## Sharing Mercurial repositories with mercurial-server

### Paul Crowley

Copyright © 2008-2010 Paul Crowley, LShift Ltd

## About mercurial-server

Home page: http://www.lshift.net/mercurial-server.html

mercurial-server gives your developers remote read/write access to centralized Mercurial repositories using SSH public key authentication; it provides convenient and fine-grained key management and access control.

Though mercurial-server is currently targeted at Debian-based systems such as Ubuntu, other users have reported success getting it running on other Unix-based systems such as Red Hat. Running it on a non-Unix system such as Windows is not supported. You will need root privileges to install it.

## Step by step

mercurial-server authenticates users not using passwords but using SSH public keys; everyone who wants access to a mercurial-server repository will need such a key. In combination with ssh-agent (or equivalents such as the Windows program Pageant), this means that users will not need to type in a password to access the repository. If you're not familiar with SSH public keys, the OpenSSH Public Key Authentication tutorial may be helpful.

### Initial access to mercurial-server

In what follows, we assume that your username is `jay`, that you usually sit at a machine called `spoon` and you have installed mercurial-server on `jeeves` using the package management system (see the README for more on installation). We assume that you have created your SSH public key, set up your SSH agent with this key, and that this key gives you access to `jeeves`.

    jay@spoon:~$ ssh -A jeeves
    jay@jeeves:~$ ssh-add -L > my-key
    jay@jeeves:~$ sudo mkdir -p /etc/mercurial-server/keys/root/jay
    jay@jeeves:~$ sudo cp my-key /etc/mercurial-server/keys/root/jay/spoon
    jay@jeeves:~$ sudo -u hg /usr/share/mercurial-server/refresh-auth
    jay@jeeves:~$ exit
    Connection to jeeves closed.
    jay@spoon:~$

You can now create repositories on the remote machine and have complete read-write access to all of them.

### Creating repositories

To store a repository on the server, clone it over.

    jay@spoon:~$ cd myproj
    jay@spoon:~/myproj$ hg clone . ssh://hg@jeeves/jays/project
    searching for changes
    remote: adding changesets
    remote: adding manifests
    remote: adding file changes
    remote: added 119 changesets with 284 changes to 61 files
    jay@spoon:~/myproj$ hg pull ssh://hg@jeeves/jays/project
    pulling from ssh://hg@jeeves/jays/project
    searching for changes
    no changes found
    jay@spoon:~/myproj$ cd ..
    jay@spoon:~$

### Adding other users

At this stage, no-one but you has any access to any repositories you create on this system. In order to give anyone else access, you'll need a copy of their SSH public key; we'll assume you have that key in `~/sam-saucer-key.pub`. To manage access, you make changes to the special `hgadmin` repository.

    jay@spoon:~$ hg clone ssh://hg@jeeves/hgadmin
    destination directory: hgadmin
    no changes found
    updating working directory
    0 files updated, 0 files merged, 0 files removed, 0 files unresolved
    jay@spoon:~$ cd hgadmin
    jay@spoon:~/hgadmin$ mkdir -p keys/users/sam
    jay@spoon:~/hgadmin$ cp ~/sam-saucer-key.pub keys/users/sam/saucer
    jay@spoon:~/hgadmin$ hg add
    adding keys/users/sam/saucer
    jay@spoon:~/hgadmin$ hg commit -m "Add Sam's key"
    jay@spoon:~/hgadmin$ hg push
    pushing to ssh://hg@jeeves/hgadmin
    searching for changes
    remote: adding changesets
    remote: adding manifests
    remote: adding file changes
    remote: added 1 changesets with 1 changes to 1 files
    jay@spoon:~/hgadmin$

Sam can now read and write to your `ssh://hg@jeeves/jays/project` repository. Most other changes to access control can be made simply by making and pushing changes to `hgadmin`, and you can use Mercurial to cooperate with other root users in the normal way.

If you prefer, you could give them access by logging into `jeeves`, putting the key in the right place under `/etc/mercurial-server/keys`, and re-running `sudo -u hg /usr/share/mercurial-server/refresh-auth`. However, using `hgadmin` is usually more convenient if you need to make more than a very few changes; it also makes it easier to share administration with others and provides a log of all changes.

## Access control

Out of the box, mercurial-server supports two kinds of users: "root" users and normal users. If you followed the steps above, you are a "root" user because your key is under `keys/root`, while the other user you gave access to is a normal user since their key is under `keys/users`. Keys that are not in either of these directories will by default have no access to anything.

Root users can edit `hgadmin`, create new repositories and read and write to existing ones. Normal users cannot access `hgadmin` or create new repositories, but they can read and write to any other repository.

