To activate specific SASL mechanisms on the server, you’ll need to do two things. First, create a [sasl] section in your repository’s svnserve.conf file with an initial key-value pair:

          [sasl]
          use-sasl = true

Second, create a main SASL configuration file called svn.conf in a place where the SASL library can find it—typically in the directory where SASL plug-ins are located. You’ll have to locate the plug-in directory on your particular system, such as /usr/lib/sasl2/ or /etc/sasl2/. (Note that this is not the svnserve.conf file that lives within a repository!)

On a Windows server, you’ll also have to edit the system registry (using a tool such as regedit) to tell SASL where to find things. Create a registry key named [HKEY_LOCAL_MACHINESOFTWARECarnegie MellonProject CyrusSASL Library], and place two keys inside it: a key called SearchPath (whose value is a path to the directory containing the SASL sasl*.dll plug-in libraries), and a key called ConfFile (whose value is a path to the parent directory containing the svn.conf file you created).

Because SASL provides so many different kinds of authentication mechanisms, it would be foolish (and far beyond the scope of this book) to try to describe every possible server-side configuration. Instead, we recommend that you read the documentation supplied in the doc/ subdirectory of the SASL source code. It goes into great detail about every mechanism and how to configure the server appropriately for each. For the purposes of this discussion, we’ll just demonstrate a simple example of configuring the DIGEST-MD5 mechanism. For example, if your subversion.conf (or svn.conf) file contains the following:

pwcheck_method: auxprop
auxprop_plugin: sasldb
sasldb_path: /etc/my_sasldb
mech_list: DIGEST-MD5

you’ve told SASL to advertise the DIGEST-MD5 mechanism to clients and to check user passwords against a private password database located at /etc/my_sasldb. A system administrator can then use the saslpasswd2 program to add or modify usernames and passwords in the database:

$ saslpasswd2 -c -f /etc/my_sasldb -u realm username

A few words of warning: first, make sure the “realm” argument to saslpasswd2 matches the same realm you’ve defined in your repository’s svnserve.conf file; if they don’t match, authentication will fail. Also, due to a shortcoming in SASL, the common realm must be a string with no space characters. Finally, if you decide to go with the standard SASL password database, make sure the svnserve program has read access to the file (and possibly write access as well, if you’re using a mechanism such as OTP).

This is just one simple way of configuring SASL. Many other authentication mechanisms are available, and passwords can be stored in other places such as in LDAP or a SQL database. Consult the full SASL documentation for details.

Remember that if you configure your server to allow only certain SASL authentication mechanisms, this forces all connecting clients to have SASL support as well. Any Subversion client built without SASL support (which includes all pre-1.5 clients) will be unable to authenticate. On the one hand, this sort of restriction may be exactly what you want (“My clients must all use Kerberos!”). However, if you still want non-SASL clients to be able to authenticate, be sure to advertise the CRAM-MD5 mechanism as an option. All clients are able to use CRAM-MD5, whether they have SASL capabilities or not.

SASL is also able to perform data encryption if a particular mechanism supports it. The built-in CRAM-MD5 mechanism doesn’t support encryption, but DIGEST-MD5 does, and mechanisms such as SRP actually require use of the OpenSSL library. To enable or disable different levels of encryption, you can set two values in your repository’s svnserve.conf file:

[sasl]
use-sasl = true
min-encryption = 128
max-encryption = 256

The min-encryption and max-encryption variables control the level of encryption demanded by the server. To disable encryption completely, set both values to 0. To enable simple checksumming of data (i.e., prevent tampering and guarantee data integrity without encryption), set both values to 1. If you wish to allow—but not require—encryption, set the minimum value to 0, and the maximum value to some bit length. To require encryption unconditionally, set both values to numbers greater than 1. In our previous example, we require clients to do at least 128-bit encryption but no more than 256-bit encryption.