<?xml version="1.0" encoding="utf-8"?>

<article xmlns="http://docbook.org/ns/docbook" version="5.0" xml:lang="en"

  xmlns:xlink="http://www.w3.org/1999/xlink">

<info>

  <title>Sharing Mercurial repositories with mercurial-server</title>

  <author><firstname>Paul</firstname><surname>Crowley</surname></author>

  <copyright><year>2009</year><holder>Paul Crowley, LShift Ltd</holder></copyright>

</info>

<section>

<title>About mercurial-server</title>

<para>

Home page: <link xlink:href="http://www.lshift.net/mercurial-server.html"/>

</para>

<para>

mercurial-server gives your developers remote read/write access to

centralized <link xlink:href="http://hg-scm.org/">Mercurial</link>

repositories using SSH public key authentication; it provides convenient

and fine-grained key management and access control.

</para>

<para>

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.

</para>

</section>

<section>

<title>Step by step</title>

<para>

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 <command>ssh-agent</command> (or

equivalents such as the Windows program <link

xlink:href="http://the.earth.li/~sgtatham/putty/0.60/htmldoc/Chapter9.html#pageant">Pageant</link>),

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 <link

xlink:href="http://sial.org/howto/openssh/publickey-auth/">OpenSSH Public

Key Authentication tutorial</link> may be helpful.

</para>

<section>

<title>Installing mercurial-server</title>

<para>

In what follows, we assume that your username is <systemitem

class="username">jay</systemitem>, that you usually sit at a machine called

<systemitem class="systemname">my-workstation</systemitem> and you wish to

install mercurial-server on <systemitem

class="systemname">repository-host</systemitem>. 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 <systemitem

class="systemname">repository-host</systemitem>.  

</para>

<para>First install mercurial-server on <systemitem

class="systemname">repository-host</systemitem>:</para>

<screen><computeroutput>jay@my-workstation:~$ </computeroutput><userinput>scp mercurial-server_0.6.1_amd64.deb repository-host:</userinput>

<computeroutput>mercurial-server_0.6.1_amd64.deb 100%

jay@my-workstation:~$ </computeroutput><userinput>ssh -A repository-host</userinput>

<computeroutput>jay@repository-host:~$ </computeroutput><userinput>sudo dpkg -i mercurial-server_0.6.1_amd64.deb</userinput>

<computeroutput>[sudo] password for jay: 

Selecting previously deselected package mercurial-server.

(Reading database ... 144805 files and directories currently installed.)

Unpacking mercurial-server (from .../mercurial-server_0.6.1_amd64.deb) ...

Setting up mercurial-server (0.6.1) ...

jay@repository-host:~$ </computeroutput></screen>

<para>

mercurial-server is now installed on the repository host.  Next, we need to give you permission to access its repositories.

</para>

<screen><computeroutput>jay@repository-host:~$ </computeroutput><userinput>ssh-add -L > my-key</userinput>

<computeroutput>jay@repository-host:~$ </computeroutput><userinput>sudo mkdir -p /etc/mercurial-server/keys/root/jay</userinput>

<computeroutput>jay@repository-host:~$ </computeroutput><userinput>sudo cp my-key /etc/mercurial-server/keys/root/jay/my-workstation</userinput>

<computeroutput>jay@repository-host:~$ </computeroutput><userinput>sudo -u hg /usr/share/mercurial-server/refresh-auth</userinput>

<computeroutput>jay@repository-host:~$ </computeroutput><userinput>exit</userinput>

<computeroutput>Connection to repository-host closed.

jay@my-workstation:~$ </computeroutput></screen>

<para>

You can now create repositories on the remote machine and have complete

read-write access to all of them.

