Kerberos and LDAP Jason Heiss February 2002

Why is everybody still using NIS? NIS is easy to setup Easy to administer Scales fairly well Widely supported (clients and servers)

Goals Replace NIS with something secure –Weakly crypted passwords (and everything else) sent over the network in the clear –Difficult to firewall –No system authentication Provide additional directory services –Replace/supplement paper staff directory

Other Options Copy local passwd file –Error-prone –Requires root-level trust between clients and server NIS+ –Complicated –Limited client support –Dead

LDAP LDAP is a directory access protocol Up to the implementation to use whatever backend it wants LDAP can be used to store any form of information, but designed for directories –Small bits of data –Mostly read access

Goals Revisited Security –Clients authenticate server –Encrypt data in transit –Simplify firewalling Administration –Easy to configure –Easy to maintain Scalability Widespread client support

LDAP Security Authentication –LDAP clients authenticate server by ensuring server has an SSL certificate signed by a CA they trust Encryption –SSL Access control –ACLs based on Kerberos principal user authenticates with –Useful for non-NIS data like home phone number

Scalability and Client Support Scalability –Similar model to NIS for simple situations Master and replicas –Hierarchical relationships possible in larger environments Client support –nss_ldap module for any OS which supports Name Service Switch (Solaris or GNU) –BIND IRS (NSS work-alike from BIND 8)

Why not LDAP? Administration –Initial configuration complicated SSL certificate management Schemas Kerberos –Ongoing management complicated NIS+ itis –No vi; add/change/delete via command line utilities –Command line utilities take bewildering array of options

Why Kerberos LDAP is designed for public information –ACLs can protect userPassword, but… Kerberos supports password security –Dictionary checks of new passwords –Password expiration Kerberos useful for other services –Windows authentication –NFS authentication and encryption –AFS

Kerberos Client Support System logins –pam_krb5 for any OS/application which supports PAM (Pluggable Authentication Modules) Many common applications require a recompile to enable PAM (OpenSSH, sudo, xlockmore) –Replacement binaries for /bin/login, etc. Many applications with native Kerberos support –Quite a few only support Kerberos IV, which requires enabling Kerberos IV support on server

Summary of Pros and Cons Vastly improved security Complicated configuration and management Do you have time to invest in initial setup? –Can you afford not to? Friendly tools can ease ongoing administration

Kerberos Basics

Kerberos Stores username/password pairs –Usernames are called principals –Kerberos database equivalent to /etc/shadow Passwords, encrypted or not, are almost never sent across the network Server encrypts keys with user’s password, other folks can’t decrypt/use them without the password

Kerberos When user authenticates, they are given a “ticket” –Tickets are generally good for 8 hours –Useful for things like authenticated NFS, IMAP, etc. Kerberos performs authentication, not authorization –Kerberos tells you if user claiming to be X really is or not –It is up to the client to decide if user X is allowed to do something

Terms Principal –Examples jheiss/admin host/foobar.example.com ldap/ldap1.example.com Realm –Typically domain name in all caps

Example Kerberos Transaction Kerberos Server Service User “Username” TGT, encrypted with user’s password TGT “Service” Service ticket, encrypted with service password Encrypted service ticket User password User password Service password Service password Service request and service ticket

LDAP Basics

Schemas LDAP uses schemas to define what attributes an object can and must have –posixAccount object class corresponds to an entry in a passwd file –posixGroup corresponds to a group The same object can implement multiple object classes –uid=jheiss,ou=people,dc=example,dc=com might be a posixAccount, inetOrgPerson and pilotPerson

Schema Examples attributetype ( NAME ( 'uid' 'userid' ) DESC 'RFC1274: user identifier' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch SYNTAX {256} ) objectclass ( NAME 'posixAccount' SUP top AUXILIARY DESC 'Abstraction of an account with POSIX attributes' MUST ( cn $ uid $ uidNumber $ gidNumber $ homeDirectory ) MAY ( userPassword $ loginShell $ gecos $ description ) )

Distinguished Names Each object in the LDAP directory has a DN –uid=jheiss,ou=people,dc=example,dc=com –cn=users,ou=group,dc=example,dc=com

LDIF Example: User dn: uid=jheiss,ou=people,dc=example,dc=com objectClass: person objectClass: inetOrgPerson objectClass: posixAccount commonName: Jason Heiss mail: homePhone: givenName: Jason surname: Heiss uid: jheiss userPassword: loginShell: /bin/bash uidNumber: 500 gidNumber: 100 homeDirectory: /home/jheiss

