$ Linux Command

System

sysctl

Configure kernel parameters at runtime.

kernelparameterssysctlsystemconfiguration

Additional Notes

sysctl reads and modifies kernel parameters at runtime. It interfaces with the /proc/sys/ virtual filesystem, where each file corresponds to a kernel parameter. Parameters control a wide range of kernel behavior: networking (IP forwarding, buffer sizes), virtual memory (swappiness, dirty page ratios), security (kernel pointers, core dumps), filesystems, and more.

Changes made with sysctl are immediate but do not persist across reboots unless written to /etc/sysctl.conf or a file under /etc/sysctl.d/. The sysctl -p command loads settings from configuration files. The kernel parameter namespace uses a dotted notation corresponding to the filesystem path (e.g., net.ipv4.ip_forward maps to /proc/sys/net/ipv4/ip_forward).

Syntax

sysctl [options] [parameter[=value] ...]

Parameters

  • parameter: The name of a kernel parameter to read (e.g., net.ipv4.ip_forward).
  • parameter=value: Set a kernel parameter to the specified value.

Common Options

  • -a, --all: Display all currently available kernel parameters.
  • -A: Display all parameters in a table format (like -a but with more detail).
  • -n, --values: Print only the values, not the parameter names.
  • -e, --ignore: Ignore errors about unknown parameters.
  • -N, --names: Print only the parameter names, not the values.
  • -q, --quiet: Suppress output when setting parameters.
  • -w, --write: Enable writing a value to a parameter.
  • -p file, --load=file: Load settings from a file (default: /etc/sysctl.conf).
  • -r, --pattern pattern: Apply operations only to parameters matching the regular expression.
  • --system: Load settings from all system sysctl configuration files.
  • -b, --binary: Print values in binary form (for parameters with binary data).

Examples

sysctl -a

Display all kernel parameters and their current values.

sysctl net.ipv4.ip_forward

Check whether IP forwarding is enabled.

sysctl -w net.ipv4.ip_forward=1

Enable IP forwarding (routing) immediately.

sysctl -w vm.swappiness=10

Reduce the kernel's tendency to swap (lower = less swapping).

sysctl -p

Load settings from /etc/sysctl.conf.

sysctl --system

Load all sysctl configuration files from /etc/sysctl.d/, /run/sysctl.d/, /usr/lib/sysctl.d/.

sysctl -n kernel.hostname

Show only the hostname value (not the parameter name).

sysctl -w fs.file-max=100000

Increase the maximum number of open file descriptors system-wide.

Common Parameters

  • net.ipv4.ip_forward: Enable IP forwarding (for routers/VPNs).
  • net.ipv4.conf.all.rp_filter: Enable reverse path filtering.
  • net.ipv4.tcp_syncookies: Enable SYN cookies (DoS protection).
  • vm.swappiness: Tendency to swap (0-100, default 60).
  • vm.dirty_ratio: Percentage of memory that can be dirty before blocking.
  • vm.dirty_background_ratio: Percentage of dirty memory before background writeback.
  • kernel.hostname: System hostname.
  • kernel.core_uses_pid: Append PID to core dump filenames.
  • fs.file-max: Maximum number of open file handles system-wide.
  • net.core.somaxconn: Maximum listen backlog.
  • net.ipv4.tcp_keepalive_time: TCP keepalive interval.

Practical Notes

  • sysctl -a output can be very long; pipe to less or grep for filtering.
  • Persistent settings go in /etc/sysctl.conf or /etc/sysctl.d/*.conf.
  • The --system flag loads files in alphanumeric order from multiple directories.
  • Boolean parameters use 0 (false/off) and 1 (true/on).
  • Changing some parameters (like network buffers) can significantly impact performance.
  • Some kernel parameters are read-only and cannot be modified at runtime.
  • Rule of thumb: set vm.swappiness=10 for desktops/servers to reduce unnecessary swapping.
  • In containers, some sysctl parameters may be restricted for security reasons.