Stateless DNS Server

This is a stateless DNS server that returns A records for sub-domains, where the sub-domain label encodes the IP-address, expiration date, a random salt and an HMAC-SHA256 signature truncated to 128 bits.

This is allows for assigning temporary sub-domains names to nodes with a public IP-address. The same problem can also be solved with dynamic DNS server, but such entries often requires clean-up. The beauty of this approach is that the DNS server is state-less, so there is no stale DNS records to discard.

In TaskCluster this is used to assign temporary sub-domain names to EC2 spot nodes, such that we can host HTTPS resources, such as live logs, without updating and cleaning up the state of the DNS server.

Notice, that with IP-address, expiration date, random salt and HMAC-SHA256 signature encoded in the sub-domain label, you cannot decide which sub-domain label you wish to have. Hence, this is only useful in cases were the hostname for your node is transmitted to clients by other means, for example in a message over RabbitMQ or as temporary entry in a database. Further more, to serve HTTPS content you'll need a wild-card SSL certificate, for domain managed by this DNS server.

Note, this obviously doesn't have many applications, as the sub-domain label is stateful. It's mostly for serving HTTPS content from nodes that come and go quickly with minimal setup, where the hostname is transmitted by other means. Generally, any case where you might consider using the default EC2 hostname.

Sub-domain Label Generation

The sub-domain label encodes the following parameters:

• ip, address to which the A record returned should point,
• expires, expiration of sub-domain as number of ms since epoch,
• salt, random salt, allowing for generation of multiple sub-domain labels for each IP-address, and,
• signature, HMAC-SHA256 signature of ip, expires and salt truncated to 128 bit.

The expires property is encoded as a big-endian 64 bit signed integer. The salt property is encoded as bit-endian 16 bit unsigned integer. All properties are concatenated and base32 (RFC 3548) encoded to form the sub-domain label.

Example pseudo code:

  ip        = a.b.c.d
  expires   = Date.now() + number of ms to expiration
  salt      = random 16 bit integer
  signature = HMAC-SHA256(ip + expires + salt).slice(0, 16);
  label     = ip + expires + salt + signature
  hostname  = label + '.' + DOMAIN

You can also load this npm package as a library and use it to generate sub-domain labels. See example below:

var statelessDNSServer = require('stateless-dns-server');

var ip        = [127, 0, 0, 1];
var expires   = new Date(Date.now() + 10 * 60 * 1000);   // 10 minutes
var secret    = '...';  // 256 bit randomness recommended
var domain    = 'taskcluster-worker.net';
var hostname  = statelessDNSServer.createHostname(ip, expires, secret, domain);
console.log(hostname);
// out: miy6hl7234h3kcycsqfhrxgnltaa2oc4owdlo5bnvbpa5mzd.taskcluster-worker.net

The resulting hostname in the example above will resolved to 127.0.0.1 for the next 10 minutes, after which only cached DNS entries may stick around, depend on the configured TTL.

Configuration

The docker image takes the following environment variables for configuration.

• PORT, port to host DNS server on (defaults to 55553),
• TTL, time-to-live for DNS records returned in seconds (defaults to 600),
• DOMAIN, domain under which to manage sub-domains (required), and
• PRIMARY_SECRET, secret token for HMAC-SHA256 signature generation (required).
• SECONDARY_SECRET, secret token for HMAC-SHA256 signature generation. (The server supports two secrets primary and secondary to support rotation).

Development & Deployment

As usual npm test will run tests over localhost. For deployment you can build a docker image that is easy to deploy. There is a very simple makefile in this repository with the following targets. You can set the environment variable REGISTRY to overwrite the default registry.

• make image, build docker image
• make test, test docker image with DOMAIN=test-domain.local and PRIMARY_SECRET=no-secret running on port 55553 of localhost.
• make push, push the image to registry.

When running locally you can test the response from the DNS server using dig under Linux. For example dig @localhost -p 55553 <label>.test-domain.local. This is useful before deploying, or just after deployment to verify that everything works.

Reporting Issues

Issues should be reported under the under the Testing :: TaskCluster component at bugzilla.mozilla.org. View issues with this saved search.