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Sepero
Asked: 2014-06-13 03:00:15 +0800 CST

Docker: ssh hangs after entering password

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My problem is almost exactly the same as the following link, except my ssh server is in a docker container.

My Dockerfile is very simple.

FROM ubuntu:12.04
ENV DEBIAN_FRONTEND noninteractive
RUN apt-get update
RUN apt-get install -y openssh-server less nano
RUN mkdir /var/run/sshd

RUN adduser chrome
RUN echo "chrome:chrome" | chpasswd
RUN echo "root:chrome" | chpasswd

EXPOSE 22
CMD ["/usr/sbin/sshd",  "-D"]

When I try to ssh login, it hangs forever starting the shell.

$ ssh -p2222 chrome@localhost /bin/bash --noprofile --norc  # trying to login with ssh
chrome@localhost's password:    <-------------hangs forever

I can login if I use the -t argument on ssh. I don't know why.

Running other programs like ls works without problems. Perhaps someone can replicate this container and figure out what the problem is?

ssh

3 Answers

  1. This is nothing to do with docker, you can get the same thing to happen when you ssh to a remote host.

    A clue to the problem is in the ssh man page

    -t Force pseudo-tty allocation. This can be used to execute arbi- trary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.

    So, ssh doesn't use a tty allocation if you provide a remote command to be run unless you use the -t switch. As bash is interactive it requires a tty.

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  2. The following will not hang

    $ ssh -p2222 chrome@localhost /bin/bash -i --noprofile --norc

    The -i option for bash is required to get an interactive shell.

    From the Bash Reference Manual 1.2 What is a shell? section:

    Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file.

    and 6.3.1 What is an Interactive Shell?:

    An interactive shell is one started without non-option arguments, unless -s is specified, without specifying the -c option, and whose input and error output are both connected to terminals (as determined by isatty(3)), or one started with the -i option.

    An interactive shell generally reads from and writes to a user’s terminal.

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  3. When you start a container with docker run you have to specify -it for an interactive terminal for the exact same reason as explained above.

    As an observation from your dockerfile you can add --net=host to your docker run command so that the container can access the native NIC configurations if you do not want to use EXPOSE 22 explicitly.

    Also, (as dumb as it sounds, I know) I have had issues at login with docker even on a local container where it appears to hang but if I just hit <enter> I then get the prompt...

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