Hey, I made this! I also made this other thing on HN today (https://news.ycombinator.com/item?id=37062422), really cool to see ArpChat as well (guess someone went through my profile). It was a super fun project and I learned a lot about networking.
I actually got a chance to use it in a totally-offline setting a bit ago, which was fun! ... however, I then found a major bug that was causing tons of latency, and I fixed it but I haven't gotten around to building new artifacts yet, so you might want to build from scratch if you're planning on testing it out in a real setting for some reason. I also added fun username colors and message times. I promise I'll upload a new release soon!
I am just grinning like hell going through all this.
Even though ArpChat would probably give network admins a heart attack, your mind is most certainly fucking rad, ma'am. =) I too am the kind of person that read books on OSI layers for funsies, lol. Keep playing with code! Reminds me a bit of justine tunney's work with things like https://justine.lol/ape.html where someone just goes way deep on a thing that few people seem to fully understand (in her case it would be "compiler binary format minutiae" among other things) and then asks a crazy question like "what if it were possible to mesh all the binary formats together into a meta-format that actually runs on all of them?" and then conjures up this crazy thing that actually works.
i see. hmmmmm ... that's not supposed to happen, and it works fine on both nixos and ubuntu for me. may be something funky going on, i'll see if i can take a look sometime soon.
Oh heck yeah now this is what I'm talking about! Data link layer shenanigans!!
For that of you who have no idea what the Address Resolution Protocol (ARP) is, to a first approximation it is the missing link between your MAC address (what Ethernet runs on) and your IP address (what Internet runs on). After your IP routing table, which you can see with `route`, decides what immediate next IP address to forward a packet to, then your ARP cache, which you can see with `arp`, will decide which MAC address that IP correspond to (usually your router's local IP address).
Knowing that ARP even exists is considered mild esoterica today, to say nothing of its IPv6 sibling the Network Discovery Protocol. But even though you don't use it much in modern cloud-based deployments, knowing it exists makes your bird's eye view of how the whole network stack works that much more holistic.
Not sure I'd agree about it being esoteric. Understanding or at least knowing about ARP is still very much essential for people in networking. arping is a very useful tool for seeing if machines on the same network segment are up and just not responding to ICMP pings. Anyone looking at tcpdump/Wireshark dumps will run into it sooner or latter.
It is true that software engineers may sooner run into it when debugging their home network than their application though as cloud and traditional networks are very different.
> For that of you who have no idea what the Address Resolution Protocol (ARP) is, to a first approximation it is the missing link between your MAC address (what Ethernet runs on) and your IP address (what Internet runs on)
More commonly, this is 802.11 nowadays, though I guess ARP would be used at any data link layer protocol when the host needs to get an IP: https://en.wikipedia.org/wiki/Data_link_layer
Oh, I see your point now. Yes, the direct parallel of "MAC is to Ethernet as IP is to Internet" is an imprecise one, intentionally so: I remembered being really confused about Ethernet vs. Internet when I learned all this stuff, and this parallel was another good-enough approximation at the time for me to continue learning.
What follows is non-expert commentary and I probably get several details wrong. But here is my deeper understanding:
In reality, Ethernet is a really old protocol (like Metcalfe started working on it back in 1973), and the way we use it today is very different from the way it was originally imagined. In the original paper, you would hook up say 10 computers to a single giant cable and just have Computer A send data to Computer B by sending that data to _everyone_ and letting anyone who wasn't Computer B just quietly drop the network traffic. That's a really elegant design, in the same way a rock is a really elegant weapon, but these days we usually plug any Ethernet devices we have into an Ethernet switch and that does most of the filtering for us -- moving us from a complete graph where everyone is connected directly to everyone back to a spoke-and-hub design with the switch as the hub. But the original Ethernet _did_ end up using MAC addresses as the way to distinguish Computer A from Computer B, and so the analogy stuck in my brain as a "wrong but useful" model.
Now 802.11, aka wifi, comes right out the box with a spoke-and-hub model. All your traffic goes through your wireless router/switch/modem/whatever it's called. Even when you want to share things with something like BitTorrent or Syncthing on the local network, Computer A's wireless network card doesn't usually directly transfer files to Computer B's wireless network card - it goes from A, to router, to B. There's nothing technical I'm aware of that stops this direct A-B transfer, since we know wireless cards can both pull and push data, but you would have to do some networking legwork to get them to be on the same subnet to do so, which probably involves a lot of manual fiddling with IP addresses or something.
