Suddenly I realized I don’t understand scp though I have used Linux for about seven years, used scp frequently, shame of me.

The story start when I was trying to run an sshd in a docker container, yes, this is the recommended way to using a docker container.

After I have start my docker container like below:

# docker run -p 12345:22 -it my_docker_image /bin/bash

After less than a second, I get the docker container shell prompt

bash-4.2#

And then I start sshd in docker like below

bash-4.2# /usr/sbin/sshd -De
Server listening on 0.0.0.0 port 22.
Server listening on :: port 22.

So the docker container is up and running, sshd is up and running and now I can login to my container through the ssh tunnel like below.

# ssh localhost -p 12345
root@localhost's password: 
Last login: Fri Jul 18 13:45:49 2014 from 172.17.42.1
root@e6335ba07a7c ~#

And I’d like to transfer some files into my docker container, so I use scp to do that, yes, why not, I’m using scp day to day.

# scp -P 12345 test.txt localhost:/
root@localhost's password: 
bash: scp: command not found
lost connection

What?! bash: scp: command not found

That’s did surprised me! As I see:

  • I have sshd started in the docker container
  • I can login into the docker container through ssh

Why scp failed?

Is there anything wrong with docker container?

How about this goes in real Linux? I didn’t saw this error in my past seven years, so I though this maybe a docker regression.

However, when I was not busy I study out why it fail and finally got my answer at wikipedia.

Normally, a client initiates an SSH connection to the remote host, and requests an SCP process to be started on the remote server. The remote SCP process can operate in one of two modes: source mode, which reads files (usually from disk) and sends them back to the client, or sink mode, which accepts the files sent by the client and writes them (usually to disk) on the remote host. For most SCP clients, source mode is generally triggered with the -f flag (from), while sink mode is triggered with -t (to).[2] These flags are used internally and are not documented outside the SCP source code.

Yes, sshd doesn’t handle any read/write from/to file for scp but just provides the secure tunnel, so a scp is necessary to start to handle file copying on both the local end and remote end.