About Sandcats.io

Sandcats.io is a free-of-cost dynamic DNS service and HTTPS certificate service run by the Sandstorm development team. In a nutshell:

  • Sandstorm users can have a free domain name of the form example.sandcats.io.

  • Sandstorm can automatically set up that domain, including a valid HTTPS certificate for it.

  • It's an official part of Sandstorm and we recommend people use it!

In more detail:

  • Users host their own servers. A hostname like example.sandcats.io points at the IP address of someone's server, and that server holds its own private keys.

  • It assumes your server should be reachable from the global Internet.

  • It's an optional service. Keep reading this page to learn how to stop using it.

  • It points at your server's public (globally routable) IP address, which it auto-detects. Read elsewhere about setting up Sandstorm to use an internal IP address.

The purpose is to help people who run their own server have a working hostname and HTTPS (TLS/SSL) certificate without having to think hard about the domain name system or public key infrastructure.


Sandcats is integrated into the Sandstorm installer so that when you install Sandstorm, you get working DNS, including wildcard DNS, as well as working HTTPS for the main Sandstorm interface.

The Sandcats backend is free, open source software under the Apache License 2.0; you can view and participate in the project.

The Sandcats DNS service provides 60-second latency for IP address updates via a custom UDP protocol to detect address changes. To achieve this low latency, when Sandcats integration is enabled, your Sandstorm server sends a UDP ping message to the central Sandcats service every 60 seconds.

The Sandcats certificate service (for providing users with valid HTTPS) provides seven-day certificates and an API for automatic renewal.

Sandcats uses HTTPS client certificates for authentication, which Sandstorm and the install script manage for users. You can find these certificates under /opt/sandstorm/var/sandcats by default. Please save these somewhere safe so you can hold onto your domain.

How the HTTPS service works

The Sandstorm install script, when it runs on your server, generates a private key and certificate signing request that it sends to the Sandcats.io service (via the /getcertificate JSON-RPC endpoint).

Sandcats verifies that the request is coming from the owner of this particular example.sandcats.io domain name, and if so, passes the request along to GlobalSign for signing. The install script receives the signed certificate and places it in /opt/sandstorm/var/sandcats/https/example.sandcats.io/.

When Sandstorm starts, it looks in the above directory for keys & certificates and uses the first certificate that is valid.

These certificates expire weekly, so Sandstorm also checks every (approximately) 2 hours if the certificate it is using is on the last 3 days of its lifetime. If so, Sandstorm takes the same action as the install script: generate new key, generate certificate signing request, send that to Sandcats.io, store the response. (As an implementation detail, these certs technically last 9 days, but we renew them every 7 days.)

Sandstorm automatically starts using new certificates without needing intervention from the server operator. You can read the code that powers that in meteor-bundle-main.js in the sandstorm git repository.

Administering your sandcats.io subdomain

Finding debugging information

By default, Sandstorm stores a log in a text file at /opt/sandstorm/var/log/sandstorm.log. You can read it by running this command:

sudo less /opt/sandstorm/var/log/sandstorm.log

This launches a tool called less; for help using less, read this tutorial.

Disabling the sandcats service

If your Sandstorm server used to use sandcats.io but you want to transition to your own domain name (with wildcard DNS), you can disable the sandcats-related code in your Sandstorm install.

To do that, open your /opt/sandstorm/sandstorm.conf file in a text editor and notice this line:


Remove that line entirely, then save and quit your editor. Run sudo service sandstorm restart to cause Sandstorm to notice your changes to its configuration file.

That will disable the sandcats-related functionality in your Sandstorm server on your system. This means your domain will stop automatically updating its IP address.

Note that this does not delete any domains you registered. That's OK with us; from our perspective, there's no need to email us to delete your domain.

Re-installing Sandstorm and keeping your sandcats domain

If you have already registered a domain like example.sandcats.io as part of installing Sandstorm, but you find yourself doing a fresh install of Sandstorm, you can use our email-based recovery system.

You won't need any files from the old Sandstorm install. Instead, run the Sandstorm install script (which we call install.sh) on a new server; follow the prompts to recover a domain by typing help at the Sandcats prompts.

Overview. This process will:

  • Ask you what sandcats.io subdomain you use.

  • Send you an email with a short-term token.

