The Linux kernel provides some basic protections against manipulated IP packets. A configuration could be:

echo 1 > /proc/sys/net/ipv4/tcp_syncookies                          # enable syn cookies (prevent against the common 'syn flood attack')
echo 0 > /proc/sys/net/ipv4/ip_forward                              # disable Packet forwarning between interfaces
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts             # ignore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicast
echo 1 > /proc/sys/net/ipv4/conf/all/log_martians                   # log packets with impossible addresses to kernel log
echo 1 > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses       # disable logging of bogus responses to broadcast frames
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter                      # do source validation by reversed path (Recommended option for single homed hosts)
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects                 # don't send redirects
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route            # don't accept packets with SRR option

Further reading:

• The file Documentation/networking/ip-sysctl.txt in your kernel source directory
• Network Security with /proc/sys/net/ipv4

firewall.sh

#!/bin/sh
# set -e
 
### BEGIN INIT INFO
# Provides:          firewall
# Required-Start:    $network $syslog
# Required-Stop:     $network $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start iptables based firewall
# Description:       Start IPv4 and IPv6 firewall
### END INIT INFO
 
# Version:      @(#)firewall  1.0.1  2006-01-22  tehpeh-web@tty1.net  First version IPv4 only
#                             1.2.0  2011-05-28  tehpeh-web@tty1.net  Added IPv6 support
#
 
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
NAME=firewall
 
 
 
iptables4=/sbin/iptables
iptables6=/sbin/ip6tables
pub_if=eth0                                 # public interface, e.g. eth0
pub_ipv4=70.85.16.194                       # public IPv4 address
pub_ipv6=2600:3c00::f03c:91ff:fe96:d898     # public IPv6 address
 
 
test -x $iptables4 || exit 0
test -x $iptables6 || exit 0
 
firewall_flush()
{
    # flush rules
    $iptables4 -F
    $iptables6 -F
    $iptables4 -F -t mangle
    $iptables6 -F -t mangle
    $iptables4 -X -t mangle
    $iptables6 -X -t mangle
    $iptables4 -F -t nat
    $iptables4 -X -t nat
    $iptables4 -X
    $iptables6 -X
}
 
 
firewall_default()
{
    default=$1
 
    # apply default rules
    $iptables4 -P INPUT   $default
    $iptables6 -P INPUT   $default
    $iptables4 -P FORWARD $default
    $iptables6 -P FORWARD $default
    $iptables4 -P OUTPUT  $default
    $iptables6 -P OUTPUT  $default
}
 
 
firewall_start()
{
    # default policy
    firewall_default DROP
 
    # accept everything from loopback
    $iptables4 -A INPUT  -i lo -j ACCEPT
    $iptables6 -A INPUT  -i lo -j ACCEPT
    $iptables4 -A OUTPUT -o lo -j ACCEPT
    $iptables6 -A OUTPUT -o lo -j ACCEPT
 
    # Allow Link-Local addresses
    $iptables6 -A INPUT -s fe80::/10 -j ACCEPT
    $iptables6 -A OUTPUT -s fe80::/10 -j ACCEPT
 
    # drop Bad Guys
    $iptables4 -A INPUT -m recent --update --seconds 60 -j DROP
    $iptables6 -A INPUT -m recent --update --seconds 60 -j DROP
 
    # Filter all packets that have RH0 headers:
    $iptables6 -A INPUT -m rt --rt-type 0 -j DROP
    $iptables6 -A FORWARD -m rt --rt-type 0 -j DROP
    $iptables6 -A OUTPUT -m rt --rt-type 0 -j DROP
 
    # drop spoofed packets (i.e. packets with local source addresses coming from outside etc.), mark as Bad Guy
    $iptables4 -A INPUT  -i $pub_if -s $pub_ipv4 -m recent --set -j DROP
    $iptables6 -A INPUT  -i $pub_if -s $pub_ipv6 -m recent --set -j DROP
 
    # accept ICMP packets (ping et.al.)
    $iptables4 -A INPUT  -p icmp   -j ACCEPT
    $iptables6 -A INPUT  -p icmpv6 -j ACCEPT
    $iptables6 -A OUTPUT -p icmpv6 -j ACCEPT
    $iptables6 -A FORWARD -p icmpv6 -j ACCEPT
 
    # internet (established and out)
    $iptables4 -A OUTPUT -o $pub_if -j ACCEPT
    $iptables6 -A OUTPUT -o $pub_if -j ACCEPT
    $iptables4 -A INPUT  -i $pub_if -m state --state ESTABLISHED,RELATED -j ACCEPT
    $iptables6 -A INPUT  -i $pub_if -m state --state ESTABLISHED,RELATED -j ACCEPT
 
    # allow public services
    $iptables4 -A INPUT -i $pub_if -p tcp -d $pub_ipv4 -m multiport --dports 25,80,143,443,465,993,8000 -j ACCEPT
    $iptables6 -A INPUT -i $pub_if -p tcp -d $pub_ipv6 -m multiport --dports 25,80,143,443,465,993,8000 -j ACCEPT
 
    # accept ssh connections (max 2/minute from the same IP address)
    $iptables4 -A INPUT -p tcp --dport 22 -m recent --update --seconds 300 --hitcount 10 --name SSH -j DROP
    $iptables6 -A INPUT -p tcp --dport 22 -m recent --update --seconds 300 --hitcount 10 --name SSH -j DROP
    $iptables4 -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set --name SSH -j ACCEPT
    $iptables6 -A INPUT -p tcp --dport 22 -m state --state NEW -m recent --set --name SSH -j ACCEPT
 
    # reject everything else in INPUT
    $iptables4 -A INPUT -j REJECT
    $iptables6 -A INPUT -j REJECT
}
 
firewall_stop()
{
    # flush rules
    firewall_flush
 
    # default policy
    firewall_default ACCEPT
}
 
case "$1" in
    start)
        echo -n "Starting $NAME: "
        firewall_start
        echo "OK."
        ;;
    stop)
        echo -n "Stopping $NAME: "
        firewall_stop
        echo "OK."
        ;;
    restart|reload)
        echo -n "Restarting $NAME: "
        firewall_stop
        sleep 1
        firewall_start
        echo "OK."
        ;;
    *)
        echo "Usage: /etc/init.d/$NAME {start|stop|restart|reload}" >&2
        exit 1
        ;;
esac
 
exit 0

https://tty1.net/blog/2007/iptables-firewall_en.html