Show HN: I wrote a BitTorrent Client from scratch
55 comments
·June 13, 2025jorkingit
Great work! Just an FYI, you might want to limit the dynamic allocation size in the bencode decoder: since it's untrusted input (either from torrent or announce), a malicious input could DoS the client by requesting extremely large allocations during string parsing. A good upper bound could be the remaining length of the input, as a well formed torrent can't contain a string longer than the rest of the file.
piyushgupta53
thanks for pointing this out. I've added this in my to-dos.
indrora
You might look into (if you only care about reading it) writing the bencode decoder using Kaitai Struct [0] to avoid some of the common pitfalls.
[0]: kaitai.io
vkaku
Excellent, looks clean and simple.
Suggestion: Add a simple usage one liner in the README on how to actually download a .torrent file with it.
./go-torrent My-Linux-Distro-Wink-ISO.torrent
Everyone should do this for their own spiritual journey.
piyushgupta53
thanks for the suggestion, appreciate it.
__jonas
Neat! There is this challenge on codecrafters that guides you through the process a little, provides tests and such, I've played around with it a bit during a free month they had, was fun:
ashirviskas
As a non go developer, may I ask why you're using older go version 1.21? Is there a reason to stay with older releases?
EDIT: It seems like it was deprecated 10 months ago
koito17
The README is likely AI-generated. The actual go.mod file lists 1.23.1 as the Go version[1], which implies a requirement of Go 1.23.1 or higher[2].
[1] https://github.com/piyushgupta53/go-torrent-client/blob/6130...
pidgeon_lover
Windows 7 support is one reason to stick to older GoLang releases. A project in Go 1.21.4 or earlier will work on every Windows release and any computer made since 2009, whereas a version bump to v1.21.5 means it will only work on more recent computers and Win10 and 11 for no benefit.
wongarsu
I think this is a reasonable take. Yes, people shouldn't be running Windows 7 as their daily driver. But if you can support it at basically no effort and without sacrifices that is the right thing to do. Supporting more platforms is a good thing, even if that platform is an old Windows version instead of an Amiga
agiron123
Very cool!
This brings me back to college. We did this as our final project for our networking class at Georgia Tech.
I've long lost the code for this, but the lessons learned have lived on :)
Projects like these are great ways to learn new languages too!
NooneAtAll3
Do you support magnet links?
Edit: ah, planned feature
piyushgupta53
not yet. I'll be adding soon.
TheEdonian
How hard would it be to add a GUI to this? I don't think I've seen a lot of GO Gui implementations in the past
thegeekpirate
There's a bunch https://github.com/go-graphics/go-gui-projects
My personal favourite is an ImGui wrapper https://github.com/AllenDang/giu
The most featureful is probably unison, although I'm uncertain if anyone uses it outside of their own project (https://gurpscharactersheet.com), meaning documentation will be sparse https://github.com/richardwilkes/unison
Gio uses a different way of thinking about GUIs, used by Tailscale and gotraceui https://gioui.org
Wails is great if you're familiar with development on the web https://wails.io
The GTK4 bindings also look nice https://github.com/diamondburned/gotk4
Cogent Core also looks neat, but I didn't have the time to play with it before I switched over to using the Odin programming language instead of Go https://www.cogentcore.org/core
I personally had nothing but issues with Fyne (especially in regard to performance, across multiple computers and operating systems), but it's probably the most popular option https://fyne.io
throwaway894345
This is cool! I’ve been thinking about something like this as well. How hard was it, and do you have a sense for how “complete” it is? Does it handle DHT and Magnet and all the crazy NAT traversal stuff?
I’m guessing the main obstacle for me has always been that I’m not sure what the complete list of features is to have a client that will just work for the majority of torrents in the wild. It seems like there are dozens of protocols associated with torrenting and I don’t even know what the full list is much less what each does.
piyushgupta53
it was challenging for sure. Took me almost a month to get acquainted with the protocol, how bencoding works etc, build a mental model and then eventually writing code.
Magnet and DHT are yet to be added.
Charon77
In my experience, magnet is pretty straightforward. Dht is quite the rabbit hole, and it might be difficult finding clients that support dht (not everyone does)
NoMoreNicksLeft
Did you do v2 and mutable torrents? Please, for the love of all that is good and wholesome, someone do mutable torrents.
