Sysctl

sysctl is a feature of some Unix-like operating systems that reads and modifies the attributes of the system kernel such as its version number, maximum limits, and security settings. It is available both as a system call for compiled programs, and an administrator command for interactive use and scripting. Linux additionally exposes sysctl as a virtual file system.

In BSD, these parameters are generally objects in a management information base (MIB) that describe tunable limits such as the size of a shared memory segment, the number of threads the operating system will use as an client, or the maximum number of processes on the system; or describe, enable or disable behaviors such as forwarding, security restrictions on the superuser (the "securelevel"), or debugging output.

In BSD, a system call or system call wrapper is usually provided for use by programs, as well as an administrative program and a configuration file (for setting the tunable parameters when the system boots).

This feature appeared in 4.4BSD. It has the advantage over hardcoded constants that changes to the parameters can be made dynamically without recompiling the kernel.

In Linux, the sysctl interface mechanism is also exported as part of procfs under the /proc/sys directory (not to be confused with the /sys directory). This difference means checking the value of some parameter requires opening a file in a virtual file system, reading its contents, parsing them and closing the file. The sysctl system call does exist on Linux, but does not have a wrapping function in glibc and is not recommended for use.

When IP forwarding is enabled, the operating system kernel will act as a router. In FreeBSD, NetBSD, OpenBSD, DragonFly BSD, and Darwin/Mac OS X, the parameter net.inet.ip.forwarding can be set to 1 to enable this behavior. In Linux's emulation of sysctl, the parameter is called net.ipv4.ip_forward.

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