The information in this section explains container communication within the Docker default bridge. This is a
bridge network named
bridge created automatically when you install Docker.
Note: The Docker networks feature allows you to create user-defined networks in addition to the default bridge network.
Whether a container can talk to the world is governed by two factors. The first factor is whether the host machine is forwarding its IP packets. The second is whether the host’s
iptables allow this particular connection.
IP packet forwarding is governed by the
ip_forward system parameter. Packets can only pass between containers if this parameter is
1. Usually you will simply leave the Docker server at its default setting
--ip-forward=true and Docker will go set
1 for you when the server starts up. If you set
--ip-forward=false and your system’s kernel has it enabled, the
--ip-forward=false option has no effect. To check the setting on your kernel or to turn it on manually:
$ sysctl net.ipv4.conf.all.forwarding net.ipv4.conf.all.forwarding = 0 $ sysctl net.ipv4.conf.all.forwarding=1 $ sysctl net.ipv4.conf.all.forwarding net.ipv4.conf.all.forwarding = 1
Note: this setting does not affect containers that use the host network stack (
Many using Docker will want
ip_forward to be on, to at least make communication possible between containers and the wider world. May also be needed for inter-container communication if you are in a multiple bridge setup.
Docker will never make changes to your system
iptables rules if you set
--iptables=false when the daemon starts. Otherwise the Docker server will append forwarding rules to the
DOCKER filter chain.
Docker will not delete or modify any pre-existing rules from the
DOCKER filter chain. This allows the user to create in advance any rules required to further restrict access to the containers.
Docker’s forward rules permit all external source IPs by default. To allow only a specific IP or network to access the containers, insert a negated rule at the top of the
DOCKER filter chain. For example, to restrict external access such that only source IP 18.104.22.168 can access the containers, the following rule could be added:
$ iptables -I DOCKER -i ext_if ! -s 22.214.171.124 -j DROP
where ext_if is the name of the interface providing external connectivity to the host.
Whether two containers can communicate is governed, at the operating system level, by two factors.
Does the network topology even connect the containers’ network interfaces? By default Docker will attach all containers to a single
docker0 bridge, providing a path for packets to travel between them. See the later sections of this document for other possible topologies.
iptables allow this particular connection? Docker will never make changes to your system
iptables rules if you set
--iptables=false when the daemon starts. Otherwise the Docker server will add a default rule to the
FORWARD chain with a blanket
ACCEPT policy if you retain the default
--icc=true, or else will set the policy to
It is a strategic question whether to leave
--icc=true or change it to
--icc=false so that
iptables will protect other containers – and the main host – from having arbitrary ports probed or accessed by a container that gets compromised.
If you choose the most secure setting of
--icc=false, then how can containers communicate in those cases where you want them to provide each other services? The answer is the
--link=CONTAINER_NAME_or_ID:ALIAS option, which was mentioned in the previous section because of its effect upon name services. If the Docker daemon is running with both
--iptables=true then, when it sees
docker run invoked with the
--link= option, the Docker server will insert a pair of
ACCEPT rules so that the new container can connect to the ports exposed by the other container – the ports that it mentioned in the
EXPOSE lines of its
Note: The value
--link=must either be an auto-assigned Docker name like
stupefied_pareor else the name you assigned with
--name=when you ran
docker run. It cannot be a hostname, which Docker will not recognize in the context of the
You can run the
iptables command on your Docker host to see whether the
FORWARD chain has a default policy of
# When --icc=false, you should see a DROP rule: $ sudo iptables -L -n ... Chain FORWARD (policy ACCEPT) target prot opt source destination DOCKER all -- 0.0.0.0/0 0.0.0.0/0 DROP all -- 0.0.0.0/0 0.0.0.0/0 ... # When a --link= has been created under --icc=false, # you should see port-specific ACCEPT rules overriding # the subsequent DROP policy for all other packets: $ sudo iptables -L -n ... Chain FORWARD (policy ACCEPT) target prot opt source destination DOCKER all -- 0.0.0.0/0 0.0.0.0/0 DROP all -- 0.0.0.0/0 0.0.0.0/0 Chain DOCKER (1 references) target prot opt source destination ACCEPT tcp -- 172.17.0.2 172.17.0.3 tcp spt:80 ACCEPT tcp -- 172.17.0.3 172.17.0.2 tcp dpt:80
Note: Docker is careful that its host-wide
iptablesrules fully expose containers to each other’s raw IP addresses, so connections from one container to another should always appear to be originating from the first container’s own IP address.
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