/etc/sysctl.conf vs /etc/sysctl.d/99-sysctl.conf
The Wiki says to make sure that "net.ipv4.ip_forward=1" is in "/etc/sysctl.conf" however I could not get it to be persistent after a reboot unless I put "net.ipv4.ip_forward=1" in "/etc/sysctl.d/99-sysctl.conf". Not sure how valid this is but it worked for me. :) I am using ArchLinuxARM (ALARM) on my ZyXel NSA325.
Type the below in an ssh session with root login. echo "net.ipv4.ip_forward=1" >> /etc/sysctl.d/99-sysctl.conf
I found that after I came across this article.