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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 allowed hosts list or request signing (HMAC-SHA256)
  • support for jpeg, png, webp (decode only), tiff, and gif image formats (including animated gifs)
  • caching in-memory, on disk, or with Amazon S3, Google Cloud Storage, Azure Storage, or Redis
  • 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 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

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/

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

The smart crop feature can best be seen by comparing crops of this source image, with and without smart crop enabled.

Options Meaning Image
150x300 150x300px, standard crop 200x400,sc
150x300,sc 150x300px, smart crop 200x400

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


Getting Started

Install the package using:

go get

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


This will start the proxy on port 8080, without any caching and with no allowed host list (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 LRU cache. By default, this is limited to 100mb. To customize the size of the cache or the max age for cached items, use the format memory:size:age where size is measured in mb and age is a duration. For example, memory:200:4h will create a 200mb cache that will cache items no longer than 4 hours.

  • directory on local disk (e.g. /tmp/imageproxy) - will cache images on disk

  • s3 URL (e.g. s3://region/bucket-name/optional-path-prefix) - 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. (Additional methods of loading credentials are documented in the aws-sdk-go session package).

    Additional configuration options (further documented here) may be specified as URL query string parameters, which are mostly useful when working with s3-compatible services:

    • "endpoint" - specify an alternate API endpoint
    • "disableSSL" - set to "1" to disable SSL when calling the API
    • "s3ForcePathStyle" - set to "1" to force the request to use path-style addressing

    For example, when working with minio, which doesn't use regions, provide a dummy region value and custom endpoint value:


    Similarly, for Digital Ocean Spaces, provide a dummy region value and the appropriate endpoint for your space:

  • gcs URL (e.g. gcs://bucket-name/optional-path-prefix) - will cache images on Google Cloud Storage. Authentication is documented in Google's Application Default Credentials docs.

  • azure URL (e.g. azure://container-name/) - will cache images on Azure Storage. This requires AZURESTORAGE_ACCOUNT_NAME and AZURESTORAGE_ACCESS_KEY environment variables to bet set.

  • redis URL (e.g. redis://hostname/) - will cache images on the specified redis host. The full URL syntax is defined by the redis URI registration. Rather than specify password in the URI, use the REDIS_PASSWORD environment variable.

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.

Multiple caches can be specified by separating them by spaces or by repeating the -cache flag multiple times. The caches will be created in a tiered fashion. Typically this is used to put a smaller and faster in-memory cache in front of a larger but slower on-disk cache. For example, the following will first check an in-memory cache for an image, followed by a gcs bucket:

imageproxy -cache memory -cache gcs://my-bucket/

Allowed Referrer List

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.

Allowed and Denied Hosts List

You can limit the remote hosts that the proxy will fetch images from using the allowHosts and denyHosts flags. 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 these flags.

Try it out by running:

imageproxy -allowHosts

Reload the codercat URL, and you should now get an error message. Alternately, try running:

imageproxy -denyHosts

Reloading the codercat URL will still return an error message.

You can specify multiple hosts as a comma separated list to either flag, or prefix a host value with *. to allow or deny all sub-domains. You can also specify a netblock in CIDR notation ( -- this is useful for blocking reserved ranges like,, etc.

If a host matches both an allowed and denied host, the request will be denied.

Allowed Content-Type List

You can limit what content types can be proxied by using the contentTypes flag. By default, this is set to image/*, meaning that imageproxy will process any image types. You can specify multiple content types as a comma separated list, and suffix values with * to perform a wildcard match. Set the flag to an empty string to proxy all requests, regardless of content type.

Signed Requests

Instead of an allowed host list, 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 "secretkey"

Reload the codercat URL, and you should see an error message. Now load a signed codercat URL (which contains the signature option) and verify that it loads properly.

Some simple code samples for generating signatures in various languages can be found in docs/ Multiple valid signature keys may be provided to support key rotation by repeating the signatureKey flag multiple times, or by providing a space-separated list of keys. To use a key with a literal space character, load the key from a file using the "@" prefix documented above.

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

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

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

WebP and TIFF support

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.

Environment Variables

All configuration flags have equivalent environment variables of the form IMAGEPROXY_$NAME. For example, an on-disk cache could be configured by calling

IMAGEPROXY_CACHE="/tmp/imageproxy" imageproxy


In most cases, you can follow the normal procedure for building a deploying any go application. For example:

  • go build
  • copy resulting binary to /usr/local/bin
  • copy etc/imageproxy.service to /lib/systemd/system and enable using systemctl.

Instructions have been contributed below for running on other platforms, but I don't have much experience with them personally.


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

Or in your Dockerfile:

ENTRYPOINT ["/app/imageproxy", "-addr"]

If running imageproxy inside docker with a bind-mounted on-disk cache, make sure the container is running as a user that has write permission to the mounted host directory. See more details in #198.


Use the proxy_pass directive to send requests to your imageproxy instance. For example, to run imageproxy at the path "/api/imageproxy/", set:

  location /api/imageproxy/ {
    proxy_pass http://localhost:4593/;

Depending on other directives you may have in your nginx config, you might need to alter the precedence order by setting:

  location ^~ /api/imageproxy/ {
    proxy_pass http://localhost:4593/;


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

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