Most CGI scripts send email using the sendmail binary. That will not work in our jail since the sendmail binary isn’t there. Adding the complete sendmail installation to the jail would defy the very purpose of having a jail in the first place. If you encounter this problem, consider installing mini_sendmail (http://www.acme.com/ software/mini_sendmail/), a sendmail replacement specifically designed for jails. Most programming languages come with libraries that allow email to be sent directly to an SMTP server. PHP can send email directly, and from Perl you can use the Mail::Sendmail library. Using these libraries reduces the number of packages that are installed in a jail.

You will probably encounter database connectivity problems when scripts in jail try to connect to a database engine running outside the jail. This happens if the program is using localhost as the host name of the database server. When a database client library sees localhost, it tries to connect to the database using a Unix domain socket. This socket is a special file usually located in /tmp, /var/run,or /var/lib, all outside the jail. One way to get around this is to use 127.0.0.1 as the host name and force the database client library to use TCP/IP. However, since a performance penalty is involved with that solution (Unix domain socket communication is much faster than communication over TCP/IP), a better way would be to have the socket file in the jail.

For PostgreSQL, find the file postgresql.conf (usually in /var/lib/pgsql/data) and change the line containing the unix_socket_directory directive to read:

  unix_socket_directory = '/chroot/apache/tmp'

Create a symbolic link from the previous location to the new one:

  # ln -s /chroot/apache/tmp/.s.PGSQL.5432 /tmp

MySQL keeps its configuration options in a file called my.cnf, usually located in /etc. In the same file, you can add a client section (if one is not there already) and tell clients where to look for a socket:

  [mysqld ]
  datadir=/var/lib/mysql 
  socket=/chroot/apache/var/lib/mysql/ mysql.sock

  [client] 
  socket=/chroot/apache/var/lib/mysql/ mysql.sock

Or, just as you did with PostgreSQL, create a symbolic link:

  # mkdir -p /chroot/apache/var/lib/mysql
  # chown mysql /chroot/apache/var/lib/mysql/
  # ln -s /chroot/apache/var/lib/mysql/mysql.sock
/var/lib/mysql

Using the chroot(2) Patch

Now that I have explained the manual chroot process, you are wondering if an easier way exists. The answer is, conditionally, yes.

The approach so far was to create the jail before the main process was started. For this approach to work, the jail must contain all shared libraries and files the process requires. This approach is also known as an external chroot.

With an internal chroot, the jail is established from within the process after the process initialization is completed. In the case of Apache, the jail must be created before request processing begins, at the latest. The process is born free and then jailed. Since the process has full access to the filesystem during the initialization phase, it is free to access any files it needs. Because of the way chrooting works, descriptors to the files opened before the call remain valid after. Therefore, we do not have to create a copy of the filesystem and we can have a “perfect” jail, the one that contains only files needed for web serving, the files in the web server tree.

Internal chroot can be dangerous. In external chroot approaches, the process is born in jail, so it has no opportunity to interact with the outside filesystem. With the internal chroot, however, the process has full access to the filesystem in the beginning and this allows it to open files outside the jail and continue to use them even after the jail is created. This opens up interesting opportunities, such as being able to keep the logs and the binaries outside jail, but is a potential problem. Some people are not comfortable with leaving open file descriptors outside jail. You can use the lsof utility to see which file descriptors Apache has open and determine whether any of them point outside jail. My recommendation is the following: If you can justify a high level of security for your installation, go for a proper external chroot approach. For installations of less importance, spending all that time is not feasible. In such cases, use the internal chroot approach.

It is obvious that internal chrooting is not a universal solution. It works only if the following is true:

1. The only functionality needed is that of Apache and its modules.
   
2. There will be no processes (such as CGI scripts) started at runtime. Alternatively, if CGI scripts are used, they will be statically compiled.
   
3. Access to files outside the web server root will be not be required at runtime. (For example, if you intend to use the piped logging mechanism, Apache must be able to access the logging binary at runtime to restart logging in case the original logging process dies for some reason. Piped logging is discussed in Chapter 8.)

Now that I have lured you into thinking you can get away from the hard labor of chrooting, I will have to disappoint you: Apache does not support internal chrooting natively. But the help comes from Arjan de Vet in the form of a chroot(2) patch. It is available for download from http://www.devet.org/apache/chroot/. After the patch is applied to the source code, Apache will support a new directive, ChrootDir . Chrooting Apache can be as easy as supplying the new root of the filesystem as the ChrootDir first parameter. The record of a successful chroot(2) call will be in the error log.

As a downside, you will have to apply the patch every time you install Apache. And there is the problem of finding the patch for the version of Apache you want to install. At the time of this writing only the patch for Apache 1.3.31 is available. But not everything is lost.