sysctl not sticking after reboot

If your system is using systemd's systemd-sysctl.service and not sysctl for the setting at boot time then things are a little different than sysctl.

systemd-sysctl sorts all of the configuration file names, ignoring the directory name, and then loads them in that order irrespective of the directory they were in. This means that if you put your setting in /etc/sysctl.d/10-mysysctl.conf and there is another file called /usr/lib/sysctl.d/50-default.conf setting the same variable, that will override your setting.

To ensure your setting isn't overridden you should put it in a config file with a name like /etc/sysctl.d/zzz-mysysctl.conf.

Note: This behaviour was tested on Ubuntu 20.04.1 and openSUSE Leap 15.2

Create a file /etc/sysctl.d/local.conf containing the values you want.

fs.suid_dumpable=0
net.ipv4.conf.all.log_martians=1 
net.ipv4.conf.all.rp_filter=1     
net.ipv4.conf.default.log_martians=1  

Reboot.

The scripts load any *.conf files from several directories, in file name collation order. See the sysctl --system option in the man page for the search paths.

In particular, check that any values are not already defined in /etc/sysctl.conf. Consider moving values you want to keep from it to sysctl.d, then removing that file.