Instant Messaging Services with ejabberd on Ubuntu 9.10 (Karmic)

Ejabberd is a Jabber daemon written in the Erlang programming language. It is extensible, flexible and very high performance. With a web-based interface, and broad support for XMPP standards, ejabberd is a great choice for a multi-purpose XMPP server. Ejabberd can be considered “heavyweight” by critics, because of the requirements of the Erlang run-times. However, it is incredibly robust and can scale to support incredibly heavy loads. Ejabberd servers are believed to be the backbone for some of the largest Jabber servers running now.

This installation process assumes that you have a working installation of Ubuntu 9.10 (Karmic), have followed the steps in the Setting Up and Securing a Compute Instance guide, and now have an up to date instance of the Ubuntu Karmic operating system. We also assume you are connected to your Linode via SSH as root. Once you’ve completed these requirements, we can begin with the installation process.

XMPP/Jabber Basics

Though you can successfully run an XMPP server with only a passing familiarity of the way the XMPP network and system works, understanding the following basic concepts will be helpful:

• The JID or “Jabber ID,” is the unique identifier for a user in the XMPP network. It often looks like an email and contains the username that identifies a specific user on a server, the hostname that identifies the server, and a resource that identifies where a given user is logged in from. The resource is optional, and is often safely omitted or ignored for most users. In following example, “username” is the username, “example.com” is the hostname, and “/office” is the resource.

  username@example/office
  

  Again, the resource is optional; although XMPP allows a single JID to be connected to the server from multiple machines (i.e. resources), the resource adds a useful amount of specificity.

• The XMPP system is federated by nature. Users with accounts on one server–if the server administrators allow it–can communicate with users on other servers. Without a centralized server, every XMPP server maintains the accounts and serves as the communication gateway for their own users. In the XMPP system there is no single point of failure, however each server administrator can decide how their server is going to participate in the federated network. For instance, to federate with Google’s “GTalk” XMPP network, server administrators need to have server-to-server (s2s) SSL/TLS encryption enabled, while other servers don’t always require this.

• XMPP takes advantage of “SRV” DNS records to support the resolution of domains to the servers which provide DNS records.

Enabling the Universe Repository

Prior to installing the ejabberd daemon, you will need to enable the universe repository. Open /etc/apt/sources.list with your favorite text editor and make sure that the following lines are present:

File: /etc/apt/sources.list

1 2 3 4 5 6 7 8

deb http://us.archive.ubuntu.com/ubuntu/ karmic universe deb-src http://us.archive.ubuntu.com/ubuntu/ karmic universe deb http://us.archive.ubuntu.com/ubuntu/ karmic-updates universe deb-src http://us.archive.ubuntu.com/ubuntu/ karmic-updates universe deb http://security.ubuntu.com/ubuntu karmic-security universe deb-src http://security.ubuntu.com/ubuntu karmic-security universe

Now issue the following commands:

apt-get update
apt-get upgrade

Install ejabberd

To install ejabberd and its required dependencies, issue the following command:

apt-get install ejabberd

The default installation is complete and functional. The installation process creates a self-signed SSL certificate. If you want to use a commercially signed certificate, place the certificate file at /etc/ejabberd/ejabberd.pem. Most of the time a self-signed certificate is sufficient for many jabber applications.

If you have not already configured your /etc/hosts as follows, please do that before you continue. This will allow your Linode to associate its hostname with the public IP. Your file should have an excerpt that looks something like this, (use your Linode’s public IP address instead of 12.34.56.78):

File: /etc/hosts

1 2	127.0.0.1 localhost.localdomain localhost 12.34.56.78 username.example.com username

With the hostname configured, you’re ready to begin configuring ejabberd.

Configure ejabberd

Ejabberd’s configuration files are written in Erlang syntax, which might be difficult to comprehend. Thankfully, the modifications we need to make are relatively minor and straightforward. The main ejabberd configuration file is located at /etc/ejabberd/ejabberd.cfg. We’ll cover each relevant option in turn.

Administrative Users

Some users will need the ability to administer the XMPP server remotely. By default this block of the config file looks like this:

File: /etc/ejabberd/ejabberd.cfg

1	%% Admin user {acl, admin, {user, "", "localhost"}}.

In Erlang, comments begin with the % sign, and the Access Control list segment contains information in the following form: {user, "USERNAME", "HOSTNAME"}. The following examples correspond to the users with the JIDs of admin@example.com and username@example.com. You only need to specify one administrator, but you can add more than one administrator simply by adding more lines, as shown below:

File: /etc/ejabberd/ejabberd.cfg

1 2	{acl, admin, {user, "admin", "example.com"}}. {acl, admin, {user, "username", "example.com"}}.

All users specified in this manner have full administrative access to the server through both the XMPP and web-based interfaces. You will have to create your administrative users (as described below) before they can log in.

