smicro

What

A small, monothreaded, mostly safe, executable helper that provides SFTP support for any SSH server (OpenSSH, dropbear, ...).

Why

• Learning about the SFTP protocol
• Writing some Rust the easy way (no concurrency, no async/await scheduling with Pin<Box<dyn Future<Output= ...>>> or Send/Sync obsctacles, hence no unexpected hair loss)
• Settling an argument we had at work about the number of lines needed to write a new SFTP helper binary in Rust (the argument was pointless anyway, because we rightly choose not to do that, but that sparked this small project)
• Why not?

How do I use it?

Prerequisite: the target computer/server must be a Linux box!

Build the binary:

$ cargo build --target x86_64-unknown-linux-musl --release

Copy it to /usr/libexec:

$ sudo cp target/x86_64-unknown-linux-musl/release/smicro /usr/libexec/smicro-sftp

Replace Subsystem sftp internal-sftp or Subsystem sftp /usr/libexec/openssh/sftp-server in /etc/ssh/sshd_config with:

Subsystem sftp /usr/libexec/smicro-sftp

This will tell OpenSSH to spawn our binary whenever a SFTP channel is opened.

Then restart sshd.

And you're done :)

Why should I use it?

That's a good question.

• You need to modify an sftp server to add/change some behaviour, and this procject is a good base for that, being small and fairly self-contained
• Because you don't trust the SFTP server of OpenSSH (but in that case, good luck finding an SSH server you trust!)
• You like to break your setup in subtle ways and want to see how this is gonna bite you later

Performance

On a contemporary CPU (i7-11700), both reading and writing files with the OpenSSH sftp binary yields over 1.2GB/s.

In fact, both reading and writing are limited by the speed of OpenSSH itself and top at around 1.6-1.7GB/s while consuming around 40-50% of a CPU core (the fact the OpenSSH is the limiting factor is quite understandable, given that it must perform cryptographic operations on the data it transfers with the client, while we only need to copy data to/from a file).