But 802.11 also emerged into a landscape where MAC addresses are ubiquitous, so it still uses ARP to translate between IP and MAC for reasons I'm not entirely sure of (probably practically that MAC addresses change a lot less often than IP addresses to).
The main reason for wireless using MACs and ARPs and the like is it makes it more directly interoperable with wired networks. You can just bridge a LAN directly to a WLAN and expect the clients to not realize they are a different type.
If it really is becoming obscure that's a shame, I'm not sure how you could pretend to understand how IP networks work without having at least cursory familiarity with it.
If I was hiring a network technician, I definitely wouldnt hire someone who didn't know what ARP is. It's too easy and fundamental to the field. It took all of like 3 weeks at the trade school I attended to cover IPv4, MAC, ARP, basic routing protocols, TCP, and UDP, and we were definitely chilling. Understanding those things isn't complicated. You just need to know what computers are, and what "networks" are, then it all very easily clicks into place.
I had a job interview today and they asked me to describe TCP and udp, and when I did they told me not everyone knows it nowadays. I guess I can see why, if you're a web dev you never have to worry about such things.
It's fun that it abuses ARP's ability to include an arbitrary payload, but just to remind everyone of networking 101, thing that the author is relying on is just the simple fact that switches flood packets addressed to ff:ff:ff:ff:ff to the entire broadcast domain. You can achieve similar message flooding without having to have superuser/raw packet permissions by directing packets to the L3 broadcast address whereupon they will (usually) be L2 flooded (Aka it's fun to ping the broadcast address.)
Using ARP doesn't offer any particular advantage, and I would note since many modern switches do thinks like snooping arp packets, putting a high amount of bogus ARP traffic on the network could actually have some negative impact. It's still fun at any rate and reminds me of the type of crap that I used to make when I was learning. As alluded to before, I personally would have chosen to abuse ICMP for this purpose; maybe it's a future avenue for development. Kudos to the fun project!
It uses another important Networking 101 principle: It's All Just Numbers.
ARP, DNS, HTTP, TLS, on the one hand they really are the things that may pop into your head when you think about those things, but they are also simultaneously Just Numbers. ARP has access to the same numbers as HTTP. UDP doesn't get access to 11-bit bytes. DNS does not have special CPU instructions only it gets access to. It's All Just Numbers. And in the end you can stuff whatever numbers you like onto the line and the rest of the world is just going to take those numbers and do... whatever they do with them.
One of the first amplification DDoS attacks I ever saw - talking 1990s era - was to ping the broadcast addresses of unwitting campus networks with the victim’s host address spoofed as the source.
This shouldn’t work today, but back then, a naive TCP/IP implementation (which is to say, many of them) would happily participate.
Ah that’s right. It’s been so long I’d forgotten the name. Although I have a feeling it was already a recognised potential vulnerability some years before being packaged as a convenient utility.
I love (ab)using protocols that are built-in to most OS'. I wrote a dumb fake samba share that let people see messages in Finder.app/Explorer.exe/Gnome/KDE since I couldn't find a cross-platform gui or cli messaging system that didn't require installing special client software. (even netcat invocations were not working when trying on our local network, and had OS-specific flag differences if we wanted to do broadcast).
You see the samba share message in your file browser, click it and can edit the filenames to update a 3-line "bulletin board".
This may be even more fun than you expect. We were doing VLANs using hardware from a major vendor to partition a network and discovered that while the VLANs worked most of the time, they were leaky with ARP traffic. It was causing us headaches because the machines were supposed to be sending all of their traffic through the router, but sometimes they were seeing ARP replies that they shouldn't and were thinking that the other machines would be reachable directly, causing them to be unable to send traffic until the ARP cache entry timed out. We couldn't get the vendor to show any interest in fixing the bug either, because they were too big to care.
It's possible this chat app could break through VLANs in certain circumstances.
This is interesting to me for basically the same reason — being able to send and receive ARP messages with arbitrary payloads would definitely make troubleshooting VLANs (and other L2 connectivity) easier!
