On the &yet Ops Team, we use Docker for various purposes on some of the servers we run. We also make extensive use of iptables so that we have consistent firewall rules to protect those servers against attacks.
Unfortunately, we recently ran into an issue that prevented us from building Dockerfiles from behind an iptables firewall.
Here's a bit more information about the problem, and how we solved it.
When trying to run
docker build on a host that provides our default DROP policy-based iptables set,
apt-get was unable to resolve repository hosts on Dockerfiles that were
FROM Ubuntu or debian.
apt-get command would result in something like this:
Step 1 : RUN apt-get update ---> Running in 64a37c06d1f4 Err http://http.debian.net wheezy Release.gpg Could not resolve 'http.debian.net' Err http://http.debian.net wheezy-updates Release.gpg Could not resolve 'http.debian.net' Err http://security.debian.org wheezy/updates Release.gpg Could not resolve 'security.debian.org'
To figure out what was going wrong, we logged all dropped packets in iptables to syslog like this:
# Log dropped outbound packets iptables -N LOGGING iptables -A OUTPUT -j LOGGING iptables -A INPUT -j LOGGING iptables -A FORWARD -j LOGGING iptables -A LOGGING -m limit --limit 2/min -j LOG --log-prefix "IPTables-Dropped: " --log-level 4 iptables -A LOGGING -j DROP
The logs quickly showed that the
docker0 interface was trying to FORWARD port 53 to the
eth0 interface. In our case, the default FORWARD policy is DROP, so essentially iptables was dropping Docker's requests to forward the DNS port to the public interface and Internet at large.
Since Docker couldn't resolve the domain names where the Dockerfiles were located, it couldn't retrieve the data it needed.
Hmm, so we needed to allow forwarding between
eth0 , eh? That's easy! We just added the following rules to our iptables set:
# Forward chain between docker0 and eth0 iptables -A FORWARD -i docker0 -o eth0 -j ACCEPT iptables -A FORWARD -i eth0 -o docker0 -j ACCEPT # IPv6 chain if needed ip6tables -A FORWARD -i docker0 -o eth0 -j ACCEPT ip6tables -A FORWARD -i eth0 -o docker0 -j ACCEPT
Add or alter these rules as needed, and you too will be able to build Dockerfiles properly behind an iptables firewall.