`sysctl`¶

The sysctl tool is used to manage kernel runtime parameters at runtime (without rebooting).

It works with parameters located under /proc/sys, and changes made via sysctl are immediate but not persistent unless written to a config file.

Table of Contents¶

Basic Usage
Config Files
- sysctl Priority Order

Basic Usage¶

View all kernel runtime parameters as they're currently set:
```
sysctl -a
```
Filter for a specific kernel setting, e.g., ipv4:
```
sysctl -a | grep -i 'ipv4'  
```
Filter for settings that start with net.ipv4:
```
sysctl net.ipv4             
```
Note that this will not do the same as grep, it will only show settings that start with the given argument.
Filter for a specific setting by name:
```
sysctl net.ipv4.ip_forward
```
This will look in /proc/sys/ for the current state of that setting.
This will follow the path: /proc/sys/net/ipv4/ip_forward
Set a runtime kernel parameter (non-persistent):
```
sysctl net.ipv4.ip_forward=0
```
This change will not persist throughout reboots.
You'll need to add a rule to a config file to persist the change.
Reload kernel runtime parameters without rebooting:
```
sysctl --system
```

Filter kernel parameters by using a regular expression (ERE):
```
sysctl -a -r '^net\.ipv[46]\>'
```
This will show all kernel parameter that match the pattern.
- start with the word net, followed by a ., then match either ipv4 or ipv6.

Config Files¶

To configure kernel runtime parameters to persist throughout reboots, you need to add them to a config file in /etc/sysctl.d/.

Custom settings should go in /etc/sysctl.d/, but there are other locations where settings are stored:

/usr/lib/sysctl.d/: Vendor settings go in here.
/etc/sysctl.d/: This is where you should put your settings.
/run/sysctl.d/

`sysctl` Priority Order¶

When kernel paremeters are loaded in, either on boot or with sysctl --system, it looks for files in this order:

/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf

Once a file has been loaded, any other files with the same name will be ignored. Since files in /etc/sysctl.d are loaded first, this is where we put our settings.

Resources¶

man 8 sysctl
man 5 sysctl.conf

sysctl¶