The easiest way to authenticate a client is via the HTTP Basic authentication mechanism, which simply uses a username and password to verify that a user is who she says she is. Apache provides an htpasswd utility for managing the list of acceptable usernames and passwords. Let's grant commit access to Sally and Harry. First, we need to add them to the password file.

$ ### First time: use -c to create the file
$ ### Use -m to use MD5 encryption of the password, which is more secure
$ htpasswd -cm /etc/svn-auth-file harry
New password: *****
Re-type new password: *****
Adding password for user harry
$ htpasswd -m /etc/svn-auth-file sally
New password: *******
Re-type new password: *******
Adding password for user sally
$

Next, you need to add some more httpd.conf directives inside your Location block to tell Apache what to do with your new password file. The AuthType directive specifies the type of authentication system to use. In this case, we want to specify the Basic authentication system. AuthName is an arbitrary name that you give for the authentication domain. Most browsers will display this name in the pop-up dialog box when the browser is querying the user for his name and password. Finally, use the AuthUserFile directive to specify the location of the password file you created using htpasswd.

After adding these three directives, your <Location> block should look something like this:

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /etc/svn-auth-file
</Location>

This <Location> block is not yet complete, and will not do anything useful. It's merely telling Apache that whenever authorization is required, Apache should harvest a username and password from the Subversion client. What's missing here, however, are directives that tell Apache which sorts of client requests require authorization. Wherever authorization is required, Apache will demand authentication as well. The simplest thing to do is protect all requests. Adding Require valid-user tells Apache that all requests require an authenticated user:

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /etc/svn-auth-file
  Require valid-user
</Location>

Be sure to read the next section (“授权选项”一节) for more detail on the Require directive and other ways to set authorization policies.

One word of warning: HTTP Basic Auth passwords pass in very nearly plain-text over the network, and thus are extremely insecure.

Another option is to not use Basic authentication but “Digest” authentication instead. Digest authentication allows the server to verify the client's identity without passing the plaintext password over the network. Assuming that the client and server both know the user's password, they can verify that the password is the same by using it to apply a hashing function to a one-time bit of information. The server sends a small random-ish string to the client; the client uses the user's password to hash the string; the server then looks to see if the hashed value is what it expected.

Configuring Apache for Digest authentication is also fairly easy, and only a small variation on our prior example. Be sure to consult Apache's documentation for full details.

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  AuthType Digest
  AuthName "Subversion repository"
  AuthDigestDomain /svn/
  AuthUserFile /etc/svn-auth-file
  Require valid-user
</Location>

If you're looking for maximum security, then public-key cryptography is the best solution. It may be best to use some sort of SSL encryption, so that clients authenticate via https:// instead of http://; at a bare minimum, you can configure Apache to use a self-signed server certificate. ^[42] Consult Apache's documentation (and OpenSSL documentation) about how to do that.

商业应用需要越过公司防火墙的版本库访问，防火墙需要小心的考虑非认证用户“吸取”他们的网络流量的情况，SSL让那种形式的关注更不容易导致敏感数据泄露。

If a Subversion client is compiled to use OpenSSL, then it gains the ability to speak to an Apache server via https:// URLs. The Neon library used by the Subversion client is not only able to verify server certificates, but can also supply client certificates when challenged. When the client and server have exchanged SSL certificates and successfully authenticated one another, all further communication is encrypted via a session key.

怎样产生客户端和服务器端证书以及怎样使用它们已经超出了本书的范围，许多书籍，包括Apache自己的文档，描述这个任务，现在我们可以覆盖的是普通的客户端怎样来管理服务器与客户端证书。

When speaking to Apache via https://, a Subversion client can receive two different types of information:

如果客户端接收了一个服务器证书，它需要去验证它是可以相信的：这个服务器是它自称的那一个吗？OpenSSL库会去检验服务器证书的签名人或者是核证机构（CA）。如果OpenSSL不可以自动信任这个CA，或者是一些其他的问题（如证书过期或者是主机名不匹配），Subversion命令行客户端会询问你是否愿意仍然信任这个证书：

$ svn list https://host.example.com/repos/project

Error validating server certificate for 'https://host.example.com:443':
 - The certificate is not issued by a trusted authority. Use the
   fingerprint to validate the certificate manually!
Certificate information:
 - Hostname: host.example.com
 - Valid: from Jan 30 19:23:56 2004 GMT until Jan 30 19:23:56 2006 GMT
 - Issuer: CA, example.com, Sometown, California, US
 - Fingerprint: 7d:e1:a9:34:33:39:ba:6a:e9:a5:c4:22:98:7b:76:5c:92:a0:9c:7b

(R)eject, accept (t)emporarily or accept (p)ermanently?

