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Implement DNS in a Weekend

implement-dns.wizardzines.com

647 pointsasicsp71 comments

I've done something along these lines with a very simple DNS and DHCP server implementation as an Arduino library: https://github.com/pkulchenko/DHCPLite/. It is fairly short and while I don't claim it being fully correct, it was tested with different clients successfully; it was great learning experience.

I love this. DNS is something that, if you're online and visiting websites or even sshing into boxes a lot of the time, you're using, unless of course you carry around a list of IP addresses like the olden times. It is one of the many things that underpin the modern web and we take it for granted.

I missed the “you’re using” at first, and was about to ask why on earth you keep maintaining host lists :P

Totally agree, and at the same time, it’s so painful that there’s a nice haiku —

    It’s not DNS
    There’s no way it’s DNS
    It was DNS

Man, it hurts how hours of struggle and frustration can be summed up in a haiku. Beautiful.

> It was DNS

To be fair, sometimes it's BGP.

At least the haiku still works, though

I still have one IP address lodged in my memory from 25+ years ago - sable, the university's key Unix server for students, on which I learned so much about Unix and the 1990s Internet.

(I'm not sure if I'd faced DNS issues that meant I needed to know that - but it was a time when dialup ISPs told you what to manually configure for your DNS rather than it being automatically assigned, so it all seemed a bit chunkier.)

This is great. Incidentally, writing toy recursive resolvers is one of the primary methods I use to help me learn new languages. If you take the time to develop an understanding of the domain, I've found its an easily repeatable exercise and can be done in a couple of hours.

Cool! I'd implemented a very low quality DNS server in rust to solve a hackattic challenge

https://github.com/DavidVentura/hackattic/blob/master/src/se...

It'll be interesting to see if the server implementation is complete enough to work with this client

I hadn't heard of hackattic [0], but the challenges look great. Thanks.

[0] https://hackattic.com/

Julia's posts are always so informative; DNS being one of those topics which I presume so many developers are like myself and have "just enough knowledge to be dangerous" :-)

I have a slightly off topic but related query to new HTTPS RR and SVCB record types of DNS. Will these records allow me to host sites without a reverse proxy since both records can include port info.

It is very well intentioned, but unlikely to actually see adoption outside of internal network service discovery maybe (where mDNS already does a good job).

Everything runs on 443 to avoid firewalls, moving services to new ports opens up the whackamole game of trying to find other unblocked ports and raises security implications.

Here is a more positive take for background on SVCB: https://www.isc.org/docs/2022-webinar-dns-scvb.pdf

I host a pi-hole instance in the cloud. It's only accessible to my Tailscale network, which means that I can't reach it on my TV - unless I write a DNS proxy. Maybe this'll be enough to get my ass in gear and actually write it for once.

What about a Pi that connects to Tailscale and have your TV work via that? https://tailscale.com/kb/1019/subnets/

That would mean getting a Pi, which I don't have...

Setting the pi-hole as your DNS server in your router would not work? TV should then use this DNS server after getting an IP-address via DHCP.

The Pi-hole isn't hosted on my local network, but on my Tailscale network because it's running on a VPS. My router can't run Tailscale, so that's not possible.

What router do you have? You'd be surprised, I have tailscale running on a 4-year old Linksys router.

If you upgrade to ZeroTier you can get it running on OpenWRT and MikroTik routers https://help.mikrotik.com/docs/display/ROS/ZeroTier

Good luck installing OpenWRT on the silly router provided by my ISP.

You can always install one daisy-chained.

You don't need a Pi to run pihole, it runs great on a low spec VM.

Isn't the DNS design effectively its own proxy by the way resolvers forward requests towards more authoritative resolvers? On whatever local-network machine you would run your DNS proxy, you just run a DNS resolver in forwarding mode where all requests are sent to your pi-hole instance.

This is fabulous.

It would be convenient if I didn't have to download the code to run it though - if it was in a GitHub repo it could provide links to hosted notebook services such as JupyterHub and Colab - then lazy people like myself could click those links to try out the notebooks in their browsers without downloading, unzipping and running Jupyter locally.

Can you copy the code into a hosted Jupyter?