LDIF Example: Group dn: cn=users,ou=group,dc=example,dc=com cn: users objectClass: posixGroup userPassword: {crypt}* gidNumber: 100 memberUid: jheiss memberUid: bob

Alphabet Soup LDAP –Lightweight Directory Access Protocol SASL –Simple Authentication and Security Layer GSSAPI –Generic Security Services Application Programming Interface PAM –Pluggable Authentication Module NSS –Name Service Switch

Kerberos Implementation

Software Servers –Kerberos MIT (Recommended) Heimdal SEAM Clients –pam_krb5 Included with Red Hat, FreeBSD, Solaris, possibly others Open Source versions available from Red Hat (recommended), Linux PAM project –See references

Kerberos Servers Edit /etc/krb5.conf –Realm, servers –Generally identical on all Kerberized systems in realm Edit /var/kerberos/krb5kdc/kdc.conf –Realm –Needed on KDCs only /usr/kerberos/sbin/kdb5_util create –s Edit /var/kerberos/krb5kdc/kadm5.acl

Kerberos Servers, cont. Configure init to start daemons –kadmin (master KDC only) –krb5kdc (all KDCs) /usr/kerberos/sbin/kadmin.local –q “addprinc jheiss/admin” Add additional principals as needed with kadmin Logs –/var/log/krb5kdc.log –/var/log/kadmind.log

Kerberos Replication Create host principals for slave KDCs –addprinc –randkey host/hostname Edit /var/kerberos/krb5kdc/kpropd.conf on slave KDCs –Add entry for every KDC host principal Configure init to start kpropd -S on slave KDCs Add cronjob on master KDC to dump database and run kprop regularly –See references for link to example script

Kerberos Packet Filtering 88/udp –Clients KDCs –Regular authentication traffic 749/tcp –Clients -> master KDC –Password changes, add/change/delete principals 754/tcp –Master KDC -> Slave KDCs –Database replication

Kerberos Client Copy /etc/krb5.conf from server –/etc/krb5/krb5.conf on Solaris using SEAM

PAM on Kerberos Clients Red Hat –Copy files as needed from /usr/share/doc/pam_krb5*/pam.d to /etc/pam.d –gdm, login, passwd, sshd, su, sudo, xdm, xlock Solaris –SEAM –See references for example pam.conf

Host Principal for PAM Some references that without it, PAM can’t verify Kerberos server Support –Red Hat’s pam_krb5 supports it keytab and required_tgs config options No evidence that RH does anything different when configured to use it –No evidence that SEAM support it

Testing As user: –kinit –klist Test admin functionality –kadmin addprinc delprinc

Kerberos Management kadmin –addprinc –delprinc –listprincs –ktadd –ktremove ktutil –rkt –list –quit Easy to integrate into existing user management tool –See references for details

User Password Management Custom centralized password program –Least confusing if you have more than one password database (NIS, Windows, Samba, etc.) –See references for more information on integrating Kerberos into one of these PAM –PAM configured to change password in Kerberos Non-PAM –Users need to use kpasswd

LDAP Implementation

Software Servers –Kerberos –OpenSSL –SASL (1.x until OpenLDAP 2.1.x is available) –OpenLDAP Clients –All of the above plus nss_ldap and pam_krb5

LDAP Servers, Prep Work Create user and group (ldap/ldap) Make/buy signed SSL certificate –CN in SSL certificate should be canonical name of server as reported by reverse DNS I.e. moonshine.example.com –If possible, list user-friendly name in x509v3 Subject Alternative Name field Within usr_cert section of openssl.cnf: –subjectAltName=DNS:ldap1.example.com OpenSSL doesn’t have support for prompting for this field, so you’ll have to edit openssl.cnf for each cert you generate –chmod 640 slapd-key.pem; chgrp ldap slapd-key.pem

LDAP Servers, Prep Work Create service principal –kadmin –q “addprinc ldap/hostname” –kadmin –q “ktadd –k /etc/openldap/ldap.keytab ldap/hostname” –chmod 640 ldap.keytab; chgrp ldap ldap.keytab

LDAP Server Configuration Edit /etc/openldap/slapd.conf/etc/openldap/slapd.conf –ACLs –SSL cert –suffix –rootdn and rootpw Configure init to start slapd –KRB5_KTNAME="FILE:/etc/openldap/ldap.keytab“ /usr/sbin/slapd -u ldap -g ldap -h "ldap:/// ldaps:///"

