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Add a new Handbook chapter about FreeBSD jails. It has been originally written

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by Matteo Riondato and further work has been done by me and my great mentors.
Despite it took months to get it in, we believe it's a right time to commit and
have a wider audience to look at it, and improve it even more. I hope you will
like it :-)

Approbed by: trhodes (mentor), keramida (mentor)
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Daniel Gerzo 2007-03-22 20:30:18 +00:00
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@ -169,6 +169,7 @@ SRCS+= firewalls/chapter.sgml
SRCS+= geom/chapter.sgml
SRCS+= install/chapter.sgml
SRCS+= introduction/chapter.sgml
SRCS+= jails/chapter.sgml
SRCS+= kernelconfig/chapter.sgml
SRCS+= l10n/chapter.sgml
SRCS+= linuxemu/chapter.sgml

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<!ENTITY % chap.users "IGNORE">
<!ENTITY % chap.kernelconfig "IGNORE">
<!ENTITY % chap.security "IGNORE">
<!ENTITY % chap.jails "IGNORE">
<!ENTITY % chap.printing "IGNORE">
<!ENTITY % chap.disks "IGNORE">
<!ENTITY % chap.geom "IGNORE">
@ -263,6 +264,7 @@
<![ %chap.boot; [ &chap.boot; ]]>
<![ %chap.users; [ &chap.users; ]]>
<![ %chap.security; [ &chap.security; ]]>
<![ %chap.jails; [ &chap.jails; ]]>
<![ %chap.mac; [ &chap.mac; ]]>
<![ %chap.audit; [ &chap.audit; ]]>
<![ %chap.disks; [ &chap.disks; ]]>

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<!ENTITY chap.boot SYSTEM "boot/chapter.sgml">
<!ENTITY chap.users SYSTEM "users/chapter.sgml">
<!ENTITY chap.security SYSTEM "security/chapter.sgml">
<!ENTITY chap.jails SYSTEM "jails/chapter.sgml">
<!ENTITY chap.mac SYSTEM "mac/chapter.sgml">
<!ENTITY chap.audit SYSTEM "audit/chapter.sgml">
<!ENTITY chap.disks SYSTEM "disks/chapter.sgml">

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<!--
The FreeBSD Documentation Project
$FreeBSD$
-->
<chapter id="jails">
<chapterinfo>
<authorgroup>
<author>
<firstname>Matteo</firstname>
<surname>Riondato</surname>
<contrib>Contributed by </contrib>
</author>
</authorgroup>
</chapterinfo>
<title>Jails</title>
<indexterm><primary>jails</primary></indexterm>
<sect1 id="jails-synopsis">
<title>Synopsis</title>
<para>This chapter will provide an explanation of what &os; jails
are and how to use them. Jails, sometimes referred to as an
enhanced replacement of <emphasis>chroot environments</emphasis>,
are very powerful tool for system administrators but their basic
usage can also be useful for advanced users.</para>
<para>After reading this chapter, you will know:</para>
<itemizedlist>
<listitem>
<para>What a jail is and what purpose it may serve in &os;
installations.</para>
</listitem>
<listitem>
<para>How to build, start, and stop a jail.</para>
</listitem>
<listitem>
<para>The basics of jail administration, both from inside
and outside the jail.</para>
</listitem>
</itemizedlist>
<para>Other sources of useful information about jails are:</para>
<itemizedlist>
<listitem>
<para>The &man.jail.8; manual page. This is the full reference
of the <command>jail</command> utility &mdash; the
administrative tool which can be used in &os; to start, stop,
and control &os; jails.</para>
</listitem>
<listitem>
<para>The mailing lists and their archives. The archives of the
&a.questions; and other mailing lists hosted by the
&a.mailman.lists; already contain a wealth of material for
jails. It should always be engaging to search the archives,
or post a new question to the &a.questions.name; mailing
list.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="jails-terms">
<title>Terms Related to Jails</title>
<para>To facilitate better understanding of parts of the &os; system
related to jails, their internals and the way they interact with
the rest of &os;, the following terms are used further in this
section:</para>
<variablelist>
<varlistentry>
<term>&man.chroot.2; (command)</term>
<listitem><para></para></listitem>
</varlistentry>
<varlistentry>
<term>&man.chroot.2; (environment)</term>
<listitem><para></para></listitem>
</varlistentry>
<varlistentry>
<term>&man.jail.8; (command)</term>
<listitem><para></para></listitem>
</varlistentry>
<varlistentry>
<term>host (system, process, user, etc.)</term>
<listitem><para></para></listitem>
</varlistentry>
