Why do LDAP users fail to authenticate against vsftpd in Red Hat Enterprise Linux 6.1?

Issue

• LDAP users can't authenticate against the vsftpd service:

May 28 12:42:38 cl01-07 vsftpd[16009]: pam_unix(vsftpd:auth): check pass; user unknown
May 28 12:42:38 cl01-07 vsftpd[16009]: pam_unix(vsftpd:auth): authentication failure; logname= uid=0 euid=0 tty=ftp ruser=ldapuser rhost=localhost.localdomain
May 28 12:42:38 cl01-07 vsftpd[16009]: pam_succeed_if(vsftpd:auth): error retrieving information about user ldapuser

• local users can authenticate against the vsftpd service.

• LDAP users can authenticate against other services (e.g. ssh).