imageproxy is a caching image proxy server written in go. It features:

• basic image adjustments like resizing, cropping, and rotation
• access control using host whitelists or request signing (HMAC-SHA256)
• support for jpeg, png, webp (decode only), tiff, and gif image formats (including animated gifs)
• on-disk caching, respecting the cache headers of the original images
• easy deployment, since it's pure go

Personally, I use it primarily to dynamically resize images hosted on my own site (read more in this post). But you can also enable request signing and use it as an SSL proxy for remote images, similar to atmos/camo but with additional image adjustment options.

imageproxy URLs are of the form http://localhost/{options}/{remote_url}.

Options are available for cropping, resizing, rotation, flipping, and digital signatures among a few others. Options for are specified as a comma delimited list of parameters, which can be supplied in any order. Duplicate parameters overwrite previous values.

See the full list of available options at https://godoc.org/willnorris.com/go/imageproxy#ParseOptions.

The URL of the original image to load is specified as the remainder of the path, without any encoding. For example, http://localhost/200/https://willnorris.com/logo.jpg.

In order to optimize caching, it is recommended that URLs not contain query strings.

The following live examples demonstrate setting different options on this source image, which measures 1024 by 678 pixels.

Options	Meaning	Image
200x	200px wide, proportional height
x0.15	15% original height, proportional width
100x150	100 by 150 pixels, cropping as needed
100	100px square, cropping as needed
150,fit	scale to fit 150px square, no cropping
100,r90	100px square, rotated 90 degrees
100,fv,fh	100px square, flipped horizontal and vertical
200x,q60	200px wide, proportional height, 60% quality
200x,png	200px wide, converted to PNG format
cx175,cw400,ch300,100x	crop to 400x300px starting at (175,0), scale to 100px wide

Transformation also works on animated gifs. Here is this source image resized to 200px square and rotated 270 degrees:

Install the package using:

go get willnorris.com/go/imageproxy/cmd/imageproxy

(Note that go1.2 and earlier may have trouble fetching the package with go get).

Once installed, ensure $GOPATH/bin is in your $PATH, then run the proxy using:

imageproxy

This will start the proxy on port 8080, without any caching and with no host whitelist (meaning any remote URL can be proxied). Test this by navigating to http://localhost:8080/500/https://octodex.github.com/images/codercat.jpg and you should see a 500px square coder octocat.

By default, the imageproxy command does not cache responses, but caching can be enabled using the -cache flag. It supports the following values:

• memory - uses an in-memory cache. (This can exhaust your system's available memory and is not recommended for production systems)
• directory on local disk (e.g. /tmp/imageproxy) - will cache images on disk
• s3 URL (e.g. s3://s3-us-west-2.amazonaws.com/my-bucket) - will cache images on Amazon S3. This requires either an IAM role and instance profile with access to your your bucket or AWS_ACCESS_KEY_ID and AWS_SECRET_KEY environmental variables be set.
• gcs URL (e.g. gcs://bucket-name/optional-path-prefix) - will cache images on Google Cloud Storage. This requires GCP_PRIVATE_KEY environmental variable be set.
• azure URL (e.g. azure://container-name/) - will cache images on Azure Storage. This requires AZURESTORAGE_ACCOUNT_NAME and AZURESTORAGE_ACCESS_KEY environmental variables set.

For example, to cache files on disk in the /tmp/imageproxy directory:

imageproxy -cache /tmp/imageproxy

Reload the codercat URL, and then inspect the contents of /tmp/imageproxy. Within the subdirectories, there should be two files, one for the original full-size codercat image, and one for the resized 500px version.

You can limit images to only be accessible for certain hosts in the HTTP referrer header, which can help prevent others from hotlinking to images. It can be enabled by running:

imageproxy  -referrers example.com

Reload the codercat URL, and you should now get an error message. You can specify multiple hosts as a comma separated list, or prefix a host value with *. to allow all sub-domains as well.

You can limit the remote hosts that the proxy will fetch images from using the whitelist flag. This is useful, for example, for locking the proxy down to your own hosts to prevent others from abusing it. Of course if you want to support fetching from any host, leave off the whitelist flag. Try it out by running:

imageproxy -whitelist example.com

Reload the codercat URL, and you should now get an error message. You can specify multiple hosts as a comma separated list, or prefix a host value with *. to allow all sub-domains as well.

