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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Path-Based Authorization</title><link rel="stylesheet" href="styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><link rel="start" href="index.html" title="Version Control with Subversion" /><link rel="up" href="svn.serverconfig.html" title="Chapter 6. Server Configuration" /><link rel="prev" href="svn.serverconfig.httpd.html" title="httpd, the Apache HTTP server" /><link rel="next" href="svn.serverconfig.multimethod.html" title="Supporting Multiple Repository Access Methods" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Path-Based Authorization</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="svn.serverconfig.httpd.html">Prev</a> </td><th width="60%" align="center">Chapter 6. Server Configuration</th><td width="20%" align="right"> <a accesskey="n" href="svn.serverconfig.multimethod.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="svn.serverconfig.pathbasedauthz"></a>Path-Based Authorization</h2></div></div></div><p>Both Apache and <span class="command"><strong>svnserve</strong></span> are capable of
granting (or denying) permissions to users. Typically this is
done over the entire repository: a user can read the repository
(or not), and she can write to the repository (or not). It's
also possible, however, to define finer-grained access rules.
One set of users may have permission to write to a certain
directory in the repository, but not others; another directory
might not even be readable by all but a few special
people.</p><p>Both servers use a common file format to describe these
path-based access rules. In the case of Apache, one needs to
load the <span class="command"><strong>mod_authz_svn</strong></span> module and then add
the <code class="literal">AuthzSVNAccessFile</code> directive (within
the <code class="filename">httpd.conf</code> file) pointing to your own
rules-file. (For a full explanation, see
<a class="xref" href="svn.serverconfig.httpd.html#svn.serverconfig.httpd.authz.perdir" title="Per-Directory Access Control">the section called “Per-Directory Access Control”</a>.) If
you're using <span class="command"><strong>svnserve</strong></span>, then you need to make
the <code class="literal">authz-db</code> variable
(within <code class="filename">svnserve.conf</code>) point to your
rules-file.</p><div class="sidebar"><p class="title"><b>Do you really need path-based access control?</b></p><p>A lot of administrators setting up Subversion for the
first time tend to jump into path-based access control without
giving it a lot of thought. The administrator usually knows
which teams of people are working on which projects, so it's
easy to jump in and grant certain teams access to certain
directories and not others. It seems like a natural thing,
and it appeases the administrator's desire to maintain tight
control of the repository.</p><p>Note, though, that there are often invisible (and
visible!) costs associated with this feature. In the visible
category, the server needs to do a lot more work to ensure
that the user has the right to read or write each specific
path; in certain situations, there's very noticeable
performance loss. In the invisible category, consider the
culture you're creating. Most of the time, while certain
users <span class="emphasis"><em>shouldn't</em></span> be committing changes to
certain parts of the repository, that social contract doesn't
need to be technologically enforced. Teams can sometimes
spontaneously collaborate with each other; someone may want to
help someone else out by committing to an area she doesn't
normally work on. By preventing this sort of thing at the
server level, you're setting up barriers to unexpected
collaboration. You're also creating a bunch of rules that
need to be maintained as projects develop, new users are
added, and so on. It's a bunch of extra work to
maintain.</p><p>Remember that this is a version control system! Even if
somebody accidentally commits a change to something they
shouldn't, it's easy to undo the change. And if a user
commits to the wrong place with deliberate malice, then it's a
social problem anyway, and that the problem needs to be dealt
with outside of Subversion.</p><p>So before you begin restricting users' access rights, ask
yourself if there's a real, honest need for this, or if it's
just something that “<span class="quote">sounds good</span>” to an
administrator. Decide whether it's worth sacrificing some
server speed for, and remember that there's very little risk
involved; it's bad to become dependent on technology as a
crutch for social problems.<sup>[<a id="id397410" href="#ftn.id397410" class="footnote">49</a>]</sup>.</p><p>As an example to ponder, consider that the Subversion
project itself has always had a notion of who is allowed to
commit where, but it's always been enforced socially. This is
a good model of community trust, especially for open-source
projects. Of course, sometimes there <span class="emphasis"><em>are</em></span>
truly legitimate needs for path-based access control; within
corporations, for example, certain types of data really can be
sensitive, and access needs to be genuinely restricted to
small groups of people.</p></div><p>Once your server knows where to find your rules-file, it's
time to define the rules.</p><p>The syntax of the file is the same familiar one used
by <span class="command"><strong>svnserve.conf</strong></span> and the runtime
configuration files. Lines that start with a hash
(<code class="literal">#</code>) are ignored. In its simplest form, each
section names a repository and path within it, and the
authenticated usernames are the option names within each
section. The value of each option describes the user's level of
access to the repository path: either
<code class="literal">r</code> (read-only) or <code class="literal">rw</code>
(read-write). If the user is not mentioned at all, no access is
allowed.</p><p>To be more specific: the value of the section-names are
either of the form <code class="literal">[repos-name:path]</code> or the
form <code class="literal">[path]</code>. If you're using the
<code class="literal">SVNParentPath</code> directive, then it's important
to specify the repository names in your sections. If you omit
them, then a section like
<code class="literal">[/some/dir]</code> will match the path
<code class="filename">/some/dir</code> in <span class="emphasis"><em>every</em></span>
repository. If you're using the <code class="literal">SVNPath</code>
directive, however, then it's fine to only define paths in your
sections—after all, there's only one repository.</p><pre class="screen">
[calc:/branches/calc/bug-142]
harry = rw
sally = r
</pre><p>In this first example, the user <code class="literal">harry</code> has
full read and write access on the
<code class="filename">/branches/calc/bug-142</code> directory in the
<code class="literal">calc</code> repository, but the user
<code class="literal">sally</code> has read-only access. Any other users
are blocked from accessing this directory.</p><p>Of course, permissions are inherited from parent to child
directory. That means that we can specify a subdirectory with a
different access policy for Sally:</p><pre class="screen">
[calc:/branches/calc/bug-142]
harry = rw
sally = r
# give sally write access only to the 'testing' subdir
[calc:/branches/calc/bug-142/testing]
sally = rw
</pre><p>Now Sally can write to the <code class="filename">testing</code>
subdirectory of the branch, but can still only read other parts.