This dialogue should look familiar; it's essentially the same question you've probably seen coming from your web browser (which is just another HTTP client like Subversion). If you choose the (p)ermanent option, the server certificate will be cached in your private run-time auth/ area in just the same way your username and password are cached (see “客户端凭证缓存”一节). If cached, Subversion will automatically trust this certificate in future negotiations.

Your run-time servers file also gives you the ability to make your Subversion client automatically trust specific CAs, either globally or on a per-host basis. Simply set the ssl-authority-files variable to a semicolon-separated list of PEM-encoded CA certificates:

[global]
ssl-authority-files = /path/to/CAcert1.pem;/path/to/CAcert2.pem

Many OpenSSL installations also have a pre-defined set of “default” CAs that are nearly universally trusted. To make the Subversion client automatically trust these standard authorities, set the ssl-trust-default-ca variable to true.

当与Apache通话时，Subversion客户端也会收到一个证书的要求，Apache是询问客户端来证明自己的身份：这个客户端是否是他所说的那一个？如果一切正常，Subversion客户端会发送回一个通过Apache信任的CA签名的私有证书，一个客户端证书通常会以加密方式存放在磁盘，使用本地密码保护，当Subversion收到这个要求，它会询问你证书的路径和保护用的密码：

$ svn list https://host.example.com/repos/project

Authentication realm: https://host.example.com:443
Client certificate filename: /path/to/my/cert.p12
Passphrase for '/path/to/my/cert.p12':  ********
…

注意这个客户端证书是一个“p12”文件，为了让Subversion使用客户端证书，它必须是运输标准的PKCS#12格式，大多数浏览器可以导入和导出这种格式的证书，另一个选择是用OpenSSL命令行工具来转化存在的证书为PKCS#12格式。

Again, the runtime servers file allows you to automate this challenge on a per-host basis. Either or both pieces of information can be described in runtime variables:

[groups]
examplehost = host.example.com

[examplehost]
ssl-client-cert-file = /path/to/my/cert.p12
ssl-client-cert-password = somepassword

Once you've set the ssl-client-cert-file and ssl-client-cert-password variables, the Subversion client can automatically respond to a client certificate challenge without prompting you. ^[43]

最简单的访问控制形式是授权特定用户为只读版本库访问或者是读/写访问版本库。

You can restrict access on all repository operations by adding the Require valid-user directive to your <Location> block. Using our previous example, this would mean that only clients that claimed to be either harry or sally, and provided the correct password for their respective username, would be allowed to do anything with the Subversion repository:

<Location /svn>
  DAV svn
  SVNParentPath /var/svn

  # how to authenticate a user
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /path/to/users/file

  # only authenticated users may access the repository
  Require valid-user
</Location>

Sometimes you don't need to run such a tight ship. For example, Subversion's own source code repository at http://svn.collab.net/repos/svn allows anyone in the world to perform read-only repository tasks (like checking out working copies and browsing the repository with a web browser), but restricts all write operations to authenticated users. To do this type of selective restriction, you can use the Limit and LimitExcept configuration directives. Like the Location directive, these blocks have starting and ending tags, and you would nest them inside your <Location> block.

The parameters present on the Limit and LimitExcept directives are HTTP request types that are affected by that block. For example, if you wanted to disallow all access to your repository except the currently supported read-only operations, you would use the LimitExcept directive, passing the GET, PROPFIND, OPTIONS, and REPORT request type parameters. Then the previously mentioned Require valid-user directive would be placed inside the <LimitExcept> block instead of just inside the <Location> block.

