curl > /dev/sda: How I made a Linux distro that runs wget | dd

https://astrid.tech/2026/03/24/0/curl-to-dev-sda/

172 65 4 months ago

M95D• 4 months ago

From the article:

The OS may stop you from unmounting /dev/sda1, but it won’t stop you from writing to /dev/sda1 or /dev/sda even if there’s something mounted!

Not always true. There's a kernel config option that allows it. CONFIG_BLK_DEV_WRITE_MOUNTED

Sophira• 4 months ago

It's worth noting, though, that that config option was only introduced in kernel version 6.8! Before then the option didn't exist and you could write with impunity to mounted devices (as root, obviously).

rwmj• 4 months ago

Unfortunately it's not safe as the kernel can still write to (what it thinks is) the old filesystem on the device, which will introduce corruption to the new disk image.

However a fun fact is that you can (do not actually do this!) boot a qemu VM from /dev/sda. You have to use an overlay (eg. qemu -drive snapshot=on flag) so that qemu won't write through to /dev/sda. I use this trick in supernested, a script I wrote that runs nested within nested within nested VMs ad infinitum until your hypervisor crashes. http://git.annexia.org/?p=supernested.git;a=blob;f=run-super...

vidarh• 4 months ago

The second part in the series deals with that by mounting it read-only from initrd.

Joker_vD3• 4 months ago

What if we remount the filesystem(s) at /dev/sda as read-only first? Then make a small ramfs with statically-linked curl in it and exec it. Hmm. Ideally, you'd also want to call reboot(2) after it's done...

akdev1l• 4 months ago

in most cases you could just drop back into the initramfs that is included in most distros

Or if you have access to the boot command line you can also usually stop the boot process before pivot_root happens (hence you’ll be left running in the initramfs environment)

On Fedora/EL it would be done by putting `rd.break` in the kernel command line

astralbijection8• 4 months ago

All of those things get covered in parts 2, 3, and 4 :)

tremon• 4 months ago

You also don't want to do this under any kind of memory pressure, because the kernel will happily drop read-only pages from memory if it thinks they can be re-read from disk when needed.

duskwuff• 4 months ago

One bit of magic you may be interested in is pivot_root, which allows another filesystem to take the place of the root filesystem (e.g. / and /mnt become /old and /). It's usually used during startup, to allow the "real" root filesystem to take the place of the initrd, but could have other uses.

ciupicri• 4 months ago

Could xfs_freeze help with this?

[0] https://www.man7.org/linux/man-pages/man8/xfs_freeze.8.html

astralbijection8• 4 months ago

That script sounds extremely unhinged, and I mean it as a compliment :)

Without spoiling too much, the command at the very end of the series does something adjacent to this.

tux3• 4 months ago

I used to dual-boot windows, but I was too lazy to actually reboot, so naturally I had Virtualbox just boot the physical Windows partition while Linux was running. Which is totally fine!

It's not a real dual boot if you don't boot both partitions at the same time.

As long as you don't install guest VBox drivers, those would make it hang when it boots as the host on physical hardware, since there's no longer someone above to answer the hypercalls.

johnisgood• 4 months ago

Yeah. That is a valid use. I mean, this is how I installed Windows to begin with, from Linux via QEMU, onto my other hard drive. I did reboot and test it out, and it worked just fine.

ahartmetz• 4 months ago

I think Windows refused to do that at some point? So I booted the physical Linux partition from Windows if I needed both at the same time. That's on a laptop that otherwise almost always ran Linux.

hdb2• 4 months ago

I had Virtualbox just boot the physical Windows partition while Linux was running. Which is totally fine!

I had no idea that this was possible, and I learned something new today. Thank you!

kyrjtejkrtys• 4 months ago

depending on the size of your disk image and your uefi+boot partitions it's still possible to safely pull off.

unmount the efi and boot partitions, write your image to the head of the disk, power cycle, then grow the last filesystem from the image to cover the rest of the disk.

you might get lucky and have all three of uefi/boot/swap to work with.

of course with the advent of uefi, you could instead just drop an installer image directly into the efi parition and boot that.

matja3• 4 months ago

How do you unmount your OS’s disk while keeping the OS running to be able to overwrite itself?

