Virtual Domains

What are virtual domains?

Virtual domains means hosting a service for more than one domain on a single server. Cyrus IMAP has the ability to host IMAP/POP mailboxes for multiple domains (for example: test@cyrusisgreat.org and test@ilovecyrus.com) on a single server or Murder.

Cyrus needs to know which domain to use when a mailbox is accessed. There are two ways in which Cyrus can determine the domain:

Fully qualified userid

The client logs in with a userid containing the domain in which the user belongs (for example: test@cyrusisgreat.org or test@ilovecyrus.com)

IP address

The server looks up the domain based on the IP address of the receiving interface (useful for servers with multiple NICs or those using IP aliasing)

If the virtdomains option is set to off (or no, 0, false), Cyrus does not know or care about domains, and only ever considers the local part of email addresses. This configuration is never recommended, but is currently the default.

If the virtdomains option is set to userid, then only the fully qualified userid is used. This is the only recommended configuration for new deployments, and in the future may become the default or only option. Existing deployments should strongly consider migrating towards this configuration.

If the virtdomains option is set to on (or yes, 1, true), Cyrus uses both mechanisms to work out the domain (with the fully qualified userid taking precedence). This configuration is not recommended.

Note

If you are providing calendaring services, you MUST use the virtdomains: userid configuration. Calendaring services require a consistent single authoritative fully-qualified email address for each user in order to function, and this is the only configuration that provides it.

The virtdomains: off and virtdomains: on configurations both allow users’ domains to be changed from outside of Cyrus without Cyrus knowing about it, which fundamentally breaks calendaring. These configurations are only suitable for IMAP-only deployments.

Concepts

Everyone is in a domain

It’s best to think of every user as existing inside a domain. Unqualified users are technically inside the defaultdomain.

Names can be qualified

Global admins can reference mailboxes and IDs by qualified names. That is, for any given mailbox command, you can add @domain to the end of the mailbox name.

Here are some examples:

  • cyradm> create user/lukecage@example.net - create a user

  • cyradm> create user/mercedesknight@example.net - create another user

  • cyradm> setquota user/lukecage@example.net 50000 - define a quota

  • cyradm> setaclmailbox user/lukecage@example.net mercedesknight@example.net read - give Mercedes Knight read access to Luke Cage’s mailbox

  • cyradm> listmailbox *@example.net - list all mailboxes in the example.net domain

Each mailbox exists in only one domain

Domains are mutually exclusive

Users only have access to mailboxes within their own domain (intra-domain). The following example will not work: setacl user/mercedesknight@herdomain.com lukecage@hisdomain.com read.

Global and Domain admins

The Cyrus virtual domains implementation supports per-domain administrators as well as global (inter-domain) administrators.

Domain-specific administrators are specified with a fully qualified userid in the admins option (e.g., admin@example.net) and only have access to mailboxes in the associated domain.

Global administrators are specified with unqualified userids.

MOST OF THIS SHOULD BE IN DEPLOYMENT GUIDE?

Quick Start

  • Add virtdomains: userid to imapd.conf(5)

  • Add a defaultdomain entry to imapd.conf(5)

  • Use cyradm (as a global or domain admin) to create mailboxes for each domain.

Configuration

Support for virtual domains is enabled by turning on the virtdomains option in imapd.conf(5).

When upgrading from a single domain installation to a virtual domain installation, the name of the existing domain (domain of the server hostname) should be specified using the defaultdomain option in imapd.conf(5). This allows users to continue to access their mailboxes using unqualified userids. For example, if the primary IP address on your server resolves to ‘www.xxx.yyy.zzz’, then set defaultdomain to ‘xxx.yyy.zzz’.

Even for new installations, set the defaultdomain to the “real” domain of the server (domain of its primary hostname). See Administrators for further discussion.

Here is a sample imapd.conf with a minimal set of configuration options:

configdirectory: /var/imap
partition-default: /var/spool/cyrus
admins: admin lukecage.admin@hisdomain.com mercedesknight.admin@herdomain.net
virtdomains: yes
defaultdomain: exampleisp.net

This example has three domains: exampleisp.net, hisdomain.com, and herdomain.net. admin can administer all three domains, while lukecage.admin@hisdomain.com and mercedesknight.admin@herdomain.net can only administer their respective domains.

Everyday users should not be administrators. In the above example, Mercedes Knight and Luke Cage have separate administrative accounts for their domains.

Multiple IP Addresses

In order to use a multiple IP address configuration, the server must be able to do a reverse lookup on the IP address to determine the hostname of the receiving interface. For example:

192.168.0.1  ->  mail.example.com
192.168.0.2  ->  mail.example.net
192.168.0.3  ->  mail.foo.bar

Once the server obtains the fully qualified hostname of the interface, it removes the localpart (i.e., ‘mail’) and uses the remainder as the domain for any user that logs in.

This address to hostname mapping would usually be done via DNS, /etc/hosts, NIS, etc. Configuration of the various naming services is beyond the scope of this document.

Delivering mail

To deliver mail to your virtual domains, configure your MTA so that the envelope recipient (RCPT TO) passed to lmtpd is fully qualified with the correct domain.

Configuring Sendmail

Follow the basic configuration instructions.

Some items to be aware of:

  • It is easiest to use the mailertable to route mail to Cyrus, rather than adding the domain to the local-host-names file ($w). This prevents Sendmail from changing the domain name to the local host name.

    example.com              cyrusv2:/var/imap/socket/lmtp

  • You’ll have to use the Cyrus mailer in LMTP mode, and you’ll have to change the mailer flags so that it provides the full domain while communicating via LMTP. Specifically these changes:

    S=EnvFromSMTP/HdrFromSMTP, R=EnvToSMTP

Mail Clients

The only changes you’ll need to make to mail clients is to change usernames to the fully qualified domain names, i.e., user@example.com. The user%example.com form of userid is also supported.

Users in the default domain will not need to reconfigure their clients (as unqualified userids are assumed to be in the default domain).

Administrators

The Cyrus virtual domains implementation supports per-domain administrators as well as “global” (inter-domain) administrators. Domain-specific administrators are specified with a fully qualified userid in the admins option (e.g., admin@example.net) and only have access to mailboxes in the associated domain. Mailbox names should be specified in the same fashion as on a single domain configuration.

Global administrators are specified with an unqualified userid in the admins option and have access to any mailbox on the server. Because global admins use unqualified userids, they belong to the defaultdomain. As a result, you CANNOT have a global admin without specifying a defaultdomain. Note that when trying to login as a global admin to a multi-homed server from a remote machine, it might be necessary to fully qualify the userid with the defaultdomain.

Global admins must use mailbox@domain syntax when specifying mailboxes outside of the defaultdomain. Examples (using cyradm):

To create a new INBOX for user ‘test’ in defaultdomain:

cm user/test

To create a new INBOX for user ‘test’ in domain ‘example.com’:

cm user/test@example.com

To list all mailboxes in domain ‘example.com’:

lm *@example.com