moby--moby/docs/sources/articles/networking.md

page_title: Network Configuration page_description: Docker networking page_keywords: network, networking, bridge, docker, documentation

Network Configuration

TL;DR

When Docker starts, it creates a virtual interface named docker0 on the host machine. It randomly chooses an address and subnet from the private range defined by RFC 1918 that are not in use on the host machine, and assigns it to docker0. Docker made the choice 172.17.42.1/16 when I started it a few minutes ago, for example — a 16-bit netmask providing 65,534 addresses for the host machine and its containers.

Note: This document discusses advanced networking configuration and options for Docker. In most cases you won't need this information. If you're looking to get started with a simpler explanation of Docker networking and an introduction to the concept of container linking see the Docker User Guide.

But docker0 is no ordinary interface. It is a virtual Ethernet bridge that automatically forwards packets between any other network interfaces that are attached to it. This lets containers communicate both with the host machine and with each other. Every time Docker creates a container, it creates a pair of “peer” interfaces that are like opposite ends of a pipe — a packet sent on one will be received on the other. It gives one of the peers to the container to become its eth0 interface and keeps the other peer, with a unique name like vethAQI2QT, out in the namespace of the host machine. By binding every veth* interface to the docker0 bridge, Docker creates a virtual subnet shared between the host machine and every Docker container.

The remaining sections of this document explain all of the ways that you can use Docker options and — in advanced cases — raw Linux networking commands to tweak, supplement, or entirely replace Docker's default networking configuration.

Quick Guide to the Options

Here is a quick list of the networking-related Docker command-line options, in case it helps you find the section below that you are looking for.

Some networking command-line options can only be supplied to the Docker server when it starts up, and cannot be changed once it is running:

• -b BRIDGE or --bridge=BRIDGE — see Building your own bridge

• --bip=CIDR — see Customizing docker0

• -H SOCKET... or --host=SOCKET... — This might sound like it would affect container networking, but it actually faces in the other direction: it tells the Docker server over what channels it should be willing to receive commands like “run container” and “stop container.”

• --icc=true|false — see Communication between containers

• --ip=IP_ADDRESS — see Binding container ports

• --ip-forward=true|false — see Communication between containers

• --iptables=true|false — see Communication between containers

• --mtu=BYTES — see Customizing docker0

There are two networking options that can be supplied either at startup or when docker run is invoked. When provided at startup, set the default value that docker run will later use if the options are not specified:

• --dns=IP_ADDRESS... — see Configuring DNS

• --dns-search=DOMAIN... — see Configuring DNS

Finally, several networking options can only be provided when calling docker run because they specify something specific to one container:

• -h HOSTNAME or --hostname=HOSTNAME — see Configuring DNS and How Docker networks a container

• --link=CONTAINER_NAME:ALIAS — see Configuring DNS and Communication between containers

• --net=bridge|none|container:NAME_or_ID|host — see How Docker networks a container

• -p SPEC or --publish=SPEC — see Binding container ports

• -P or --publish-all=true|false — see Binding container ports

The following sections tackle all of the above topics in an order that moves roughly from simplest to most complex.

Configuring DNS

How can Docker supply each container with a hostname and DNS configuration, without having to build a custom image with the hostname written inside? Its trick is to overlay three crucial /etc files inside the container with virtual files where it can write fresh information. You can see this by running mount inside a container:

$$ mount
...
/dev/disk/by-uuid/1fec...ebdf on /etc/hostname type ext4 ...
/dev/disk/by-uuid/1fec...ebdf on /etc/hosts type ext4 ...
tmpfs on /etc/resolv.conf type tmpfs ...
...

This arrangement allows Docker to do clever things like keep resolv.conf up to date across all containers when the host machine receives new configuration over DHCP later. The exact details of how Docker maintains these files inside the container can change from one Docker version to the next, so you should leave the files themselves alone and use the following Docker options instead.

Four different options affect container domain name services.

• -h HOSTNAME or --hostname=HOSTNAME — sets the hostname by which the container knows itself. This is written into /etc/hostname, into /etc/hosts as the name of the container's host-facing IP address, and is the name that /bin/bash inside the container will display inside its prompt. But the hostname is not easy to see from outside the container. It will not appear in docker ps nor in the /etc/hosts file of any other container.

• --link=CONTAINER_NAME:ALIAS — using this option as you run a container gives the new container's /etc/hosts an extra entry named ALIAS that points to the IP address of the container named CONTAINER_NAME. This lets processes inside the new container connect to the hostname ALIAS without having to know its IP. The --link= option is discussed in more detail below, in the section Communication between containers.