Not OP but

> then lazy people like myself could click those links to try out the notebooks in their browsers without downloading, unzipping and running Jupyter locally.

https://github.com/alanhamlett/dns-weekend

Tangential, but this made me remember:

- Some time ago on HN I saw a free ad blocking DNS server (https://controld.com/free-dns)

- I had wondered what protocol the 'Private DNS' setting on my Pixel uses, but never got around to checking.

It turns out that it setting the 'Private DNS provider hostname' to x-oisd.freedns.controld.com just works: DNS-over-TLS and ad blocking.

This looks great. I've seen a few things in rest of comments but if anyone could share anything similar about networking or computer science topics in general I'd really appreciate it. It's weird looking back I learned so much stuff as a kid in the 90s that I wish I knew better now, from trying to be a wannabe hacker and always breaking stuff I had to figure out how to fix if I wanted to use it again. I never went to college for Comp Science or worked in IT or coding but now I want to make a career change. Even though I always had some personal projects or occasional freelance type web dev, I guess the more complicated stuff was abstracted away over the years and it was easier to do more with knowing less.

Same here. 1998 was the first time I touched a pc. Next 3 to 4 years were c & c++. Then WYSIWYG html editors. Then php. Then javascript & html n static stuff. No formal education related to computers. Although I am comfortable with my current job, but somewhere in the back of my mind I wish I was in IT, earning IT money.

I love all the series about writing your own X in 100 lines of code. It gives you the understanding of technology and removes a lot of unnecessary details.

The great examples of this are 'A from-scratch tour of Bitcoin in Python' https://karpathy.github.io/2021/06/21/blockchain/ and 'Let's build GPT: from scratch, in code, spelled out' https://youtu.be/kCc8FmEb1nY from Andrej Karpathy

I wonder if anybody tried to collect all such projects together and built his own 'Internet in just 100 lines of code'

Maybe not only 100 lines of code though, I think of Code Crafters. https://github.com/codecrafters-io/build-your-own-x

i subscribed to codecrafters for a bit. it was ok, but it was super annoying to have to use their tooling around git and ci. too much hand holding.

> I love all the series about writing your own X in 100 lines of code.

Me too, they're a really good resource.

What are some other ones you've come across?

Another one: Write yourself a Git: https://wyag.thb.lt/

I've enjoyed doing stuff like this myself. I wrote an IP stack up to being able to ping an IP address. I had learnt all of this in university, but doing it myself really cemented the knowledge. Using a notebook and doing literate programming is a must. I pretend that I'm teaching someone else, even though I don't plan on ever sharing it really.

Bocker is in this same category...docker clone in bash that's helpful in seeing what's really happening underneath with nsenter, namespaces, network bridging, cgroups, etc.

https://github.com/p8952/bocker

Adding Liz Rice's superb "Containers from scratch" to the list. https://www.youtube.com/watch?v=_TsSmSu57Zo

The series is great, wish I had more time to try out the projects!

I just submitted this as its own post, because I thought it was so cool, but here's a complete operating system in 2000 lines of code: https://github.com/yhzhang0128/egos-2000

Egos is really neat, and super approachable. I did some documentation work for it last fall, and despite only having a weak grasp of operating systems I could easily understand the whole thing. I only needed to figure out a few common acronyms and magic numbers that weren’t explained.

Plug: I am a big fan of Build Your Own X educational projects. I have a build your own KV Store project. I have set up this project in TDD fashion with the tests. So, you start with simple functions, pass the tests, and the difficulty level goes up. There are hints if you get stuck (e.g. link). When all the tests pass, you will have written a persistent key-value store.

go - https://github.com/avinassh/go-caskdb

python - https://github.com/avinassh/py-caskdb

Check out the book "500 Lines or Less: Experienced programmers solve interesting problems"

I especially like the chapter "A Python Interpreter Written in Python": https://aosabook.org/en/500L/a-python-interpreter-written-in...

Amazingly the DNS operator community doesn't offer a checklist for DNS implementations. I asked on dns-operations@.

I really enjoyed this short series on making a dns server in python. It's very to the point and watchable. You can get through it in an evening or two.

https://www.youtube.com/playlist?list=PLBOh8f9FoHHhvO5e5HF_6...

