Transfer files over an Ethernet patch cable

(maurycyz.com)

52 points | by jllyhill 4 hours ago

10 comments

  • rmunn 39 minutes ago
    That used to require a crossover cable; I've done precisely that (with a crossover cable) back before https://en.wikipedia.org/wiki/Medium-dependent_interface#Aut... became widespread. With a straight-through cable, you'd be connecting the Transmit (TX) pin of one adapter to the TX pin of the other one, and the Receive (RX) pin of one to the RX pin of the other — and neither device would "hear" the messages the other one was sending. A crossover cable flipped those wires, so each device's TX pin was connected to the RX pin on the other end, and both devices could "hear" each other.

    But with auto MDI-X, each device would notice "hey, I'm sending but not receiving anything," and would try flipping its Transmit and Receive functions around (transmitting on the RX pin and receiving on the TX pin). Since each device waited a random period before doing that, it was very unlikely (nigh-impossible) that they would both flip at the exact same moment. And if they did, the second interval would most likely not be identical either.

    I'm simplifying a bit in the explanation above, but that's the broad strokes. And that's how my carefully-labeled crossover cables started gathering dust. (And then I realized "hey wait, I can just use these as normal cables now", and pulled them back out of storage and mixed them with my normal patch cables).

    • rootsudo 1 minute ago
      Basically beat me to writing this, but I wouldn’t have made it so eloquent of an explanation.
    • soundworlds 37 minutes ago
      That wasn't that long ago was it..?

      /looks longingly out the window, and catch my aging face in the reflection

      • abrookewood 3 minutes ago
        Yup ... still have some old 'xover' cables lying around - all of which are RED in colour to explicitly declare their usage
  • yjftsjthsd-h 42 minutes ago
    I am 95% confident you don't actually need to assign IPs. Do this:

      # ping all link-local devices on an interface:
      ping ff02::1%eth0
    
    And then do your socat/rsync/whatever to the only IP that responds.
    • steve1977 18 minutes ago
      I would assume so as well, I'm not sure if all host operating systems act the same though. But you could probably just wait a couple of seconds and the interfaces would have assigned themselves link-local addresses.
  • steve1977 48 minutes ago
    People discovering that network cables can be used to network things. And then calling it a "trick" (I'm surprised it's not a "hack" actually).
    • concerned_user 46 minutes ago
      Back in the day you needed a modified cable for this with RX/TX swapped on one end but nowadays probably all nic chips have auto-detect so you can take a normal patch cable and do it.
      • bronlund 44 minutes ago
        Yeah, I remember those. When setting up an ad-hoc network, you were like; "where the eff is the crossover cable!" :D
        • luz666 42 minutes ago
          it was the red one, right?
          • bronlund 38 minutes ago
            Mine was just a custom made gray one, but I have seen those red ones, yes :D
      • steve1977 44 minutes ago
        Yeah so called crossover cables. But that's like decades ago.
    • yjftsjthsd-h 37 minutes ago
      If you're used to a "normal" network with a router, it's a revelation that you can go direct.
  • nkurz 45 minutes ago
    It would be nice if this article defined a "ethernet patch cable". I think he's just using "patch" as a slang term for a short cable and it actually works with any length of standard cable, but I'm not certain.

    Traditionally (pre-2000?), one had to use a special "crossover cable" to do direct connections like this, but apparently modern Gigabit ethernet adapters are able to detect this situation automatically?

    • rmunn 37 minutes ago
      https://en.wikipedia.org/wiki/Medium-dependent_interface#Aut... is what allowed the switchover to happen automatically, and it got folded into the 1000BASE-T standard. So yes, crossover cables are no longer required because the two Ethernet adapters at either end can negotiate between them to have one of them "flip" the meaning of its Receive (RX) and Transmit (TX) pins.
    • bronlund 39 minutes ago
      The way I learned this, is that patch cables use stranded wires, but ethernet cables use solid wires.
      • globular-toast 11 minutes ago
        They are all just twisted pair cables, probably Cat5e or Cat6 etc. The solid ones are usually used for structured cabling, inside walls and trunking, that doesn't often move. Structured cabling is terminated into keystones or patch panels, the "female" end.

        Patch cables are outside of walls and designed to be moved around and plugged and replugged. They are terminated with 8p8c "male" connectors, commonly called rj45. Stranded cable is used because it's much more flexible.

    • lstodd 37 minutes ago
      IIRC crossovers are pre-2000, yes. "Fast Ethernet" or 100Base-T adapters usually could talk over straight link.

      edit: as for length, it depends. you won't get a gigabit over 150m of crap cat3, but you will get a link, even if it's 10 half-duplex.

