Easy Debian 9 Server Firewall

If you read our previous article Easy Debian Server Firewall, then you may have noted that on Debian Stretch the described method no longer works. This is due to systemd. In the article below we will walk through creating a persistent IPTables based firewall on Debian Stretch. First we need to install some required software packages. As seen in the command below, install iptables-persistent. Next we will make netfilter-persistent run at boot. This is the most important step as it will ensure your rules are reloaded at boot time.

# Install IPTables Persistent Package
apt-get install -y iptables-persistent
# Add netfilter-persistent Startup
invoke-rc.d netfilter-persistent save
# Start netfilter-persistent Service
service netfilter-persistent start

Once the packages above are installed, you will have a new directory at /etc/iptables/. This directory holds the IPTables filter rules that will be reloaded at boot time. These files are named rules.v4 and rules.v6 respectively. IPV4 rules are loaded into rules.v4 and IPV6 rules are loaded into rules.v6. For the purpose of this article we will focus on IPV4 rules. Next we will want to copy the rules below into our rules.v4 file. Of course the rules will need to be modified to fit your environment.

# Generated by iptables-save v1.6.0 on Tue Apr 10 13:56:08 2018
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]

# Allow internal loopback connections
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT

# Allow pinging
-A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT

# Allow any outbound data, and any inbound data related to a connection that is already in use
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT

# =========BEGIN SERVER SPECIFIC PORT OPEN RULES=========
# Allow SCP/SSH Access from Green & Blue Subnet
-A INPUT -s 172.16.12.0/255.255.255.0 -p tcp -m tcp --dport 22 -j ACCEPT
-A INPUT -s 10.10.12.0/255.255.255.0 -p tcp -m tcp --dport 22 -j ACCEPT
# =========END SERVER SPECIFIC PORT OPEN RULES=========

# Drop everything that hasn't been picked up by one of the rules above
-A INPUT -j DROP
-A FORWARD -j DROP
-A OUTPUT -j DROP

COMMIT
# Completed on Tue Apr 10 13:56:08 2018

Lastly, in order for our new rules to take affect, we simply need to restart the netfilter-persistent service as seen below. That’s it, you now have a fully functional IPTables based firewall.

# Restart netfilter-persistent Service
service netfilter-persistent restart
# Check if IPTables were applied
iptables -L
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