Harry, meanwhile, continues to have complete read-write access
to the whole branch.</p><p>It's also possible to explicitly deny permission to someone
via inheritance rules, by setting the username variable to
nothing:</p><pre class="screen">
[calc:/branches/calc/bug-142]
harry = rw
sally = r
[calc:/branches/calc/bug-142/secret]
harry =
</pre><p>In this example, Harry has read-write access to the
entire <code class="filename">bug-142</code> tree, but has absolutely no
access at all to the <code class="filename">secret</code> subdirectory
within it.</p><p>The thing to remember is that the most specific path always
matches first. The server tries to match the path itself, and
then the parent of the path, then the parent of that, and so on.
The net effect is that mentioning a specific path in the
accessfile will always override any permissions inherited from
parent directories.</p><p>By default, nobody has any access to the repository at all.
That means that if you're starting with an empty file, you'll
probably want to give at least read permission to all users at
the root of the repository. You can do this by using the
asterisk variable (<code class="literal">*</code>), which means “<span class="quote">all
users</span>”:</p><pre class="screen">
[/]
* = r
</pre><p>This is a common setup; notice that there's no repository
name mentioned in the section name. This makes all repositories
world readable to all users. Once all users have read-access to
the repositories, you can give explicit
<code class="literal">rw</code> permission to certain users on specific
subdirectories within specific repositories.</p><p>The asterisk variable (<code class="literal">*</code>) is also worth
special mention here: it's the
<span class="emphasis"><em>only</em></span> pattern which matches an anonymous
user. If you've configured your server block to allow a mixture
of anonymous and authenticated access, all users start out
accessing anonymously. The server looks for a
<code class="literal">*</code> value defined for the path being accessed;
if it can't find one, then it demands real authentication from
the client.</p><p>The access file also allows you to define whole groups of
users, much like the Unix <code class="filename">/etc/group</code>
file:</p><pre class="screen">
[groups]
calc-developers = harry, sally, joe
paint-developers = frank, sally, jane
everyone = harry, sally, joe, frank, sally, jane
</pre><p>Groups can be granted access control just like users.
Distinguish them with an “<span class="quote">at</span>”
(<code class="literal">@</code>) prefix:</p><pre class="screen">
[calc:/projects/calc]
@calc-developers = rw
[paint:/projects/paint]
@paint-developers = rw
jane = r
</pre><p>Groups can also be defined to contain other groups:</p><pre class="screen">
[groups]
calc-developers = harry, sally, joe
paint-developers = frank, sally, jane
everyone = @calc-developers, @paint-developers
</pre><div class="sidebar"><p class="title"><b>Partial Readability and Checkouts</b></p><p>If you're using Apache as your Subversion server and have
made certain subdirectories of your repository unreadable to
certain users, then you need to be aware of a possible
non-optimal behavior with <span class="command"><strong>svn
checkout</strong></span>.</p><p>When the client requests a checkout or update over HTTP, it
makes a single server request, and receives a single (often
large) server response. When the server receives the request,
that is the <span class="emphasis"><em>only</em></span> opportunity Apache has to
demand user authentication. This has some odd side-effects.
For example, if a certain subdirectory of the repository is only
readable by user Sally, and user Harry checks out a parent
directory, his client will respond to the initial authentication
challenge as Harry. As the server generates the large response,
there's no way it can re-send an authentication challenge when
it reaches the special subdirectory; thus the subdirectory is
skipped altogether, rather than asking the user to
re-authenticate as Sally at the right moment. In a similar way,
if the root of the repository is anonymously world-readable,
then the entire checkout will be done without
authentication—again, skipping the unreadable directory,
rather than asking for authentication partway through.</p></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id397410" href="#id397410" class="para">49</a>] </sup>A common theme in
this book!</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="svn.serverconfig.httpd.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="svn.serverconfig.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="svn.serverconfig.multimethod.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">httpd, the Apache HTTP server </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Supporting Multiple Repository Access Methods</td></tr></table></div></body></html>
