The rancher.sysctl cloud-config key can be used to control sysctl parameters. This works in a manner similar to /etc/sysctl.conf for other Linux distros.
#cloud-config
rancher:
sysctl:
net.ipv4.conf.default.rp_filter: 1
You can either add these settings to your cloud-init.yml, or use sudo ros config merge -i somefile.yml to merge settings into your existing system.