I went down a similar rabbit-hole myself, with the goal of safely replacing the Linux installation on a disk that a machine is already running from (e.g. replace a VPS's setup image with one of your own) without needing a KVM-style remote access tool to the console.

The problem there is if you directly modify the disk when a filesystem is mounted on that disk then all bets are off in terms of corruption of the filesystem that's already on there and also the filesystem(s) you're writing over the top.

My solution was to kexec into a new kernel+initramfs which has a DHCP client and cURL in it - that effectively stops any filesystem access while the image is being written over the disk, then to just reboot.

lloydatkinson• 4 months ago

The gymnastics VPS providers force people to go through just so they can have some dumb "wizard" with a limited number of OS choices is maddening. Just allow people to upload an ISO!

codeflo• 4 months ago

My solution was to kexec into a new kernel+initramfs which has a DHCP client and cURL in it - that effectively stops any filesystem access while the image is being written over the disk, then to just reboot.

That's what I was expecting from the article.

Update: It's not obvious, but it turns out that this is a multipart article, and kexec is reserved for part 3: https://astrid.tech/2026/03/24/2/how-to-pass-secrets-between...

matja3• 4 months ago

I totally missed part 2/3, thanks for linking!

kees99• 4 months ago

Keeping with the YOLO spirit of the article, one can be even lazier, and do emergency R/O remount using this little thing:

https://www.kernel.org/doc/html/latest/admin-guide/sysrq.htm...

It's technically not an unmount, but still a pretty strong guarantee OS will not corrupt the image being written.

When done, reboot has to be done from the same sysrq handler, of course.

rkeene22• 4 months ago

I usually just move all the files to a new directory (/oldroot) and pivot_root -- any open files reference the new paths. Then install into the newly empty root directory of the filesystem, reboot and delete the /oldroot.

matja3• 4 months ago

That sounds like the best way if keeping the filesystem is an option. In my case I wanted to also change filesystems and apply FDE, which is possible to do if the original filesystem supports online shrinking but many do not.

arboles2• 4 months ago

Don't you get any errors even if you race immediately to start pivot_root? pivot_root also won't modify all open file descriptors at once. Seems it's not fatal, but have you managed to do this over ssh and not be disconnected?

dizhn3• 4 months ago

Reminded me of how to install Alpine linux (which isn't available) on Oracle cloud over an ubuntu install. It uses dd and has the advantage of having a console.

I had found it in a github gist when I used it but here's a similar blog post.

https://alextsang.net/articles/20191006-063049/index.html

mbana• 4 months ago

Wait hold on, can you not simply just access the underlying volume/block device using an API? The VMs in OCI have a boot volume that is attached, so I reckon it's possible to "mount" this somehow and overwrite it with whatever data you want.

dizhn3• 4 months ago

I am not sure. Maybe it's a thing about not being able to download the iso (no network on the console?) or not having space for it or something. I wouldn't know about the API thing. I am not a cloud user.

Made me think though.

astralbijection8• 4 months ago

From what it sounds like, because you have a console and therefore aren't dependent on SSHD not getting overwritten, you can just dd the live running system here?

dizhn3• 4 months ago

Like I said in the other comment I am not really familiar with the various clouds. I self host. The console is a weird web based thing that isn't the same environment as your VMs (or their hypervisor). It's a barebones shell where you can mount your volumes and such, not actually boot or enter the vm. (Edit: This was incorrect) And if I remember correctly I probably did have to do some mounting there to create device files and such. I didn't really have much use for the console after that either. I will try to find the actual gist I followed.

Here's the gist I had used. It's really simple. https://gist.github.com/unixfox/05d661094e646947c4b303f19f9b...

