Docker nsinit
===
`nsinit` provides a handy way to access a shell inside a running container's namespace.  This is useful for learning about how containers work, debugging your system without worrying about sshd daemons, and even hot fixes in production all you sad pandas!

:p

Running the docker daemon with the lxc driver allows you to use `lxc-attach` to do this.  But now that docker deafults to the new native libcontainer driver, `nsinit` is probably the best way to go.  [jpetazzo's blog](http://jpetazzo.github.io/2014/03/23/lxc-attach-nsinit-nsenter-docker-0-9/) has a great high level summary.  

The new issue is that the libcontainer .json format is under heavy development so you need to keep an `nsinit` binary built from the exact release tag from the the docker github repo.  The tricky part for me was understanding that golang is designed with certain assumptions about dependencies vs versioning with regards to when you push to master, push to a topic branch, and fork a project.

Long story short, I just modified crosbymichael's Dockerfile for skydock to build nsinit in a container and then copy it out to my Linux host system.  Here is the procedure:

### Clone
Get yourself the docker repo and select the tag that matches the daemon version installed on your host system.

    $ cd ~/projects
    $ git clone https://github.com/dotcloud/docker.git
    $ cd docker
    $ git checkout -b v1.0.0 tags/v1.0.0 # create a local branch from the tagged version

### Create Dockerfile
Put this Dockerfile in the nsinit directory.

    $ cd pkg/libcontainer/nsinit
    $ vi Dockerfile

Dockerfile:

    FROM crosbymichael/golang
  
    # setup build environment
    RUN apt-get update && \
        apt-get install -y \
        build-essential  

    # install just the deps
    RUN go get -d github.com/dotcloud/docker/pkg/libcontainer/nsinit
  
    # stuff in the specifically tagged code from the current directory 
    ADD . /go/src/github.com/dotcloud/docker/pkg/libcontainer/nsinit
    
    # build a binary image
    RUN cd /go/src/github.com/dotcloud/docker/pkg/libcontainer/nsinit && \
        go install . ./...
  

### Build and Run

    $ docker build -t ubergarm/nsinit .
    $ docker run --name nsinit -d ubergarm/nsinit
    $ docker cp nsinit:/go/bin/nsinit ./outdir/                                 
    $ docker rm nsinit
    $ docker rmi ubergarm/nsinit

### Test
Confirm you can run the binary on your host system by checking with `ldd ./outdir/nsinit.new` and `./outdir/nsinit.new`.

### Install
Do this however you'd like.  Something like this should be close enough.

    $ sudo chmod 755 ./outdir/nsinit
    $ sudo chown root:root ./outdir/nsinit
    $ sudo mv ./outdir/nsinit /usr/local/bin/nsinit-1.0.0
    $ sudo ln -s /usr/local/bin/nsinit-1.0.0 /usr/local/bin/nsinit


### Nifty Script
Since it is a pain to manually look up the container hash and find the directory I put together a script:

    #!/bin/bash
    [[ -n "$1" ]] || { echo "Usage: `basename $0` <partial container ID or name>"; exit 0 ; }
    FULL_ID=$(docker inspect --format='{{.Id}}' $1)

    # LXC driver method:
    # sudo lxc-attach -n $FULL_ID /bin/bash 
  
    # libcontainer driver method:
    sudo bash -c "cd /var/lib/docker/execdriver/native/$FULL_ID && nsinit exec bash"


### Done
Now hop in your favorite running container and have some fun!