  • Ask you for the token, then pass it to the sandcats.io service.

The install will continue and your new Sandstorm install will be bound to example.sandcats.io.

Full details for those who are curious.

  • When you run the install.sh script, if you choose mode 1 for a full server, and you say yes to the defaults, install.sh prepares to enable sandcats.io (even if you end up not using the sandcats.io service).

  • Specifically, install.sh looks for an existing client certificate on your system in /opt/sandstorm/var/sandcats/id_rsa.private.combined. sandcats.io uses client certificates to identify a Sandstorm server as controlling a specific domain like example.sandcats.io. If install.sh does not find one, it generates one using openssl.

  • install.sh asks via the console what sandcats domain you want to register. At this point, you can type help. This changes the question - install.sh now asks what domain you want to recover. Provide your sandcats hostname.

  • install.sh then uses curl to ask sandcats.io to send an email to the address that you provided when first registering the domain. The email contains a small bit of text that serves as a one-time-use recovery token.

  • install.sh waits for you to receive the email and asks via the console for your recovery token.

  • install.sh sends that recovery token to sandcats.io using curl, while also providing the client certificate currently on your system (/opt/sandstorm/var/sandcats). If the recovery token matches what the server expects, then the server updates your user registration to trust the client certificate on your system.

Manually moving sandcats client certificates to a new Sandstorm install

If you prefer, you can move your sandcats.io credentials to a new Sandstorm install without running the install.sh script. We call that file-based recovery. Here are the steps.

  • Find your three three id_rsa certificate files (usually /opt/sandstorm/var/sandcats) and keep them safe somewhere. Also keep a copy of /opt/sandstorm/var/sandcats/https if it exists.

  • Do a new Sandstorm install, presumably on a new server somewhere. It will install to /opt/sandstorm. You should choose a non-sandcats.io host name during this process, such as using literally example.com.

  • Copy those three id_rsa certificate files from the old server to the new server's Sandcats directory, /opt/sandstorm/var/sandcats. Do the same for /opt/sandstorm/var/sandcats/https if you backed it up.

  • In your new Sandstorm install, ensure you have your BASE_URL and WILDCARD_HOST set properly. If your sandcats.io subdomain is example, then you'll need BASE_URL=example.sandcats.io and WILDCARD_HOST=*.example.sandcats.io. Consider copying these values from the old server's sandstorm.conf.

  • Edit the new server's sandstorm.conf to contain this line: SANDCATS_BASE_DOMAIN=sandcats.io

  • Now restart Sandstorm by running sudo service sandstorm stop ; sudo service sandstorm start, and wait at least 60 seconds.

  • Your DNS hostname should have auto-updated. Check that DNS is working with nslookup <myname>.sandcats.io from another machine. This will help eliminate DNS as an issue when trying to access your server.

Note that if you are using sandcats.io free HTTPS certificates, we suggest also backing up and restoring the contents of /opt/sandstorm/var/sandcats/https. This is a suggestion rather than a hard requirement; Sandstorm will request new certificates at startup. However, if your server makes lots of requests, you will run afoul of the sandcats.io anti-abuse protections. See the Diagnosing "Not Authorized" problems section for details.

Diagnosing "Not Authorized" problems

If you see Not Authorized in your log files, the sandcats.io service is returning HTTP code 403 for at least one request from your server.

One reason this occurs is if you have the wrong id_rsa* certificate files in /opt/sandstorm/var/sandcats. You can fix this problem using the email-based recovery system; for now, this requires using install.sh on a throwaway VM. Once your new certificate files are registered with sandcats.io, you can move them to whichever server you want using file-based recovery.

Another reason you might see Not Authorized in the log files is if your server has run afoul of sandcats.io's defense in depth against Sandstorm bugs. The HTTPS certificate service within sandcats.io will reject new certificate requests if your server has more than approximately 5 active certificates per week; this code exists to prevent a Sandstorm bug from requesting many thousands of certificates. If you are constantly requesting new certificates, you can request only about 5 before being automatically blocked in this way. Typically, your server will keep retrying and the sandcats.io service will permit it to get certificates again when one of your certificates expire.

In either case, if you need further help, please email support@sandstorm.io!

Terms of service, privacy policy, & contact information

Sandcats.io has the following formal documents:

If you have more questions, or are having trouble, email: