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freebsd/usr.sbin/jail/jail.8
Robert Watson a5ca5c1a75 A variety of content cleanups: 
(1) Document the notion of using jail(8) to run "virtual servers" or
    just to constrain specific applications.  If only running specific
    applications, some configuration steps are unnecessary (such as
    editing rc.conf).

(2) Add some more subsection headers to break up the bigger chunks of
    text.

(3) Clarify the problems associated with applications binding all IP
    addresses in the host, and attempt to be more specific about
    potential application problems.  Document how to force sshd to
    bind the the right socket.

(4) Suggest that in a jailed application scenario, you might want to
    have the host syslogd listen on the socket in the jail, rather
    than running syslogd in the jail.

(5) Catch another reference to /stand/sysinstall.

Approved by:	re (bmah implicitly)
2003-11-20 03:47:50 +00:00

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.\"
.\" Copyright (c) 2000, 2003 Robert N. M. Watson
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.\" ----------------------------------------------------------------------------
.\" "THE BEER-WARE LICENSE" (Revision 42):
.\" <phk@FreeBSD.ORG> wrote this file. As long as you retain this notice you
.\" can do whatever you want with this stuff. If we meet some day, and you think
.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
.\" ----------------------------------------------------------------------------
.\"
.\" $FreeBSD$
.\"
.Dd April 8, 2003
.Dt JAIL 8
.Os
.Sh NAME
.Nm jail
.Nd "imprison process and its descendants"
.Sh SYNOPSIS
.Nm
.Op Fl i
.Op Fl u Ar username
.Ar path hostname ip-number command ...
.Sh DESCRIPTION
The
.Nm
utility imprisons a process and all future descendants.
.Pp
The options are as follows:
.Bl -tag -width ".Fl u Ar username"
.It Fl i
Output the jail identifier of the newly created jail.
.It Fl u Ar username
The user name as whom the
.Ar command
should run.
.It Ar path
Directory which is to be the root of the prison.
.It Ar hostname
Hostname of the prison.
.It Ar ip-number
IP number assigned to the prison.
.It Ar command
Pathname of the program which is to be executed.
.El
.Pp
Jails are typically set up using one of two philosophies: either to
constrain a specific application (possibly running with privilege), or
to create a "virtual system image" running a variety of daemons and
services.
In both cases, a fairly complete file system install of FreeBSD is
required, so as to provide the necessary command line tools, daemons,
libraries, application configuration files, etc are available.
However, for a virtual server configuration, a fair amount of
additional work is required so as to configure the "boot" process.
This man page documents the configuration steps necessary to support
either of these steps, althoguh the configuration steps may be
refined based on local requirements.
.Pp
Please see the
.Xr jail 2
man page for further details.
.Sh EXAMPLES
.Ss "Setting up a Jail Directory Tree"
This example shows how to setup a jail directory tree
containing an entire
.Fx
distribution:
.Bd -literal
D=/here/is/the/jail
cd /usr/src
mkdir -p $D
make world DESTDIR=$D
cd etc
make distribution DESTDIR=$D
mount_devfs devfs $D/dev
cd $D
ln -sf dev/null kernel
.Ed
.Pp
NOTE: It is important that only appropriate device nodes in devfs be
exposed to a jail; access to disk devices in the jail may permit processes
in the jail to bypass the jail sandboxing by modifying files outside of
the jail.
See
.Xr devfs 8
for information on how to use devfs rules to limit access to entries
in the per-jail devfs.
.Pp
In many cases this example would put far more stuff in the jail than is needed.
In the other extreme case a jail might contain only one single file:
the executable to be run in the jail.
.Pp
We recommend experimentation and caution that it is a lot easier to
start with a
.Dq fat
jail and remove things until it stops working,
than it is to start with a
.Dq thin
jail and add things until it works.
.Ss "Setting Up a Jail"
Do what was described in
.Sx "Setting Up a Jail Directory Tree"
to build the jail directory tree.
