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Sunday, 17 July 2016


a small http proxy to simplify routing images through an SSL host。
Camo is all about making insecure assets look secure. This is an SSL image proxy to prevent mixed content warnings on secure pages served from GitHub.
We want to allow people to keep embedding images in comments/issues/READMEs/google charting.
There's more info on the GitHub blog.
Using a shared key, proxy URLs are encrypted with hmac so we can bust caches/ban/rate limit if needed.
Camo currently runs on node version 0.10.13 at GitHub on heroku.


  • Proxy google charts
  • Proxy images under 5 MB
  • Follow redirects to a configurable depth
  • Proxy remote images with a content-type of image/*
  • 404s for anything other than a 200, 301, 302, 303, 304 or 307 HTTP response
  • Disallows proxying to private IP ranges
At GitHub we render markdown and replace all of the src attributes on the img tags with the appropriate URL to hit the proxies. There's example code for creating URLs in the tests.

URL Formats

Camo supports two distinct URL formats:<digest>?url=<image-url><digest>/<image-url>
The <digest> is a 40 character hex encoded HMAC digest generated with a shared secret key and the unescaped <image-url> value. The <image-url> is the absolute URL locating an image. In the first format, the <image-url> should be URL escaped aggressively to ensure the original value isn't mangled in transit. In the second format, each byte of the <image-url> should be hex encoded such that the resulting value includes only characters [0-9a-f].

Testing Functionality

Bundle Everything

% rake bundle

Start the server

% coffee

In another shell

% rake


To see the full URL restclient is hitting etc, try this.
% RESTCLIENT_LOG=stdout rake


You can see an example god config here.
To enable useful line numbers in stacktraces you probably want to compile the file to native javascript when deploying.
% coffee -c
% /usr/bin/env PORT=9090 CAMO_KEY="<my application key>" node server.js