A simple Docker image that uses mitmproxy to convert Slack block kit webhook payloads to the legacy format. This is useful for sending webhooks to Mattermost, which doesn't yet support the new format.

Messages are still susceptible to the limitations described in the Mattermost docs about Slack Compatibility.

Quickstart

Point the webhook source to the proxy port (18080 in this example) and set proxy_dest to the Mattermost host (including protocol & port).

docker run \
    -p 18080:8080 \
    -e proxy_dest=http://mattermost:8065/
    thinkmassive/mattermost-webhook-slack-proxy

Test the connection using curl:

MM_HOST=http://localhost:18080
MM_SECRET=REPLACE_WITH_YOUR_WEBHOOK_SECRET

curl -i -X POST -H 'Content-Type: application/json' \
  -d '{"blocks": [{"text": {"text": "Hello :tada:"}}]}' \
  $MM_HOST/hooks/$MM_SECRET

If you run the proxy in an ad-hoc container, you may want to attach it to an existing docker-compose network with the --network parameter. Then your other compose hosts can refer to it by its alias, which can be found using docker inspect on the ad-hoc container.

Docker Compose

  mattermost:
    image: mattermost/mattermost-preview:latest
    ports:
      - 8065:8065

  mattermost-proxy:
    image: thinkmassive/mattermost-webhook-slack-proxy:latest
    environment:
      - listen_port=8080
      - proxy_dest=http://mattermost:8065
    ports:
      - 8080:8080

Interactive Use

You can exec into the container to experiment with mitmproxy and run it in interactive mode.

docker-compose exec -- mattermost-proxy /bin/sh

Kubernetes

I will add a kubernetes manifest soon.

---

Development Notes

This initial implementation is extremely simple. It rewrites the payload as a single text field with the contents of blocks[0][text][text].

The Slack "blocks" message looks like this:

{
  "blocks": [
    {
      "text": {
        "text": "This is where your chat message goes"
      }
    }
  ]
}

The legacy message looks like this:

{"text": "This is where your chat message goes"}