Installation et configuration de FireWall.

Nous allons utilisé un outil permettant la persistance des configuration iptables

sudo apt-get install -y iptables-persistent

Dans le fichier /etc/iptables/rules.v6

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
COMMIT

Dans le fichier /etc/iptables/rules.v4

*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT ACCEPT [687:218631]

# Allow internal traffic on the loopback device
-A INPUT -i lo -j ACCEPT

# Continue connections that are already established or related to an established connection
-A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT

# Drop non-conforming packets, such as malformed headers, etc.
-A INPUT -m conntrack --ctstate INVALID -j DROP

# SSH
-A INPUT -p tcp -m tcp --dport 22 -j ACCEPT

# DHCP used by OVH
-A INPUT -p udp --dport 67:68 --sport 67:68 -j ACCEPT

# DNS (bind)
-A OUTPUT -p tcp --dport 53 -j ACCEPT
-A OUTPUT -p udp --dport 53 -j ACCEPT
-A INPUT -p tcp --dport 53 -j ACCEPT
-A INPUT -p udp --dport 53 -j ACCEPT

# HTTP + HTTPS
-A INPUT -p tcp -m multiport --dports 80,443 -j ACCEPT

# Email (postfix + devecot)
# 25 = smtp (internal only), 587 = submission and 993 = IMAPS
-A INPUT -p tcp --dport 25 -j ACCEPT
-A INPUT -p tcp --dport 587 -j ACCEPT
-A INPUT -p tcp --dport 993 -j ACCEPT

# Chain for preventing ping flooding - up to 6 pings per second from a single
# source, again with log limiting. Also prevents us from ICMP REPLY flooding
# some victim when replying to ICMP ECHO from a spoofed source.
-N ICMPFLOOD
-A ICMPFLOOD -m recent --name ICMP --set --rsource
-A ICMPFLOOD -m recent --name ICMP --update --seconds 1 --hitcount 6 --rsource --rttl -m limit --limit 1/sec --limit-burst 1 -j LOG --log-prefix "iptables[ICMP-flood]: "
-A ICMPFLOOD -m recent --name ICMP --update --seconds 1 --hitcount 6 --rsource --rttl -j DROP
-A ICMPFLOOD -j ACCEPT

# Permit useful IMCP packet types.
# Note: RFC 792 states that all hosts MUST respond to ICMP ECHO requests.
# Blocking these can make diagnosing of even simple faults much more tricky.
# Real security lies in locking down and hardening all services, not by hiding.
-A INPUT -p icmp --icmp-type 0  -m conntrack --ctstate NEW -j ACCEPT
-A INPUT -p icmp --icmp-type 3  -m conntrack --ctstate NEW -j ACCEPT
-A INPUT -p icmp --icmp-type 8  -m conntrack --ctstate NEW -j ICMPFLOOD
-A INPUT -p icmp --icmp-type 11 -m conntrack --ctstate NEW -j ACCEPT

# Drop all incoming malformed NULL packets
-A INPUT -p tcp --tcp-flags ALL NONE -j DROP

# Drop syn-flood attack packets
-A INPUT -p tcp ! --syn -m conntrack --ctstate NEW -j DROP

# Drop incoming malformed XMAS packets
-A INPUT -p tcp --tcp-flags ALL ALL -j DROP

COMMIT

Pour rafraîchir le firewall

sudo service netfilter-persistent restart

Pour désactiver le firewall, écrire le fichier /tmp/stop_firewall

iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT

sudo bash /tmp/stop_firewall