To Fix The Docker and UFW Security Flaw Without Disabling Iptables

• English

TL;DR

Please take a look at Solving UFW and Docker issues.

Problem

UFW is a popular iptables front end on Ubuntu that makes it easy to manage firewall rules. But when Docker is installed, Docker bypass the UFW rules and the published ports can be accessed from outside.

The issue is:

1. UFW is enabled on a server that provides external services, and all incoming connections that are not allowed are blocked by default.
2. Run a Docker container on the server and use the -p option to publish ports for that container on all IP addresses. For example: docker run -d --name httpd -p 0.0.0.0:8080:80 httpd:alpine, this command will run an httpd service and publish port 80 of the container to port 8080 of the server.
3. UFW will not block all external requests to visit port 8080. Even the command ufw deny 8080 will not prevent external access to this port.
4. This problem is actually quite serious, which means that a port that was originally intended to provide services internally is exposed to the public network.

Searching for "ufw docker" on the web can find a lot of discussion:

• https://github.com/moby/moby/issues/4737
• https://forums.docker.com/t/running-multiple-docker-containers-with-ufw-and-iptables-false/8953
• https://www.techrepublic.com/article/how-to-fix-the-docker-and-ufw-security-flaw/
• https://blog.viktorpetersson.com/2014/11/03/the-dangers-of-ufw-docker.html
• https://askubuntu.com/questions/652556/uncomplicated-firewall-ufw-is-not-blocking-anything-when-using-docker
• https://chjdev.com/2016/06/08/docker-ufw/
• https://askubuntu.com/questions/652556/uncomplicated-firewall-ufw-is-not-blocking-anything-when-using-docker
• https://my.oschina.net/abcfy2/blog/539485
• https://www.v2ex.com/amp/t/466666
• https://blog.36web.rocks/2016/07/08/docker-behind-ufw.html
• ...

Almost all of these solutions are similar. It requires to disable docker's iptables function first, but this also means that we give up docker's network management function. This causes containers will not be able to access the external network. It is also mentioned in some articles that you can manually add some rules in the UFW configuration file, such as -A POSTROUTING ! -o docker0 -s 172.17.0.0/16 -j MASQUERADE. But this only allows containers that belong to network 172.17.0.0/16 can access outside. If we create a new docker network, we must manually add such similar iptables rules for the new network.

Expected goal

The solutions that we can find on internet are very similar and not elegant, I hope a new solution can:

• Don't need to disable Docker's iptables and let Docker to manage it's network. We don't need to manually maintain iptables rules for any new Docker networks, and avoid potential side effects after disabling iptables in Docker.
• The public network cannot access ports that published by Docker. Even if the port is published on all IP addresses using an option like -p 8080:80. Containers and internal networks can visit each other normally. Although it is possible to have Docker publish a container's port to the server's private IP address, the port will not be accessed on the public network. But, this server may have multiple private IP addresses, and these private IP addresses may also change.
• In a very convenient way to allow/deny public networks to access container ports without additional software and extra configurations. Just like using command ufw allow 8080 to allow external access port 8080, then using command ufw delete allow 8080 to deny public networks visit port 8080.

How to do?

Revoke the original modification

If you have modified your server according to the current solution that we find on the internet, please rollback these changes first, including:

• Enable Docker's iptables feature. Remove all changes like --iptables=false , including configuration file /etc/docker/daemon.json.
• UFW's default FORWARD rule changes back to the default DROP instead of ACCEPT.
• Remove the rules related to the Docker network in the UFW configuration file /etc/ufw/after.rules.
• If you have modified Docker configuration files, restart Docker first. We will modify the UFW configuration later and we can restart it then.

Solving UFW and Docker issues

This solution needs to modify only one UFW configuration file, all Docker configurations and options remain the default.

