How to verify if the content of two Docker images

Question

how can we determine that two Docker images have exactly the same file system structure, and that the content of corresponding files is the same, irrespective of file timestamps?

I tried the image ids but they differ when building from the same Dockerfile and a clean local repository: I did this test by building one image, cleaning the local repository, then touching one of the files to change its modification date, then building the second image, and their image ids do not match. I used Docker 17.06 (the latest version I believe).

Thanks

Answer 1

After some research I came up with a solution which is fast and clean per my tests.

The overall solution is this:

1. Create a container for your image via docker create ...
2. Export its entire file system to a tar archive via docker export ...
3. Pipe the archive directory names, symlink names, symlink contents, file names, and file contents, to an hash function (e.g., MD5)
4. Compare the hashes of different images to verify if their contents are equal or not

And that's it.

Technically, this can be done as follows:

1) Create file md5docker, and give it execution rights, e.g., chmod +x md5docker:

#!/bin/sh
dir=$(dirname "$0")
docker create $1 | { read cid; docker export $cid | $dir/tarcat | md5; docker rm $cid > /dev/null; }

2) Create file tarcat, and give it execution rights, e.g., chmod +x tarcat:

#!/usr/bin/env python3
# coding=utf-8

if __name__ == '__main__':
    import sys
    import tarfile
    with tarfile.open(fileobj=sys.stdin.buffer, mode="r|*") as tar:
        for tarinfo in tar:
            if tarinfo.isfile():
                print(tarinfo.name, flush=True)
                with tar.extractfile(tarinfo) as file:
                    sys.stdout.buffer.write(file.read())
            elif tarinfo.isdir():
                print(tarinfo.name, flush=True)
            elif tarinfo.issym() or tarinfo.islnk():
                print(tarinfo.name, flush=True)
                print(tarinfo.linkname, flush=True)
            else:
                print("\33[0;31mIGNORING:\33[0m ", tarinfo.name, file=sys.stderr)

3) Now invoke ./md5docker <image>, where <image> is your image name or id, to compute an MD5 hash of the entire file system of your image.

To verify if two images have the same contents just check that their hashes are equal as computed in step 3).

Note that this solution only considers as content directory structure, regular file contents, and symlinks (soft and hard). If you need more just change the tarcat script by adding more elif clauses testing for the content you wish to include (see Python's tarfile, and look for methods TarInfo.isXXX() corresponding to the needed content).

The only limitation I see in this solution is its dependency on Python (I am using Python3, but it should be very easy to adapt to Python2). A better solution without any dependency, and probably faster (hey, this is already very fast), is to write the tarcat script in a language supporting static linking so that a standalone executable file was enough (i.e., one not requiring any external dependencies, but the sole OS). I leave this as a future exercise in C, Rust, OCaml, Haskell, you choose.

Note, if MD5 does not suit your needs, just replace md5 inside the first script with your hash utility.

Hope this helps anyone reading.

Answer 2

If you want to compare content of images you can use docker inspect <imageName> command and you can look at section RootFS

docker inspect redis

    "RootFS": {
        "Type": "layers",
        "Layers": [
            "sha256:eda7136a91b7b4ba57aee64509b42bda59e630afcb2b63482d1b3341bf6e2bbb",
            "sha256:c4c228cb4e20c84a0e268dda4ba36eea3c3b1e34c239126b6ee63de430720635",
            "sha256:e7ec07c2297f9507eeaccc02b0148dae0a3a473adec4ab8ec1cbaacde62928d9",
            "sha256:38e87cc81b6bed0c57f650d88ed8939aa71140b289a183ae158f1fa8e0de3ca8",
            "sha256:d0f537e75fa6bdad0df5f844c7854dc8f6631ff292eb53dc41e897bc453c3f11",
            "sha256:28caa9731d5da4265bad76fc67e6be12dfb2f5598c95a0c0d284a9a2443932bc"
        ]
    }

if all layers are identical then images contains identical content

Answer 3

There doesn't seem to be a standard way for doing this. The best way that I can think of is using the Docker multistage build feature. For example, here I am comparing the apline and debian images. In yourm case set the image names to the ones you want to compare

I basically copy all the file from each image into a git repository and commit after each copy.

FROM alpine as image1

FROM debian as image2

FROM ubuntu
RUN apt-get update && apt-get install -y git
RUN git config --global user.email "[e-mail]" &&\
 git config --global user.name "Your Name"

RUN mkdir images
WORKDIR images
RUN git init

COPY --from=image1 / .
RUN git add . && git commit -m "image1"

COPY --from=image2 / .
RUN git add . && git commit -m "image2"

CMD tail > /dev/null

This will give you an image with a git repository that records the differences between the two images.

docker build -t compare .
docker run -it compare bash

Now if you do a git log you can see the logs and you can compare the two commits using git diff <commit1> <commit2>

Note: If the image building fails at the second commit, this means that the images are identical, since a git commit will fail if there are no changes to commit.