Sysctl Parameters – How to Customize Sysctl Parameters by User

You can use sysctl to set some of the kernel parameters, specifically the ones under /proc/sys. These can be set in the file /etc/sysctl.conf or added to a single file (the preferred method on some distro's such as Fedora) in the directory /etc/sysctl.d. On distros that have this directory it's meant for customization's.

excerpt from sysctl's man page

   sysctl - configure kernel parameters at runtime

Example

You can get a partial list of what kernel parameters are currently set using this command:

$ sudo sysctl -a | head -5
abi.vsyscall32 = 1
debug.exception-trace = 1
debug.kprobes-optimization = 1
dev.cdrom.autoclose = 1
dev.cdrom.autoeject = 0

Making a change

/etc/sysctl.conf

Simply add rules to the file sysctl.conf.

# sysctl.conf sample
#
kernel.domainname = example.com
; this one has a space which will be written to the sysctl!
kernel.modprobe = /sbin/mod probe

You can also use the sysctl.conf command line to make edits to this file without having to edit it directly.

$ sysctl -w kernel.domainname="example.com"

After making any changes be sure to make them active.

$ sysctl -p

/etc/sysctl.d

To add your override of this parameter simply put it in a file named similarly to the files that are already present in the /etc/sysctl.d directory.

$ ls -l /etc/sysctl.d
total 40
-rw-r--r-- 1 root root   77 Jul 16  2012 10-console-messages.conf
-rw-r--r-- 1 root root  490 Jul 16  2012 10-ipv6-privacy.conf
-rw-r--r-- 1 root root  726 Jul 16  2012 10-kernel-hardening.conf
-rw-r--r-- 1 root root 1184 Jul 16  2012 10-magic-sysrq.conf
-rw-r--r-- 1 root root  509 Jul 16  2012 10-network-security.conf
...

In a file named something like 99-myparam.conf.

$ more 10-console-messages.conf 

# the following stops low-level messages on console
kernel.printk = 4 4 1 7

Where the name of the parameter is on the left, and it's corresponding value is on the right.

See sysctl's man page for more details.

Try all and default:

# sysctl -w "net.ipv4.conf.all.rp_filter=0"
# sysctl -w "net.ipv4.conf.default.rp_filter=0"

This is explained in the Kernel documentation:

conf/default/*:    Change the interface-specific default settings.
conf/all/*:        Change all the interface-specific settings.

However note that putting a new value in all doesn't change the value you read from the interface, but it only gets computed and used internally. See this email for more details.