<varlistentry>
<term>hosted (system, process, user, etc.)</term>
<listitem><para></para></listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="jails-intro">
<title>Introduction</title>
<para>Since the system administration is a difficult and perplexing
task, many powerful tools were developed to make life easier for
the administrator. That mostly provide enhancements of some sort
to the way systems are installed, configured and then maintained
without problems. Part of these tasks which an administrator is
expected to do is to properly configure the security of a system,
so that it can continue serving its real purpose, without allowing
security violations.</para>
<para>One of the tools which can be used to enhance the security of
a &os; system are <emphasis>jails</emphasis>. Jails were
introduced in &os; 4.X by &a.phk;, but were greatly improved in
&os; 5.X to make them a powerful and flexible subsystem. Their
development still goes on to zest their usefulness, performance
and security.</para>
<sect2 id="jails-what">
<title>What is a Jail</title>
<para>BSD-like operating systems have had &man.chroot.2; since the
time of 4.2BSD. The &man.chroot.2; utility can be used to
change the <filename role="directory">/</filename> directory
of a set of processes, creating a safe environment, separate
from the rest of the system: processes created in the chrooted
environment can not access files or resources outside of it.
For that reason, compromising a service running in a chrooted
environment should not allow the attacker to compromise the
entire system. The &man.chroot.2; utility is good for easy
tasks, which do not require a lot of flexibility or complex and
advanced features. Furthermore, since the inception of the
chroot concept, many ways have been found to escape from a
chrooted environment and, although they have been fixed in
modern versions of the &os; kernel, it was clear that
&man.chroot.2; was not the ideal solution for securing services.
A new subsystem had to be implemented.</para>
<para>These are the one of the main reasons why
<emphasis>jails</emphasis> were developed.</para>
<para>Jails improve on the concept of the traditional
&man.chroot.2; environment, in several ways. In a traditional
&man.chroot.2; environment, processes are only limited in the
part of the file system they can access. The rest of the system
resources (like the set of system users, the running processes,
or the networking subsystem) are shared by the chrooted
processes and the processes of the host system. Jails expand
this model by virtualizing not only access to the file system,
but also the set of users, the networking subsystem of the &os;
kernel and a few other things. A more complete set of
fine-grained controls available for tuning the access of a
jailed environment is described in <xref
linkend="jails-tuning">.</para>
<para>A jail is characterized by four elements:</para>
<itemizedlist>
<listitem>
<para>A directory subtree &mdash; the starting point from
which a jail is entered. Once inside the jail, a process
is not permitted to escape outside of this subtree.
Traditional security issues which plagued the original
&man.chroot.2; design will not affect &os; jails.</para>
</listitem>
<listitem>
<para>A hostname &mdash; the hostname which will be used
within the jail. Jails are mainly used for hosting network
services, therefore having a descriptive hostname for each
jail can really help the system administrator.</para>
</listitem>
<listitem>
<para>An <acronym>IP</acronym> address &mdash; this will be
assigned to the jail and cannot be changed in any way during
the jail's life span. It will usually be an alias address
for an existing network interface.</para>
</listitem>
<listitem>
<para>A command &mdash; the path name of an executable to run
inside the jail. This is relative to the root directory of
the jail environment, and may vary a lot, depending on the
type of the specific jail environment.</para>
</listitem>
</itemizedlist>
<para>Apart from these, jails can have their own set of users and
their own <username>root</username> user. Naturally, the powers
of the <username>root</username> user are limited within the
jail environment and, from the point of view of the host system,
the jail <username>root</username> user is not omnipotent user.