For the sake of this example, we will
assume you built it in
.Pa /data/jail/192.168.11.100 ,
named for the jailed IP address.
Substitute below as needed with your
own directory, IP address, and hostname.
.Ss "Setting up the Host Environment"
First, you will want to set up your real system's environment to be
.Dq jail-friendly .
For consistency, we will refer to the parent box as the
.Dq "host environment" ,
and to the jailed virtual machine as the
.Dq "jail environment" .
Because jail is implemented using IP aliases, one of the first things to do
is to disable IP services on the host system that listen on all local
IP addresses for a service.
If a network service is present in the host environment that binds all
available IP addresses rather than specific IP addresses, it may service
requests sent to jail IP addresses.
This means changing
.Xr inetd 8
to only listen on the
appropriate IP address, and so forth.
Add the following to
.Pa /etc/rc.conf
in the host environment:
.Bd -literal -offset indent
sendmail_enable="NO"
inetd_flags="-wW -a 192.168.11.23"
rpcbind_enable="NO"
.Ed
.Pp
.Li 192.168.11.23
is the native IP address for the host system, in this example.
Daemons that run out of
.Xr inetd 8
can be easily set to use only the specified host IP address.
Other daemons
will need to be manually configured\(emfor some this is possible through
the
.Xr rc.conf 5
flags entries, for others it is necessary to modify per-application
configuration files, or to recompile the application.
The following frequently deployed services must have their individual
configuration files modified to limit the application to listening
to a specific IP address:
.Pp
To configure
.Xr sshd 8 ,
it is necessary to modify
.Pa /etc/ssh/sshd_config .
.Pp
To configure
.Xr sendmail 8 ,
it is necessary to modify
.Pa /etc/mail/sendmail.cf .
.Pp
For
.Xr named 8 ,
it is necessary to modify
.Pa /etc/namedb/named.conf .
.Pp
In addition, a number of services must be recompiled in order to run
them in the host environment.
This includes most applications providing services using
.Xr rpc 3 ,
such as
.Xr rpcbind 8,
.Xr nfsd 8 ,
and
.Xr mountd 8 .
In general, applications for which it is not possible to specify which
IP address to bind should not be run in the host environment unless they
should also service requests sent to jail IP addresses.
Attempting to serve
NFS from the host environment may also cause confusion, and cannot be
easily reconfigured to use only specific IPs, as some NFS services are
hosted directly from the kernel.
Any third party network software running
in the host environment should also be checked and configured so that it
does not bind all IP addresses, which would result in those services also
appearing to be offered by the jail environments.
.Pp
Once
these daemons have been disabled or fixed in the host environment, it is
best to reboot so that all daemons are in a known state, to reduce the
potential for confusion later (such as finding that when you send mail
to a jail, and its sendmail is down, the mail is delivered to the host,
etc.)
.Ss Configuring the Jail
.Pp
Start any jails for the first time without configuring the network
interface so that you can clean it up a little and set up accounts.
As
with any machine (virtual or not) you will need to set a root password, time
zone, etc.
Some of these steps apply only if you intend to run a full virtual server
inside the jail; others apply for both constraining a particular application
or for a virtual server.
.Pp
Start a shell in the jail:
.Pp
.Dl "jail /data/jail/192.168.11.100 testhostname 192.168.11.100 /bin/sh"
.Pp
You will end up with a shell prompt, assuming no errors, within the jail.
You can now run
.Pa /usr/sbin/sysinstall
and do the post-install configuration to set various configuration options,
or perform these actions manually by editing
.Pa /etc/rc.conf ,
etc.
.Pp
.Bl -bullet -offset indent -compact
.It
Create an empty
.Pa /etc/fstab
to quell startup warnings about missing fstab (virtual server only)
.It
Disable the port mapper
.Pa ( /etc/rc.conf :
.Li rpcbind_enable="NO" )
(virtual server only)
.It
Run
.Xr newaliases 1
to quell
.Xr sendmail 8
warnings.