Instead of a host whitelist, you can require that requests be signed. This is useful in preventing abuse when you don't have just a static list of hosts you want to allow. Signatures are generated using HMAC-SHA256 against the remote URL, and url-safe base64 encoding the result:

base64urlencode(hmac.New(sha256, <key>).digest(<remote_url>))

The HMAC key is specified using the signatureKey flag. If this flag begins with an "@", the remainder of the value is interpreted as a file on disk which contains the HMAC key.

Try it out by running:

imageproxy -signatureKey "secret key"

Reload the codercat URL, and you should see an error message. Now load a signed codercat URL and verify that it loads properly.

Some simple code samples for generating signatures in various languages can be found in URL Signing.

If both a whiltelist and signatureKey are specified, requests can match either. In other words, requests that match one of the whitelisted hosts don't necessarily need to be signed, though they can be.

Typically, remote images to be proxied are specified as absolute URLs. However, if you commonly proxy images from a single source, you can provide a base URL and then specify remote images relative to that base. Try it out by running:

imageproxy -baseURL https://octodex.github.com/

Then load the codercat image, specified as a URL relative to that base: http://localhost:8080/500/images/codercat.jpg. Note that this is not an effective method to mask the true source of the images being proxied; it is trivial to discover the base URL being used. Even when a base URL is specified, you can always provide the absolute URL of the image to be proxied.

By default, the imageproxy won't scale images beyond their original size. However, you can use the scaleUp command-line flag to allow this to happen:

imageproxy -scaleUp true

Imageproxy can proxy remote webp images, but they will be served in either jpeg or png format (this is because the golang webp library only supports webp decoding) if any transformation is requested. If no format is specified, imageproxy will use jpeg by default. If no transformation is requested (for example, if you are just using imageproxy as an SSL proxy) then the original webp image will be served as-is without any format conversion.

Because so few browsers support tiff images, they will be converted to jpeg by default if any transformation is requested. To force encoding as tiff, pass the "tiff" option. Like webp, tiff images will be served as-is without any format conversion if no transformation is requested.

Run imageproxy -help for a complete list of flags the command accepts. If you want to use a different caching implementation, it's probably easiest to just make a copy of cmd/imageproxy/main.go and customize it to fit your needs... it's a very simple command.

You can build and deploy imageproxy using any standard go toolchain, but here's how I do it.

I use goxc to build and deploy to an Ubuntu server. I have a $GOPATH/willnorris.com/go/imageproxy/.goxc.local.json file which limits builds to 64-bit linux:

 {
   "ConfigVersion": "0.9",
   "BuildConstraints": "linux,amd64"
 }

I then run goxc which compiles the static binary and creates a deb package at build/0.2.1/imageproxy_0.2.1_amd64.deb (or whatever the current version is). I copy this file to my server and install it using sudo dpkg -i imageproxy_0.2.1_amd64.deb, which is installed to /usr/bin/imageproxy.

Ubuntu uses upstart to manage services, so I copy etc/imageproxy.conf to /etc/init/imageproxy.conf on my server and start it using sudo service imageproxy start. You will certainly want to modify that upstart script to suit your desired configuration.

It's easy to vendorize the dependencies with Godep and deploy to Heroku. Take a look at this GitHub repo

A docker image is available at willnorris/imageproxy.

You can run it by

docker run -p 8080:8080 willnorris/imageproxy -addr 0.0.0.0:8080

Or in your Dockerfile:

ENTRYPOINT ["/go/bin/imageproxy", "-addr 0.0.0.0:8080"]

You can use follow config to prevent URL overwritting:

  location ~ ^/api/imageproxy/ {
    # pattern match to capture the original URL to prevent URL
    # canonicalization, which would strip double slashes
    if ($request_uri ~ "/api/imageproxy/(.+)") {
      set $path $1;
      rewrite .* /$path break;
    }
    proxy_pass http://localhost:8080;
  }

imageproxy is copyright Google, but is not an official Google product. It is available under the Apache 2.0 License.