OccamsMirror
Could this be used as a library?
tomhow
Stub for offtopicness
blibble
[flagged]
spuz
Yes, very strange. There's no problem with using AI to build your first app and leaving the generated comments in the code is fine. But the number of comments on this thread that begin "This is so cool" is very suspicious.
throwaway894345
It doesn't seem wild to me that people would post "This is cool" on a ShowHN post. I did [here](https://news.ycombinator.com/item?id=44265915), and as far as I'm aware, I'm not an AI.
Would you like more information about how to identify AI comments? (kidding)
WhyIsItAlwaysHN
Or like a go beginner, which is fine
OtherShrezzing
Scanning around their other repositories the persons been programming for a few years now. There are ‘.cursor/rules’ directories in some recent repos.
I think it’s a reasonable hypothesis that “I wrote a BitTorrent client from scratch” may be “I produced a BitTorrent client from cursor”.
diagraphic
"convert length string into an integer" is a machine generated comment?
I've been writing code for 15+ years, this made me laugh my ass off. Comments are great, I don't read comments but I write them for others, especially for open source code. Atoi may be something you and I and a whole bunch of others know but people who don't it's a fine comment. Relax! :)
imiric
That comment is a strong sign that this was AI-generated. LLMs have the tendency to leave superfluous comments even when the code is self-explanatory. In this case, strconv is a well-known stdlib package, and anyone reading this in their IDE would get the documentation for what it does. In fact, all of the comments in this function and in most of the file are redundant, and I would point this out in a code review.
But, of course, this was vibe coded, so it's unlikely a human actually reviewed it.
rvnx
In the tests it more obvious:
You can see here for example: https://github.com/piyushgupta53/go-torrent-client/commit/61...
and some strings coming from crawled resources like: lengthi12345e4 but slightly different tokens (like 25 instead of 35 etc).
Gemini Pro 2.5 even gave me the prompt:
> If you asked me, "Generate Go unit tests for a Bencode decoder function called Decode that takes an io.Reader and returns an interface{} and an error. Cover strings, integers, lists, and dictionaries, including common error cases and nested structures" the output I would strive to produce would look very much like the code you've shown.
> It's a good example of well-written Go tests, and that's the kind of pattern I've learned to recognize and replicate.
and a lot actually matches when you ask from a fresh conversation.
So most likely Cursor + Gemini 2.5 Pro, but I cannot blame, I spend 100% of my time with Claude, and I take ownership of the code.
diagraphic
Clean code! Very nice :)
ivanjermakov
No seeding, no DHT, no magnet links, no uTP, no extensions. At this stage it is BitTorrent downloader, not a client.
Using P2P networks in download-only mode, so called leeching or free-riding, is discouraged.
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Moosdijk
what's up with the amount of new accounts praising this project?
ivanjermakov
Seems like someone (OP or not) is testing how good they can use HN for free advertisement.
throwaway894345
I only see two green usernames. Have others been deleted already?
diagraphic
odd indeed
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blibble
[flagged]
alexpadula
I always wondered how the heck do people get away with that. HN mods lacking allowing those sort of projects to the top and legit bot likes and comments. Craziness. Put's all the projects and posts worthy of eyes to the dead bottom.
pvg
You're probably getting downvoted because there are local conventions against astroturfing/shillage/botting accusations described in https://news.ycombinator.com/newsguidelines.html
If you think there's something wrong with the post email the mods at hn@ycombinator.com
hwmrocker
[dead]
Omarbev
[flagged]
man_is_obsolete
[flagged]
man_is_obsolete
[flagged]
b0a04gl
pretty solid attempt. but no mention of crash recovery, encrypted peer handshakes, or even basic uTP support. no idea how it behaves with NAT either. no memory guardrails during parsing, feels risky in real swarm. not production safe without those. would love to see it modular too, like usable as lib not just cli. tracking roadmap would've helped too.
chrisrickard
[flagged]
I picked up programming in late 2023 and been enjoying it now. Wanted to challenge myself and set a stretch goal, so set out to build a bittorrent client.