.It
Disable interface configuration to quell startup warnings about
.Xr ifconfig 8
.Pq Li network_interfaces=""
(virtual server only)
.It
Configure
.Pa /etc/resolv.conf
so that name resolution within the jail will work correctly
.It
Set a root password, probably different from the real host system
.It
Set the timezone
.It
Add accounts for users in the jail environment
.It
Install any packages the environment requires
.El
.Pp
You may also want to perform any package-specific configuration (web servers,
SSH servers, etc), patch up
.Pa /etc/syslog.conf
so it logs as you would like, etc.
If you are not using a virtual server, you may wish to modify
.Xr syslogd 8
in the host environment to listen on the syslog socket in the jail
environment; in this example, the syslog socket would be stored in
.Pa /data/jail/192.168.11.100/var/run/log .
.Pp
Exit from the shell, and the jail will be shut down.
.Ss "Starting the Jail"
You are now ready to restart the jail and bring up the environment with
all of its daemons and other programs.
If you are running a single application in the jail, substitute the
command used to start the application for
.Pa /etc/rc
in the examples below.
To start a virtual server environment,
.Pa /etc/rc
is run to launch various daemons and services.
To do this, first bring up the
virtual host interface, and then start the jail's
.Pa /etc/rc
script from within the jail.
.Pp
NOTE: If you plan to allow untrusted users to have root access inside the
jail, you may wish to consider setting the
.Va security.jail.set_hostname_allowed
sysctl variable to 0.
Please see the management discussion later in this document as to why this
is a good idea.
If you do decide to set this variable,
it must be set before starting any jails, and once each boot.
.Bd -literal -offset indent
ifconfig ed0 inet alias 192.168.11.100/32
mount -t procfs proc /data/jail/192.168.11.100/proc
jail /data/jail/192.168.11.100 testhostname 192.168.11.100 \\
/bin/sh /etc/rc
.Ed
.Pp
A few warnings will be produced, because most
.Xr sysctl 8
configuration variables cannot be set from within the jail, as they are
global across all jails and the host environment.
However, it should all
work properly.
You should be able to see
.Xr inetd 8 ,
.Xr syslogd 8 ,
and other processes running within the jail using
.Xr ps 1 ,
with the
.Ql J
flag appearing beside jailed processes.
To see an active list of jails, use the
.Xr jls 8
utility.
You should also be able to
.Xr telnet 1
to the hostname or IP address of the jailed environment, and log
in using the accounts you created previously.
.Ss "Managing the Jail"
Normal machine shutdown commands, such as
.Xr halt 8 ,
.Xr reboot 8 ,
and
.Xr shutdown 8 ,
cannot be used successfully within the jail.
To kill all processes in a
jail, you may log into the jail and, as root, use one of the following
commands, depending on what you want to accomplish:
.Pp
.Bd -literal -offset indent
kill -TERM -1
kill -KILL -1
.Ed
.Pp
This will send the
.Dv SIGTERM
or
.Dv SIGKILL
signals to all processes in the jail from within the jail.
Depending on
the intended use of the jail, you may also want to run
.Pa /etc/rc.shutdown
from within the jail.
To kill processes from outside the jail, use the
.Xr jexec 8
utility in conjuction with the one of the
.Xr kill 1
commands above, or use the
.Xr killall 1
utility with the
.Fl j
option.
.Pp
The
.Pa /proc/ Ns Ar pid Ns Pa /status
file contains, as its last field, the hostname of the jail in which the
process runs, or
.Dq Li -
to indicate that the process is not running within a jail.
The
.Xr ps 1
command also shows a
.Ql J
flag for processes in a jail.
However, the hostname for a jail may be, by
default, modified from within the jail, so the
.Pa /proc
status entry is unreliable by default.