    • facepalmz 32 minutes ago
      That's exactly what a patch cable is. /facepalm
      • steve1977 22 minutes ago
        No, traditionally a patch cable is a cable that you would use in a networking rack to connect (to "patch") from say a network switch to the desk outlets or to another switch. A patch cable would explicitly be NOT a crossover.

        The main difference between patch and "normal" is that the former is using stranded conductors and the latter solid conductors.

        That makes the patch cable more flexible (physically)

  • 3r7j6qzi9jvnve 25 minutes ago
    tbstream looks fun, but hard to use for the casual user -- not sure if it'll get simpler but I don't think I'll remember messing in configfs just to transfer a file:

    https://git.kernel.org/pub/scm/linux/kernel/git/westeri/thun...

  • ltbarcly3 2 minutes ago
    Ok is there a version for usb-c? Because that would be 10x or more faster and I have a usb-c port and a usb-c cable at all times.
  • yitchelle 41 minutes ago
    This remind myself the good old 8bit days of using a cross-over RS232 cable to send a file from one computer to another. even at 30bps, it was much more reliable than write my data to a cassette tape on one computer and then reading on the other.
  • lstodd 54 minutes ago
    I remember we played warcraft 2 over ~15m of cat3 cable stretched outside between two flats on 6th floor. Those were the times, lol.
    • bronlund 33 minutes ago
      I remember playing over a 9-pin RS232 null cable - but I guess that is telling about my age :D

      Not Warcraft though - some ancient DOS game.

    • ButlerianJihad 32 minutes ago
      For anyone today who's got copper wire stretched between any buildings, you are in danger and your configuration is quite unwise.

      For every standalone building that has a ground, that building also has an electrical potential. This potential is not necessarily the same between two neighboring buildings. When you stretch a copper line between them, you are tempting fate, electrically, not to mention the dangers of lightning strikes and the elements attacking that connection.

      It is generally recognized that fiber optics are not only very efficient and durable, but also impervious to electrical troubles of the type that may plague inter-building connections. Simply grab switches or routers that support at least one fiber-optic connection, and link up!

      • EvanAnderson 23 minutes ago
        I did work for a small community college back in the 90s that had expanded their building multiple times since the 70s. The electrical potential between different parts of the building was significant enough to make wired Ethernet unreliable. (I never did understand why this was-- multiple electric service entrances with separate grounds, I assume.)

        Because of this they were an early adopter of fiber-based networking. I remember being wowed by ATM over fiber running between closets at a blazing 155Mbps (with tons of 10BASE-T clients connected to FORE chassis switches).

      • lstodd 25 minutes ago
        Who cared, we were 15 at the time.

        Besides you worry too much. Entire cities were wired with cat5 hanged off steel cable between 9 to 15 story buildings in early 2000s, before fiber. Nothing burned down.

  • ButlerianJihad 43 minutes ago
    It seems a strange choice, for the demo and for the script, to manually configure addresses on both ends. If your TCP/IP stack is functioning properly, this will not be necessary. Once DHCP fails, you should get a pair of 169.254.0.0/16 (APIPA) addresses, and then Bob's your uncle.

    Configuring all this manually adds extra complexity when it seems that the goal is simply to connect up your cable and let 'er fly.

    • rmunn 28 minutes ago
      Back in my college days, 169.254.x.y addresses were the bane of my existence, because my job was getting students' computers set up with the brand-new Ethernet connection in their dorm room. This usually required a tech to come out to their room and configure Windows 95 properly, with IP and DHCP instead of Novell Netware and IPX (I don't remember why Netware/IPX was the default on so many Windows 95 installs, but that was definitely what I found most often when I went to look at someone's computer that "wasn't connecting to the Internet".)

      Occasionally we had someone who knew what they were doing and had their Windows 95 settings set up correctly, and then all we had to do was activate their room's Ethernet port and add their MAC address to the DHCP server's list of authorized MACs. But most often I or one of the other techs had to head down to the person's dorm room and set up their computer's network settings correctly before it would work. Once the 169.254 address was replaced by 192.168, my job was finally done. But there were times when that took some doing.

  • riobard 54 minutes ago
    For the time spent finding the ethernet adapter, plugging the cable, configuring IP addresses, and testing with pings, any decent portable SSD drives should have finished copying tens of GB of data.
    • yjftsjthsd-h 46 minutes ago
      As the article says,

      > Removable storage is quite slow unless you are willing to spend a lot of money.

      An SSD good enough to copy tens of GB quickly is not cheap.

      • riobard 33 minutes ago
        At this particular point in time, yes SSD is expensive. But 5~10Gbps USB 3.0 SSD had been very affordable for a long time before 2025.