<Location /svn>
  DAV svn
  SVNParentPath /var/svn

  # how to authenticate a user
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /path/to/users/file

  # For any operations other than these, require an authenticated user.
  <LimitExcept GET PROPFIND OPTIONS REPORT>
    Require valid-user
  </LimitExcept>
</Location>

These are only a few simple examples. For more in-depth information about Apache access control and the Require directive, take a look at the Security section of the Apache documentation's tutorials collection at http://httpd.apache.org/docs-2.0/misc/tutorials.html.

It's possible to set up finer-grained permissions using a second Apache httpd module, mod_authz_svn. This module grabs the various opaque URLs passing from client to server, asks mod_dav_svn to decode them, and then possibly vetoes requests based on access policies defined in a configuration file.

If you've built Subversion from source code, mod_authz_svn is automatically built and installed alongside mod_dav_svn. Many binary distributions install it automatically as well. To verify that it's installed correctly, make sure it comes right after mod_dav_svn's LoadModule directive in httpd.conf:

LoadModule dav_module         modules/mod_dav.so
LoadModule dav_svn_module     modules/mod_dav_svn.so
LoadModule authz_svn_module   modules/mod_authz_svn.so

To activate this module, you need to configure your Location block to use the AuthzSVNAccessFile directive, which specifies a file containing the permissions policy for paths within your repositories. (In a moment, we'll discuss the format of that file.)

Apache非常的灵活，你可以从三种模式里选择一种来配置你的区块，作为开始，你选择一种基本的配置模式。（下面的例子非常简单；见Apache自己的文档中的认证和授权选项来查看更多的细节。）

最简单的区块是允许任何人可以访问，在这个场景里，Apache决不会发送认证请求，所有的用户作为“匿名”对待。

<Location /repos>
  DAV svn
  SVNParentPath /var/svn

  # our access control policy
  AuthzSVNAccessFile /path/to/access/file
</Location>
          

On the opposite end of the paranoia scale, you can configure your block to demand authentication from everyone. All clients must supply credentials to identify themselves. Your block unconditionally requires authentication via the Require valid-user directive, and defines a means to authenticate.

<Location /repos>
  DAV svn
  SVNParentPath /var/svn

  # our access control policy
  AuthzSVNAccessFile /path/to/access/file

  # only authenticated users may access the repository
  Require valid-user

  # how to authenticate a user
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /path/to/users/file
</Location>
          

A third very popular pattern is to allow a combination of authenticated and anonymous access. For example, many administrators want to allow anonymous users to read certain repository directories, but want only authenticated users to read (or write) more sensitive areas. In this setup, all users start out accessing the repository anonymously. If your access control policy demands a real username at any point, Apache will demand authentication from the client. To do this, you use both the Satisfy Any and Require valid-user directives together.

<Location /repos>
  DAV svn
  SVNParentPath /var/svn

  # our access control policy
  AuthzSVNAccessFile /path/to/access/file

  # try anonymous access first, resort to real
  # authentication if necessary.
  Satisfy Any
  Require valid-user

  # how to authenticate a user
  AuthType Basic
  AuthName "Subversion repository"
  AuthUserFile /path/to/users/file
</Location>
          

Once you've settled on one of these three basic httpd.conf templates, you need to create your file containing access rules for particular paths within the repository. This is described in “基于路径的授权”一节.

The mod_dav_svn module goes through a lot of work to make sure that data you've marked “unreadable” doesn't get accidentally leaked. This means that it needs to closely monitor all of the paths and file-contents returned by commands like svn checkout or svn update commands. If these commands encounter a path that isn't readable according to some authorization policy, then the path is typically omitted altogether. In the case of history or rename tracing—e.g. running a command like svn cat -r OLD foo.c on a file that was renamed long ago—the rename tracking will simply halt if one of the object's former names is determined to be read-restricted.

All of this path-checking can sometimes be quite expensive, especially in the case of svn log. When retrieving a list of revisions, the server looks at every changed path in each revision and checks it for readability. If an unreadable path is discovered, then it's omitted from the list of the revision's changed paths (normally seen with the --verbose option), and the whole log message is suppressed. Needless to say, this can be time-consuming on revisions that affect a large number of files. This is the cost of security: even if you haven't configured a module like mod_authz_svn at all, the mod_dav_svn module is still asking Apache httpd to run authorization checks on every path. The mod_dav_svn module has no idea what authorization modules have been installed, so all it can do is ask Apache to invoke whatever might be present.