• --dns=IP_ADDRESS... — sets the IP addresses added as server lines to the container's /etc/resolv.conf file. Processes in the container, when confronted with a hostname not in /etc/hosts, will connect to these IP addresses on port 53 looking for name resolution services.

• --dns-search=DOMAIN... — sets the domain names that are searched when a bare unqualified hostname is used inside of the container, by writing search lines into the container's /etc/resolv.conf. When a container process attempts to access host and the search domain example.com is set, for instance, the DNS logic will not only look up host but also host.example.com.

Note that Docker, in the absence of either of the last two options above, will make /etc/resolv.conf inside of each container look like the /etc/resolv.conf of the host machine where the docker daemon is running. The options then modify this default configuration.

Communication between containers

Whether two containers can communicate is governed, at the operating system level, by three factors.

1. 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.

2. Is the host machine willing to forward IP packets? This 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 ip_forward to 1 for you when the server starts up. To check the setting or turn it on manually:

   # Usually not necessary: turning on forwarding,
   # on the host where your Docker server is running
   
   $ cat /proc/sys/net/ipv4/ip_forward
   0
   $ sudo echo 1 > /proc/sys/net/ipv4/ip_forward
   $ cat /proc/sys/net/ipv4/ip_forward
   1
   

3. Do your iptables allow this particular connection to be made? 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 DROP if --icc=false.

Nearly everyone using Docker will want ip_forward to be on, to at least make communication possible between containers. But it is a strategic question whether to leave --icc=true or change it to --icc=false (on Ubuntu, by editing the DOCKER_OPTS variable in /etc/default/docker and restarting the Docker server) 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:ALIAS option, which was mentioned in the previous section because of its effect upon name services. If the Docker daemon is running with both --icc=false and --iptables=true then, when it sees docker run invoked with the --link= option, the Docker server will insert a pair of iptables 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 Dockerfile. Docker has more documentation on this subject — see the linking Docker containers page for further details.

Note

: The value CONTAINER_NAME in --link= must either be an auto-assigned Docker name like stupefied_pare or 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 --link= option.

You can run the iptables command on your Docker host to see whether the FORWARD chain has a default policy of ACCEPT or DROP:

# When --icc=false, you should see a DROP rule:

$ sudo iptables -L -n
...
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
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
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
DROP       all  --  0.0.0.0/0            0.0.0.0/0

Note

: Docker is careful that its host-wide iptables rules 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.

Binding container ports to the host

By default Docker containers can make connections to the outside world, but the outside world cannot connect to containers. Each outgoing connection will appear to originate from one of the host machine's own IP addresses thanks to an iptables masquerading rule on the host machine that the Docker server creates when it starts:

# You can see that the Docker server creates a
# masquerade rule that let containers connect
# to IP addresses in the outside world:

$ sudo iptables -t nat -L -n
...
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  172.17.0.0/16       !172.17.0.0/16
...

But if you want containers to accept incoming connections, you will need to provide special options when invoking docker run. These options are covered in more detail in the Docker User Guide page. There are two approaches.

First, you can supply -P or --publish-all=true|false to docker run which is a blanket operation that identifies every port with an EXPOSE line in the image's Dockerfile and maps it to a host port somewhere in the range 49000–49900. This tends to be a bit inconvenient, since you then have to run other docker sub-commands to learn which external port a given service was mapped to.

More convenient is the -p SPEC or --publish=SPEC option which lets you be explicit about exactly which external port on the Docker server — which can be any port at all, not just those in the 49000–49900 block — you want mapped to which port in the container.

Either way, you should be able to peek at what Docker has accomplished in your network stack by examining your NAT tables.

# What your NAT rules might look like when Docker
# is finished setting up a -P forward:

$ iptables -t nat -L -n
...
Chain DOCKER (2 references)
target     prot opt source               destination
DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:49153 to:172.17.0.2:80

# What your NAT rules might look like when Docker
# is finished setting up a -p 80:80 forward:

Chain DOCKER (2 references)
target     prot opt source               destination
DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:80 to:172.17.0.2:80

You can see that Docker has exposed these container ports on 0.0.0.0, the wildcard IP address that will match any possible incoming port on the host machine. If you want to be more restrictive and only allow container services to be contacted through a specific external interface on the host machine, you have two choices. When you invoke docker run you can use either -p IP:host_port:container_port or -p IP::port to specify the external interface for one particular binding.

