ZMODEM saves the day! Or, why my firmware for a machine with a CPU from 2017 contains a serial file transfer protocol from the 1980s

Recently, I added the package lrzsz to op-build in this commit. This package provides the rz and sz commands – for receive zmodem and send zmodem respectively. For those who don’t know, op-build builds a firmware image for OpenPOWER machines, and adding this package adds the commands to the petitboot shell (the busybox environment you get when you “exit to shell” from the boot menu).

For those who aren’t familiar with ZMODEM, you should first get off my lawn, and secondly, know that it’s a method for sending/receiving files over something akin to a serial port, e.g. a modem. The basic protocol is “I want to send you this file named FOO”, “okay, I would like to receive it”, “here’s some data and a checksum, did you get it and does it match the checksum?”, “yes!”, “okay, great, here’s the next bit” until the file is transferred. Importantly, it has a provision for “no, I didn’t get that right” and for bits to be resent.

The one thing that pretty much always somewhat works on a computer is a serial port (or something that looks like a serial port to software). When you connect to the IPMI console (“ipmitool sol activate”), the host sees this as a serial port that it pumps bytes over. With OpenBMC, you can actually connect to this serial port via SSH.

When diagnosing weird problems during firmware bringup such as “why doesn’t PCI work” or “why does my network adapter not work” (or, perhaps, somebody helpfully didn’t plug the network cable in), it can be useful to copy off a bunch of debug information from the machine.

You may say “can’t you just print the log file to the screen and save it?” and you’d be right, you can do that for text – it’s really annoying for binary data though. Plus, there have been bugs in the console implementations on pretty much every BMC I’ve ever used that makes them not as reliable as you’d like.

So, how could we transfer a file over the serial connection we have to the machine? The same way we did on a BBS! Enter ZMODEM. The error recovery properties are perfect in this situation.

So, how do you use it? I’ve found two ways that work well: GNU screen and zssh. For GNU Screen, you’ll want to configure it to catch zmodem traffic by doing “control-a:zmodem catch<ENTER>” (you need the colon there). After that if you execute “sz” on the host and the rest should be obvious! If you wanted to send a file to the host, run “rz” rather than “sz”.

2 thoughts on “ZMODEM saves the day! Or, why my firmware for a machine with a CPU from 2017 contains a serial file transfer protocol from the 1980s

  1. Remembering statistical multiplexors and other non-transparent connections, I would have suggested kermit… which doesn’t need an 8-bit clean connection.

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