### Using access.conf

mercurial-server offers much more fine-grained access control than this division into two classes of users. Let's suppose you wish to give Pat access to the `widget` repository, but no other. We first copy Pat's SSH public key into the `keys/pat` directory in `hgadmin`. This tells mercurial-server about Pat's key, but gives Pat no access to anything because the key is not under either `keys/root` or `keys/users`. To grant this key access, we must give mercurial-server a new access rule, so we create a file in `hgadmin` called `access.conf`, with the following contents:

    # Give Pat access to the "widget" repository
    write repo=widget user=pat/*

Pat will have read and write access to the `widget` repository as soon as we add, commit, and push these files.

Each line of `access.conf` has the following syntax:

    rule condition condition...

Blank lines and lines that start with `#` are ignored. Rule is one of

- `init`: allow reads, writes, and the creation of new repositories
- `write`: allow reads and writes
- `read`: allow only read operations
- `deny`: deny all requests

A condition is a globpattern matched against a relative path. The two most important conditions are

- `user=`globpattern``: path to the user's key
- `repo=`globpattern``: path to the repository

`*` only matches one directory level, where `**` matches as many as you want. More precisely, `*` matches zero or more characters not including `/` while `**` matches zero or more characters including `/`. So `projects/*` matches `projects/foo` but not `projects/foo/bar`, while `projects/**` matches both.

When considering a request, mercurial-server steps through all the rules in `/etc/mercurial-server/access.conf` and then all the rules in `access.conf` in `hgadmin` looking for a rule which matches on every condition. The first match determines whether the request will be allowed; if there is no match in either file, the request will be denied.

By default, `/etc/mercurial-server/access.conf` has the following rules:

    init user=root/**
    deny repo=hgadmin
    write user=users/**

These rules ensure that root users can do any operation on any repository, that no other users can access the `hgadmin` repository, and that those with keys in `keys/users` can read or write to any repository but not create repositories. Some examples of how these rules work:

- User `root/jay` creates a repository `foo/bar/baz`. This matches the first rule and so will be allowed.
- User `root/jay` changes repository `hgadmin`. Again, this matches the first rule and so will be allowed; later rules have no effect.
- User `users/sam` tries to read repository `hgadmin`. This does not match the first rule, but matches the second, and so will be denied.
- User `users/sam` tries to create repository `sams-project`. This does not match the first two rules, but matches the third; this is a `write` rule, which doesn't grant the privilege to create repositories, so the request will be denied.
- User `users/sam` writes to existing repository `projects/main`. Again, this matches the third rule, which allows the request.
- User `pat` tries to write to existing repository `widget`. Until we change the `access.conf` file in `hgadmin`, this will match no rule, and so will be denied.
- Any request from a user whose key not under the `keys` directory at all will always be denied, no matter what rules are in effect; because of the way SSH authentication works, they will be prompted to enter a password, but no password will work. This can't be changed.

### /etc/mercurial-server and hgadmin

mercurial-server consults two distinct locations to collect information about what to allow: `/etc/mercurial-server` and its own `hgadmin` repository. This is useful for several reasons:

- Some users may not need the convenience of access control via mercurial; for these users updating `/etc/mercurial-server` may offer a simpler route.
- `/etc/mercurial-server` is suitable for management with tools such as Puppet
- If a change to `hgadmin` leaves you "locked out", `/etc/mercurial-server` allows you a way back in.
- At install time, all users are "locked out", and so some mechanism to allow some users in is needed.

Rules in `/etc/mercurial-server/access.conf` are checked before those in `hgadmin`, and keys in `/etc/mercurial-server/keys` will be present no matter how `hgadmin` changes.

We anticipate that once mercurial-server is successfully installed and working you will usually want to use `hgadmin` for most access control tasks. Once you have the right keys and `access.conf` set up in `hgadmin`, you can delete `/etc/mercurial-server/access.conf` and all of `/etc/mercurial-server/keys`, turning control entirely over to `hgadmin`.

`/etc/mercurial-server/remote-hgrc.d` is in the `HGRCPATH` for all remote access to mercurial-server repositories. This directory contains the hooks that mercurial-server uses for access control and logging. You can add hooks to this directory, but obviously breaking the existing hooks will disable the relevant functionality and isn't advisable.

### File and branch conditions

mercurial-server supports file and branch conditions, which restrict an operation depending on what files it modifies and what branch the work is on.

### Caution

The way these conditions work is subtle and can be counterintuitive. Unless you need what they provide, ignore this section, stick to user and repo conditions, and then things are likely to work the way you would expect. If you do need what they provide, read what follows very carefully.

File and branch conditions are added to the conditions against which a rule matches, just like user and repo conditions; they have this form:

- `file=`globpattern``: file within the repo
- `branch=`globpattern``: Mercurial branch name

However, in order to understand what effect adding these conditions will have, it helps to understand how and when these rules are applied.