Julia's zines[0] are great. Got mine this week and it's a delight to read.

0 - https://wizardzines.com

agreed, have several printed ones and love the combination of whimsy and concise technical content.

The style fits pretty well with GitHub.com/charmbracelet

On Firefox mobile, I get some element overflow blocking the speech bubble. Maybe a flex wrap gone wrong?

Just a heads up incase Julia is reading :)

thanks, I'll take a look

That is an excellent website. Thank you for bringing my attention to it

The cute hand-drawn aesthetic reminded me of fun times reading Forrest Mims' electronics books.

Some of them definitely remind me of a late 70s/early 80s style.

To be clear it's a DNS resolver, i.e. the client.

Very important detail

A DNS resolver is both a client and a server -- for example Google's 8.8.8.8 (which this is a toy version of) is a server (you can query it with `dig @8.8.8.8 example.com`), but also a client of the various authoritative DNS servers that it fetches and caches records from.

I implemented this as a command line tool because that's much easier to do in a Jupyter notebook environment, but you can also pretty easily transform it into a UDP server running on localhost and query it with dig in the same way that you would with 8.8.8.8. That's one of the bonus exercises at the end (Exercise 7).

I might end up bringing "convert it into a server" into the main content though because it's pretty easy to do and I think it makes the whole thing seem more "real".

Nope, Google’s quad eights is “recursive DNS server”.

Resolver is usually a part of operating system (sometimes) implementing DNS client functionality and serving as a link between a userland library providing, say, getaddrinfo(), and DNS implementation.

This one is even less than a resolver, it does not implement a library link. It’s a toy DNS client, implementing minimal functionality.

Quad8 is a recursive resolver.

You are referring to a system's local resolver (which is obviously also a resolver, so you are right on this regard).

But still, this project is _also_ a resolver (although obviously not a complete one, it's a learning project).

Just wanted to thank you for this great project, it was a lot of fun :)

I'll dig up my old comment about how easy DNS is to work with using DNS4J:

  Message query = new Message(data);
  Header header = query.getHeader();
  Record question = query.getQuestion();
  Message response = new Message(query.getHeader().getID());
  response.getHeader().setFlag(Flags.QR);
  response.addRecord(question, Section.QUESTION);
  Name name = question.getName();
  int type = question.getType();
  int dclass = question.getDClass();
  String host = name.toString(true).toLowerCase();
  ...
  response.addRecord(new ARecord(name, dclass, 300, "someIP"), Section.ANSWER);
  ...
  response.getHeader().setFlag(Flags.AA);
  return response.toWire(512);

Can't tell if this is sarcasm or not :)

On one hand, like all Java code, this is really really verbose. But on the other, it's not that complicated—every line seems like it corresponds to some part of the DNS spec.

It’s verbose but not complicated.

Verboseness doesn’t make something uneasy, it means you tell your IDE to autocomplete.

Yeah, I'll take a verbose but uncomplicated API any day of the week. Straight-forwardly named classes/functions, no surprising behavior, one concept == one name, and a good reference … that's how stuff gets done.

… always better than something that tries to hide things from you and does the wrong thing sometimes.

Enjoyed. I’ve been experimenting with parsing network protocols a lot lately with an eye to separating the parsing from the “logic” (usually combined in one handler). DNS (and DHCP) are the two I started with - having the flexibility to easily extend/alter the logic is useful from time to time (ala PiHole).

Yes. We need more of those little tutorials demystifying things we take for granted.

Over time I've learned to stop taking people like Julia Evans for granted. Sometimes it feels like the Internet is an endless supply of brilliance and generosity and talent. But its not true! Not everyone creates, and not everyone who creates shares, and not everyone who shares shares freely. I feel overwhelmed by gratitude for Julia and (what I estimate to be) the mere ~100 creators in the world like her.

I just have to say I love everything Julia produces. Very inspiring!

I can't check the code on my phone right now, but does it handle mDNS queries (xyz.local)? It's kind of a special case but really useful in a home setup.

Julia Evans's writing is just so much fun.

Julia is the best ever.

Url changed from https://jvns.ca/blog/2023/05/12/introducing-implement-dns-in..., which points to this.

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