[dark-star]

Yeah, I was really confused when I learned that every device was simply a file in /dev, except the network interfaces. I never understood why there is no /dev/eth0 ...

That was back in the mid-90s but even today I still don't understand why network interfaces are treated differently than other devices

[simonh]

It's probably because ethernet and early versions of what became TCP/IP were not originally developed on Unix, and weren't tied to it's paradigms, they were ported to it.

[MisterTea]

Plan 9 does exactly this but all networking protocols live in /net - ethernet, tcp, udp, tls, icmp, etc. The dial string in the form of "net!address!service" abstracts the protocol from the application. A program can dial tcp!1.2.3.4!7788 or maybe udp!1.2.3.4!7788. How about raw Ethernet? /net/ether1!aabbccddeeff!12345. The dial(2) routine takes a dial string and returns an fd you read() and write(). Very simple networking API.

[mixmastamyk]

What would it mean to write to a network interface? Blast everyone as multicast? Not that useful. But Plan9 had connections as files, though I’ve never tried.

[dark-star]

That's a bad argument. What does it mean to write to a mouse device? To the audio mixer? To the i2c bus device? To a raw SCSI device (scanner or whatever)? Those are all not very useful either.

Especially since there actually is a very useful thing that writing to /dev/eth0 would do: Put a raw frame on the wire, and reading from it would read raw frames.

[mixmastamyk]

You haven't thought through what you're asking. That's the bad argument. Network packets are not viable without a destination address. Nor does anyone want unaddressed (garbage) packets on their network.

[dark-star]

I certainly have thought this through.

Network packets don't need a destination address. Broadcast addresses exist. Also, packets to invalid/unknown destinations exist. You can send network packets with invalid source or destination addresses already anyway.

Taking a raw chunk of data and putting it on the wire as-is is the most logical interpretation of "writing to the ethernet device". Does it make sense to allow everyone to do that? Certainly not, that's why you restrict access to devices anyway.

The fact that not every chunk of data "makes sense" for every device in /dev is certainly nothing new, since that is the case for all other devices already (I mentioned a few in my post above).

[mixmastamyk]

My first comment mentions multicast. Invalid packets won’t get routed, unless lucky enough to be accidentally valid.

[dark-star]

Packets don't need to be routed. Sometimes you just want to communicate with a host on the same Layer-2 network. I said "Broadcast" (not Multicast) on purpose.

Sometimes you don't even want TCP/IP on the wire. Heck, sometimes you maybe don't even want DIX Ethernet on the wire.

Anyway, this discussion is going nowhere. Handcrafting packets is possible (it's basically what the kernel does anyway), sometimes it's useful, and if you could write a user-space program that could just open /dev/eth0 and write its own handcrafted packets to that stream would be helpful.