To disable the setting of the hostname
from within a jail, set the
.Va security.jail.set_hostname_allowed
sysctl variable in the host environment to 0, which will affect all jails.
You can have this sysctl set on each boot using
.Xr sysctl.conf 5 .
Just add the following line to
.Pa /etc/sysctl.conf :
.Pp
.Dl security.jail.set_hostname_allowed=0
.Ss "Sysctl MIB Entries"
Certain aspects of the jail containments environment may be modified from
the host environment using
.Xr sysctl 8
MIB variables.
Currently, these variables affect all jails on the system, although in
the future this functionality may be finer grained.
.Bl -tag -width XXX
.It Va security.jail.set_hostname_allowed
This MIB entry determines whether or not processes within a jail are
allowed to change their hostname via
.Xr hostname 1
or
.Xr sethostname 3 .
In the current jail implementation, the ability to set the hostname from
within the jail can impact management tools relying on the accuracy of jail
information in
.Pa /proc .
As such, this should be disabled in environments where privileged access to
jails is given out to untrusted parties.
.It Va security.jail.socket_unixiproute_only
The jail functionality binds an IPv4 address to each jail, and limits
access to other network addresses in the IPv4 space that may be available
in the host environment.
However, jail is not currently able to limit access to other network
protocol stacks that have not had jail functionality added to them.
As such, by default, processes within jails may only access protocols
in the following domains:
.Dv PF_LOCAL , PF_INET ,
and
.Dv PF_ROUTE ,
permitting them access to
.Ux
domain sockets,
IPv4 addresses, and routing sockets.
To enable access to other domains, this MIB variable may be set to
0.
.It Va security.jail.sysvipc_allowed
This MIB entry determines whether or not processes within a jail have access
to System V IPC primitives.
In the current jail implementation, System V primitives share a single
namespace across the host and jail environments, meaning that processes
within a jail would be able to communicate with (and potentially interfere
with) processes outside of the jail, and in other jails.
As such, this functionality is disabled by default, but can be enabled
by setting this MIB entry to 1.
.El
.Pp
There are currently two MIB related variables that have per-jail settings.
Changes to these variables by a jailed process do not effect the host
environment, only the jail environment.
The variables are
.Va kern.securelevel
and
.Va kern.hostname .
.Sh SEE ALSO
.Xr killall 1 ,
.Xr newaliases 1 ,
.Xr ps 1 ,
.Xr chroot 2 ,
.Xr jail 2 ,
.Xr jail_attach 2 ,
.Xr procfs 5 ,
.Xr rc.conf 5 ,
.Xr sysctl.conf 5 ,
.Xr devfs 8 ,
.Xr halt 8 ,
.Xr inetd 8 ,
.Xr jexec 8 ,
.Xr jls 8 ,
.Xr mount_devfs 8 ,
.Xr named 8 ,
.Xr reboot 8 ,
.Xr rpcbind 8 ,
.Xr sendmail 8 ,
.Xr shutdown 8 ,
.Xr sysctl 8 ,
.Xr syslogd 8
.Sh HISTORY
The
.Nm
utility appeared in
.Fx 4.0 .
.Sh AUTHORS
.An -nosplit
The jail feature was written by
.An Poul-Henning Kamp
for R&D Associates
.Pa http://www.rndassociates.com/
who contributed it to
.Fx .
.Pp
.An Robert Watson
wrote the extended documentation, found a few bugs, added
a few new features, and cleaned up the userland jail environment.
.Sh BUGS
Jail currently lacks the ability to allow access to
specific jail information via
.Xr ps 1
as opposed to
.Xr procfs 5 .
Similarly, it might be a good idea to add an
address alias flag such that daemons listening on all IPs
.Pq Dv INADDR_ANY
will not bind on that address, which would facilitate building a safe
host environment such that host daemons do not impose on services offered
from within jails.
Currently, the simplist answer is to minimize services
offered on the host, possibly limiting it to services offered from
.Xr inetd 8
which is easily configurable.
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