imageproxy

imageproxy is a caching image proxy server written in golang. It supports dynamic image resizing and URL whitelisting.

This project was inspired by, and is designed to be an alternative to, WordPress's photon service. Photon is a great free service, but is limited to sites hosted on WordPress.com, or that use the Jetpack plugin. If you don't want to use Jetpack, then you're asked to use a different service. If you're looking for an alternative hosted service, I'd recommend resize.ly, embed.ly, or cloudinary. I decided to try building my own for fun.

URL Structure

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

Options

Options are specified as a comma delimited list of parameters, which can be supplied in any order. Duplicate parameters overwrite previous values.

The format is a superset of resize.ly's options.

Size

The size option takes the general form {width}x{height}, where width and height are numbers. Integer values greater than 1 are interpreted as exact pixel values. Floats between 0 and 1 are interpreted as percentages of the original image size. If either value is omitted or set to 0, it will be automatically set to preserve the aspect ratio based on the other dimension. If a single number is provided (with no "x" separator), it will be used for both height and width.

Crop Mode

Depending on the options specified, an image may be cropped to fit the requested size. In all cases, the original aspect ratio of the image will be preserved; imageproxy will never stretch the original image.

When no explicit crop mode is specified, the following rules are followed:

• If both width and height values are specified, the image will be scaled to fill the space, cropping if necessary to fit the exact dimension.

• If only one of the width or height values is specified, the image will be resized to fit the specified dimension, scaling the other dimension as needed to maintain the aspect ratio.

If the fit option is specified together with a width and height value, the image will be resized to fit within a containing box of the specified size. As always, the original aspect ratio will be preserved. Specifying the fit option with only one of either width or height does the same thing as if fit had not been specified.

Rotate

The r{degrees} option will rotate the image the specified number of degrees, counter-clockwise. Valid degrees values are 90, 180, and 270. Images are rotated after being resized.

Flip

The fv option will flip the image vertically. The fh option will flip the image horizontally. Images are flipped after being resized and rotated.

Quality

The q{percentage} option can be used to specify the output quality (JPEG only). If not specified, the default value of 95 is used.

Remote URL

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.

Examples

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
0.15x	15% original width, proportional height
x100	100px tall, 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

Getting Started

Install the package using:

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

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.

Disk cache

By default, the imageproxy command uses an in-memory cache that will grow unbounded. To cache images on disk instead, include the cacheDir flag:

imageproxy -cacheDir /tmp/imageproxy

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

Host whitelist

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.

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.

Default Base URL

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.

Deploying

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.

License

This application is distributed under the Apache 2.0 license found in the LICENSE file.