trying to change settings such as:
net.ipv4.tcp_max_syn_backlog=2048 by net.ipv4.tcp_max_syn_backlog=256
net.ipv4.conf.all.accept_redirects=1 by net.ipv4.conf.all.accept_redirects=0
among others to try to prevent DDoS attacks, after reboot once again have the default values.
How I can avoid this?
You seem to think that we are very well known to you, your systems and what you are doing. This is not the case. That means that you have to tell us as much as possible and not such a minimal description.
Which openSUSE level?
You “try to change settings”. That is very vague. Tell us exactly what you change where and how. How can we e.g. tell you you did it in the wrong place when we do not know where you did it?
To change values on the fly:
# /sbin/sysctl -w net.ipv4.tcp_max_syn_backlog=256
and, to make it permanent, edit /etc/sysctl.conf
thanks this was what I needed