I have two Kerberos realms I can authenticate against. One of them I can control, and the other one is external from my point of view. I also have an internal user database in LDAP. Let's say the realms are INTERNAL.COM and EXTERNAL.COM. In ldap I have user entries like this: 1054 uid=testuser,ou=People,dc=tml,dc=hut,dc=fi shadowFlag: 0 shadowMin: -1 loginShell: /bin/bash shadowInactive: -1 displayName: User Test objectClass: top objectClass: account objectClass: posixAccount objectClass: shadowAccount objectClass: person objectClass: organizationalPerson objectClass: inetOrgPerson uidNumber: 1059 shadowWarning: 14 uid: testuser shadowMax: 99999 gidNumber: 1024 gecos: User Test sn: Test homeDirectory: /home/testuser mail: [email protected] givenName: User shadowLastChange: 15504 shadowExpire: 15522 cn: User.Test userPassword: {SASL}[email protected] What I would like to do, somehow, is to specify per-user basis to which authentication server / realm the user is authenticated against. Configuring kerberos to handle multiple realms is easy. But how to I configure other instances, like PAM, to handle the fact that some users are from INTERNAL.COM and some from EXTERNAL.COM? There needs to be an LDAP lookup of some kind where the realm and the authentication name is fetched from, and then the actual authentication itself. Is there a standardized way to add this information to LDAP, or look it up? Are there some other workarounds for a multi-realm user base? I might be ok with a single realm solution, too, as long as I can specify the user name - realm -combination for the user separately.