gifserver

gifserver is a service written in Go that transcodes GIFs to videos on the fly. Useful for displaying user uploaded GIFs on a website without incurring slowdowns from excess bandwidth.

The server is a wrapper around ffmpeg, you must have the ffmpeg command installed on your system.

The server will automatically cache transcoded GIFs to disk so subsequent requests will avoid the initial conversion time. The server is aware of what's currently being converted so multiple requests to the same GIF during a conversion do not trigger multiple conversions.

Install

go get github.com/leafo/gifserver

gifserver -help

Running

gifserver

By default the command will look for a config file named gifserver.json in the current directory. You can override the config file's path with the -config command line flag.

To transcode a GIF just request the server at the /transcode path with the URL of the GIF you would like converted: (The http:// is optional):

http://localhost:9090/transcode?url=leafo.net/dump/test.gif

The type of video will be detected from the Accept header. If nothing could be detected a MP4 is returned by default. If there are any problems an HTTP 500 error is returned and the server returns an error message.

The /transcode path takes the following query parameters:

• url the url of the gif to load (required)
• format the format to transcode to, either mp4, ogv, png (optional)

Using png as the format will return an image that has the first frame of the GIF.

Config

A JSON file is used to configure the server. The default config is as follows:

{
	"Address": ":9090",
	"Secret": "",
	"CacheDir": "gifcache",
	"MaxBytes": 5242880,
	"MaxWidth": 512,
	"MaxHeight": 512,
	"MaxConcurrency": 0
}

Your config can replace any combination of the defaults with your own values.

• Address the address to bind the server to
• Secret secret key to use for singed URLs. If "" is the secret key then signed URLs are disabled
• CacheDir where to cache transcoded GIFs
• MaxBytes the max size of GIF in bytes allowed to be processed, setting to 0 disabled
• MaxWidth the max width of GIF that can be processed, setting to 0 disables
• MaxHeight the max width of GIF that can be processed, setting to 0 disables
• MaxConcurrency the max number of transcodes that can be in process at once, additional ones are queued. Setting to 0 disables

Signed URLs

You should used signed URLs whenever possible to avoid malicious users from triggering a denial-of-service-attack on your server by sending a large amount of conversion requests.

To enable signed URLs provide a Secret in your config file. The server uses SHA1 HMAC to generate signatures for URLs. Generating the signature is relatively simple:

Given the following request to transcode a GIF:

http://localhost:9090/transcode?url=leafo.net/dump/test.gif

Take the entire path and query from the URL an perform a SHA1 HMAC digest on it. Base64 encode the sum. Then append the URL escaped signature to the end of the URL as the query parameter sig:

sum = hmac_sha1(theSecret, "/transcode?url=leafo.net/dump/test.gif")
signature = encode_base64(sum)

signed_url = url + "&sig=" + url_escape(signature)

Then perform the request with the signed URL. You should always append the signature at the end of the URL. You must not change the order of the original query parameters in any way when appending the signature otherwise the signature is invalid.

Preventing Abuse

A few config options to prevent malicious GIFs from blocking the server are included.

MaxBytes, MaxWidth, and MaxHeight should all be set to reasonable values to prevent large images from being loaded and taking up a large amount of memory.

MaxConcurrency should be set to prevent an influx of trascode requests from taking over the CPU.

About

Author: Leaf Corcoran (leafo) (@moonscript)
Email: leafot@gmail.com
Homepage: http://leafo.net
License: MIT, Copyright (C) 2014 by Leaf Corcoran