Modify the UFW configuration file /etc/ufw/after.rules and add the following rules at the end of the file:

# BEGIN UFW AND DOCKER
*filter
:ufw-user-forward - [0:0]
:ufw-docker-logging-deny - [0:0]
:DOCKER-USER - [0:0]
-A DOCKER-USER -j ufw-user-forward

-A DOCKER-USER -j RETURN -s 10.0.0.0/8
-A DOCKER-USER -j RETURN -s 172.16.0.0/12
-A DOCKER-USER -j RETURN -s 192.168.0.0/16

-A DOCKER-USER -p udp -m udp --sport 53 --dport 1024:65535 -j RETURN

-A DOCKER-USER -j ufw-docker-logging-deny -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 192.168.0.0/16
-A DOCKER-USER -j ufw-docker-logging-deny -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 10.0.0.0/8
-A DOCKER-USER -j ufw-docker-logging-deny -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 172.16.0.0/12
-A DOCKER-USER -j ufw-docker-logging-deny -p udp -m udp --dport 0:32767 -d 192.168.0.0/16
-A DOCKER-USER -j ufw-docker-logging-deny -p udp -m udp --dport 0:32767 -d 10.0.0.0/8
-A DOCKER-USER -j ufw-docker-logging-deny -p udp -m udp --dport 0:32767 -d 172.16.0.0/12

-A DOCKER-USER -j RETURN

-A ufw-docker-logging-deny -m limit --limit 3/min --limit-burst 10 -j LOG --log-prefix "[UFW DOCKER BLOCK] "
-A ufw-docker-logging-deny -j DROP

COMMIT
# END UFW AND DOCKER

Using command sudo systemctl restart ufw or sudo ufw reload to restart UFW after changing the file. Now the public network can't access any published docker ports, the container and the private network can visit each other normally, and the containers can also access the external network from inside. There may be some unknown reasons cause the UFW rules will not take effect after restart UFW, please reboot servers.

If you want to allow public networks to access the services provided by the Docker container, for example, the service port of a container is 80. Run the following command to allow the public networks to access this service:

ufw route allow proto tcp from any to any port 80

This allows the public network to access all published ports whose container port is 80.

Note: If we publish a port by using option -p 8080:80, we should use the container port 80, not the host port 8080.

If there are multiple containers with a service port of 80, but we only want the external network to access a certain container. For example, if the private address of the container is 172.17.0.2, use the following command:

ufw route allow proto tcp from any to 172.17.0.2 port 80

If the network protocol of a service is UDP, for example a DNS service, you can use the following command to allow the external network to access all published DNS services:

ufw route allow proto udp from any to any port 53

Similarly, if only for a specific container, such as IP address 172.17.0.2:

ufw route allow proto udp from any to 172.17.0.2 port 53

How it works?

The following rules allow the private networks to be able to visit each other. Normally, private networks are more trusted than public networks.

-A DOCKER-USER -j RETURN -s 10.0.0.0/8
-A DOCKER-USER -j RETURN -s 172.16.0.0/12
-A DOCKER-USER -j RETURN -s 192.168.0.0/16

The following rules allow UFW to manage whether the public networks are allowed to visit the services provided by the Docker container. So that we can manage all firewall rules in one place.

-A DOCKER-USER -j ufw-user-forward

For example, we want to block all outgoing connections from inside a container whose IP address is 172.17.0.9 which means to block this container to access internet or external networks. Using the following command:

ufw route deny from 172.17.0.9 to any

The following rules block connection requests initiated by all public networks, but allow internal networks to access external networks. For TCP protocol, it prevents from actively establishing a TCP connection from public networks. For UDP protocol, all accesses to ports which is less then 32767 are blocked. Why is this port? Since the UDP protocol is stateless, it is not possible to block the handshake signal that initiates the connection request as TCP does. For GNU/Linux we can find the local port range in the file /proc/sys/net/ipv4/ip_local_port_range. The default range is 32768 60999. When accessing a UDP protocol service from a running container, the local port will be randomly selected one from the port range, and the server will return the data to this random port. Therefore, we can assume that the listening port of the UDP protocol inside all containers are less then 32768. This is the reason that we don't want public networks to access the UDP ports that less then 32768.

-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 192.168.0.0/16
-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 10.0.0.0/8
-A DOCKER-USER -j DROP -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -d 172.16.0.0/12
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 192.168.0.0/16
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 10.0.0.0/8
-A DOCKER-USER -j DROP -p udp -m udp --dport 0:32767 -d 172.16.0.0/12

-A DOCKER-USER -j RETURN

If a docker container doesn't follow the OS's settings when receiving data, that is to say, the minimal port number less than 32768. For example, we have a Dnsmasq container. The minimal port number that Dnsmasq uses for receiving data is 1024. We can use the following command to allow a bigger port range used for receiving DNS packages.

ufw route allow proto udp from any port 53 to any port 1024:65535

Because DNS is a very common service, so there is already a firewall rule to allow a bigger port range to receive DNS packages.