Why would not have not done the dd bit on the console? I have no idea. Again possibly can't download the iso there?

irishcoffee• 4 months ago

I've been dd-ing A/B partitions for embedded yocto distributions for years and years. read-only-rootfs (/var/log is its own writable partition), dd the "other partition", sed fstab, reboot.

The neat part was the whole process kicked off when you scp'd the rootfs and inotifywait kicked off the whole process.

BirAdam• 4 months ago

Yeah, make /home, /var/log, and /usr/local rw and everything else ro. Makes a great "immutable" that's not as annoying as truly "immutable" systems.

PunchyHamster2• 4 months ago

and we've gone full circle, back in the day you installed os on diskettes like that!

saidnooneever• 4 months ago

please insert diskette 34...

now go back to diskette 2...

now please put diskette 15 again....

aa-jv• 4 months ago

That was Windows. Linux floppy-based installs usually just got through the whole series and then asked for disk 1 prior to reboot ..

PunchyHamster2• 4 months ago

Well, what can we try instead? write to the mounted disk anyways. fuck you

Stupid penguin trick I learned: Add a file inside ramdisk (i use /dev/shm) as LVM PV.

pvmove off the hard drive

Boom, now your OS lives entirely in RAM

You can now even replace the hard disk, put a new one and migrate back.

Or migrate to network storage (nbd,iSCSI etc.), re-sequence disks into whatever RAID you need, and migrate back

Need to fix /boot after that tho, and probably make sure to not have power failure in meantime

pzmarzly• 4 months ago

You will run into problems if destination drive has different sector size than your VM, as GPT header won't be aligned.

QEMU defaults to 512B sectors, which isn't true for many NVMe drives. There are some flags to change that. https://unix.stackexchange.com/a/722450

I think it should be possible to make an image with many headers at different locations, so that it works on all types of disks at once, but I don't think any tools do it for you by default.

ma2kx• 4 months ago

Why not just use netboot?

kotaKat• 4 months ago

you may be in a restricted environment with no boot option selections, like on some VPS and dedi server providers.

i've seen similar techniques used to shove windows on "linux" VPS/dedis boxes by booting into rescue mode and then applying a raw Windows boot image that's preconfigured and rebooting back to the Windows install and hoping you stood the image up right.

good ol' days of getting Windows up on Kimsufi boxen.

alexellisuk• 4 months ago

This reminds me of netbooting workflows from things like MaaS, Tinkerbell, and Dan's old Plunder tool.

They'd netboot.. not mount the disks, then download an ISO/IMG and write it directly to the primary boot disk.

If netbooting is a heavy lift, why not boot into a custom initramfs you built, with i.e. dd/curl installed, and flash the disk that way, without mounting / at all? Then kexec/chroot into it?

I'd much prefer this as a way to provision Raspberry Pis.

astralbijection8• 4 months ago

Part 2 presents a fully automated proof of concept that does all of this: https://astrid.tech/2026/03/24/2/how-to-pass-secrets-between...

klinch• 4 months ago

Sounds cursed. But I'm not judging, given that I use nixos-anywhere[0] on an almost weekly basis.

[0] https://github.com/nix-community/nixos-anywhere

astralbijection8• 4 months ago

Does it make it more cursed that the distro was built off of NixOS Anywhere, and then it theseus shipped NixOS Anywhere out of it?

poppafuze• 4 months ago

Looking forward to seeing a device with a short image that has the string "404" on it.

igtztorrero• 4 months ago

Can I run a Windows qcow2 disk imagen on a Contabo Vps ?

SamWhited• 4 months ago

Reminds me of the first company I worked for out of school.