Or if you always want Docker port forwards to bind to one specific IP address, you can edit your system-wide Docker server settings (on Ubuntu, by editing DOCKER_OPTS in /etc/default/docker) and add the option --ip=IP_ADDRESS. Remember to restart your Docker server after editing this setting.

Again, this topic is covered without all of these low-level networking details in the Docker User Guide document if you would like to use that as your port redirection reference instead.

Customizing docker0

By default, the Docker server creates and configures the host system's docker0 interface as an Ethernet bridge inside the Linux kernel that can pass packets back and forth between other physical or virtual network interfaces so that they behave as a single Ethernet network.

Docker configures docker0 with an IP address and netmask so the host machine can both receive and send packets to containers connected to the bridge, and gives it an MTU — the maximum transmission unit or largest packet length that the interface will allow — of either 1,500 bytes or else a more specific value copied from the Docker host's interface that supports its default route. Both are configurable at server startup:

• --bip=CIDR — supply a specific IP address and netmask for the docker0 bridge, using standard CIDR notation like 192.168.1.5/24.

• --mtu=BYTES — override the maximum packet length on docker0.

On Ubuntu you would add these to the DOCKER_OPTS setting in /etc/default/docker on your Docker host and restarting the Docker service.

Once you have one or more containers up and running, you can confirm that Docker has properly connected them to the docker0 bridge by running the brctl command on the host machine and looking at the interfaces column of the output. Here is a host with two different containers connected:

# Display bridge info

$ sudo brctl show
bridge name     bridge id               STP enabled     interfaces
docker0         8000.3a1d7362b4ee       no              veth65f9
                                                        vethdda6

If the brctl command is not installed on your Docker host, then on Ubuntu you should be able to run sudo apt-get install bridge-utils to install it.

Finally, the docker0 Ethernet bridge settings are used every time you create a new container. Docker selects a free IP address from the range available on the bridge each time you docker run a new container, and configures the container's eth0 interface with that IP address and the bridge's netmask. The Docker host's own IP address on the bridge is used as the default gateway by which each container reaches the rest of the Internet.

# The network, as seen from a container

$ sudo docker run -i -t --rm base /bin/bash

$$ ip addr show eth0
24: eth0: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 32:6f:e0:35:57:91 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.3/16 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::306f:e0ff:fe35:5791/64 scope link
       valid_lft forever preferred_lft forever

$$ ip route
default via 172.17.42.1 dev eth0
172.17.0.0/16 dev eth0  proto kernel  scope link  src 172.17.0.3

$$ exit

Remember that the Docker host will not be willing to forward container packets out on to the Internet unless its ip_forward system setting is 1 — see the section above on Communication between containers for details.

Building your own bridge

If you want to take Docker out of the business of creating its own Ethernet bridge entirely, you can set up your own bridge before starting Docker and use -b BRIDGE or --bridge=BRIDGE to tell Docker to use your bridge instead. If you already have Docker up and running with its old bridge0 still configured, you will probably want to begin by stopping the service and removing the interface:

# Stopping Docker and removing docker0

$ sudo service docker stop
$ sudo ip link set dev docker0 down
$ sudo brctl delbr docker0

Then, before starting the Docker service, create your own bridge and give it whatever configuration you want. Here we will create a simple enough bridge that we really could just have used the options in the previous section to customize docker0, but it will be enough to illustrate the technique.

# Create our own bridge

$ sudo brctl addbr bridge0
$ sudo ip addr add 192.168.5.1/24 dev bridge0
$ sudo ip link set dev bridge0 up

# Confirming that our bridge is up and running

$ ip addr show bridge0
4: bridge0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state UP group default
    link/ether 66:38:d0:0d:76:18 brd ff:ff:ff:ff:ff:ff
    inet 192.168.5.1/24 scope global bridge0
       valid_lft forever preferred_lft forever

# Tell Docker about it and restart (on Ubuntu)

$ echo 'DOCKER_OPTS="-b=bridge0"' >> /etc/default/docker
$ sudo service docker start

The result should be that the Docker server starts successfully and is now prepared to bind containers to the new bridge. After pausing to verify the bridge's configuration, try creating a container — you will see that its IP address is in your new IP address range, which Docker will have auto-detected.

Just as we learned in the previous section, you can use the brctl show command to see Docker add and remove interfaces from the bridge as you start and stop containers, and can run ip addr and ip route inside a container to see that it has been given an address in the bridge's IP address range and has been told to use the Docker host's IP address on the bridge as its default gateway to the rest of the Internet.