</para>

</section>

<section>

<title>Creating repositories</title>

<para>

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

</para>

<screen><computeroutput>jay@my-workstation:~$ </computeroutput><userinput>cd my-mercurial-project</userinput>

<computeroutput>jay@my-workstation:~/my-mercurial-project$ </computeroutput><userinput>hg clone . ssh://hg@repository-host/repository/name</userinput>

<computeroutput>searching for changes

remote: adding changesets

remote: adding manifests

remote: adding file changes

remote: added 119 changesets with 284 changes to 61 files

jay@my-workstation:~/my-mercurial-project$ </computeroutput><userinput>hg pull ssh://hg@repository-host/repository/name</userinput>

<computeroutput>pulling from ssh://hg@repository-host/repository/name

searching for changes

no changes found

<computeroutput>jay@my-workstation:~/my-mercurial-project$ </computeroutput><userinput>cd ..</userinput>

jay@my-workstation:~$ </computeroutput></screen>

</section>

<section>

<title>Adding other users</title>

<para>

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

<filename>~/sam-key.pub</filename>.  To manage access, you make changes to the special <literal>hgadmin</literal> repository.

</para>

<screen><computeroutput>jay@my-workstation:~$ </computeroutput><userinput>hg clone ssh://hg@repository-host/hgadmin</userinput>

<computeroutput>destination directory: hgadmin

no changes found

updating working directory

0 files updated, 0 files merged, 0 files removed, 0 files unresolved

jay@my-workstation:~$ </computeroutput><userinput>cd hgadmin</userinput>

<computeroutput>jay@my-workstation:~/hgadmin$ </computeroutput><userinput>mkdir -p keys/users/sam</userinput>

<computeroutput>jay@my-workstation:~/hgadmin$ </computeroutput><userinput>cp ~/sam-key.pub keys/users/sam/their-workstation</userinput>

<computeroutput>jay@my-workstation:~/hgadmin$ </computeroutput><userinput>hg add</userinput>

<computeroutput>adding keys/users/sam/their-workstation

jay@my-workstation:~/hgadmin$ </computeroutput><userinput>hg commit -m "Add Sam's key'"</userinput>

<computeroutput>jay@my-workstation:~/hgadmin$ </computeroutput><userinput>hg push</userinput>

<computeroutput>pushing to ssh://hg@repository-host/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@my-workstation:~/hgadmin$ </computeroutput></screen>

<para>

Sam can now read and write to your

<literal>ssh://hg@repository-host/repository/name</literal> repository.

Most other changes to access control can be made simply by making and

pushing changes to <literal>hgadmin</literal>, and you can use Mercurial to

cooperate with other root users in the normal way.

</para>

<para>

If you prefer, you could give them access by

logging into <systemitem class="systemname">repository-host</systemitem>,

putting the key in the right place under <filename

class='directory'>/etc/mercurial-server/keys</filename>, and re-running

<userinput>sudo -u hg /usr/share/mercurial-server/refresh-auth</userinput>.

However, using <literal>hgadmin</literal> 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.

</para>

</section>

</section>

<section>

<title>Access control</title>

<para>

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 <filename class='directory'>keys/root</filename>, while the other user you gave access to is a normal user since their key is under <filename class='directory'>keys/users</filename>.  Keys that are not in either of these directories will by default have no access to anything.

</para>

<para>

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

</para>

<section>

<title>Using access.conf</title>

<para>

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 <literal>widget</literal> repository, but no other.  We first copy Pat's SSH public key into the <filename

class='directory'>keys/widget/pat</filename> directory in <literal>hgadmin</literal>.  Now mercurial-server knows about Pat's key, but will give Pat no access to anything because the key is not under either <filename

class='directory'>keys/root</filename> or <filename

class='directory'>keys/users</filename>.  To grant this key access, we must give mercurial-server a new access rule, so we create a file in <literal>hgadmin</literal> called <filename>access.conf</filename>, with the following contents:</para>

<programlisting>write repo=widget user=widget/**

</programlisting>

<para>

Pat will have read and write access as soon as we add, commit, and push these files.

