JAIL(2) MidnightBSD System Calls Manual JAIL(2)


jail, jail_attach — imprison current process and future descendants


Standard C Library (libc, −lc)


#include <sys/param.h>
#include <sys/jail.h>


jail(struct jail *jail);


jail_attach(int jid);


The jail() system call sets up a jail and locks the current process in it.

The argument is a pointer to a structure describing the prison:

struct jail {







‘‘version’’ defines the version of the API in use. It should be set to zero at this time.

The ‘‘path’’ pointer should be set to the directory which is to be the root of the prison.

The ‘‘hostname’’ pointer can be set to the hostname of the prison. This can be changed from the inside of the prison.

The ‘‘ip_number’’ can be set to the IP number assigned to the prison.

The jail_attach() system call attaches the current process to an existing jail, identified by jid.


If successful, jail() returns a non-negative integer, termed the jail identifier (JID). It returns −1 on failure, and sets errno to indicate the error.

The jail_attach() function returns the value 0 if successful; otherwise the value −1 is returned and the global variable errno is set to indicate the error.


Once a process has been put in a prison, it and its descendants cannot escape the prison.

Inside the prison, the concept of ‘‘superuser’’ is very diluted. In general, it can be assumed that nothing can be mangled from inside a prison which does not exist entirely inside that prison. For instance the directory tree below ‘‘path’’ can be manipulated all the ways a root can normally do it, including ‘‘rm -rf /*’’ but new device special nodes cannot be created because they reference shared resources (the device drivers in the kernel). The effective ‘‘securelevel’’ for a process is the greater of the global ‘‘securelevel’’ or, if present, the per-jail ‘‘securelevel’’.

All IP activity will be forced to happen to/from the IP number specified, which should be an alias on one of the network interfaces.

It is possible to identify a process as jailed by examining ‘‘/proc/<pid>/status’’: it will show a field near the end of the line, either as a single hyphen for a process at large, or the hostname currently set for the prison for jailed processes.


The jail() system call will fail if:


The version number of the argument is not correct.

Further jail() calls chroot(2) internally, so it can fail for all the same reasons. Please consult the chroot(2) manual page for details.


chdir(2), chroot(2)


The jail() system call appeared in FreeBSD 4.0. The jail_attach() system call appeared in FreeBSD 5.1.


The jail feature was written by Poul-Henning Kamp for R&D Associates ‘‘http://www.rndassociates.com/’’ who contributed it to FreeBSD.

MidnightBSD 0.3 April 8, 2003 MidnightBSD 0.3