6.7 来了！
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Add hierarchical jails. A jail may further virtualize its environment by creating a child jail, which is visible to that jail and to any parent jails. Child jails may be restricted more than their parents, but never less. Jail names reflect this hierarchy, being MIB-style dot-separated strings.
http://svn.freebsd.org/viewvc/base?view=revision&revision=192895