We had a big drive with the source of truth image used to boot all our machines on it, and we added rsync to the init image. When each machine booted init would rsync everything from the storage box to the local machine. We'd keep the storage machine up to date and when we wanted to update other machines in the fleet we'd just do a reboot and it would sync up the latest files (provisioning for whatever each machine was supposed to do happened later, can't remember how that was handled now). The storage machine was running ZFS so we also took a snapshot before doing any rolling reboots, so if anything did go wrong you could just revert to the previous snapshot and reboot again as long as you didn't break the init image.

Sounds jank saying it out loud, but I don't remember it ever causing us any problems.

zoobab• 4 months ago

I used netcat and dd via the network to clone machines that has the same HDD:

https://support.tools/dd-over-netcat-clone-drive-remote-back...

But I like the curl approach very much!

megous2• 4 months ago

Instead of applying some sense to the problem, and using a solution that actually allows you to kill all running processes of the original distro at runtime, incl. getting rid of the original init process, to be able to pivot_root somewhere else amd umount the original system's filesystems and free the block device for re-installation, this ridiculous approach gets promoted to a front page, lol.

astralbijection8• 4 months ago

I did learn about systemctl switch-root after finishing my kexec solution, but I think if you're at the point of reimaging a server over the network, you probably don't care enough to shut it down correctly :)

megous2• 4 months ago

Actually you do, bacause you can't re-image safely while kernel is also writing to your target block device via fs driver. And if original server has nything more complex going on, like md/lvm you're in even more trouble.

creantum• 4 months ago

Just because you can doesn’t mean you should.

tosti• 4 months ago

If you have a swap partition, swapoff it and install there. Or at least a minimal kernel and initramfs. Set as default in grub and there you go.

Also, I once burned an iso straight from ftp using a fifo. I was low on disk space and really needed that CD. Worked fine because the Internet was already faster than the CDR.

indigodaddy• 4 months ago

NOC techs have been doing these tricks for tens of years

anshulbasia27• 4 months ago

Happened with me as well

em3rgent0rdr• 4 months ago

"download a pre-prepared disk image directly to your disk"

Well not quite direct; the bits go through your RAM in between.

e12e• 4 months ago

Nice series! Really takes me back to the days of Linux 1.x kernel, Lilo and trying to fit a kernel and initrd on a single floppy disk.

So ending up at:

From a 292MB initramfs, we now have a 6.1MB initramfs, smaller than almost every other distro's initramfs and made entirely to run busybox wget dd.

Is pretty great achievement today - but way bigger than something that can fit on a floppy.

astralbijection8• 4 months ago

To be honest, even this has plenty of room to go down. I get the feeling I could have squeezed a couple more MB off if I had actually cut things off of the default Nixpkgs busybox, and possibly also cut a couple of kernel drivers out.

justsomehnguy• 4 months ago

Modern "floppy" is just any USB thumbdrive you found lying around. Just giggles checked my 'to go' retailer - there are whopping 3 variants of 4GB no-name drives for $6. For $12 you would get a Kingston one with 64GB.

"Floppy" image size is not important now nor for the last 15 years.

motrm• 4 months ago

Mildly pedantic, and of course ignores how wild this whole thing is, but I don't think this bit is correct:

  After waiting for a little while, the program terminated with the following output:
  
  astrid@chungus infra  gzip -vc result/nixos.img | ssh root@myhost.example -- bash -c 'gunzip -vc > /dev/sda'
  root@myhost.example's password:
   77.8% -- replaced with stdout
  
  What happened here?

The 77.8% bit is gunzip -v reporting that it finished decompressing the data to stdout and that the compression ratio was 77.8%... so this invocation may well have succeeded. Assuming, as rwmj points out, nothing else stomped on any of the written blocks.

I do like this idea - with sufficient prep of the system before writing the image, namely stopping as many processes as possible especially those that might do some writing, it's a quick and dirty way to replace a stock OS with a ready-made image. Could perhaps be safer doing it twice, once into a minimal image that does very little beyond network bringup & runs ssh, followed by final OS replacement in a (more) controlled manner.

curl > /dev/sda: How I made a Linux distro that runs wget | dd

Related Stories