In addition, the <username>root</username> user will not be
allowed to perfom critical operations to the system outside of
their respective &man.jail.8; environment. More information
about capabilities and restrictions of the
<username>root</username> user will be discussed in <xref
linkend="jails-tuning"> below.</para>
</sect2>
</sect1>
<sect1 id="jails-build">
<title>Creating, and Controlling the jail</title>
<para>Some administrators divide jails into the following two types:
<quote>complete</quote> jails, which resemble a real &os; system,
and <quote>service</quote> jails, dedicated to one application or
service, possibly running with privileges. This is only a
conceptual division and the process of building a jail is not
affected by it. The &man.jail.8; manual page is quite clear about
the procedure for building a jail:</para>
<screen>&prompt.root; <userinput>setenv D <replaceable>/here/is/the/jail</replaceable></userinput>
&prompt.root; <userinput>mkdir -p $D</userinput> <co id="jailpath">
&prompt.root; <userinput>cd /usr/src</userinput>
&prompt.root; <userinput>make world DESTDIR=$D</userinput> <co id="jailworld">
&prompt.root; <userinput>cd etc/</userinput> <footnote><para>This step
is not required on &os; 6.0 and later.</para></footnote>
&prompt.root; <userinput>make distribution DESTDIR=$D</userinput> <co id="jaildistrib">
&prompt.root; <userinput>mount_devfs devfs $D/dev</userinput> <co id="jaildevfs"></screen>
<calloutlist>
<callout arearefs="jailpath">
<para>Selecting a location for jail is the best starting point.
This is where a jail will physically reside on the host
system. A good choice can be <filename
role="directory">/usr/jail/<replaceable>jailname</replaceable></filename>,
where <replaceable>jailname</replaceable> is the hostname
identifying the jail. The <filename
role="directory">/usr/</filename> file system usually has
enough space for the jail file system, which is, in practice,
a replication of every file present in a default installation
of the &os; base system.</para>
</callout>
<callout arearefs="jailworld">
<para>This command will populate the directory subtree chosen
as jail's physical location on the file system with the
necessary binaries, libraries, manual pages and so on.
Everything is done in the typical &os; style &mdash; first
everything is built/compiled, then installed to the
destination path.</para>
</callout>
<callout arearefs="jaildistrib">
<para>The <maketarget>distribution</maketarget> target for
<application>make</application> installs every needed
configuration file, in poor words, it copies
<filename role="directory">/usr/src/etc/</filename> to
<filename role="directory">$D/etc/</filename>.</para>
</callout>
<callout arearefs="jaildevfs">
<para>Mounting the &man.devfs.8; file system inside the jail is
not required. On the other hand, any, or almost any
application requires access to at least one device, depending
on the purpose of the given application. It is very important
to control access to devices from inside a jail, as improper
settings could permit an attacker to do nasty things in the
jail. Control over &man.devfs.8; is managed through rulesets
which are described in the &man.devfs.8; and
&man.devfs.conf.5; manual pages.</para>
</callout>
</calloutlist>
<para>Once a jail is installed, it can be started by using the
&man.jail.8; utility. The &man.jail.8; utility takes four
mandatory arguments which are described in the <xref
linkend="jails-what"> section. Other arguments may be
specified and give the ability to run the program as a specific
user. The <command>command</command> argument depends on
the type of the jail; for a <emphasis>virtual system</emphasis>,
<filename>/etc/rc</filename> is a good choice, since it will
replicate the startup sequence of a real &os; system. For a
<emphasis>service</emphasis> jail, it depends on the service or
application that will run within the jail.</para>
<para>Jails are often started at boot time and the &os;
<filename>rc</filename> mechanism provides an easy way to do
this.</para>
<procedure>
<step>
<para>A list of the jails which are enabled to start at boot
time should be added to the &man.rc.conf.5; file:</para>
<programlisting>jail_enable="YES" # Set to NO to disable starting of any jails
jail_list="<replaceable>www</replaceable>" # Space separated list of names of jails</programlisting>
</step>
<step>
<para>For each jail listed in <varname>jail_list</varname>, a
group of &man.rc.conf.5; settings, which describe the
particular jail, should be added:</para>
<programlisting>jail_<replaceable>www</replaceable>_rootdir="/usr/jail/www" # jail's root directory
jail_<replaceable>www</replaceable>_hostname="<replaceable>www</replaceable>.example.org" # jail's hostname
jail_<replaceable>www</replaceable>_ip_="192.168.0.10" # jail's IP address
jail_<replaceable>www</replaceable>_devfs_enable="YES" # mount devfs in the jail
jail_<replaceable>www</replaceable>_devfs_ruleset="<replaceable>www_ruleset</replaceable>" # devfs ruleset to apply to jail</programlisting>
<para>The default startup of jails configured in
&man.rc.conf.5;, will run the <filename>/etc/rc</filename>
