```eval_rst
:orphan:
```

# Hosting multiple organizations

The vast majority of Zulip servers host just a single organization (or
"realm", as the Zulip code calls organizations).  This article
documents what's involved in hosting multiple Zulip organizations on a
single server.

Throughout this article, we'll assume you're working on a zulip server
with hostname `zulip.example.com`.  You may also find the more
[technically focused article on realms](../subsystems/realms.html) to be useful
reading.

## Subdomains

Zulip's approach for supporting multiple organizations on a single
Zulip server is for each organization to be hosted on its own
subdomain.  E.g. you'd have `org1.zulip.example.com` and
`org2.zulip.example.com`.

Web security standards mean that one subdomain per organization is
required to support a user logging into multiple organizations on a
server at the same time.

When you want to create a new organization, you need to do a few
things:

* If you're using Zulip older than 1.7, you'll need to set
  `REALMS_HAVE_SUBDOMAINS=True` in your `/etc/zulip/settings.py`
  file.  That setting is the default in 1.7 and later.
* Make sure you have SSL certificates for all of the subdomains you're
  going to use.  If you're using
  [our LetsEncrypt instructions](ssl-certificates.html), it's easy to
  just specify multiple subdomains in your certificate request.
* If necessary, modify your `nginx` configuration to use your new
  certificates.
* Use `./manage.py generate_realm_creation_link` again to create your
  new organization.  Review
  [the install instructions](install.html) if you need a
  refresher on how this works.
* If you're planning on using GitHub auth or another social
  authentication method, review
  [the notes on `SOCIAL_AUTH_SUBDOMAIN` below](#social-authentication).

For servers hosting a large number of organizations, like
[zulipchat.com](https://zulipchat.com), one can set
`ROOT_DOMAIN_LANDING_PAGE = True` in `/etc/zulip/settings.py` so that
the homepage for the server is a copy of the Zulip homepage.

### SSL Certificates

You'll need to install an SSL certificate valid for all the
(sub)domains you're using your Zulip server with.  You can get an SSL
certificate covering several domains for free by using
[our Certbot wrapper tool](../production/ssl-certificates.html#after-zulip-is-already-installed),
though if you're going to host a large number of organizations, you
may want to get a wildcard certificate.  You can also get a wildcard
certificate for
[free using Certbot](https://community.letsencrypt.org/t/getting-wildcard-certificates-with-certbot/56285),
but because of the stricter security checks for acquiring a wildcard
cert, it isn't possible for a generic script like `setup-certbot` to
create it for you; you'll have to do some manual steps with your DNS
provider.

### Other hostnames

If you'd like to use hostnames that are not subdomains of each other,
you can set the `REALM_HOSTS` setting in `/etc/zulip/settings.py` to a
Python dictionary, like this:

```
REALM_HOSTS = {
    'mysubdomain': 'hostname.example.com',
}
```

What this will do is map the hostname `hostname.example.com` to the
realm whose `subdomain` in the Zulip database is `mysubdomain`.

In a future version of Zulip, we expect to move this configuration
into the database.

### The root domain

Most Zulip servers host a single Zulip organization on the root domain
(i.e. `zulip.example.com`).  The way this is implemented internally
involves the organization having the empty string (`''`) as its
"subdomain".

You can mix having an organization on the root domain and some others
on subdomains (e.g. `subdivision.zulip.example.com`), but this only
works well if there are no users in common between the two
organizations, because the auth cookies for the root domain are
visible to the subdomain (so it's not possible for a single
browser/client to be logged into both).  So we don't recommend that
configuration.

### Social authentication

If you're using GitHub authentication (or any other authentication
backend that we implement using python-social-auth), you will likely
want to set the `SOCIAL_AUTH_SUBDOMAIN` setting to something (`'auth'`
is a good choice) and update the GitHub authentication callback URL to
be that subdomain.  Otherwise, your users will experience confusing
behavior where attempting to login using a social authentication
backend will appear to log them out of the other organizations on your
server.

### The system bot realm

This is very much an implementation detail, but worth documenting to
avoid confusion as to why there's an extra realm when inspecting the
Zulip database.

Every Zulip server comes with 1 realm that isn't created by users: the
`zulip` realm.  By default, this realm only contains the Zulip "system
bots".  You can get a list of these on your system via
`./scripts/get-django-setting INTERNAL_BOTS`, but this is where bots
like "Notification Bot", "Welcome Bot", etc. exist.  In the future,
we're considering moving these bots to exist in every realm, so that
we wouldn't need the system realm anymore.