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Values stored in the /proc filesystem can, in most cases, be tuned by echoing values directly to the file. For example:

# echo 1 > /proc/sys/net/ipv4/ip_forward

However, upon rebooting this value will be reset to the default. A clean way to make changes to the /proc filesystem so they are permanent is to set them as variables in /etc/sysctl.conf.

Variable names in this file are derived from their proc filename. So to determine a variable's name, take the path of the proc file, drop the leading /proc/sys/ and then replace the remain /'s with .'s. Take for example the above:

# echo 1 > /proc/sys/net/ipv4/ip_forward

in /etc/sysctl.conf, this would look like:

net.ipv4.ip_forward = 1

Changes to sysctl.conf take effect upon booting. If you want changes to take effect without rebooting, use the following:

# sysctl -p

See man sysctl for more details.