The reason for choosing ufw-user-forward, not ufw-user-input

using ufw-user-input

Pro:

Easy to use and understand, supports older versions of Ubuntu.

For example, to allow the public to visit a published port whose container port is 8080, use the command:

ufw allow 8080

Con:

It not only exposes ports of containers but also exposes ports of the host.

For example, if a service is running on the host, and the port is 8080. The command ufw allow 8080 allows the public network to visit the service and all published ports whose containers' port is 8080. But we just want to expose the service running on the host, or just the service running inside containers, not the both.

To avoid this problem, we may need to use a command similar to the following for all containers:

ufw allow proto tcp from any to 172.16.0.3 port 8080

using ufw-user-forward

Pro:

Cannot expose services running on hosts and containers at the same time by the same command.

For example, if we want to publish the port 8080 of containers, use the following command:

ufw route allow 8080

The public network can access all published ports whose container ports are 8080.

But the port 8080 of the host is still not be accessed by the public network. If we want to do so, execute the following command to allow the public access the port on the host separately:

ufw allow 8080

Con:

Doesn't support older versions of Ubuntu, and the command is a bit more complicated. But you can use my script.

Conclusion

If we are using an older version of Ubuntu, we can use ufw-user-input chain. But be careful to avoid exposing services that should not be exposed

If we are using a newer version of Ubuntu which is support ufw route sub-command, we'd better use ufw-user-forward chain, and use ufw route command to manage firewall rules for containers.

ufw-docker util

This script also supports Docker Swarm mode.

Install

Download ufw-docker script

sudo wget -O /usr/local/bin/ufw-docker \
  https://github.com/chaifeng/ufw-docker/raw/master/ufw-docker
sudo chmod +x /usr/local/bin/ufw-docker

Then using the following command to modify the after.rules file of ufw

ufw-docker install

This command does the following things:

• Back up the file /etc/ufw/after.rules
• Append the rules of UFW and Docker at the end of the file

Install for Docker Swarm mode

We can only use this script on manager nodes to manage firewall rules when using in Swarm mode.

• Modifying all after.rules files on all nodes, including managers and workers
• Deploying this script on manager nodes

Running in Docker Swarm mode, this script will add a global service ufw-docker-agent. The image chaifeng/ufw-docker-agent is also automatically built from this project.

Usage

Show help

ufw-docker help

Check the installation of firewall rules in UFW configurations

ufw-docker check

Update UFW configurations, add the necessary firewall rules

ufw-docker install

Show the current firewall allowed forward rules

ufw-docker status

List all firewall rules related to container httpd

ufw-docker list httpd

Expose the port 80 of the container httpd

ufw-docker allow httpd 80

Expose the 443 port of the container httpd and the protocol is tcp

ufw-docker allow httpd 443/tcp

Expose the 443 port of the container httpd and the protocol is tcp and the network is foobar-external-network when the container httpd is attached to multiple networks

ufw-docker allow httpd 443/tcp foobar-external-network

Expose all published ports of the container httpd

ufw-docker allow httpd

Remove all rules related to the container httpd

ufw-docker delete allow httpd

Remove the rule which port is 443 and protocol is tcp for the container httpd

ufw-docker delete allow httpd 443/tcp

Expose the port 80 of the service web

docker service create --name web --publish 8080:80 httpd:alpine

ufw-docker service allow web 80
# or
ufw-docker service allow web 80/tcp

Remove rules from all nodes related to the service web

ufw-docker service delete allow web

Try it out

We use Vagrant to set up a local testing environment.

Run the following command to create 1 master node and 2 worker nodes

vagrant up

Log into the master node

vagrant ssh master

After logging in, create a web service

docker service create --name web --publish 8080:80 httpd:alpine

We shouldn't visit this web service from our host

curl -v http://192.168.56.131:8080

On the master node, run the command to allow the public access port 80 of the web service.

sudo ufw-docker service allow web 80

We can access the web service from our host now

curl "http://192.168.56.13{0,1,2}:8080"

Discussions

• What is the best practice of docker + ufw under Ubuntu - Stack Overflow
• docker and ufw serious problems · Issue #4737 · moby/moby