Paranoid Penguin - Authenticate with LDAP, Part III
This option also may be used with ldapadd.
The ldaps:// URL is required in the above example. I've specified the -x option for simple clear-text authentication, so I definitely need to connect to the server with TLS encryption rather than in the clear. Last month, I showed how to set up an LDAP server to accept TLS connections.
Having said all that, however, I must point out that password management for end users is one of LDAP's problem areas. On the one hand, if your users all have access to the ldappasswd command, you can use a combination of local /etc/ldap.conf files and scripts/front ends for ldappasswd to make it reasonably simple for users to change their own passwords.
But for users who run some other OS, you must manage passwords centrally and have all users contact the e-mail administrator every time they need to change their password, or you must install LDAP client software for their OS. For client systems running Microsoft Windows, you can configure Samba to let users change their LDAP password with the Windows password tool. See the article “OpenLDAP Everywhere” in LJ, December 2002.
Technically, we've covered or touched on all the tasks needed to build an LDAP server using OpenLDAP (excluding, necessarily, the sometimes lengthy step of actually getting your various server applications to authenticate users against it successfully). In the interests of robust security, a concept not alien to readers of this column, we need to discuss one more thing: OpenLDAP access control lists (ACLs).
As with most other things affecting the slapd dæmon, ACLs are set in /etc/openldap/slapd.conf. And, like most other things involving LDAP, ACLs can be confusing to say the least and usually require some tinkering to get right. Listing 2 shows a sample set of ACLs.
Listing 2. ACLs in /etc/slapd.conf
access to attrs=userPassword by dn="cn=ldapguy,dc=wiremonkeys,dc=org" write by self write by * compare access to * by dn="cn=ldapguy,dc=wiremonkeys,dc=org" write by users read by * auth
ACLs are described in detail in the slapd.conf(5) man page, but in Listing 2 you can see generally how they work. For each LDAP element to which you wish to control access, you specify who may access it and with what level of access. Technically, an entire ACL can be listed on one line, but by convention we list each “by...” statement on its own line. slapd is smart enough to know that the string “access to” marks the beginning of the next ACL.
Space doesn't permit my describing ACL syntax in detail, but remember a few important points. First, ACLs are parsed from top to bottom, and first match wins; they act like a stack of filters. Therefore, it's crucial that you put specific ACLs and by statements above more general ones. For example, in Listing 2 we see an ACL restricting access to the userPassword attribute, followed by one applicable to *, that is, the entire LDAP database. Putting the userPassword ACL first means the rule that allows users to change their own passwords (access to attrs=userPassword by self write) is an exception to the more general rule stating users may read anything (access to * by users read).
Another important point is access levels are hierarchical. Possible levels are none, auth, compare, search, read and write, where none is the lowest level of access and write is the highest, and where each level includes the rights of all levels lower than it. These two points, the first match wins rule and the inclusive nature of access levels, are crucial to understanding how ACLs are parsed. They also are important for making sure your ACLs don't lead to either greater or lesser levels of access than you intend in a given situation.
LDAP is one of the most complicated technologies I personally have worked with lately. To make it work the way you need, you have to spend a lot of time testing while watching logs and fine-tuning the configurations of both the LDAP server and the applications you wish to authenticate against it. But, having such a flexible, powerful and widely supported authentication and directory mechanism is well worth the trouble. I hope this series of articles has helped you get there or at least pointed you in the right direction.
OpenLDAP software and documentation, including the important “OpenLDAP Administrator's Guide”: www.openldap.org.
List of error codes used in LDAP error messages. This is essential in interpreting LDAP log messages: www-user.tu-chemnitz.de/~fri/web500gw/errors.html.
The Exchange Replacement HOWTO, which describes how to use LDAP as the authentication mechanism for Cyrus-IMAPD: www.arrayservices.com/projects/Exchange-HOWTO/html/book1.html.
Carter, Gerald. LDAP System Administration. Sebastopol, California: O'Reilly & Associates, 2003. An excellent new book with detailed coverage of OpenLDAP.
Mick Bauer, CISSP, is Linux Journal's security editor and an IS security consultant for Upstream Solutions LLC in Minneapolis, Minnesota. Mick spends his copious free time chasing little kids (strictly his own) and playing music, sometimes simultaneously. Mick is author of Building Secure Servers With Linux (O'Reilly & Associates, 2002).
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