Docker

Notes on building, managing and running Docker containers.

Minimal Image for HTTP Testing

The other day I was working on a project to automate provisioning of infrastructure to host container applications. As part of this effort, I wanted to have a minimal container image without external dependencies (e.g. a database connection), to test the deployment process and infrastructure.

To achieve this, I created the following Dockerfile which exposes a very basic web service on PORT. While this is a completely non-interesting image, it may be useful for testing.

FROM busybox:latest
ENV PORT 8080
EXPOSE ${PORT}
CMD echo "Fish go moo" > index.html; httpd -f -p ${PORT}

Usage

Build

% docker build -t example .
Sending build context to Docker daemon  2.048kB
Step 1/4 : FROM busybox:latest
latest: Pulling from library/busybox
61c5ed1cbdf8: Pull complete 
Digest: sha256:4f47c01fa91355af2865ac10fef5bf6ec9c7f42ad2321377c21e844427972977
Status: Downloaded newer image for busybox:latest
 ---> 018c9d7b792b
Step 2/4 : ENV PORT 8080
 ---> Running in f10f13b46cde
Removing intermediate container f10f13b46cde
 ---> 2fe842562e7a
Step 3/4 : EXPOSE ${PORT}
 ---> Running in fd413d604f9b
Removing intermediate container fd413d604f9b
 ---> 73125953451d
Step 4/4 : CMD echo "Fish go moo" > index.html; httpd -f -p ${PORT}
 ---> Running in a7f467f0703d
Removing intermediate container a7f467f0703d
 ---> eaafb3bad78b
Successfully built eaafb3bad78b
Successfully tagged example:latest

Run and Test

% docker run -d -p 8080:8080 example
...
% curl -i http://127.0.0.1:8080
HTTP/1.0 200 OK
Date: Sun, 02 Aug 2020 18:16:50 GMT
Connection: close
Content-type: text/html
Accept-Ranges: bytes
Last-Modified: Sun, 02 Aug 2020 18:15:34 GMT
Content-Length: 12

Fish go moo

Tiny!

The resulting image is very small:

% docker image ls example --format {{.Size}}
1.22MB

Docker Image from Live ISO

I've been thinking about picking up Protostar again. My previous setup was running the live image using VirtualBox, but having recently switched to WSL2, I can no longer run my VM on some machines. As such, I decided to try and create a Docker image.

Prerequisites

First, the protostar image is downloaded into a working directory.

• Confirm the unsquashfs tool is available
• Verify the hash to ensure the image was successfully downloaded

$ which unsquashfs
/usr/bin/unsquashfs
$ echo "d030796b11e9251f34ee448a95272a4d432cf2ce " $(ls) | shasum -c -
exploit-exercises-protostar-2.iso: OK

Converting the Image

1. Create some working folders:

   $ mkdir rootfs unsquashfs
   $ ls
   exploit-exercises-protostar-2.iso  rootfs  unsquashfs
   

2. Mount the ISO file to the rootfs folder as a loop device:

   $ sudo mount -o loop exploit-exercises-protostar-2.iso rootfs
   mount: /.../protostar/rootfs: WARNING: device write-protected, mounted read-only.
   

3. Locate the filesystem.squashfs file:

   $ find . -type f | grep filesystem.squashfs
   ./rootfs/live/filesystem.squashfs
   

4. Run unsquashfs to extract the file system:

   $ sudo unsquashfs -f -d unsquashfs/ rootfs/live/filesystem.squashfs
   Parallel unsquashfs: Using 8 processors
   33614 inodes (33227 blocks) to write
   
   [===============================================================\                ] 26696/33227  80%
   create_inode: socket unsquashfs//var/run/acpid.socket ignored
   [==============================================================================| ] 33226/33227  99%
   
   created 29619 files
   created 2997 directories
   created 3472 symlinks
   created 0 devices
   created 0 fifos
   

   Too many levels of symbolic links

   If you run into this error message, you may be unsquashing the file system into a location that is a symbolic link itself. For example, if running this from WSL you may have a symbolic link for your working folders to ensure your projects stay in sync (I obviously did, and ran into this message). The command also runs very slowly if run from a location on the mounted Windows file system (/mnt/c/...).

5. Compress the file system. If needed, copy this file to the host where docker is installed:

   $ sudo tar -C unsquashfs -c . -f protostar.tar
   

6. Import the file system to a docker image:

   $ docker import protostar.tar protostar
   sha256:f43616a6b520b1c11b26da6e5e39d08736959ddf08d1eb531f776eb629846fbf
   $ docker images protostar
   REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
   protostar           latest              f43616a6b520        16 minutes ago      670MB
   

Running the Image

The image can be started using docker:

docker run -h protostar -i -t protostar bash
root@protostar:/#

And a shell as the user for performing the exercises:

docker run --rm -h protostar -it --user 1001:1001 protostar bash
user@protostar:/$ cd opt/protostar/bin/
user@protostar:/opt/protostar/bin$ ./format0 $(python -c 'print "A" * 64 + "\xef\xbe\xad\xde"')
you have hit the target correctly :)