My wife, kids and I all work or study mostly from home, so often all on our computers in our separate crates^H^H^H^H^H^Hrooms. We use Beebeep to be able to message back an forth when needed. It doesn't need a server and has both Linux (me) and Windows (everyone else) clients which was the deal clincher.
It works pretty well, although sometimes you have to restart after your laptop wakes up before it sees others on the LAN. That and not being able to open a message window to someone direct from the tray icon are really my only grips. Can cut and paste pictures (of funny cats) and even send files and other capabilities I've never used. (This is not a paid advertisement, I've just started rambling.)
Oh, agreed. Just while reading about it, it made me think of Beebeep so thought I'd give it a quick review in case anyone else was reading thinking, "I could use something like that."
> I found that trick and showed it to some friends... But it's hard to keep a secret, and somehow a larger non-friend group was using it to cheat in a computer-lab class. I was offended by the blatant cheating, and a little by the idea that certain people I didn't particularly like were profiting from what I'd found.
> So I spoofed the system administrator's username (not hard on Win9x) and sent off a stern warning, which must've surprised them since suddenly all the messages stopped. :)
> I also described ARP as a layer 3 protocol. There’s some contention on this — while I think this is the most accurate and easiest explanation, networking is more nuanced than the OSI model might convey. Some would describe ARP as a level 2 or even level 2.5 protocol. For our purposes, I believe calling it level 3 makes the most sense.
TCP/IP doesn't follow the OSI model, so the nuance of OSI isn't directly applicable. ARP is practically defined (ref Stevens) as being in layer 2.5 (of the TCP/IP model), so i believe calling it 2.5 would make the most sense.
and i mean, ARP frames aren't routable. so they clearly aren't L3.
Back when IPv4 NAT was sort of standard (still is but so is IPv6 as well as GCNAT now), I created a thing I called NATChat. It would basically just create a chat room for you based on your public IPv4 address, allowing you to chat with anyone on your network. Was kinda neat to use in office with others (back when that was also a thing).
That reminds me of a chat software that I did in Delphi 3 back in 1999/2000 (being not that much older than OP) for our LAN parties, that was basically broadcasting on 255.255.255.255 [0]
It was inspired by Kali (remember Kali? [1]) and IRC.
I think it worked until we moved from hubs to switches ;)
I don't even understand the network code all that much (actually written by a friend) but it does use INADDR_ANY, INADDR_BROADCAST and SO_BROADCAST.
edit: Tried it on two old Windows 7 computers on my mixed rj45 / wifi home network and some chat data are received by the other party and some don't, or it crashes. Fun.
edit2: Actually works great on a version where I could set a specific broadcast address like 192.168.1.255.
I've been to ski resorts where there's no phone reception, and I love it that way. No one's on their phones. But it's easy to lose someone for an entire day (or until your agreed meeting time).
Is it possible to use something like this to setup a wireless network which would facilitate chat without a connection to the outside world?
Ok now that I typed it it sounds crazy but I always thought this should be possible
There's a similar app designed for cruises, where you can talk peer-to-peer over the ship's wifi without actually being online. It could work if you have a wifi antenna with you.
Recently my wife and I had separate seats in an airplane and we didn't want to pay for texting. I thought about how I could set up my phone as a hotspot with a captive portal which just presents a chat interface. Nothing came of it, but it was something interesting to think about on a flight, at least.
I vaguely remember commercial from 90s. It was Ericsson? phone that could message to another nearby Ericsson phone even without GSM. Or was it Nokia? Google gives my just recent products.
I did something similar to this before in my old job, because installing of chat applications is unauthorized, the solution I come up with is I created a batch script with easily to identify computer network map so regular users will have problem using it, it utilizes net send command.
I actually got a chance to use it in a totally-offline setting a bit ago, which was fun! ... however, I then found a major bug that was causing tons of latency, and I fixed it but I haven't gotten around to building new artifacts yet, so you might want to build from scratch if you're planning on testing it out in a real setting for some reason. I also added fun username colors and message times. I promise I'll upload a new release soon!
If you're interested, I wrote an article about a year ago about how ArpChat works: https://kognise.dev/writing/arp