Path-Based Authorization

Both Apache and svnserve 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. As files are paths, too, it's even possible to restrict access on a per file basis.

Both servers use a common file format to describe these path-based access rules. In the case of Apache, one needs to load the mod_authz_svn module and then add the AuthzSVNAccessFile directive (within the httpd.conf file) pointing to your own access rules file. (For a full explanation, see the section called “Per-directory access control”.) If you're using svnserve, you need to make the authz-db variable (within svnserve.conf) point to your access rules file.

Once your server knows where to find your access file, it's time to define the rules.

The syntax of the file is the same familiar one used by svnserve.conf and the runtime configuration files. Lines that start with a hash (#) are ignored. In its simplest form, each section names a repository and path within it, as well as 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 r (read-only) or rw (read/write). If the user is not mentioned at all, no access is allowed.

To be more specific: the value of the section names is either of the form [repos-name:path] or of the form [path].

[Warning] Warning

Subversion treats repository names and paths in a case-insensitive fashion for the purposes of access control, converting them to lower case internally before comparing them against the contents of your access file. Use lower case for the contents of the section headers in your access file.

If you're using the SVNParentPath directive, it's important to specify the repository names in your sections. If you omit them, a section such as [/some/dir] will match the path /some/dir in every repository. If you're using the SVNPath directive, however, it's fine to only define paths in your sections—after all, there's only one repository.

[calc:/branches/calc/bug-142]
harry = rw
sally = r

In this first example, the user harry has full read and write access on the /branches/calc/bug-142 directory in the calc repository, but the user sally has read-only access. Any other users are blocked from accessing this directory.

[Warning] Warning

mod_dav_svn offers a directive, SVNReposName, which allows administrators to define a more human-friendly name, of sorts, for a repository:

<Location /svn/calc>
  SVNPath /var/svn/calc
  SVNReposName "Calculator Application"
…

This allows mod_dav_svn to identify the repository by something other than merely its server directory basename—calc, in the previous example—when providing directory listings of repository content. Be aware, however, that when consulting the access file for authorization rules, Subversion uses this repository basename for comparison, not any configured human-friendly name.

Of course, permissions are inherited from parent to child directory. That means we can specify a subdirectory with a different access policy for Sally:

[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

Now Sally can write to the testing subdirectory of the branch, but can still only read other parts. Harry, meanwhile, continues to have complete read/write access to the whole branch.

It's also possible to explicitly deny permission to someone via inheritance rules, by setting the username variable to nothing:

[calc:/branches/calc/bug-142]
harry = rw
sally = r

[calc:/branches/calc/bug-142/secret]
harry =

In this example, Harry has read/write access to the entire bug-142 tree, but has absolutely no access at all to the secret subdirectory within it.

[Tip] Tip

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 access file will always override any permissions inherited from parent directories.

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 (*), which means all users:

[/]
* = r

This is a common setup; notice that no repository name is 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 rw permission to certain users on specific subdirectories within specific repositories.

The access file also allows you to define whole groups of users, much like the Unix /etc/group file:

[groups]
calc-developers = harry, sally, joe
paint-developers = frank, sally, jane
everyone = harry, sally, joe, frank, jane

Groups can be granted access control just like users. Distinguish them with an at (@) prefix:

[calc:/projects/calc]
@calc-developers = rw

[paint:/projects/paint]
jane = r
@paint-developers = rw

Another important fact is that group permissions are not overridden by individual user permissions. Rather, the combination of all matching permissions is granted. In the prior example, Jane is a member of the paint-developers group, which has read/write access. Combined with the jane = r rule, this still gives Jane read/write access. Permissions for group members can only be extended beyond the permissions the group already has. Restricting users who are part of a group to less than their group's permissions is impossible.

Groups can also be defined to contain other groups:

[groups]
calc-developers = harry, sally, joe
paint-developers = frank, sally, jane
everyone = @calc-developers, @paint-developers

Subversion 1.5 brought several useful features to the access file syntax—username aliases, authentication class tokens, and a new rule exclusion mechanism—all of which further simplify the maintenance of the access file. We'll describe first the username aliases feature.

Some authentication systems expect and carry relatively short usernames of the sorts we've been describing here—harry, sally, joe, and so on. But other authentication systems—such as those which use LDAP stores or SSL client certificates—may carry much more complex usernames. For example, Harry's username in an LDAP-protected system might be CN=Harold Hacker,OU=Engineers,DC=red-bean,DC=com. With usernames like that, the access file can become quite bloated with long or obscure usernames that are easy to mistype. Fortunately, username aliases allow you to have to type the correct complex username only once, in a statement which assigns to it a more easily digestable alias.

[aliases]
harry = CN=Harold Hacker,OU=Engineers,DC=red-bean,DC=com
sally = CN=Sally Swatterbug,OU=Engineers,DC=red-bean,DC=com
joe = CN=Gerald I. Joseph,OU=Engineers,DC=red-bean,DC=com
…

Once you've defined a set of aliases, you can refer to the users elsewhere in the access file via their aliases in all the same places you could have instead used their actual usernames. Simply prepend an ampersand to the alias to distinguish it from a regular username:

[groups]
calc-developers = &harry, &sally, &joe
paint-developers = &frank, &sally, &jane
everyone = @calc-developers, @paint-developers

You might also choose to use aliases if your users' usernames change frequently. Doing so allows you to need to update only the aliases table when these username changes occur, instead of doing global-search-and-replace operations on the whole access file.

Subversion also supports some magic tokens for helping you to make rule assignments based on the user's authentication class. One such token is the $authenticated token. Use this token where you would otherwise specify a username, alias, or group name in your authorization rules to declare the permissions granted to any user who has authenticated with any username at all. Similarly employed is the $anonymous token, except that it matches everyone who has not authenticated with a username.

[calendar:/projects/calendar]
$anonymous = r
$authenticated = rw

Finally, another handy bit of access file syntax magic is the use of the tilde (~) character as an exclusion marker. In your authorization rules, prefixing a username, alias, group name, or authentication class token with a tilde character will cause Subversion to apply the rule to users who do not match the rule. Though somewhat unnecessarily obfuscated, the following block is equivalent to the one in the previous example:

[calendar:/projects/calendar]
~$authenticated = r
~$anonymous = rw

A less obvious example might be as follows:

[groups]
calc-developers = &harry, &sally, &joe
calc-owners = &hewlett, &packard
calc = @calc-developers, @calc-owners

# Any calc participant has read-write access...
[calc:/projects/calc]
@calc = rw

# ...but only allow the owners to make and modify release tags.
[calc:/projects/calc/tags]
~@calc-owners = r

All of the above examples use directories, because defining access rules on them is the most common case. But is similarly able to restrict access on file paths, too.

[calendar:/projects/calendar/manager.ics]
harry = rw
sally = r


[53] A common theme in this book!

copyright  ©  November 24 2014 sean dreilinger url: http://durak.org/sean/pubs/software/version-control-with-subversion-1.6/svn.serverconfig.pathbasedauthz.html