unicorn: Rack HTTP server for fast clients and Unix

unicorn is an HTTP server for Rack applications designed to only serve fast clients on low-latency, high-bandwidth connections and take advantage of features in Unix/Unix-like kernels. Slow clients should only be served by placing a reverse proxy capable of fully buffering both the the request and response in between unicorn and slow clients.



unicorn is copyright 2009-2018 by all contributors (see logs in git). It is based on Mongrel 1.1.5. Mongrel is copyright 2007 Zed A. Shaw and contributors.

unicorn is licensed under (your choice) of the GPLv2 or later (GPLv3+ preferred), or Ruby (1.8)-specific terms. See the included LICENSE file for details.

unicorn is 100% Free Software (including all development tools used).


The library consists of a C extension so you’ll need a C compiler and Ruby development libraries/headers.

You may install it via RubyGems on RubyGems.org:

gem install unicorn

You can get the latest source via git from the following locations (these versions may not be stable):

https://repo.or.cz/unicorn.git (mirror)

You may browse the code from the web:

See the HACKING guide on how to contribute and build prerelease gems from git.


Rack (including Rails 3+) applications

In APP_ROOT, run:


unicorn will bind to all interfaces on TCP port 8080 by default. You may use the --listen/-l switch to bind to a different address:port or a UNIX socket.

Configuration File(s)

unicorn will look for the config.ru file used by rackup in APP_ROOT.

For deployments, it can use a config file for unicorn-specific options specified by the --config-file/-c command-line switch. See ::Unicorn::Configurator for the syntax of the unicorn-specific options. The default settings are designed for maximum out-of-the-box compatibility with existing applications.

Most command-line options for other Rack applications (above) are also supported. Run ‘unicorn -h` to see command-line options.


There is NO WARRANTY whatsoever if anything goes wrong, but href="ISSUES.html">ISSUES.html let us know and we’ll try our best to fix it.

unicorn is designed to only serve fast clients either on the local host or a fast LAN. See the PHILOSOPHY and DESIGN documents for more details regarding this.

Due to its ability to tolerate crashes and isolate clients, unicorn is unfortunately known to prolong the existence of bugs in applications and libraries which run on top of it.


All feedback (bug reports, user/development dicussion, patches, pull requests) go to the mailing list/newsgroup. See the ISSUES document for information on the mailing list.

The mailing list is archived at yhbt.net/unicorn-public/

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For the latest on unicorn releases, you may also finger us at unicorn@yhbt.net or check our NEWS page (and subscribe to our Atom feed).