Packet filtering and firewalls
On Linux, Docker manipulates iptables
rules to provide network isolation.
While this is an implementation detail and you should not modify the rules
Docker inserts into your iptables
policies, it does have some implications
on what you need to do if you want to have your own policies in addition to
those managed by Docker.
If you're running Docker on a host that is exposed to the Internet, you will probably want to have iptables policies in place that prevent unauthorized access to containers or other services running on your host. This page describes how to achieve that, and what caveats you need to be aware of.
Add iptables policies before Docker's rules
Docker installs two custom iptables
chains named DOCKER-USER
and DOCKER
,
and it ensures that incoming packets are always checked by these two chains
first. These chains are part of the FORWARD
chain.
All of Docker's iptables
rules are added to the DOCKER
chain. Do not
manipulate this chain manually. If you need to add rules which load before
Docker's rules, add them to the DOCKER-USER
chain. These rules are applied
before any rules Docker creates automatically.
Other rules added to the FORWARD
chain, either manually, or by another
iptables-based firewall, are evaluated after the DOCKER-USER
and DOCKER
chains.
This means that if you publish a port through Docker,
this port gets published no matter what rules your firewall has configured.
If you want rules to apply even when a port gets published through Docker,
you must add these rules to the DOCKER-USER
chain.
Match the original IP and ports for requests
When packets arrive to the DOCKER-USER
chain, they have already passed through
a Destination Network Address Translation (DNAT) filter. That means that the
iptables
flags you use can only match internal IP addresses and ports of
containers.
If you want to match traffic based on the original IP and port in the network
request, you must use the
conntrack
iptables extension.
For example:
$ sudo iptables -I DOCKER-USER -p tcp -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
$ sudo iptables -I DOCKER-USER -p tcp -m conntrack --ctorigsrc 1.2.3.4 --ctorigdstport 80 -j ACCEPT
Important
Using the
conntrack
extension may result in degraded performance.
Restrict connections to the Docker host
By default, all external source IPs are allowed to connect to the Docker host.
To allow only a specific IP or network to access the containers, insert a
negated rule at the top of the DOCKER-USER
filter chain. For example, the
following rule restricts external access from all IP addresses except 192.168.1.1
:
$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.1 -j DROP
You will need to change ext_if
to correspond with your
host's actual external interface. You could instead allow connections from a
source subnet. The following rule only allows access from the subnet 192.168.1.0/24
:
$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.0/24 -j DROP
Finally, you can specify a range of IP addresses to accept using --src-range
(Remember to also add -m iprange
when using --src-range
or --dst-range
):
$ iptables -I DOCKER-USER -m iprange -i ext_if ! --src-range 192.168.1.1-192.168.1.3 -j DROP
You can combine -s
or --src-range
with -d
or --dst-range
to control both
the source and destination. For instance, if the Docker daemon listens on both
192.168.1.99
and 10.1.2.3
, you can make rules specific to 10.1.2.3
and leave
192.168.1.99
open.
iptables
is complicated and more complicated rules are out of scope for this
topic. See the
Netfilter.org HOWTO
for a lot more information.
Docker on a router
Docker also sets the policy for the FORWARD
chain to DROP
. If your Docker
host also acts as a router, this will result in that router not forwarding
any traffic anymore. If you want your system to continue functioning as a
router, you can add explicit ACCEPT
rules to the DOCKER-USER
chain to
allow it:
$ iptables -I DOCKER-USER -i src_if -o dst_if -j ACCEPT
Prevent Docker from manipulating iptables
It is possible to set the iptables
key to false
in the Docker engine's configuration file at /etc/docker/daemon.json
, but this option is not appropriate for most users. It is not possible to completely prevent Docker from creating iptables
rules, and creating them after-the-fact is extremely involved and beyond the scope of these instructions. Setting iptables
to false
will more than likely break container networking for the Docker engine.
For system integrators who wish to build the Docker runtime into other applications, explore the
moby
project.
Setting the default bind address for containers
By default, the Docker daemon binds published container ports to the 0.0.0.0
address. When you publish a container's ports as follows:
docker run -p 8080:80 nginx
This publishes port 8080 to all network interfaces on the host, potentially making them available to the outside world. Unless you've disabled IPv6 at the kernel level, the port gets published on both IPv4 and IPv6.
You can change the default binding address for published container ports so that
they're only accessible to the Docker host by default. To do that, you can
configure the daemon to use the loopback address (127.0.0.1
) instead.
To do so, configure the "ip"
key in the daemon.json
configuration file:
{
"ip": "127.0.0.1"
}
This changes the default binding address to 127.0.0.1
for published container
ports on the default bridge network.
Restart the daemon for this change to take effect.
Alternatively, you can use the dockerd --ip
flag when starting the daemon.
Note
Changing the default bind address doesn't have any effect on Swarm services. Swarm services are always exposed on the
0.0.0.0
network interface.
To configure this setting for user-defined bridge networks, use
the com.docker.network.bridge.host_binding_ipv4
driver option when you create the network.
$ docker network create mybridge \
-o "com.docker.network.bridge.host_binding_ipv4=127.0.0.1"
Integration with firewalld
If you are running Docker with the iptables
option set to true
, and
firewalld is enabled on your system, Docker
automatically creates a firewalld
zone called docker
and inserts all the
network interfaces it creates (for example, docker0
) into the docker
zone
to allow seamless networking.
Docker and ufw
Uncomplicated Firewall (ufw) is a frontend that ships with Debian and Ubuntu, and it lets you manage firewall rules. Docker and ufw use iptables in ways that make them incompatible with each other.
When you publish a container's ports using Docker, traffic to and from that
container gets diverted before it goes through the ufw firewall settings.
Docker routes container traffic in the nat
table, which means that packets
are diverted before it reaches the INPUT
and OUTPUT
chains that ufw uses.
Packets are routed before the firewall rules can be applied,
effectively ignoring your firewall configuration.