script of the jail, which assumes the jail is a complete
virtual system. For service jails, the default startup
command of the jail should be changed, by setting the
<varname>jail_<replaceable>jailname</replaceable>_exec_start</varname>
option appropriately.</para>
<note>
<para>For full list of available options, please see the
&man.rc.conf.5; manual page.</para>
</note>
</procedure>
<para>The <filename>/etc/rc.d/jail</filename> script can be used to
start or stop a jail by hand, if an entry for it exists in
<filename>rc.conf</filename>:</para>
<screen>&prompt.root; <userinput>/etc/rc.d/jail start <replaceable>www</replaceable></userinput>
&prompt.root; <userinput>/etc/rc.d/jail stop <replaceable>www</replaceable></userinput></screen>
<para>A clean way to shut down a &man.jail.8; is not available at
the moment. This is because commands normally used to accomplish
a clean system shutdown cannot be used inside a jail. The best
way to shut down a jail is to run the following command from
within the jail itself or using the &man.jexec.8; utility from
outside the jail:</para>
<screen>&prompt.root; <userinput>sh /etc/rc.shutdown</userinput></screen>
<para>More information about this can be found in the &man.jail.8;
manual page.</para>
</sect1>
<sect1 id="jails-tuning">
<title>Fine Tuning and Administration</title>
<para>There are several options which can be set for any jail, and
various ways of combining a host &os; system with jails, to produce
higher level applications. This section presents:</para>
<itemizedlist>
<listitem>
<para>Some of the options available for tuning the behavior and
security restrictions implemented by a jail
installation.</para>
</listitem>
<listitem>
<para>Some of the high-level applications for jail management,
which are available through the &os; Ports Collection, and can
be used to implement overall jail-based solutions.</para>
</itemizedlist>
<sect2 id="jails-tuning-utilities">
<title>System tools for jail tuning in &os;</title>
<para>Fine tuning of a jail's configuration is mostly done by
setting &man.sysctl.8; variables. A special subtree of sysctl
exists as a basis for organizing all the relevant options; the
<varname>security.jail.*</varname> hierarchy of &os; kernel
options. Here is a list of the main jail-related sysctls,
complete with their default value. Names should be
self-explanatory, but for more information about them, please
refer to the &man.jail.8; and &man.sysctl.8; manual
pages.</para>
<itemizedlist>
<listitem>
<para><varname>security.jail.set_hostname_allowed:
1</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.socket_unixiproute_only:
1</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.sysvipc_allowed:
0</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.enforce_statfs:
2</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.allow_raw_sockets:
0</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.chflags_allowed:
0</varname></para>
</listitem>
<listitem>
<para><varname>security.jail.jailed: 0</varname></para>
</listitem>
</itemizedlist>
<para>These variables can be used by the system administrator of
the <emphasis>host system</emphasis> to add or remove some of
the limitations imposed by default on the
<username>root</username> user. Note that there are some
limitations which cannot be removed. The
<username>root</username> user is not allowed to mount or
unmount file systems from within a &man.jail.8;. The
<username>root</username> inside a jail may not load or unload
&man.devfs.8; rulesets, set firewall rules, or do many other
administration tasks which require modifications of in-kernel
data, such as setting the <varname>securelevel</varname> of the
kernel.</para>
<para>The base system of &os; contains a basic set of tools for
viewing information about the active jails, and attaching to a
jail to run administrative commands. The &man.jls.8; and
&man.jexec.8; are part of the base &os; system, and can be used
to perform the following simple tasks:</para>
<itemizedlist>
<listitem>
<para>Print a list of active jails and their corresponding
jail identifier (<acronym>JID</acronym>),
<acronym>IP</acronym> address, hostname and path.</para>
</listitem>
<listitem>
<para>Attach to a running jail, from its host system, and run
a command inside the jail administrative tasks inside the
jail itself). This is especially useful when the
<username>root</username> user wants to cleanly shut down a
jail. The &man.jexec.8; utility can also be used to start a
shell in a jail to do administration in it; for
example:</para>
<screen>&prompt.root; <userinput>jexec <replaceable>1</replaceable> tcsh</userinput></screen>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="jails-tuning-admintools">
<title>High-level administrative tools in &os; Ports
Collection</title>
<para>Among the many third-party utilities for jail administration,
one of the most complete and useful is <filename
role="package">sysutils/jailutils</filename>. It is a set of
small applications that contribute to &man.jail.8; management.
Please refer to its web page for more information.</para>
</sect2>
</sect1>
</chapter>