sysctl reload without reboot [100% Working]

Perform sysctl reload - Introduction

• A sysctl variable represents file locations under the /proc/sys directory.
• Everytime you make changes to your sysctl configuration file, you must reload it to activate the changes.
• Now sysctl is not controlled by some daemon such as systemd, supervisord, monit etc wherein you restart the service and then expect that your sysctl configuration changes have been activated
• We have to explicitly reload the sysctl changes. But then there are also times when some of the sysctl changes are activated during boot up stage, such as reserving hugepages, memory etc which can not be done runtime and requires a reboot

In this tutorial I will cover different Dos and Don'ts covering the following areas:

• multiple ways to update sysctl configuration file
• performing sysctl reload without reboot
• reverting sysctl changes to default value

Pre-requisites

Since sysctl modified the kernel level parameters, so it is important that you have root level privilege on the server when you intend to modify and perform sysctl reload. Alternatively you can also use sudo privilege to execute the sysctl commands or else you will get "Permission denied" error message.

List of sysctl configuration files

You can add your custom sysctl changes at different locations in different files. In most Linux distributions, following files can be used to modify your sysctl configurations:

/etc/sysctl.conf
/etc/sysctld.d/*.conf
/usr/lib/sysctl.d/*.conf

There may be some more files whoose availability is based on individual distributions:

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

Vendors settings live in /usr/lib/sysctl.d/. To override a whole file, create a new file with the same in /etc/sysctl.d/ and put new settings there. To override only specific settings, add a file with a lexically later name in /etc/sysctl.d/ and put new settings there.

For example, to override the changes from /usr/lib/sysctl.d/80-broadcom.conf, you can create /etc/sysctld.d/80-broadcom.conf with different setting and the values from /etc/sysctld.d/80-broadcom.conf will override /usr/lib/sysctl.d/80-broadcom.conf.

Get the existing value of sysctl parameters

Method-1: Using sysctl --all

You can use sysctl with -a or --all argument to list all the sysctl parameters with the applied value:

sysctl -a

OR

sysctl --all

This will print a long list of parameters, so you can combine this command with grep to only capture the required parameter. For example:

 ~]# sysctl -a | grep somaxconn
net.core.somaxconn = 128

Method-2: Using sysctl [parameter]

You can also provide the parameter name along with the sysctl command to print it's value. For example:

~]# sysctl net.core.somaxconn
net.core.somaxconn = 128

Now this can be challenging as you must know the exact name of the sysctl parameter to be able to get the value. But this method is preferred in scripts when we are trying to automate the process.

Method-3: Get the value from /proc/sys

Since these sysctl values are ultimately applied to /proc/sys, so we can also query the respective location to fetch the value. For example the path of net.core.somaxconn would be /proc/sys/net/core/somaxconn:

~]# cat /proc/sys/net/core/somaxconn
128

How to properly edit or update sysctl values

Scenario-1: Make non-persistent (temporary) changes using sysctl -w

You may wish to make temporary changes which are non-persistent i.e. it will not survive the reboot. In such case we can utilize -w or --write argument of sysctl command.

Use the following syntax to temporarily update the value of any sysctl parameter:

sysctl -w <parameter>=<value>

For example, we update the value of net.core.somaxconn to 240 from it's default value of 128:

~]# sysctl -w net.core.somaxconn=240
net.core.somaxconn = 240

~]# sysctl -a | grep somaxconn
net.core.somaxconn = 240

Sample Output:

Scenario-2: Make non-persistent changes by modifying /proc/sys

We know that making any changes to sysctl values are defined under /proc/sys path at the respective parameter section. So we can also directly modify the values of these sysctl parameters, directly inside /proc/sys path:

~]# echo 250 > /proc/sys/net/core/somaxconn

~]# sysctl -a | grep somaxconn
net.core.somaxconn = 250

As you can see, we have updated the value of net.core.somaxconn from 240 to 250 by directly adding the new value inside /proc/sys/net/core/somaxconn

Scenario-3: Make persistent changes using /etc/sysctl.d/*.conf

You can add your custom sysctl configuration inside /etc/sysctl.d/XXXX.conf file as shown below:

~]# echo "net.core.somaxconn=1024" >> /etc/sysctl.d/99-sysctl.conf

To update the changes use sysctl -p or --load along with the file name with the newly added sysctl changes:

~]# sysctl -p /etc/sysctl.d/99-sysctl.conf
net.core.somaxconn = 1024

Verify the changes

~]# sysctl -a | grep somaxconn
net.core.somaxconn = 1024

Scenario-4: Assign/Revert to default values of sysctl parameters

If you wish to revert any sysctl parameter to it's default value then you can manually add the same inside /etc/sysctl.d/XXX.conf file. But in such case, you should actually know the default value of the respective parameter.

In most cases we may not remember the default value, so in such scenario you must delete all the entries of the respective sysctl parameter from any configuration file. You can search for the parameter using following command:

find PATH -type f -name "*.conf" -exec grep -ri PARAMETER {} +

Here replace PATH and PARAMETER as per your requirement.

For example suppose we want to find any sysctl config file where we have modified somaxconn value, so we can use:

~]# find /etc/ /usr/lib/sysctl.d/* -type f -name "*.conf" -exec grep -i net.core.somaxconn {} +
/etc/sysctl.conf:net.core.somaxconn=1024

Now we need to delete these entries found in the above command and reboot the node. Just by deleting the entries and executing sysctl -p will not revert the parameter's value to default.

For example, I have deleted the above entry from /etc/sysctl.d/99-sysctl.conf and refreshed the sysctl values:

~]# sysctl --system

~]# sysctl -a | grep somax
net.core.somaxconn = 1024

But still the system shows net.core.somaxconn value as 1024. Now let's reboot the node and re-verify:

~]# reboot
login as: root
root@127.0.0.1's password:

[root@controller ~]# sysctl -a | grep somax
net.core.somaxconn = 128

So, after the reboot now as you can see the value of net.core.somaxconn has been reverted back to it's default value i.e. 128.

How to perform sysctl reload without reboot

This is the most crucial part of this article. You have to understand there are many such sysctl parameters "SHOULD NOT" be reloaded runtime. For example some of them are:

vm.nr_hugepages
vm.nr_overcommit_hugepages
kernel.shmall
kernel.shmmax
kernel.shmmni

Now you can definitely modify the values of these parameters runtime, but they deal with the system memory. So if we want to increase or decrease the amount of hugepages or memory reserved by the kernel then the same should be done at boot up stage when no other process would be using the memory.

As runtime, it is possible one or the other process may be using certain block of memory and if we try to modify these sysctl parameters runtime then it can cause adverse effects to the running processes.

Having said that, most of the other sysctl parameters can be updated and modified runtime. So to properly perform sysctl reload use

sysctl -p <path/of/conf/file>

OR

sysctl --system

If you execute sysctl -p without the file path, then it will only update the changes from /etc/sysctl.conf or /etc/sysctl.d/99-sysctl.conf which is actually a symbolic link of /etc/sysctl.conf:

~]# ls -l /etc/sysctl.d/99-sysctl.conf
lrwxrwxrwx. 1 root root 14 Feb 24  2020 /etc/sysctl.d/99-sysctl.conf -> ../sysctl.conf

To perform sysctl reload immediately for all the available configuration files, execute:

sysctl --system

The sysctl values are loaded early in boot via initramfs, so finally, rebuild initramfs to override any previous persistent sysctl settings in the initial ramdisk image.

On RHEL, CentOS, SuSE, Fedora, Rocky Linux execute:

dracut -v -f

On Ubuntu, Debian execute:

update-initramfs -u

Summary

In this tutorial we covered different areas related to sysctl configuration. Some of the topics which we learned are:

• Different sysctl configuration files available in Linux
• How to make temporary non-persistent changes to sysctl values
• How to make permanent persistent changes to sysctl values
• How to properly perform sysctl reload without reboot

What's Next

How to suppress sysctl: reading key "net.ipv6.conf.all.stable_secret" in Linux
Linux disable IPv6 properly (with or without reboot)

Further Reading

man page for sysctl(8)
man page for sysctl(5)