</para>

<para>

Each line of <filename>access.conf</filename> has the following syntax:

</para>

<programlisting><replaceable>rule</replaceable> <replaceable>condition</replaceable> <replaceable>condition...</replaceable>

</programlisting>

<para>

Blank lines and lines that start with <literal>#</literal> are ignored. Rule is one of

</para>

<itemizedlist>

<listitem>

<literal>init</literal>: allow reads, writes, and the creation of new repositories

</listitem>

<listitem>

<literal>write</literal>: allow reads and writes

</listitem>

<listitem>

<literal>read</literal>: allow only read operations

</listitem>

<listitem>

<literal>deny</literal>: deny all requests

</listitem>

</itemizedlist>

<para>

When considering a request, mercurial-server steps through all the rules in <filename>/etc/mercurial-server/access.conf</filename> and then all the rules in <filename>access.conf</filename> in <literal>hgadmin</literal> looking for a rule which matches on every condition.  If it does not find such a rule, it denies the request; otherwise it checks whether the rule grants sufficient privilege to allow it.

</para>

<para>

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

</para>

<programlisting>init user=root/**

deny repo=hgadmin

write user=users/**

</programlisting>

<para>

These rules ensure that root users can do any operation on any repository, that no other users can access the <literal>hgadmin</literal> repository, and that those with keys in <filename class='directory'>keys/users</filename> can read or write to any repository but not create repositories.

</para>

<para>

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

important conditions are

</para>

<itemizedlist>

<listitem>

<code><literal>user=</literal><replaceable>globpattern</replaceable></code>: path to the user's key

</listitem>

<listitem>

<code><literal>repo=</literal><replaceable>globpattern</replaceable></code>: path to the repository

</listitem>

</itemizedlist>

<para>

"*" 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 "/".

</para>

</section>

<section>

<title>/etc/mercurial-server and hgadmin</title>

<para>

mercurial-server consults two distinct locations to collect information about what to allow: <filename

class='directory'>/etc/mercurial-server</filename> and its own <literal>hgadmin</literal> repository.  This is useful for several reasons:

</para>

<itemizedlist>

<listitem>

Users may not need the sophistication of access control via mercurial; for these users updating <filename

class='directory'>/etc/mercurial-server</filename> may offer a simpler route.

</listitem>

<listitem>

<filename

class='directory'>/etc/mercurial-server</filename> is suitable for management by some other route, such as with  <link

xlink:href="http://reductivelabs.com/products/puppet">Puppet</link>

</listitem>

<listitem>

If a change to <literal>hgadmin</literal> leaves you "locked out", <filename

class='directory'>/etc/mercurial-server</filename> allows you a way back in.

</listitem>

<listitem>

At install time, all users are "locked out", and so some mechanism to allow some users in is needed.

</listitem>

</itemizedlist>

<para>

Rules in <filename>/etc/mercurial-server/access.conf</filename> take precedence over those in <literal>hgadmin</literal>, and obviously keys in <filename class='directory'>/etc/mercurial-server/keys</filename> cannot be affected by changes to <literal>hgadmin</literal>.

</para>

<para>

We anticipate that once mercurial-server is successfully installed and

working most users will want to use <literal>hgadmin</literal> for most

access control tasks. Once you have the right keys and

<filename>access.conf</filename> set up in <literal>hgadmin</literal>, you

can delete <filename>/etc/mercurial-server/access.conf</filename> and all

of <filename class='directory'>/etc/mercurial-server/keys</filename>,

turning control entirely over to <literal>hgadmin</literal>.

</para>

<para>

<filename>/etc/mercurial-server/remote-hgrc</filename> is in the

<systemitem>HGRCPATH</systemitem> for all remote access to mercurial-server

repositories. This file contains the hooks that mercurial-server uses for

access control and logging. You can add hooks to this file, but obviously

breaking the existing hooks will disable the relevant functionality and

isn't advisable.

