To run Sandstorm, you must assign it a wildcard host, in which it can generate new hostnames as-needed. For instance, you might set up Sandstorm to run at example.com and assign it the wildcard *.example.com.

This page explains how to configure this and why it is needed.

How to configure and test your own wildcard DNS record for Sandstorm

If your Sandstorm server is at example.com you might have the following lines in your /opt/sandstorm/sandstorm.conf.

BASE_URL=https://example.com
WILDCARD_HOST=*.example.com

In order for Sandstorm grains to load, DNS lookups for domains within the WILDCARD_HOST need to resolve to your Sandstorm server. You can manually test this on many systems. Open a terminal and run this comand.

host arbitrary.example.com

You should see a message like:

arbitrary.example.com has address 93.184.216.34

This looks good: in the example, we see that a WILDCARD_HOST of *.example.com exists. The IP address printed by the host command should match the IP address for your Sandstorm server. If host tells you the domain is not found: 3(NXDOMAIN), then you probably need to adjust your DNS zone to create a wildcard record. If your system does not have host, you can try dig or ping which serve similar functions.

To learn how to add a new wildcard DNS record, consider reading this tutorial on setting up Sandstorm using the DigitalOcean DNS control panel. Your domain's DNS configuration tool might look different, but the fundamentals are probably the same. Text-based DNS configuration systems might need a record like the following, for a Sandstorm server running at 93.184.216.34.

* IN A 93.184.216.34

Once you add a wildcard record, you can re-run the test with host to see if your wildcard record is working. You can try looking up other subdomains than arbitrary, such as arbitrary2, to make sure that all subdomains resolve to the right IP address.

Sandstorm can use a BASE_URL within a wildcard DNS record. If *.example.com maps to the right IP address, then you can configure Sandstorm to use a BASE_URL like sandstorm.example.com that is part of a wildcard DNS record; you could set WILDCARD_HOST to ss-*.example.com or sandstorm-*.example.com, etc.. This can be convenient if you already have one wildcard DNS record. Note that the WILDCARD_HOST and the BASE_URL must be within the same domain name because otherwise web browsers may prevent Sandstorm from setting a cookie with the wildcard subdomains, as part of a web browser privacy protection feature (third-party cookies). One such configuration would look like this.

BASE_URL=https://sandstorm.example.com
WILDCARD_HOST=sandstorm-*.example.com

If you can't set up wildcard DNS, try xip.io. If your Sandstorm BASE_URL needs to contain a numeric IP address because you cannot configure wildcard DNS, consider reading about how to use Sandstorm with an internal IP address and xip.io.

Testing wildcard HTTPS

If your server uses SSL or HTTPS, you will also need working HTTPS for all possible subdomains of your server's domain name. This is also known as wildcard HTTPS.

If your server is at https://example.com/, you can visit https://testing.example.com/ and https://testing2.example.com/ in your browser. If you see any kind of certificate warning or error, then note that you need to adjust your configuration for Sandstorm to work properly. Read more in our SSL topic guide.

local.sandstorm.io and sandcats.io provide wildcard DNS

If you are using vagrant-spk to develop Sandstorm apps, or are developing Sandstorm itself, you will likely use local.sandstorm.io as the BASE_URL for your Sandstorm server. Sandstorm.io (the company behind Sandstorm) maintains local.sandstorm.io as a wildcard domain where both local.sandstorm.io and all of its subdomains (*.local.sandstorm.io) point to 127.0.0.1, the same as localhost. This allows you to run Sandstorm for development without needing to own a domain name or configure wildcard DNS for a subdomain.

Within the sandcats.io DNS service, each domain is also a wildcard domain. This allows a self-hosted Sandstorm domain to operate correctly.

Why Sandstorm needs wildcard DNS

Sandstorm is designed to implement strong sandboxing of apps, such that users need not worry about the risk that a malicious -- or simply buggy -- app might interfere with other apps or the rest of the network. Our goal is for the network admins to be able to say: "As long as it's on Sandstorm, you can run whatever apps you want, because we trust Sandstorm to keep things secure."

Using a wildcard host is just one part of Sandstorm's security model.

Frequently-asked questions about wildcards

Here are some common questions about Sandstorm's use of wildcards.

How does Sandstorm use its wildcard?

Every time you open a grain (an instance of a Sandstorm app), you begin a new "session" with that grain. Every session is assigned a unique, cryptographically-random hostname. The session ends shortly after you close the tab; after that, the hostname is disabled and is never used again.

Each session -- and thus each hostname -- belongs to a single user. Using a cookie, Sandstorm ensures that no other user can access the session's hostname. This cookie is enforced by Sandstorm, not the app; Sandstorm accomplishes this by acting as a proxy in front of the app that checks the cookie.

Why can't Sandstorm map every app under one domain, like example.com/app-name?

Due to "same-origin policy" as specified in web standards, it is impossible to isolate apps hosted within the same "origin", i.e. hostname. If two tabs or frames are in the same origin, then Javascript in each is permitted to arbitrarily modify the contents of the other, including executing arbitrary code. This makes it totally impossible for these apps to defend themselves from each other even if they tried. Hence, it is absolutely necessary for every app to be on a separate host.

Why can't Sandstorm use one hostname per app, like app-name.example.com? Then admins could configure specifically those hosts.

Multiple reasons:

  • Sandstorm aims to make app installation a one-click process. If every time a new app is installed, the user must edit DNS records and issue a new SSL certificate, one of the platform's most valuable features is lost.

  • Sandstorm does not just isolate apps from each other, but isolates individual resources within an app. For example, with Etherpad (a document editor), Sandstorm creates a new Etherpad instance in its own isolated container for every Etherpad document you create. This protects against potential security problems in Etherpad which may have allowed a user with access to one document to improperly obtain access to other documents -- Etherpad has in fact had several such bugs in the last few months, but none of them affected Etherpad on Sandstorm.

Why a new hostname for every session, rather than for every document?

Randomized, unguessable hostnames can help mitigate certain common security bugs in apps, such as XSRF, reflected XSS, and clickjacking attacks. All of these attacks involve an attacker tricking the user's browser into performing actions on another site using the user's credentials. But for any of these attacks to work, the attacker must know the address to attack. If every user gets a different hostname, and indeed the hostnames change frequently, then it is much harder to launch these kinds of attacks.

Note that because DNS requests are made in cleartext, random hostnames will not defend against an attacker who has the ability to snoop network traffic coming from the user's machine. Therefore, apps should still implement their own defenses against these attacks as they always have. But, when a bug slips through (as they commonly do), randomized hostnames make an attack much, much more difficult to pull off, which is still a big win.

Can Sandstorm contact the DNS and SSL servers to request creation of each new host on-demand, rather than require a wildcard?

While theoretically possible, Sandstorm would need to do generate a new hostname every time the user opens a document. This would lead to undesirably high load on DNS and SSL systems.