linuxexpert-practice-labs/labs/lfcs/1-1-kernel-parameters/README.md

LFCS 1.1: Kernel parameters

Changing kernel parameters now and for good, a per-interface IPv6 setting, a boot parameter, and module options.

Before you start

The lab turns IPv4 forwarding off and makes sure vm.swappiness isn't 10. It adds two linked network interfaces, lab0 and lab1, which come back at every boot; NetworkManager and networkd leave them alone. The dummy module is unloaded. Reboot when a task says so; the check looks at the running system.

Tasks

1. Turn on IPv4 forwarding until the next reboot only. Check it yourself: sysctl net.ipv4.ip_forward shows 1 now and 0 after a reboot. Do this one before task 2.
2. Make IPv4 forwarding permanent, and set vm.swappiness to 10, in a file named /etc/sysctl.d/90-lab.conf, without rebooting.
3. Disable IPv6 on interface lab0 only, persistently. After a reboot, lab0 has no IPv6 addresses while lab1 and the other interfaces keep theirs.
4. Add the boot parameter audit=1 to every installed kernel, then reboot.
5. Make the dummy module load at every boot with two devices, then reboot. ip -br link | grep -c dummy should print 2.

Hints

• sysctl -w, /proc/sys.
• A .conf file in /etc/sysctl.d/, then sysctl --system.
• Per-interface keys look like net.ipv6.conf.NAME.disable_ipv6.
• grubby on RHEL relatives; /etc/default/grub and update-grub on Ubuntu. /proc/cmdline after the reboot.
• modinfo -p dummy, /etc/modprobe.d/, /etc/modules-load.d/. Files in every modprobe.d directory are read in name order and the last setting wins; modprobe --show-depends dummy shows what modprobe will pass.