9 October 2018
I recently deployed a small hack on my personal website. In short it enables this:
[[email protected] ~]$ dig LOC iss.morsmachine.dk ; <<>> DiG 9.11.4-P1-RedHat-9.11.4-2.P1.fc27 <<>> LOC iss.morsmachine.dk ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62528 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 512 ;; QUESTION SECTION: ;iss.morsmachine.dk. IN LOC ;; ANSWER SECTION: iss.morsmachine.dk. 4 IN LOC 43 46 46.715 S 145 49 45.069 W 417946.18m 1m 1m 1m ;; Query time: 42 msec ;; SERVER: 126.96.36.199#53(188.8.131.52) ;; WHEN: Tue Oct 09 12:04:02 BST 2018 ;; MSG SIZE rcvd: 75
I am tracking the location of the International Space station via DNS. This might raise some questions like "Why does DNS have the ability to represent location?", "how the hell are you finding the location of the ISS?" and "Why would you willingly run DNS infrastructure in the Year of Our Lord 2018?".
DNS LOC records
DNS LOC records are mostly a historical oddity that lets you specify a location on earth. Back in the infancy of computer networking, when everybody wasn't sold on the whole TCP/IP thing yet, ferrying email between computers was handled through UUCP. UUCP worked by periodically talking to nearby systems and exchanging any email that needed to be forwarded.
To send an email, you'd have to look up a path from your location to the computer system of the recipient.
You'd then address the email to
host!foo!bar!dest_host!recipient. To figure out the path, map files were created that included basic information like the administrative contact, the phone number to call when your email was delayed and in some cases, the location of the server.
Meanwhile, the DNS system was being developed. DNS was meant to replace the
HOSTS.TXT file used in TCP/IP networks that mapped user-readable names to IP addresses. To establish feature parity with old UUCP maps, an RFC was written that let you point out where your server is on a map. These LOC records are 16 byte binary representations of latitude, longitude, height above sea level and precision information
I don't know why they included the altitude of the server in the format. I guess people were going to use it to say whether a server was in the basement or on the third floor. For our purposes, the important thing is the range it allows. With a maximum of 42849 kilometers, this easily allows us to represent the location of low-earth orbiting objects.
The international Space Station is one of those objects, so how do we find it?
A brief introduction to the world of orbital mechanics
Tracking satellites in orbit is hard. It usually involves people at NORAD with radars. The good news is that satellites follow the laws of physics and the folks at NORAD are a kindly bunch, who lets us look over their notes.
Every day, they publish new information about the objects in the earths orbit that they track. This data is known as a two-line element set (TLE). The ISS is one of those objects and given a data set, we can predict where the ISS will be sometime in the future.
An aside: TLEs include the year a satellite was launched in its data set and it is specified to have 2 digits. In the leadup to Y2K, there was discussions about changing the TLE format such that it would be able to handle satellites launched after 1999. However, since no artificial satellites existed before 1957, it was decided that any year before 57 would be in the 21st century and any year after would be in the 20th century, solving the problem once and for all!
For the purposes of modeling a satellite orbiting earth, we consider earth as a point mass, with a reference coordinate system where the Z axis points positive through the true north pole and X and Y sits in a plane through the equator. This is known as an earth-centered inertial (ECI) coordinate system and does not rotate with the earth. The model for calculating the future orbit is known as the SGP4/SDP4 algorithms and take into account a whole slew of things, like gravitational effects, atmospheric drag and the shape of the earth. Given the TLE data, we know the orbit and can calculate a future position in the ECI.
The ECI makes orbital calculations easier, because we don't have to handle the rotation of the earth, but we want latitude, longitude and altitude (LLA) for our DNS positioning and latitude is defined by an angular offset to the international meridian. Since we can't stop the world turning, we're forced to do More Maths. Luckily, the angular offset between the reference plane and the meridian can be defined by where the earth is in its rotation around itself and where the earth is in its rotation around the sun. If that all remains the same, then we're all set!
So, we have the position of the ISS, calculated from a TLE in the ECI, converted to LLA and shoved into DNS. Now that we've run out of 3 letter acronyms, let's see how we manage to actually get this thing deployed.
Deploying this thing
Despite my enthusiasm for this project, I do not want to develop a DNS server. The protocol has become a vast, complex mess and I want to offload the effort as much as possible.
PowerDNS provides a good authoritative DNS server that can use remote backends via HTTP. These backends can use a JSON format and I wrote a small HTTP server in Go that responds with custom LOC records that follow the ISS, calculated from a TLE.
Since the SGP4 model is just a model, we have to periodically match it with the physical data to track the ISS accurately. Additionally, the ISS may use its boosters to push it into a higher orbit as it counteracts its orbital decay. Because of this, we fetch new TLEs every day from CelesTrak to make sure that we're following the ISS properly.
The PowerDNS server and my backend HTTP server are deployed as 2 containers in a pod, running on Google Kubernetes Engine. Since my website is managed by Cloudflare, we add an NS record for iss.morsmachine.dk pointing at ns1.morsmachine.dk and an A record for ns1.morsmachine.dk to delegate the responsibility for the subdomain to my new DNS server.
You can try it out by running
dig LOC iss.morsmachine.dk in your terminal. You may have to change nameservers if the one on your router doesn't handle DNS esoterica properly.
Sadly, there are a couple of things that I'd like to have added to this hack that weren't possible. GKE doesn't allow you to have a load balanced service that listens on UDP and TCP both on the same IP address which breaks certain DNS operations. Since the LOC record is fairly small, we never actually hit a truncated DNS record and the lack of TCP is mostly a nuisance.
Cloudflare doesn't allow you to specify an A record and NS record for the same domain. Additionally, they don't allow you to proxy wildcard domains unless you are an enterprise customer. Because of this, I couldn't add a small HTTP server to iss.morsmachine.dk that redirected to an explanation and a graphical view like the ISS tracker.
All in all, I'm pretty happy with how this turned out and I learned a bunch about orbital mechanics.
As you've probably guessed by me blogging again: I am looking for work. If you want to work with someone who has knowledge of DNS, Go internals and knows where the HTTP bodies are buried, have a look at my CV and send me an email on [email protected]