Highly Available LDAP
As an organization adds applications and services, centralizing authentication and password services can increase security and decrease administrative and developer headaches. However, consolidating any service on a single server creates reliability concerns. High availability is especially critical for enterprise authentication services, because in many cases the entire enterprise comes to a stop when authentication stops working.
This article describes one method of creating a reliable authentication server cluster. We use an LDAP (Lightweight Directory Access Protocol) server to provide authentication services that can be subscribed to by various applications. To provide a highly available LDAP server, we use the heartbeat package from the Linux-HA initiative (www.linux-ha.org).
We are using the OpenLDAP package (www.openldap.org), which is part of several Linux distributions, including Red Hat 7.1. Version 2.0.9 ships with Red Hat 7.1, and the current download version (as of this writing) is 2.0.11. The OpenLDAP Foundation was created as “a collaborative effort to develop a robust, commercial-grade, fully featured and open-source LDAP suite of applications and development tools” (from www.openldap.org). OpenLDAP version 1.0 was released in August 1998. The current major version is 2.0, which was released at the end of August 2000 and adds LDAPv3 support.
LDAP, like any good network service, is designed to run across multiple servers. LDAP uses two major features: replication and referral. The referral mechanism lets you split the LDAP namespace across multiple servers and arrange LDAP servers in a hierarchy. LDAP allows only one master server for a particular directory namespace (see Figure 1).
Replication is driven by the OpenLDAP replication dæmon, slurpd, which periodically wakes up and checks a log file on the master for any updates. The updates are then pushed to the slave servers. Read requests can be answered by either server; updates can be performed only on the master. Update requests to a slave generate a referral message that gives the address of the master server. It is the client's responsibility to chase the referral and retry the update. OpenLDAP has no built-in way of distributing queries across replicated servers; you must use an IP sprayer/fanout program, such as balance.
To achieve our reliability goals we cluster together a pair of servers. We could use shared storage between these servers and maintain one copy of the data. For simplicity, however, we choose to use a shared-nothing implementation, where each server has its own storage. LDAP databases typically are small, and their update frequency is low. (Hint: if your LDAP dataset is large, consider dividing the namespace into smaller pieces with referrals.) The shared-nothing setup does require some care when restarting a failed node: any new changes must be added to the database on the failed node before restart. We'll show an example of that situation later.
To start, let's clear up a minor confusion. Most HA (high availability) clusters have a system keep-alive function called the heartbeat. A heartbeat is used to monitor the health of the nodes in the cluster. The Linux-HA (www.linux-ha.org) group provides open-source clustering software named, aptly enough, Heartbeat. This naming situation can lead to some confusion. (Well, it confuses us sometimes.) In this paper, we refer to the Linux-HA package as Heartbeat and the general concept as heartbeat. Clear, yes?
The Linux-HA Project began in 1998 as an outgrowth of the Linux-HA HOWTO, written by Harald Milz. The project is currently led by Alan Robertson and has many other contributors. Version 0.4.9 of Heartbeat was released in early 2001. Heartbeat monitors node health through communication media, usually serial and Ethernet links. It is a good idea to have multiple redundant media. Each node runs a dæmon process called heartbeat. The master dæmon forks child read and write processes to each heartbeat media, along with a status process. When a node death is detected, Heartbeat runs shell scripts to start or stop services on the secondary node. By design, these scripts use the same syntax as the system init scripts (normally found in /etc/init.d). Default scripts are furnished for filesystem, web server and virtual IP failovers.
Starting with two identical LDAP servers, several configurations can be used. First we could do a “cold standby”, where the master node would have a virtual IP and a running server. The secondary node would be sitting idle. When the master node fails, the server instance and IP would move to the cold node. This is a simple setup to implement, but data synchronization between the master and secondary servers could be a problem. To solve that, we can instead configure the cluster with live servers on both nodes. This way, the master node runs the master LDAP server, and the secondary node runs a slave instance. Updates to the master are immediately pushed to the slave via slurpd (Figure 2).
Failure of the master node leaves our secondary node available to respond to queries, but now we cannot update. To accommodate updates, on a failover we'll restart the secondary server and promote it to the master server position (Figure 3).
This second configuration gives us full LDAP services, but adds one gotcha. If updates are made to the secondary server we'll have to fix the primary one before allowing it to restart. Heartbeat supports a nice failback option that bars a failed node from re-acquiring resources after a failover, an option that would be preferable. So, we'll show a restart by hand. Our sample configuration uses the Heartbeat-supplied virtual IP facility.
If heavy query loads need to be supported, the virtual IP could be replaced with an IP sprayer that distributes queries to both master and slave servers. In this case, update requests made to the slave would result in a referral. Follow-up on referrals is not automatic, so the functionality must be built into the client application. The master and slave nodes are identically configured except for the replication directives [see the Sidebar on the LJ FTP site, ftp.linuxjournal.com/pub/lj/listings/issue104/5505.tgz]. The master configuration file indicates the location of the replication log file and contains a listing of the slave servers, which are replication targets with credential information:
replica host=slave5:389 binddn="cn=Manager,dc=lcc,dc=ibm,dc=com"; bindmethod=simple credentials=secret
The slave configuration file does not indicate the master server. Rather it lists the credentials needed for replication: