This topic is about how to limit access to your docker containers from outside world using iptables.
For impatient people, you can check the examples. For the others, please read the remark section to understand how to build new rules.
|ext_if||Your external interface on Docker host.|
|XXX.XXX.XXX.XXX||A particular IP on which Docker containers access should be given.|
|YYY.YYY.YYY.YYY||Another IP on which Docker containers access should be given.|
|ipfriends||The ipset name defining the IPs allowed to access your Docker containers.|
Configuring iptables rules for Docker containers is a bit tricky. At first, you would think that "classic" firewall rules should do the trick.
For example, let's assume that you have configured a nginx-proxy container + several service containers to expose via HTTPS some personal web services. Then a rule like this should give access to your web services only for IP XXX.XXX.XXX.XXX.
$ iptables -A INPUT -i eth0 -p tcp -s XXX.XXX.XXX.XXX -j ACCEPT $ iptables -P INPUT DROP
It won't work, your containers are still accessible for everyone.
Indeed, Docker containers are not host services. They rely on a virtual network in your host, and the host acts as a gateway for this network. And concerning gateways, routed traffic is not handled by the INPUT table, but by the FORWARD table, which makes the rule posted before uneffective.
But it's not all. In fact, Docker daemon creates a lot of iptables rules when it starts to do its magic concerning containers network connectivity. In particular, a DOCKER table is created to handle rules concerning containers by forwarding traffic from the FORWARD table to this new table.
$ iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy DROP) target prot opt source destination DOCKER-ISOLATION all -- anywhere anywhere DOCKER all -- anywhere anywhere ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere DOCKER all -- anywhere anywhere ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere Chain OUTPUT (policy ACCEPT) target prot opt source destination Chain DOCKER (2 references) target prot opt source destination ACCEPT tcp -- anywhere 172.18.0.4 tcp dpt:https ACCEPT tcp -- anywhere 172.18.0.4 tcp dpt:http Chain DOCKER-ISOLATION (1 references) target prot opt source destination DROP all -- anywhere anywhere DROP all -- anywhere anywhere RETURN all -- anywhere anywhere
If you check the official documentation (https://docs.docker.com/v1.5/articles/networking/), a first solution is given to limit Docker container access to one particular IP.
$ iptables -I DOCKER -i ext_if ! -s 188.8.131.52 -j DROP
Indeed, adding a rule at the top of the DOCKER table is a good idea. It does not interfere with the rules automatically configured by Docker, and it is simple. But two major lacks :
For the first observation, we can use ipset. Instead of allowing one IP in the rule above, we allow all IPs from the predefined ipset. As a bonus, the ipset can be updated without the necessity to redefine the iptable rule.
$ iptables -I DOCKER -i ext_if -m set ! --match-set my-ipset src -j DROP
For the second observation, this is a canonical problem for firewalls : if you are allowed to contact a server through a firewall, then the firewall should authorize the server to respond to your request. This can be done by authorizing packets which are related to an established connection. For the docker logic, it gives :
$ iptables -I DOCKER -i ext_if -m state --state ESTABLISHED,RELATED -j ACCEPT
The last observation focuses on one point : iptables rules is essential. Indeed, additional logic to ACCEPT some connections (including the one concerning ESTABLISHED connections) must be put at the top of the DOCKER table, before the DROP rule which deny all remaining connections not matching the ipset.
As we use the -I option of iptable, which inserts rules at the top of the table, previous iptables rules must be inserted by reverse order :
// Drop rule for non matching IPs $ iptables -I DOCKER -i ext_if -m set ! --match-set my-ipset src -j DROP // Then Accept rules for established connections $ iptables -I DOCKER -i ext_if -m state --state ESTABLISHED,RELATED -j ACCEPT $ iptables -I DOCKER -i ext_if ... ACCEPT // Then 3rd custom accept rule $ iptables -I DOCKER -i ext_if ... ACCEPT // Then 2nd custom accept rule $ iptables -I DOCKER -i ext_if ... ACCEPT // Then 1st custom accept rule
With all of this in mind, you can now check the examples which illustrate this configuration.