Please take a look at [Solving UFW and Docker issues](#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:
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
:DOCKER-USER - [0:0]
-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 -j ufw-user-forward
-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
COMMIT
# END UFW AND DOCKER
Using command `sudo systemctl restart ufw` 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.
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 internal port is 80.
Note: If we publish a port by using option `-p 8080:80`, we should use the internal port `80`, not the mapping 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
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
