How to connect to Heroku rendezvous url with openssl

In this post we are looking into how to run a command on a heroku one-off dyno with curl and consume the rendezvous url over tcp.

Running a command

If you are familiar with the heroku API, you know that you can easily query it from the shell with curl.

For example, in order to run a rake task, you can do something like this:

# $1 is your heroku app name, $2 is the command
curl -H "Authorization: Bearer ${HEROKU_OAUTH_TOKEN}" \
         -H "Accept: application/vnd.heroku+json; version=3"  \
         -H "Content-Type: application/json" \
         -d "{\"attach\": true, \"command\" : \"$2\", \"type\" : \"run\"}" \
        "https://api.heroku.com/apps/$1/dynos

Heroku API will return you a json object that contains a rendezvous URL.

{ "attach_url":"rendezvous://rendezvous.runtime.heroku.com:5000/<secret>" ... }

Connecting to the rendezvous url

One nice little trick, is to get your command output stream by connecting to the rendezvous url. Since it's a tcp connection with ssl, you can use openssl for that.

openssl s_client -connect rendezvous.runtime.heroku.com:5000
# Once the connection is established, you can past the <secret> in order to finish the handshake.

# here is a one-liner to do that
(echo "<secret>"; sleep 1) | openssl s_client -connect rendezvous.runtime.heroku.com:5000

Try it out

If you want to try it out, I made a small script that allows for running a command on heroku and meeting at the rendezvous with openssl.