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+# git-mirror: Sync your git repositories
+
+## Introduction
+
+[git-mirror](https://www.ralfj.de/projects/git-mirror) is a tool to keep 
+multiple git repositories of the same project in sync. Whenever something is 
+pushed to any repository, the commits will immediately be forwarded to all the 
+others. The tool assumes to run on a server hosting one of these repositories - 
+so there has to be at least one you can control. A typical use-case would be 
+your own [gitolite](http://gitolite.com/gitolite/index.html) installation, that 
+you want to keep in sync with [GitHub](https://github.com/).
+
+## Setup (gitolite)
+
+This describes how you set up git-mirror on a server running gitolite. For other 
+git hosting software, please consult the respective documentation on adding git 
+hooks. I will assume that gitolite is installed to `/home/git/gitolite`, that 
+the repositories are sitting in `/home/git/repositories`, and that git-mirror 
+has been cloned to `/home/git/git-mirror`.
+
+First of all, you need to create a file called `git-mirror.conf` in the 
+`git-mirror` directory. For now, it only needs to contain a single line:
+
+    mail-sender = git@example.com
+
+We will also need to add hooks to the git repositories you want to sync. The 
+easiest way to manage these hooks is to put them into your `gitolite-admin` 
+repository, so enable the following line in `/home/git/.gitolite.rc`:
+
+    LOCAL_CODE                =>  "$rc{GL_ADMIN_BASE}/local",
+
+Make sure you read the [security note](http://gitolite.com/gitolite/non-core.html#pushcode)
+concerning this configuration.
+
+Now add a file called `local/hooks/repo-specific/git-mirror` to your 
+`gitolite-admin` repository, make ii executable, and give it the following 
+content:
+
+    #!/bin/sh
+    exec ~/git-mirror/githook.py
+
+For every repository you want to be synced, you can enable the hook by adding 
+the following line to its configuration in `conf/gitolite.conf`:
+
+    option hook.post-receive = git-mirror
+
+(If you need multiple hooks here, you can separate them by spaces.)
+
+Finally, you need to tell git-mirror where to sync incoming changes to this 
+repository to. Add a block like the following to `git-mirror.conf`:
+
+    [repo-name]
+    owner = email@example.com
+    local = /home/git/repositories/repo-name.git
+    deploy-key = ssh-key
+    mirror-a = git@server2.example.com:repo-name.git
+    mirror-b = git@server2.example.org:the-repo.git
+
+Here, `local` has to be set to the path where the repository is stored 
+locally. `deploy-key` is the name of the SSH key used for pushing the changes 
+to other repositories. `owner` is the e-mail-address that errors occurring 
+during synchronization are sent to. And finally, the URLs to push to are given 
+by `mirror-<something>`. If these other servers also run gitolite and have a 
+symmetric setup, then no matter where a change is pushed, git-mirror will 
+forward it to all the other repositories.
+
+## Setup (GitHub)
+
+If one of the to-be-synced repositories is on GitHub, you can obviously not use 
+the procedure above to sync changes that are arriving at GitHub, to the other 
+repositories. Instead, we will use a webhook, such that GitHub tells your server 
+that a change happened, and then your server can pull the changes to its local 
+repository and synchronize all the others. This assumes that the server running 
+the webhook also hosts one of the copies of the git repository.
+
+First of all, you will have to configure your webserver to run `webhook.py` as 
+CGI script. Consult the webserver documentation for more details.
+
+Secondly, `webhook.py` needs to be able to find the main git-mirror scripts, 
+and it needs to be able to execute them as the `git` user. For the first 
+point, open `webhook.py` and change `webhook_core` to point to the file 
+`webhook-core.py` in your git-mirror clone. If your installation matches the 
+paths I used above, that should already be the case. For the second point, 
+`webhook.py` is using `sudo` to elevate its privileges. You need to tell 
+`sudo` that this is all right, by creating a file 
+`/etc/sudoers.d/git-mirror` with content:
+
+    www-data        ALL=(git) NOPASSWD: /home/git/git-mirror/webhook-core.py
+
+Now, if you visit `https://example.com/git-mirror/webhook.py` (replace with 
+your URL), the script should run and tell you `Repository missing or not 
+found.`.
+
+The next step is to add this as a webhook to the GitHub repository you want to 
+sync with, to create a fresh SSH key and configure it as deployment key for the 
+repository, and to configure git-mirror accordingly. For additional security, 
+one should also configure a shared HMAC secret, such that the webhook can verify 
+that the data indeed comes from GitHub.
+
+To make your job easier, there is a script `github-add-hooks.py` that can do 
+all this for you. It assumes that the repository exists on the GitHub side, but 
+has not yet been configured for git-mirror at all.
+
+To give the script access to your repositories, you need to create an access 
+token for it. Go to "Personal Access Tokens" in your GitHub configuration, and 
+create a new token with the permissions `admin:repo_hook` and `public_repo`. 
+Add the token and the webhook URL to the top part of `git-mirror.conf` (right 
+below `mail-sender`):
+
+    github-token = pastethetokenhere
+    webhook-url = https://example.com/git-mirror/webhook.py
+
+Now you can call the automatic setup script as follows:
+
+    ./github-add-hooks.py -o UserName -e email@example.com \
+      -l ~/repositories/repo-name.git/ -n github-repo-name
+
+Notice that the username is case-sensitive! This will do all the setup
+on the GitHub side, and it will add an appropriate configuration block
+to your local `git-mirror.conf`. You still have to manually add the
+local git hook to gitolite. Once you are done, any push happening to
+either gitolite or GitHub will be visible on the other side
+immediately. This applies even to pull requests that you merge in the
+GitHub web interface.
+
+## Source, License
+
+You can find the sources in the [git
+repository](http://www.ralfj.de/git/git-mirror.git) (also available
+[on GitHub](https://github.com/RalfJung/git-mirror)).  Guess what, the
+two are synced with this tool ;-) . They are provided under a
+[2-clause BSD
+license](http://opensource.org/licenses/bsd-license.php). See the file
+`LICENSE-BSD` for more details.
+
+## Contact
+
+If you found a bug, or want to leave a comment, please [send me a
+mail](mailto:post-AT-ralfj-DOT-de). I'm also happy about pull requests
+:)