The rules file is used to make three decisions:

- Whether to allow a repository to be created
- Whether to allow any access to a repository
- Whether to allow a changeset

When the first two of these decisions are being made, nothing is known about any changsets that might be pushed, and so all file and branch conditions automatically succeed for the purpose of such decisions. For the third condition, every file changed in the changeset must be allowed by a `write` or `init` rule for the changeset to be allowed.

This means that doing tricky things with file conditions can have counterintuitive consequences:

- You cannot limit read access to a subset of a repository with a `read` rule and a file condition: any user who has access to a repository can read all of it and its full history. Such a rule can only have the effect of masking a later `write` rule, as in this example:

      read repo=specialrepo file=dontwritethis
      write repo=specialrepo

  allows all users to read `specialrepo`, and to write to all files except that any changeset which writes to `dontwritethis` will be rejected.

- For similar reasons, don't give `init` rules file conditions.

- Don't try to deny write access to a particular file on a particular branch—a developer can write to the file on another branch and then merge it in. Either deny all writes to the branch from that user, or allow them to write to all the files they can write to on any branch.

      write user=docs/* branch=docs file=docs/*

  This rule grants users whose keys are in the `docs` subdirectory the power to push changesets into any repository only if those changesets are on the `docs` branch and they affect only those files directly under the `docs` directory. However, the rules below have more counterintuitive consequences.

      write user=docs/* branch=docs
      write user=docs/* file=docs/*
      read user=docs/*

  These rules grant users whose keys are in the `docs` subdirectory the power to change any file directly under the `docs` directory, or any file at all in the `docs` branch. Indirectly, however, this adds up to the power to change any file on any branch, simply by making the change on the docs branch and then merging the change into another branch.

## In detail

### How mercurial-server works

All of the repositories controlled by mercurial-server are owned by a single user, the `hg` user, which is why all URLs for mercurial-server repositories start with `ssh://hg@...`. Each SSH key that has access to the repository has an entry in `~hg/.ssh/authorized_keys`; this is how the SSH daemon knows to give that key access. When the user connects over SSH, their commands are run in a custom restricted shell; this shell knows which key was used to connect, determines what the user is trying to do, checks the access rules to decide whether to allow it, and if allowed invokes Mercurial internally, without forking.

This restricted shell also ensures that certain Mercurial extensions are loaded when the user acts on a repository; these extensions check the access control rules for any changeset that the user tries to commit, and log all pushes and pulls into a per-repository access log.

refresh-auth recurses through the `/etc/mercurial-server/keys` and the `keys` directory in the `hgadmin` repository, creating an entry in `~hg/.ssh/authorized_keys` for each one. This is redone automatically whenever a change is pushed to `hgadmin`.

### Security

mercurial-server relies entirely on sshd to grant access to remote users. As a result, it runs no daemons, installs no setuid programs, and no part of it runs as `root` except the install process: all programs run as the user `hg`. Any attack on mercurial-server can only be started if the attacker already has a public key in `~hg/.ssh/authorized_keys`, otherwise sshd will bar the way.

No matter what command the user tries to run on the remote system via SSH, mercurial-server is run. It parses the command line the user asked for, and interprets and runs the corresponding operation itself if access is allowed, so users can only read and add to history within repositories; they cannot run any other command. In addition, every push and pull is logged with a datestamp, changeset ID and the key that performed the operation.

However, while the first paragraph holds no matter what bugs mercurial-server contains, the second depends on the relevant code being correct; though the entire codebase is short, like all software mercurial-server may harbour bugs. Backups are essential!

### Logging

Every successful access is logged in a file called `~hg/repos/`repository`/.hg/mercurial-server.log`. This file is in YAML format for easy parsing, but if you don't like YAML, simply treat each line as a JSON data structure prepended with `-` . The log records the time as a UTC ISO 8601 time, the operation ("push" or "pull"), the path to the key as used in the access rules, the SSH connection information (including the source IP address), and the hex changeset IDs.

### Paths and configuration

For security reasons, all mercurial-server code runs as the `hg` user. The first thing this code reads when it starts is `~hg/.mercurial-server`; if this file is absent or corrupt the code won't run. This file specifies all of the file paths that mercurial-server uses. In particular, it specifies that mercurial-server always uses `HGRCPATH = /etc/mercurial-server/remote-hgrc.d` for remote operations, overriding any system `HGRCPATH`.

By creating such a file with suitable entries, you can run mercurial-server as a user other than `hg`, or install it without root privileges; however I strongly recommend that if you need to do this, you use a user account that is used for no other purpose, and take the time to thoroughly understand how mercurial-server works before you attempt it.

### License

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

### Thanks

Thanks for reading this far. If you use mercurial-server, please tell me about it.

Paul Crowley, `<paul@lshift.net>`, 2010