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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, 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.

URL Structure

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


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's options.


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.


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.


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


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


The s{signature} option specifies an optional base64 encoded HMAC used to
sign the remote URL in the request. The HMAC key used to verify signatures is
provided to the imageproxy server on startup.

See URL Signing
for examples of generating signatures.

Remote URL

The URL of the original image to load is specified as the remainder of the
path, without any encoding. For example,

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


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 <a href=""><img src="" alt="200x"></a>
0.15x 15% original width, proportional height <a href=""><img src="" alt="0.15x"></a>
x100 100px tall, proportional width <a href=""><img src="" alt="x100"></a>
100x150 100 by 150 pixels, cropping as needed <a href=""><img src="" alt="100x150"></a>
100 100px square, cropping as needed <a href=""><img src="" alt="100"></a>
150,fit scale to fit 150px square, no cropping <a href=",fit/"><img src=",fit/" alt="150,fit"></a>
100,r90 100px square, rotated 90 degrees <a href=",r90/"><img src=",r90/" alt="100,r90"></a>
100,fv,fh 100px square, flipped horizontal and vertical <a href=",fv,fh/"><img src=",fv,fh/" alt="100,fv,fh"></a>
200x,q60 200px wide, proportional height, 60% quality <a href=",q60/"><img src=",q60/" alt="200x,q60"></a>

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

<a href=",r270/"><img src=",r270/" alt="200,r270"></a>

Getting Started

Install the package using:

go get

(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


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/ 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:// - 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 parameters 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

Referrer Whitelist

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

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.

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

imageproxy -whitelist

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.

Signed Requests

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.

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

imageproxy -baseURL

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.

Scaling beyond original size

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


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/ 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

Deploying to Heroku

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


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

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