</para>

</section>

<section>

<title>File and branch conditions</title>

<para>

mercurial-server supports file and branch conditions, which restrict an

operation depending on what files it modifies and what branch the work is

on. </para>

<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.

</caution>

<para>

File and branch conditions are added to the conditions against which a rule

matches, just like user and repo conditions; they have this form:

</para>

<itemizedlist>

<listitem>

<code><literal>file=</literal><replaceable>globpattern</replaceable></code>: file within the repo

</listitem>

<listitem>

<code><literal>branch=</literal><replaceable>globpattern</replaceable></code>: Mercurial branch name

</listitem>

</itemizedlist>

<para>

However, in order to understand what effect adding these conditions will

have, it helps to understand how and when these rules are applied.

</para>

<para>

The rules file is used to make three decisions:

</para>

<itemizedlist>

<listitem>

Whether to allow a repository to be created

</listitem>

<listitem>

Whether to allow any access to a repository

</listitem>

<listitem>

Whether to allow a changeset

</listitem>

</itemizedlist>

<para>

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

<literal>write</literal> or <literal>init</literal> rule for the changeset

to be allowed.

</para>

<para>

This means that doing tricky things with file conditions can have

counterintuitive consequences:

</para>

<itemizedlist>

<listitem>

<para>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:</para>

<programlisting>read repo=specialrepo file=dontwritethis

write repo=specialrepo

</programlisting>

<para>

allows all users to read specialrepo, and to write to all files

<emphasis>except</emphasis> that any changeset which writes to

<filename>dontwritethis</filename> will be rejected.

</para>

</listitem>

<listitem>

For similar reasons, don't give <literal>init</literal> rules file conditions.

</listitem>

<listitem>

<para>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. In other words,

something like this will have the intended effect:

</para>

<programlisting>write user=docs/* branch=docs file=docs/*

</programlisting>

<para>

But something like this will not have the intended effect; it will

effectively allow these users to write to any file on any branch, by

writing it to "docs" first:

</para>

<programlisting>write user=docs/* branch=docs

write user=docs/* file=docs/*

read user=docs/*

</programlisting>

</listitem>

</itemizedlist>

</section>

</section>

<section>

<title>How mercurial-server works</title>

<para>

All of the repositories controlled by mercurial-server are owned by a

single user, the <literal>hg</literal> user, which is why all URLs for

mercurial-server repositories start with <literal>ssh://hg@...</literal>.

Each SSH key that has access to the repository has an entry in

<filename>~hg/.ssh/authorized_keys</filename>; this is how the SSH daemon

knows to give that key access. When the user connects over SSH, their

commands are run in a specially crafted restricted shell; this shell knows

which key was used to connect, determines what the user is trying to do,

and checks the access rules to decide whether to allow it.  

</para>

<para>

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.

</para>

<para>

<command>refresh-auth</command> recurses through the <filename

class='directory'>/etc/mercurial-server/keys</filename> and the <filename

class='directory'>keys</filename> directory in the

<literal>hgadmin</literal> repository, creating an entry in

<filename>~hg/.ssh/authorized_keys</filename> for each one. This is redone

automatically whenever a change is pushed to <literal>hgadmin</literal>.

</para>

</section>

<section>

<title>Security</title>

<para>

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 <filename>~hg/.ssh/authorized_keys</filename>,

otherwise sshd will bar the way.

</para>

<para>

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 hg operation itself if access is

allowed, so users can only read and add to history within repositories;

they cannot run any other hg command. In addition, every push and pull is

logged with a datestamp, changeset ID and the key that performed the

operation.

</para>

<para>

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, mercurial-server is a fairly

new program and may harbour bugs. Backups are essential!

</para>

</section>

<section>

<title>Legalese</title>

<para>

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.

</para>

<para>

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.

</para>

<para>

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.

</para>

</section>

<section>

<title>Thanks</title>

<para>

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

it.

</para>

<para>

Paul Crowley, <email>paul@lshift.net</email>, 2009

</para>

</section>

</article>