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10.9 Setuid Programs

Some programs need to run with “root” privileges, even when they are launched by unprivileged users. A notorious example is the passwd program, which users can run to change their password, and which needs to access the /etc/passwd and /etc/shadow files—something normally restricted to root, for obvious security reasons. To address that, these executables are setuid-root, meaning that they always run with root privileges (see How Change Persona in The GNU C Library Reference Manual, for more info about the setuid mechanism).

The store itself cannot contain setuid programs: that would be a security issue since any user on the system can write derivations that populate the store (see The Store). Thus, a different mechanism is used: instead of changing the setuid bit directly on files that are in the store, we let the system administrator declare which programs should be setuid root.

The setuid-programs field of an operating-system declaration contains a list of G-expressions denoting the names of programs to be setuid-root (see Using the Configuration System). For instance, the passwd program, which is part of the Shadow package, can be designated by this G-expression (see G-Expressions):

#~(string-append #$shadow "/bin/passwd")

A default set of setuid programs is defined by the %setuid-programs variable of the (gnu system) module.

Scheme Variable: %setuid-programs

A list of G-expressions denoting common programs that are setuid-root.

The list includes commands such as passwd, ping, su, and sudo.

Under the hood, the actual setuid programs are created in the /run/setuid-programs directory at system activation time. The files in this directory refer to the “real” binaries, which are in the store.

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