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Setting Up a Jail Directory Tree

This example shows how to setup a jail directory tree containing an entire DragonFly distribution, provided that you built world before already:

D=/here/is/the/jail
cd /usr/src
mkdir -p $D
make installworld DESTDIR=$D
cd etc
make distribution DESTDIR=$D
cd $D
ln -sf dev/null boot/kernel

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.

We recommend experimentation and caution that it is a lot easier to start with a “fat” jail and remove things until it stops working, than it is to start with a “thin” jail and add things until it works.

Setting Up a Jail

Do what was described in 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 /data/jail/192.168.11.100, named for the external jail IP address. Substitute below as needed with your own directory, IP addresses, and hostname.

First, you will want to set up your real system's environment to be “jail-friendly”. For consistency, we will refer to the parent box as the “host environment”, and to the jailed virtual machine as the “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. This means changing inetd(8) to only listen on the appropriate IP address, and so forth. Add the following to /etc/rc.conf in the host environment:

sendmail_enable="NO"
inetd_flags="-wW -a 192.168.11.23"
rpcbind_enable="NO"

192.168.11.23 is the native IP address for the host system, in this example. Daemons that run out of inetd(8) can be easily set to use only the specified host IP address. Other daemons will need to be manually configured—for some this is possible through the rc.conf(5) flags entries, for others it is not possible without munging the per-application configuration files, or even recompiling. For those applications that cannot specify the IP they run on, it is better to disable them, if possible.

A number of daemons ship with the base system that may have problems when run from outside of a jail in a jail-centric environment. This includes sendmail(8), named(8), and rpcbind(8). While sendmail(8) and named(8) can be configured to listen only on a specific IP using their configuration files, in most cases it is easier to simply run the daemons in jails only, and not in the host environment. 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.

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.)

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.

Now start the jail:

You will end up with a shell prompt, assuming no errors, within the jail. You can now do the post-install configuration to set various configuration options by editing /etc/rc.conf, etc.

• Disable the port mapper (/etc/rc.conf: rpcbind_enable="NO")
• Run newaliases(1) to quell sendmail(8) warnings.
• Disable interface configuration to quell startup warnings about ifconfig(8) (network_interfaces="")
• Configure /etc/resolv.conf so that name resolution within the jail will work correctly
• Set a root password, probably different from the real host system
• Set the timezone with tzsetup(8)
• Add accounts for users in the jail environment
• Install any packages that you think the environment requires

You may also want to perform any package-specific configuration (web servers, SSH servers, etc), patch up /etc/syslog.conf so it logs as you would like, etc.

Exit from the shell, and the jail will be shut down.

Starting the Jail

You are now ready to restart the jail and bring up the environment with all of its daemons and other programs. To do this, first bring up the virtual host interface, and then start the jail's /etc/rc script from within the jail.

NOTE: If you plan to allow untrusted users to have root access inside the jail, you may wish to consider setting the jail.set_hostname_allowed to 0. Please see the management reasons 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.

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 127.0.0.1,192.168.11.100 \
	/bin/sh /etc/rc

A few warnings will be produced, because most 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 inetd(8), syslogd(8), and other processes running within the jail using ps(1), with the ‘J’ flag appearing beside jailed processes. You should also be able to telnet(1) to the hostname or IP address of the jailed environment, and log in using the accounts you created previously.

Managing the Jail

Normal machine shutdown commands, such as halt(8), reboot(8), and 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:

kill -TERM -1
kill -KILL -1

This will send the SIGTERM or 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 /etc/rc.shutdown from within the jail. Currently there is no way to insert new processes into a jail, so you must first log into the jail before performing these actions.

To kill processes from outside the jail, you must individually identify the PID of each process to be killed. The /proc/pid/status file contains, as its last field, the hostname of the jail in which the process runs, or “-” to indicate that the process is not running within a jail. The ps(1) command also shows a ‘J’ flag for processes in a jail. However, the hostname for a jail may be, by default, modified from within the jail, so the /proc status entry is unreliable by default. To disable the setting of the hostname from within a jail, set the 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 sysctl.conf(5). Just add the following line to /etc/sysctl.conf:

In a future version of DragonFly, the mechanisms for managing jails may be more refined.

Sysctl MIB Entries

Certain aspects of the jail containments environment may be modified from the host environment using sysctl(8) MIB variables. Currently, these variables affect all jails on the system, although in the future this functionality may be finer grained.

jail.set_hostname_allowed

This MIB entry determines whether or not processes within a jail are allowed to change their hostname via hostname(1) or 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 /proc. As such, this should be disabled in environments where privileged access to jails is given out to untrusted parties.

jail.socket_unixiproute_only

The jail functionality binds IPv4 and IPv6 addresses to each jail, and limits access to other network addresses in the IPv4 and IPv6 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: PF_LOCAL, PF_INET, PF_INET6, and PF_ROUTE, permitting them access to UNIX domain sockets, IPv4 and IPv6 addresses, and routing sockets. To enable access to other domains, this MIB variable may be set to 0.

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.