multi server setup guide - installation and filesystem replication

PROBLEM
I'm not sure how to install atmail mail server in a multi server environment.

ENVIRONMENT

• atmail on-premises mail server installations: current version

CAUSE
atmail has not yet been installed.

RESOLUTION

Before continuing with this guide, setup MySQL master-master replication or MariaDB master-slave replication first.

Installing atmail's First Instance

You can now install atmail. You will need to do this on only one of the servers first, followed by the NFS replication setup. For the purposes of this guide, we will use the server1.local as configured above (192.168.0.2).

 Mail Server Installation

Email Replication Across Servers

There are three methods for replicating email files across servers: NFS, GlusterFS and Unison. Instructions on how to implement your preferred replication format follow below

GlusterFS

GlusterFS is an open source clustered file system capable of scaling to several petabytes and handling thousands of clients. GlusterFS clusters together storage building blocks, aggregating disk and memory resources and managing data in a single global namespace.

A key advantage is the GlusterFS modular architecture that allows modules to be stacked to match user requirements. You can use GlusterFS to quickly configure a standalone server system and later expand the system as your needs grow. For more information, see: http://gluster.org

⚠ Note: There are two sub-tutorials - one for GlusterFS 3.0 and one for GlusterFS 2.0. We highly recommend that you use the GlusterFS 3.0 documentation.

For this tutorial, there will be two servers - server 1 (192.168.0.2) and server 2 (192.168.0.3). This tutorial, with its usage of yum, is primarily for CentOS 5.4. Feel free to use the equivalent package manager for your operating system.

GlusterFS 3.3

First, make sure that the fuse module is installed:

Check that it's installed by running this command:

The above command should produce output similar to this:

Once the fuse module is installed, you are now ready to install GlusterFS 3.3. Note that this installation will require the deactivation of previous GlusterFS installations (i.e., 3.2 or older). Make sure your backups are in place.

On both machines, download the GlusterFS 3.3 source from: http://download.gluster.org/pub/gluster/glusterfs/LATEST/

Untar, install:

The output of the end of the installation should be similar to:

Start the GlusterFS service on both machines:

Once the service is started, run the following on server 1 (Where 192.168.0.3 is the other server):

The output should be similar to:

You can test the peer status further by executing:

Which should yield similar output to the following:

Afterwards, create the GlusterFS volume on server 1. This command will also create the GlusterFS volume on server 2:

In the above command, replace the IP addresses with the IP addresses of your GlusterFS servers; replace /export/atmail/ with your preferred GlusterFS export directory.

Start the volume with this command:

Mount the directories afterward to your desired mount points. For this example, we will use the atmail users/ directory.

On server 1, mount the GlusterFS volume:

Do the same for server 2:

Create the tmp directory for server1:

Set a symlink to the /usr/local/atmail/webmail/tmp/ directory:

Do the same for server 2:

Congratulations. Your installation is now setup to use GlusterFS 3.3.0.

Unison As A File-Replication System

Unison allows two replicas of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other. In other words, it can be used for two-way synchronization between your servers.

For illustrative purposes in this example, the Unison Master hostname will be called "master-foo-01"; the Client hostname will be called "slave-foo-02".

Installing Unison

To setup Unison on your machine, do the following for both machines. Download the dependency, OCaml from: http://caml.inria.fr/download.en.html

Install emacs (a dependency):

Unpack, install OCaml:

Download Unison from: http://www.cis.upenn.edu/~bcpierce/unison/download.html

Untar, install:

Move the Unison binary to your preferred prefix. We suggest /usr/bin/:

Creating login keys for Unison

You will then need to set the keys, so the servers can automatically talk to each other. Create a SSH public key on the Master server:

This will produce a public key in /home/atmail/.ssh/id_rsa.pub. Copy the .pub file to the Slave machine:

Pipe the public key into a file called authorized_keys, in your .ssh directory.

Make sure that the authorized_keys file, and all of the contents of the .ssh directory have 600 permissions:

Try to login, sans password, from the Master machine to the Client machine.

This should allow you to login.

Running Unison

Now, you can try to sync the differences between the users/ directory of the master and slave machines. From any of the machines, execute:

The output should be similar to:

You will need to set this in cron. Create a file called /home/atmail/unison.sh. In the file:

⚠ A Note about Unison preferences

Set permissions:

Add an entry to your /etc/crontab. It will look like:

This will synchronize your /usr/local/atmail/users directory for both machines.

NFS Replication

NFS is another viable alternative. NFS allows a user on a client server to access files over a network in a manner similar to how local storage is accessed. One potential drawback is that there is a single point of failure. However, this can be minimized by splitting directories across two servers. The tutorial below will cover basic server-client NFS setup.
To get started, open up the /etc/exports file on the first server (server1.local). Add the following lines:

This adds a permission entry for 192.168.0.3 (server2.local) to access your NFS share. Afterwards, restart NFS:

Login afterwards to server2.local, then restart NFS services:

Try to mount the mount point from server1.local afterwards, you will need to create the /usr/local/atmail/users directory if it does not already exist:

Now the directory is shared between the servers.

Installing The Second Instance Of atmail

Now you can install the second atmail instance. The steps are the same as the first atmail instance.

Mail Server Installation

Using Rsync to synchronize the atmail application between frontends

After installing atmail on the second instance, you are required to setup a cron job to rsync the atmail application directory between servers. This ensures any updates to the atmail application on the master frontend, are replicated between the other server instances.

On each frontend for atmail append the following script to /root/rsync-atmail.sh

After applying the script, set execute permissions:

Next, edit the /etc/crontab and define an rsync push every 10 minutes to synchronize the application directory folder.

Once setup

Configuring the Dovecot IMAP server

⚠ Note: These configuration parameters are only necessary for multiservers setups that use NFS. If using GlusterFS or Unison, skip this step.

Dovecot needs additional configuration parameters before it can work with NFS. You will need to add this at the top of your /usr/local/atmail/mailserver/etc/dovecot.conf file:

Add this to both files on both servers. Restart atmail afterwards on both machines:

DNS Round Robin

Load Balancing

Load balancing can take many forms. Some may require specialized load balancing hardware to split the load between servers, while some may prefer sharing the load via DNS tweaks. This portion of the guide will guide you through setting up a DNS round robin.

The DNS round robin works on the principle that both machines have equal priority. This means that when a user sends a request for Web, SMTP, or POP/IMAP service, a server is randomly picked to handle the load.

For this guide, we will have three machines: server1.domain, server2.domain and server3.domain.

First, each machine must be defined in your zone file:

The hostname for each domain is placed on the left side. The IP addresses are in the right side. Set your hostname to your preferences.

DNS MX records can now be assigned. You will need to assign the MX records equal priority:

Replace "domain.com" with your domain-name. Next, you will need to add a subdomain for all POP3, IMAP, or Web requests to point to. For this example, we will use "mail":

You can now restart your nameserver. This will then apply the changes.

To test, type this command:

Replace "nameserver" with your nameserver's hostname or IP. Replace "domain.com" with your domain name. This should show something similar to:

Make sure that the "ANSWER SECTION" corresponds to your MX records. Next, we will test the domain name settings for your subdomain. It can be done via this command:

Replace "domain.com" with your domain. It will show something similar to:

Once verified, you can then set the servers to accept connections.