 schema implementation: objectclass ( 1.3.6.1.1.1.2.0 NAME 'posixAccount' DESC 'Abstraction of an account with POSIX attributes' SUP top AUXILIARY MUST ( cn $ uid $ uidNumber $ gidNumber $ homeDirectory ) MAY ( userPassword $ loginShell $ gecos $ description ) ) attributetype ( 1.3.6.1.1.1.1.3 NAME 'homeDirectory' DESC 'The absolute path to the home directory' EQUALITY caseExactIA5Match SYNTAX 1.3.6.1.4.1.1466.115.121.1.26 SINGLE-VALUE )

  9. Directory Information Tree (DIT)

  10. DN and RDN of a directory entry • RDN – a unique attribute among all siblings of a single parent in the DIT(eg. “uid=jatestar”) • DN – concatenation of RDNs when following the path from the entry (node) to the root of the DIT(eg. “uid=jatestar, ou=people, dc=ldap, dc=student, dc=cs, dc=uwaterloo, dc=ca)

  11. Directory Entry Example Entry in /etc/passwd: jatestar:x:1449:1449:Jason Testart [CSCF],DC2555B,x37174,,:/u4/jatestart:/xhbin/tcsh Becomes the following LDIF: dn: uid=jatestar, ou=ldap, ou=people, dc=student, dc=cs, dc=uwaterloo, dc=ca objectClass: top objectClass: person objectClass: posixAccount cn: Jason Testart sn: Testart uid: jatestar uidNumber: 1449 gidNumber: 1449 homeDirectory: /u4/jatestart loginShell: /xhbin/tcsh gecos: Jason Testart [CSCF],DC2555B,x37174,,

  12. Some LDAP Interfaces • LDIF - ldapsearch, ldapmodify, ldp.exe(see “man ldif, man ldapsearch, etc…) • Perl - Net::LDAP(see http://ldap.perl.org/) • C/C++ - OpenLDAP API (likely others…)(see “man 3 ldap”) • ADSI - Windows specific(see http://www.microsoft.com/windows2000/techinfo/howitworks/activedirectory/adsilinks.asp)

  13. When interfacing, you need… • Server hostname • Bind DN • Base DN (if searching) • SSL? Note: An Active Directory domain controller will accept the value of theuserPrincipalName attribute as the binddn. (eg. “jatestar@nexus.uwaterloo.ca” is friendlier than “CN=Jason J Testart (jatestar),OU=CSCF,OU=Staff,OU=Accounts,OU=Computer Science,OU=Faculties,DC=NEXUS,DC=UWATERLOO,DC=CA”)

  14. What object classes does AD use for users? Query: ldapsearch -x -W -H "ldaps://canadenis.student.cs.uwaterloo.ca“ \ -D "jatestar@student.cs.uwaterloo.ca" \-b "dc=student,dc=cs,dc=uwaterloo,dc=ca“ \ "(cn=jatestar)" objectClass Yields: dn: CN=jatestar,OU=Users,OU=CS,DC=student,DC=cs,DC=uwaterloo,DC=ca objectClass: top objectClass: person objectClass: organizationalPerson objectClass: user

  15. Need Unix attributes in AD • The user objectClass is missing needed attributes that RFC 2307 provides • So, extend the schema in AD, but be careful! • Differences between MS-SFU-2.0, MS-SFU-3.5 and schema provided by Windows Server 2003 R2 • Maximize use of client attribute mappings!

  16. Attribute Mappings • Example: “When I ask for the gecos attribute value, fetch the value of cn from the directory instead” • Minimize duplication of data in the directory (ie. redundant attributes) • In some cases, doing this allows you to avoid extending the schema of a directory

  17. Groups • posixGroup uses memberUid, which is the uid of the member • groupofUniqueNames uses the member attribute, which is the DN of the member • Attribute mappings may be inappropriate, redundancy may be unavoidable • Netgroups may give additional functionality, with additional complexity

  18. Applying the knowledge (client-side) • Make sure you know what directory attributes that you are using! • Tell /etc/nsswitch.conf to use the nss_ldap library from padl.com • Edit the ldap.conf appropriately to point to AD and define the attribute maps • No need to add users/groups in /etc/passwd or /etc/group!

  19. Kerberos Using Active Directory Kerberos for Unix/Linux authentication

  20. What is Kerberos? • Authentication protocol • Secure • SSO • Trusted 3rd party • Mutual Authentication

  21. Some Kerberos Terminology • User Principal • Host/Service Principal • Instance • Realm • KDC • TGT • Credential cache

  22. Principals • username[/instance]@REALM • servicename/FQDN@REALM Examples: • jatestar@NEXUS.UWATERLOO.CA • nfs/gl01.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA • host/cpu14.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA • imap/services02.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA

  23. Credential Cache (on a Mac) $ klist Ticket cache: FILE:/tmp/krb5cc_1000 Default principal: jatestar@STUDENT.CS.UWATERLOO.CA Valid starting Expires Service principal 12/13/06 01:06:50 12/13/06 11:05:03 krbtgt/STUDENT.CS.UWATERLOO.CA@STUDENT.CS.UWATERLOO.CA renew until 12/14/06 01:06:50 12/13/06 01:06:38 12/13/06 11:05:03 fs02$@STUDENT.CS.UWATERLOO.CA renew until 12/14/06 01:06:50 12/13/06 01:10:23 12/13/06 11:05:03 host/cpu20.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA renew until 12/14/06 01:06:50

  24. AD Domain == Kerberos v5 Realm • Domain controllers provide KDC functionality • A “domain” is synonymous with “realm” • Joining a Windows computer to a domain means you are creating a host principal in the realm • No multipart principal names in AD, so mappings are needed for instances

  25. Adding a Unix host to AD • Create a user account for the host/service (eg. cpu04-host) • Map the account to a service principal:host/cpu04.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA • Set the account password • Generate a krb5.keytab file • Stick the keytab file in /etc/krb5 on cpu04 • Done on all student.cs CPU servers

  26. Adding a Unix host to AD (sans GUI) Create the account using LDIF (from a Linux box): % ldapmodify -x -W -H ldaps://canadenis -D “Administrator@student.cs.uwaterloo.ca” dn: cn=cpu04-host,OU=Service Principals,DC=student,DC=cs,DC=uwaterloo,DC=ca changetype: add cn: cpu04-host objectClass: user sAMAccountName: cpu04-host displayName: cpu04-host description: Kerberos host service principal for cpu04 userAccountControl: 2097664 Do the mapping and generate a keytab file (on the domain controller): ktpass –princ host/cpu04.student.cs.uwaterloo.ca@STUDENT.CS.UWATERLOO.CA –mapuser cpu04-host –password S0m3Rand0mPaZZw0rd –out cpu04-host.keytab

  27. Tell Unix login to authenticate against AD • Set-up an appropriate /etc/krb5.conf • Modify the PAM authentication stack to use the pam_krb5 module

  28. Lots of work for simple authentication! • Could have used ldap, or radius, etc… • Kerberos gives us Single Sign-On • Can take advantage of domain trusts! • Most apps use SASL and GSSAPI to support Kerberos 5

  29. SSH and SSO • Configure SSH clients and servers to use GSSAPI for authentication • Mac Lab user can ssh to a CPU server without a password (no ssh keys or .shosts required) • Honours .k5login file (handy for course accounts) • Possibilities with NFS (v3+), IMAP, SMTP AUTH, HTTP, etc…

  30. SSO Demo (Linux client to AD) • Show krb5.conf • Login to realm (kinit) • Show file shares on NetApp • Query our entry on domain controller • Show resulting credential cache (klist) • Logout of realm (kdestroy)

  31. Using slapd (OpenLDAP) with AD • OpenLDAP supports Kerberos via SASL/GSSAPI • Can map entities in a realm with entries in the directory • Use authz-regexp directive in slapd.conf • See:http://www.openldap.org/doc/admin23/sasl.html

  32. References • LDAP System Administration by Gerald Carter (O’Reilly) • Kerberos The Definitive Guide by Jason Garman (O’Reilly) • Unified Windows® and UNIX® Authorization Using Microsoft® Active Directory LDAP as a Directory Store by Ellie Berriman (Network Appliance Inc.) • Unified Windows® and UNIX® Authentication Using Microsoft® Active Directory Kerberos by Ellie Berriman (Network Appliance Inc.)

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