SSL and TLS SSL/TLS is a generic method of encrypting application-layer network traffic using x.509 certs for authentication “Netscape” way of connecting –Application connects to alternate port for SSL communication I.e. HTTPS IETF-approved way of connecting –Application connects to standard port, requests SSL –Commonly called “StartTLS”

Additional LDAP Server Config Packet Filtering –LDAP, LDAP w/ TLS 389/tcp –LDAPS 636/tcp

LDAP Replication slurpd watches for changes, pushes to replicas Acts as LDAP client, and thus needs Kerberos ticket, not keytab –Need cronjob to keep ticket current Replicas must have ACLs which allow modification by whatever principal slurpd is configured to use

LDIF Example dn: dc=example,dc=com objectclass: organization o: Example, Inc. dn: ou=people,dc=example,dc=com objectclass: organizationalUnit ou: People dn: uid=jheiss,ou=people,dc=example,dc=com objectClass: posixAccount commonName: Jason Heiss surname: Heiss uid: jheiss userPassword: loginShell: /bin/bash uidNumber: 500 gidNumber: 100 homeDirectory: /home/jheiss

Initial Database Population ldapadd -x -D “cn=Manager,dc=example,dc=com” -W -f initial.ldif Remove rootdn and rootpw from slapd.conf and restart All future edits should be authorized via ACLs in slapd.conf

Testing Server Test in stages –kinit –ldapsearch -H ldap://hostname/ -x –ldapsearch -H ldaps://hostname/ -x –ldapsearch -H ldap://hostname/ -ZZ -x –ldapsearch -H ldap://hostname/ –ldapsearch -H ldaps://hostname/ –ldapsearch -H ldap://hostname/ -ZZ

LDAP Clients Install nss_ldap Edit /etc/ldap.conf hostldap1.example.com ldap2.example.com basedc=example,dc=com sslstart_tls tls_checkpeeryes tls_cacertfile/etc/ssl/ca-cert.pem Edit /etc/openldap/ldap.conf URI ldaps://ldap1.example.com/ ldaps://ldap2.example.com/ BASE dc=example,dc=com

Testing Client ldapsearch –Makes sure /etc/openldap/ldap.conf is setup properly and that connection to server is good id username getent passwd username If things don’t work –Try turning of checkpeer in /etc/ldap.conf –Try setting ssl to no in /etc/ldap.conf –Try turning off nscd

Troubleshooting Sample error messages –ldap_sasl_interactive_bind_s: Local error ldap/hostname service principal not setup User doesn’t have ticket or ticket has expired –ldap_sasl_interactive_bind_s: Can't contact LDAP server Checking hostname from CN field of SSL cert failed See my web page in references for more

Controlling Access Linux –Add to /etc/pam.d/whatever account required /lib/security/pam_access.so –Edit /etc/security/access.conf See /usr/share/doc/pam-*/txts/README.pam_access for syntax Solaris –Add entries to /etc/project after removing default entries (except user.root) user.username:uid::::

LDAP Management OpenLDAP tools –ldapadd, ldapmodify, ldapdelete –Not very user friendly Jason’s tools –ldapcat, ldapedit, ldapposixadd –Useful for folks used to NIS Integration into centralized tools –Perl and Net::LDAP Sample code on web page

Support Kerberos –comp.protocols.kerberos OpenLDAP –echo subscribe | mail openldap-software- subscribe | mail openldap-software- nss_ldap –echo subscribe | mail nssldap- subscribe | mail nssldap-

References –Kerberos replication script –Sample SEAM pam.conf –Examples of integrating Kerberos management into existing tools –Sample slapd.conf –Sample nss_ldap and OpenLDAP ldap.conf’s –Sample LDIF –List of OpenLDAP error messages –LDAP tools and sample Net::LDAP code

References Friendly Kerberos introduction:

References Kerberos –MIT: –Heimdal: –SEAM: seam/ seam/ Encryption modules necessary for Kerberized NFS: Full SEAM package:

References pam_krb5 –Red Hat /usr/share/doc/pam_krb5-*/README on a Red Hat box –Linux PAM Project: SASL: library.htmlhttp://asg.web.cmu.edu/sasl/sasl- library.html LDAP –OpenLDAP: