3.3.8 Ensure TCP SYN Cookies is enabled

Audit

Run the following script to verify net.ipv4.tcp_syncookies = 1:

#!/usr/bin/env bash
{
 l_output="" l_output2=""
 l_parlist="net.ipv4.tcp_syncookies=1"
 l_searchloc="/run/sysctl.d/*.conf /etc/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf /etc/sysctl.conf $([ -f /etc/default/ufw ] && awk -F= '/^\s*IPT_SYSCTL=/{print $2}' /etc/default/ufw)"
 KPC()
 {
 l_krp="$(sysctl "$l_kpname" | awk -F= '{print $2}' | xargs)"
 l_pafile="$(grep -Psl -- "^\h*$l_kpname\h*=\h*$l_kpvalue\b\h*(#.*)?$" $l_searchloc)"
 l_fafile="$(grep -s -- "^\s*$l_kpname" $l_searchloc | grep -Pv -- "\h*=\h*$l_kpvalue\b\h*" | awk -F: '{print $1}')"
 if [ "$l_krp" = "$l_kpvalue" ]; then
 l_output="$l_output\n - \"$l_kpname\" is set to \"$l_kpvalue\" in the running configuration"
 else
 l_output2="$l_output2\n - \"$l_kpname\" is set to \"$l_krp\" in the running configuration"
 fi
 if [ -n "$l_pafile" ]; then
 l_output="$l_output\n - \"$l_kpname\" is set to \"$l_kpvalue\" in \"$l_pafile\""
 else
 l_output2="$l_output2\n - \"$l_kpname = $l_kpvalue\" is not set in a kernel parameter configuration file"
 fi
 [ -n "$l_fafile" ] && l_output2="$l_output2\n - \"$l_kpname\" is set incorrectly in \"$l_fafile\""
 }
 for l_kpe in $l_parlist; do
 l_kpname="$(awk -F= '{print $1}' <<< "$l_kpe")"
 l_kpvalue="$(awk -F= '{print $2}' <<< "$l_kpe")"
 KPC
 done
 if [ -z "$l_output2" ]; then
 echo -e "\n- Audit Result:\n ** PASS **\n$l_output\n"
 else
 echo -e "\n- Audit Result:\n ** FAIL **\n - Reason(s) for audit failure:\n$l_output2\n"
 [ -n "$l_output" ] && echo -e "\n- Correctly set:\n$l_output\n"
 fi
}

Remediation

Run the following script to set net.ipv4.tcp_syncookies = 1:

#!/usr/bin/env bash
{
 l_output="" l_output2=""
 l_parlist="net.ipv4.tcp_syncookies=1"
 l_searchloc="/run/sysctl.d/*.conf /etc/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf /etc/sysctl.conf $([ -f /etc/default/ufw ] && awk -F= '/^\s*IPT_SYSCTL=/{print $2}' /etc/default/ufw)"
 l_kpfile="/etc/sysctl.d/60-netipv4_sysctl.conf"
 KPF()
 {
 # comment out incorrect parameter(s) in kernel parameter file(s)
 l_fafile="$(grep -s -- "^\s*$l_kpname" $l_searchloc | grep -Pv -- "\h*=\h*$l_kpvalue\b\h*" | awk -F: '{print $1}')"
 for l_bkpf in $l_fafile; do
 echo -e "\n - Commenting out \"$l_kpname\" in \"$l_bkpf\""
 sed -ri "/$l_kpname/s/^/# /" "$l_bkpf"
 done
 # Set correct parameter in a kernel parameter file
 if ! grep -Pslq -- "^\h*$l_kpname\h*=\h*$l_kpvalue\b\h*(#.*)?$" $l_searchloc; then
 echo -e "\n - Setting \"$l_kpname\" to \"$l_kpvalue\" in \"$l_kpfile\""
 echo "$l_kpname = $l_kpvalue" >> "$l_kpfile"
 fi
 # Set correct parameter in active kernel parameters
 l_krp="$(sysctl "$l_kpname" | awk -F= '{print $2}' | xargs)"
 if [ "$l_krp" != "$l_kpvalue" ]; then
 echo -e "\n - Updating \"$l_kpname\" to \"$l_kpvalue\" in the active kernel parameters"
 sysctl -w "$l_kpname=$l_kpvalue"
 sysctl -w "$(awk -F'.' '{print $1"."$2".route.flush=1"}' <<< "$l_kpname")"
 fi
 }
 for l_kpe in $l_parlist; do
 l_kpname="$(awk -F= '{print $1}' <<< "$l_kpe")"
 l_kpvalue="$(awk -F= '{print $2}' <<< "$l_kpe")"
 KPF
 done
}

Default Value:

net.ipv4.tcp_syncookies = 1