On the other hand, there's also an escape-hatch of sorts, one which allows you to trade security features for speed. If you're not enforcing any sort of per-directory authorization (i.e. not using mod_authz_svn or similar module), then you can disable all of this path-checking. In your httpd.conf file, use the SVNPathAuthz directive:

<Location /repos>
  DAV svn
  SVNParentPath /var/svn

  SVNPathAuthz off
</Location>
          

The SVNPathAuthz directive is “on” by default. When set “off”, all path-based authorization checking is disabled; mod_dav_svn stops invoking authorization checks on every path it discovers.

One of the most useful benefits of an Apache/WebDAV configuration for your Subversion repository is that the youngest revisions of your versioned files and directories are immediately available for viewing via a regular web browser. Since Subversion uses URLs to identify versioned resources, those URLs used for HTTP-based repository access can be typed directly into a Web browser. Your browser will issue an HTTP GET request for that URL, and based on whether that URL represents a versioned directory or file, mod_dav_svn will respond with a directory listing or with file contents.

因为URL不能确定你所希望看到的资源的版本，mod_dav_svn会一直返回最新的版本，这样会有一些美妙的副作用，你可以直接把Subversion的URL传递给文档作为引用，这些URL会一直指向文档最新的材料，当然，你也可以在别的网站作为超链使用这些URL。

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  SVNIndexXSLT "/svnindex.xsl"
  …
</Location>

<Location /svn>
  DAV svn
  SVNParentPath /var/svn
  SVNListParentPath on
  …
</Location>

Because Apache is an HTTP server at heart, it contains fantastically flexible logging features. It's beyond the scope of this book to discuss all ways logging can be configured, but we should point out that even the most generic httpd.conf file will cause Apache to produce two logs: error_log and access_log. These logs may appear in different places, but are typically created in the logging area of your Apache installation. (On Unix, they often live in /usr/local/apache2/logs/.)

The error_log describes any internal errors that Apache runs into as it works. The access_log file records every incoming HTTP request received by Apache. This makes it easy to see, for example, which IP addresses Subversion clients are coming from, how often particular clients use the server, which users are authenticating properly, and which requests succeed or fail.

Unfortunately, because HTTP is a stateless protocol, even the simplest Subversion client operation generates multiple network requests. It's very difficult to look at the access_log and deduce what the client was doing—most operations look like a series of cryptic PROPPATCH, GET, PUT, and REPORT requests. To make things worse, many client operations send nearly-identical series of requests, so it's even harder to tell them apart.

mod_dav_svn, however, can come to your aid. By activating an “operational logging” feature, you can ask mod_dav_svn to create a separate log file describing what sort of high-level operations your clients are performing.

To do this, you need to make use of Apache's CustomLog directive (which is explained in more detail in Apache's own documentation). Be sure to invoke this directive outside of your Subversion Location block:

<Location /svn>
  DAV svn
  …
</Location>

CustomLog logs/svn_logfile "%t %u %{SVN-ACTION}e" env=SVN-ACTION

In this example, we're asking Apache to create a special logfile svn_logfile in the standard Apache logs directory. The %t and %u variables are replaced by the time and username of the request, respectively. The really important part are the two instances of SVN-ACTION. When Apache sees that variable, it substitutes the value of the SVN-ACTION environment variable, which is automatically set by mod_dav_svn whenever it detects a high-level client action.

So instead of having to interpret a traditional access_log like this:

[26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc/!svn/vcc/default HTTP/1.1" 207 398
[26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc/!svn/bln/59 HTTP/1.1" 207 449
[26/Jan/2007:22:25:29 -0600] "PROPFIND /svn/calc HTTP/1.1" 207 647
[26/Jan/2007:22:25:29 -0600] "REPORT /svn/calc/!svn/vcc/default HTTP/1.1" 200 607
[26/Jan/2007:22:25:31 -0600] "OPTIONS /svn/calc HTTP/1.1" 200 188
[26/Jan/2007:22:25:31 -0600] "MKACTIVITY /svn/calc/!svn/act/e6035ef7-5df0-4ac0-b811-4be7c823f998 HTTP/1.1" 201 227
…

… you can instead peruse a much more intelligible svn_logfile like this:

[26/Jan/2007:22:24:20 -0600] - list-dir '/'
[26/Jan/2007:22:24:27 -0600] - update '/'
[26/Jan/2007:22:25:29 -0600] - remote-status '/'
[26/Jan/2007:22:25:31 -0600] sally commit r60

For an exhaustive list of all actions logged, see “High Level Logging”一节.

