====== Systems - Media Server - Secure the Server - Harden Linux kernel configuration parameters ======

The Linux kernel is flexible, and the way it works can be modified on the fly by dynamically changing some of its parameters using the **sysctl** command.

  * **sysctl** allows the viewing and changing of kernel settings on a running system.
    * The parameters available are those listed under /proc/sys/.
  * Changes take effect immediately.
  * The related **/etc/sysctl.conf** file is used to ensure that the settings persist after a reboot.

<WRAP alert>
**IMPORTANT NOTE:**  Editing the sysctl.conf file might break the system - this is for advanced users only.
</WRAP>

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===== Make a backup of the existing /etc/sysctl.conf file =====

<code bash>
sudo cp /etc/sysctl.conf /etc/sysctl.conf.orig
</code>

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===== Modify the sysctl file =====

Add the following entries to the bottom of the **/etc/sysctl.conf** file to stop some spoofing attacks and enhance other security measures:

<file bash /etc/sysctl.conf>
...
...
# Network Security
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.log_martians = 1
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_max_syn_backlog = 2048
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 5

# IPv6 Security (if enabled)
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0

# Process Security
kernel.randomize_va_space = 2
kernel.kptr_restrict = 2
kernel.dmesg_restrict = 1
kernel.perf_event_paranoid = 3
kernel.yama.ptrace_scope = 2
kernel.panic_on_oops = 1
kernel.panic = 60
kernel.sysrq = 0


# File System Security
fs.protected_hardlinks = 1
fs.protected_symlinks = 1
fs.suid_dumpable = 0
fs.protected_fifos = 2
fs.protected_regular = 2

# Additional Security Measures
#dev.tty.ldisc_autoload = 0
#kernel.modules_disabled = 1
kernel.core_uses_pid = 1
kernel.panic_on_unrecovered_nmi = 1
kernel.panic_on_io_nmi = 1
kernel.unprivileged_bpf_disabled = 1
net.core.bpf_jit_harden = 2
</file>

Save the /etc/sysctl.conf file.

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===== Activate the kernel settings that have been modified =====

This reloads the sysctl parameters:

<code bash>
sudo sysctl -p
</code>

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