Hostnames and Virtual Hosting

A single ejabberd instance can provide XMPP services for multiple domains at once, as long as those domains (or subdomains) are hosted by the server. To add a hostname for virtual hosting in ejabberd, modify the hosts option. By default, ejabberd is only configured to host the “localhost” domain:

File: /etc/ejabberd/ejabberd.cfg

1	{hosts, ["localhost"]}.

In the following example, ejabberd has been configured to host a number of additional domains. In this case “username.example.com,” “example.com,” and “example.com.”

File: /etc/ejabberd/ejabberd.cfg

1	{hosts, ["username.example.com", "example.com", "example.com"]}.

You can specify any number of hostnames in the host list, but you should be careful to avoid adding inserting a line break as this will cause ejabberd to fail.

Listening Ports

TCP port number 5222 is the conventional “XMPP” port. If you want to change the port, this is the section of the configuration that needs to be modified.

Additionally, you may want to enable SSL access for client-to-server (c2s) SSL/TLS connections if you or the other users of you are using a client that requires legacy support for secured connections on port 5223. Uncomment the following stanza.

File: /etc/ejabberd/ejabberd.cfg

1 2 3 4 5 6

{5223, ejabberd_c2s, [ {access, c2s}, {shaper, c2s_shaper}, {max_stanza_size, 65536}, tls, {certfile, "/etc/ejabberd/ejabberd.pem"} ]},

Additional Functionality

The ejabberd.cfg file is complete and well commented, and from this point forward your server should run. However, you should take the time to become familiar with the options provided in this file.

By default, MUCs or Multi-User-Chats (chatrooms) are accessible on the “conference.[hostname]” sub-domain. If you want the public to be able to access MUCs on your domain, you need to create an “A Record” pointing the conference hostname (e.g. subdomain) to the IP address where the ejabberd instance is running.

Using Ejabberd

Once installed, the use and configuration of ejabberd is uncomplicated. To start, stop, or restart the server issue the appropriate command from the following:

/etc/init.d/ejabberd start
/etc/init.d/ejabberd stop
/etc/init.d/ejabberd restart

By default, ejabberd is configured to disallow “in-band-registrations,” which prevents Internet users from getting accounts on your server without your consent. To register a new user, issue a command in the following form:

ejabberdctl register lollipop example.com man

In this example, lollipop is the username, example.com is the domain, and man is the password. This will create a JID for lollipop@example.com with the password of “man.” Use this form to create the administrative users specified above.

To remove a user from your server, issue a command in the following form:

ejabberdctl unregister lollipop example.com

The above command would unregister the lollipop@example.com account from the server.

To set or reset the password for a user, issue the following command:

ejabberdctl set-password lollipop example.com morris

This command changes the password for the lollipop@example.com user to morris.

To back up ejabberd’s database, issue the following command:

ejabberdctl dump ejabberd-backup.db

This command dumps the contents of the internal ejabberd database into a file located in the “/var/lib/ejabberd/” directory. To restore from the backup, issue the following command:

ejabberdctl load ejabberd-backup.db

For more information about the ejabberdctl command, issue ejabberdctl help or man ejabberdctl.

If you would prefer to administer your ejabberd instance via the web-based interface, log in to http://example.com:5280/admin/, where “example.com” is the domain where ejabberd is running. Log in with the full JID as the username and the password of one of the administrators specified in the /etc/ejabberd/ejabberd.cfg file.

XMPP Federation and DNS

To ensure that your ejabberd instance will federate properly with the rest of the XMPP network, particularly with Google’s “GTalk” service (i.e. the "@gmail.com" chat tool,) we must set the SRV records for the domain to point to the server where the ejabberd instance is running. We need three records, which can be created in the DNS Management tool of your choice:

1. Service: _xmpp-server Protocol: TCP Port: 5269
2. Service: _xmpp-client Protocol: TCP Port: 5222
3. Service: _jabber Protocol: TCP Port: 5269

The “target” of the SRV record should point to the publicly routable hostname for that machine (e.g. “username.example.com”). The priority and weight should both be set to 0.

Troubleshooting

If you’re having problems getting ejabberd to start, or are getting obscure errors on the console don’t be discouraged: the errors generated by Erlang are often abstruse at best. The logs for ejabberd are located in the /var/log/ejabberd/ directory. If you’re getting error messages look in these files, particularly ejabberd.log and sasl.log. Additionally, if ejabberd crashes, the “image dump” of Erlang will be saved in this directory. Begin your investigations for error messages in these files.

Furthermore, ejabberd’s “Mnesia” database is stored in the /var/lib/ejabberd/ directory. If you think the database has become corrupted, delete the files in this directory (e.g. rm /var/lib/ejabberd/*) and reload from a backup if necessary. This is sometimes required if the hostname of the local machine changes.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Ejabberd Community Site
• XMPP Standards Foundation
• XMPP Client Software