One of the nice advantages of using Apache as a Subversion server is that it can be set up for simple replication. For example, suppose that your team is distributed across four offices around the globe. The Subversion repository can only exist in one of those offices, and that means the other three offices will not enjoy accessing it—they're likely to experience significantly slower traffic and response times when updating and committing code. A powerful solution is to set up a system consisting of one master Apache server and several slave Apache servers. If you place a slave server in each office, then users can check out a working copy from whichever slave is closest to them. All read requests go to their local slave. Write requests get automatically routed to the single master server. When the commit completes, the master then automatically “pushes” the new revision to each slave server using the svnsync replication tool.

This configuration creates a huge perceptual speed increase for your users, because Subversion client traffic is typically 80-90% read requests. And if those requests are coming from a local server, it's a huge win.

In this section, we'll walk you through a standard setup of this single-master/multiple slave system. However, keep in mind that your servers must be running at least Apache 2.2.0 (with mod_proxy loaded) and Subversion (mod_dav_svn) 1.5.

<Location /svn>
  DAV svn
  SVNPath /var/svn/repos
  SVNMasterURI http://master.example.com/svn
  …
</Location>

<Location /svn-proxy-sync>
  DAV svn
  SVNPath /var/svn/repos
  Order deny,allow
  Deny from all
  # Only let the server's IP address access this Location:
  Allow from 10.20.30.40
  …
</Location>

$ svnsync init http://slave1.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.
$ svnsync init http://slave2.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.
$ svnsync init http://slave3.example.com/svn-proxy-sync file://var/svn/repos
Copied properties for revision 0.

# Perform the initial replication

$ svnsync sync http://slave1.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
…

$ svnsync sync http://slave2.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
…

$ svnsync sync http://slave3.example.com/svn-proxy-sync
Transmitting file data ....
Committed revision 1.
Copied properties for revision 1.
Transmitting file data .......
Committed revision 2.
Copied properties for revision 2.
…

#!/bin/sh
# Post-commit script to replicate newly-committed revision to slaves

svnsync sync http://slave1.example.com/svn-proxy-sync > /dev/null 2>&1
svnsync sync http://slave2.example.com/svn-proxy-sync > /dev/null 2>&1
svnsync sync http://slave3.example.com/svn-proxy-sync > /dev/null 2>&1

#!/bin/sh
# Post-revprop-change script to replicate revprop-changes to slaves

REV=${2}
svnsync copy-revprops http://slave1.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1
svnsync copy-revprops http://slave2.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1
svnsync copy-revprops http://slave3.example.com/svn-proxy-sync ${REV} > /dev/null 2>&1

Several of the features already provided by Apache in its role as a robust Web server can be leveraged for increased functionality or security in Subversion as well. The Subversion client is able to use SSL, (the Secure Socket Layer, discussed earlier). If your Subversion client is built to support SSL, then it can access your Apache server using https:// and enjoy a high-quality encrypted network session.

Equally useful are other features of the Apache and Subversion relationship, such as the ability to specify a custom port (instead of the default HTTP port 80) or a virtual domain name by which the Subversion repository should be accessed, or the ability to access the repository through an HTTP proxy.

Finally, because mod_dav_svn is speaking a subset of the WebDAV/DeltaV protocol, it's possible to access the repository via third-party DAV clients. Most modern operating systems (Win32, OS X, and Linux) have the built-in ability to mount a DAV server as a standard network “shared folder”. This is a complicated topic, but also wondrous when implemented. For details, read 附录 C, WebDAV 和自动版本.

Note that there are number of other small tweaks one can make to mod_dav_svn that are too obscure to mention in this chapter. For a complete list of all httpd.conf directives that mod_dav_svn responds to, see “指示”一节.