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Project Notes

Sinatra Echo Tools

A Sinatra app that provides a number of remote request debugging tools, and can be run with Docker

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Notes

This is a simple Sinatra application that responds with a number of diagnostic calls:

  • /time/now - Returns server UTC time in iso8601 format.
  • /ip - Returns the IP address of the calling device (as it appears to the server).
  • /request - Returns the request details as seen on the server, including request headers and the Sinatra/Rack environment.
  • /:code: - Returns a response with the specified HTTP status code (:code:, e.g. 200, 401, 500)

By default, they all return an HTML response, but adding a path extension can be used to return a plain text, XML or JSON response:

  • /:path:.txt - with a plain text response
  • /:path:.json - with a JSON response
  • /:path:.xml - with an XML response
  • /:path:.html - with an HTML response

Loading the root page returns a menu:

landing

Running the Example

To run the app locally:

$ gem install bundler
$ bundle install
$ ruby app.rb
== Sinatra (v2.0.1) has taken the stage on 4567 for development with backup from Thin
Thin web server (v1.7.2 codename Bachmanity)
Maximum connections set to 1024
Listening on localhost:4567, CTRL+C to stop

Example Calls (Local Server)

Here are some examples of calling the status code endpoint with curl:

Default HTML format:

$ curl -v localhost:4567/500.html
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 4567 (#0)
> GET /500.html HTTP/1.1
> Host: localhost:4567
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/html;charset=utf-8
< Content-Length: 218
< X-XSS-Protection: 1; mode=block
< X-Content-Type-Options: nosniff
< X-Frame-Options: SAMEORIGIN
< Connection: keep-alive
< Server: thin
<
<html>
<head>
  <title>Sinatra Echoplex</title>
  <link rel="stylesheet" type="text/css" href="/site.css">
</head>
<body>
  <h1>Custom Status Response</h1>
<p>
  Returning code 500 in html format
</p>
</body>
</html>

Plain text format:

$ curl -v localhost:4567/500.txt
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 4567 (#0)
> GET /500.txt HTTP/1.1
> Host: localhost:4567
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain;charset=utf-8
< Content-Length: 32
< X-Content-Type-Options: nosniff
< Connection: keep-alive
< Server: thin
<
Returning code 500 in txt format

XML format:

$ curl -v localhost:4567/302.xml
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 4567 (#0)
> GET /302.xml HTTP/1.1
> Host: localhost:4567
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 302 Moved Temporarily
< Content-Type: application/xml;charset=utf-8
< Content-Length: 77
< X-Content-Type-Options: nosniff
< Connection: keep-alive
< Server: thin
<
<response>
  <message>Returning code 302 in xml format</message>
</response>

JSON format:

$ curl -v localhost:4567/502.json
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 4567 (#0)
> GET /502.json HTTP/1.1
> Host: localhost:4567
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 502 Bad Gateway
< Content-Type: application/json
< Content-Length: 47
< X-Content-Type-Options: nosniff
< Connection: keep-alive
< Server: thin
<
{"message":"Returning code 502 in json format"}

Running with Docker

The dockerfile and dockerfile-compose.yml configurations are fairly straight-forward.

The main trick to ensure the Sinatra app can run from within Docker is to ensure it binds to the correct interface (else it will only be available locally within the container!). The default binds to localhost.

This can be done in a few ways:

  • set :bind, '0.0.0.0' in th application code
  • ruby app.rb -o 0.0.0.0 from the command line

In this case, I’ve done it with command line parameters in the dockerfile-compose configuration.

Running the app with docker:

docker-compose build
docker-compose up

Or to run in the background:

$docker-compose up -d
Starting sinatra_echo_web_1 ... done

$ docker-compose logs
Attaching to sinatra_echo_web_1
web_1  | == Sinatra (v2.0.1) has taken the stage on 4567 for development with backup from Thin
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:34 +0000] "GET / HTTP/1.1" 200 2069 0.0145
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:34 +0000] "GET /site.css HTTP/1.1" 200 482 0.0008
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:34 +0000] "GET /favicon.ico HTTP/1.1" 0 71 0.0026
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:48 +0000] "GET / HTTP/1.1" 200 2069 0.0040
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:48 +0000] "GET /site.css HTTP/1.1" 200 482 0.0010
web_1  | 172.18.0.1 - - [21/Jul/2019:06:46:48 +0000] "GET /favicon.ico HTTP/1.1" 0 71 0.0006
web_1  | 192.168.0.46 - - [21/Jul/2019:06:48:48 +0000] "GET /ip.txt HTTP/1.1" 200 12 0.0006
web_1  | 172.18.0.1 - - [21/Jul/2019:06:48:54 +0000] "GET /ip.txt HTTP/1.1" 200 10 0.0006
web_1  | == Sinatra (v2.0.1) has taken the stage on 4567 for development with backup from Thin

$ docker-compose down
Stopping sinatra_echo_web_1 ... done
Removing sinatra_echo_web_1 ... done
Removing network sinatra_echo_default

Request IPs in a Docker Environment

When running the app in Docker, there’s an interesting gotcha when it comes to identifying the IP of the client. Consider these responses:

$ curl 0.0.0.0/ip.txt
172.18.0.1
$ curl -H "X-Forwarded-For: 192.168.0.46" 0.0.0.0/ip.txt
192.168.0.46

In the first case, the IP 172.18.0.1 is actually not the client IP address, but the address used by the Docker userland-proxy. Docker has proxied the request, but not forwarded the original request. This is an issue that has been raised on Docker for some time with no fix in sight (see here and here ).

When running Docker in Linux, it is possible to bypass the userland-proxy and bind directly to the host interface (solving this issue). This is descibed in the docs. However it does not work for Mac and Windows.

I have added support for X-Forwarded-For headers in the app, as demonstrated above. I have not tested this yet, but I suspect that as long as an IP load balancer (AWS Elastic LB or haproxy) sits in front of the dockerized application it should be possible to have the X-Forwarded-For header inserted and forwarded.

Credits and References

About LCK#83 rubySinatraDocker
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

These range from the trivial to the complex and serious. Many are inspired by existing work and I'll note credits and references where applicable. The focus is quite scattered, as I variously work on things new and important in the moment, or go back to revisit things from the past.

This is primarily a personal collection for my own edification and learning, but anyone who stumbles by is welcome to borrow, steal or reference the work here. And if you spot errors or issues I'd really appreciate some feedback - create an issue, send me an email or even send a pull-request.