Docker Behind UFW Firewall

Docker in Firewall
Ubuntu has a great firewall called the 'Uncomplicated Firewall' or UFW for short. This is a great option if you want to secure your system.

Docker, by default, will update your iptables and punch holes in your firewall. This guide will show you how to constrain Docker behind UFW, providing consistent system firewall security.

Default UFW settings

sudo ufw allow ssh
sudo ufw default deny incoming
sudo ufw default allow outgoing
sudo ufw enable

Configure default forward policy

sudo nano /etc/default/ufw

Edit Line:

DEFAULT_FORWARD_POLICY="ACCEPT"

Reload UFW

sudo ufw reload

Allow docker port (default: 2375)

sudo ufw allow 2375

Enable daemon.json

sudo mkdir -p /lib/systemd/system/docker.service.d
sudo nano /lib/systemd/system/docker.service.d/override.conf

Set Content:

[Service]
ExecStart=
ExecStart=/usr/bin/dockerd

Setup daemon.json

sudo nano /etc/docker/daemon.json

Set Content:

{
  "hosts": ["fd://"],
  "dns": ["8.8.8.8", "8.8.4.4"],
  "iptables": false
}

Restart Docker

sudo systemctl daemon-reload
sudo systemctl restart docker

Configure NAT

sudo nano /etc/ufw/before.rules

Add lines before "filter"*

*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING ! -o docker0 -s 172.17.0.0/16 -j MASQUERADE
COMMIT

Reboot system

sudo reboot now

Now ports exposed by docker have to explicitly be enabled in UFW