Each jail in FreeBSD needs its own IP to be run. This presents a problem when jails need to be run on a host which does DHCP i.e. a laptop.

Fortunately the solution is not that difficult. In this example we assume we do dhcp on rl0, use pf as firewall and use the 10.1.1.0/24 range for the jails. Basic knowledge of jails and pf is assumed. The following steps have to performed:

• ifconfig lo1 create
• ifconfig lo1 inet 10.1.1.1 netmask 255.255.255.0
• install the jail as you usually do.
• add the line: nat on rl0 from 10.1.1.0/24 to any -> (rl0)
  to your pf.conf and load the new rule set. This allows the jail to make connections to the outside world. To reach the jail from the outside you have to use rdr rules to forward ports.
• Start the jail with the 10.1.1.1 address.

After all this work we want to make sure the configuration is done automatically at boot time. This can be done by doing the following:

• list all network interfaces you want to use in network_interfaces= in /etc/rc.conf. This must include lo1. In my case this means network_interfaces="lo0 lo1 rl0 wi0"
• Create the file /etc/start_if.lo1. This file should contain: ifconfig lo1 create
• Add ifconfig_lo1="inet 10.1.1.1 netmask 255.255.255.0" to /etc/rc.conf
• Add the options to start the jail to /etc/rc.conf