How Docker networks a container

While Docker is under active development and continues to tweak and improve its network configuration logic, the shell commands in this section are rough equivalents to the steps that Docker takes when configuring networking for each new container.

Let's review a few basics.

To communicate using the Internet Protocol (IP), a machine needs access to at least one network interface at which packets can be sent and received, and a routing table that defines the range of IP addresses reachable through that interface. Network interfaces do not have to be physical devices. In fact, the lo loopback interface available on every Linux machine (and inside each Docker container) is entirely virtual — the Linux kernel simply copies loopback packets directly from the sender's memory into the receiver's memory.

Docker uses special virtual interfaces to let containers communicate with the host machine — pairs of virtual interfaces called “peers” that are linked inside of the host machine's kernel so that packets can travel between them. They are simple to create, as we will see in a moment.

The steps with which Docker configures a container are:

1. Create a pair of peer virtual interfaces.

2. Give one of them a unique name like veth65f9, keep it inside of the main Docker host, and bind it to docker0 or whatever bridge Docker is supposed to be using.

3. Toss the other interface over the wall into the new container (which will already have been provided with an lo interface) and rename it to the much prettier name eth0 since, inside of the container's separate and unique network interface namespace, there are no physical interfaces with which this name could collide.

4. Give the container's eth0 a new IP address from within the bridge's range of network addresses, and set its default route to the IP address that the Docker host owns on the bridge.

With these steps complete, the container now possesses an eth0 (virtual) network card and will find itself able to communicate with other containers and the rest of the Internet.

You can opt out of the above process for a particular container by giving the --net= option to docker run, which takes four possible values.

• --net=bridge — The default action, that connects the container to the Docker bridge as described above.

• --net=host — Tells Docker to skip placing the container inside of a separate network stack. In essence, this choice tells Docker to not containerize the container's networking! While container processes will still be confined to their own filesystem and process list and resource limits, a quick ip addr command will show you that, network-wise, they live “outside” in the main Docker host and have full access to its network interfaces. Note that this does not let the container reconfigure the host network stack — that would require --privileged=true — but it does let container processes open low-numbered ports like any other root process. It also allows the container to access local network services like D-bus. This can lead to processes in the container being able to do unexpected things like restart your computer. You should use this option with caution.

• --net=container:NAME_or_ID — Tells Docker to put this container's processes inside of the network stack that has already been created inside of another container. The new container's processes will be confined to their own filesystem and process list and resource limits, but will share the same IP address and port numbers as the first container, and processes on the two containers will be able to connect to each other over the loopback interface.

• --net=none — Tells Docker to put the container inside of its own network stack but not to take any steps to configure its network, leaving you free to build any of the custom configurations explored in the last few sections of this document.

To get an idea of the steps that are necessary if you use --net=none as described in that last bullet point, here are the commands that you would run to reach roughly the same configuration as if you had let Docker do all of the configuration:

# At one shell, start a container and
# leave its shell idle and running

$ sudo docker run -i -t --rm --net=none base /bin/bash
root@63f36fc01b5f:/#

# At another shell, learn the container process ID
# and create its namespace entry in /var/run/netns/
# for the "ip netns" command we will be using below

$ sudo docker inspect -f '{{.State.Pid}}' 63f36fc01b5f
2778
$ pid=2778
$ sudo mkdir -p /var/run/netns
$ sudo ln -s /proc/$pid/ns/net /var/run/netns/$pid

# Check the bridge's IP address and netmask

$ ip addr show docker0
21: docker0: ...
inet 172.17.42.1/16 scope global docker0
...

# Create a pair of "peer" interfaces A and B,
# bind the A end to the bridge, and bring it up

$ sudo ip link add A type veth peer name B
$ sudo brctl addif docker0 A
$ sudo ip link set A up

# Place B inside the container's network namespace,
# rename to eth0, and activate it with a free IP

$ sudo ip link set B netns $pid
$ sudo ip netns exec $pid ip link set dev B name eth0
$ sudo ip netns exec $pid ip link set eth0 up
$ sudo ip netns exec $pid ip addr add 172.17.42.99/16 dev eth0
$ sudo ip netns exec $pid ip route add default via 172.17.42.1

At this point your container should be able to perform networking operations as usual.

When you finally exit the shell and Docker cleans up the container, the network namespace is destroyed along with our virtual eth0 — whose destruction in turn destroys interface A out in the Docker host and automatically un-registers it from the docker0 bridge. So everything gets cleaned up without our having to run any extra commands! Well, almost everything:

# Clean up dangling symlinks in /var/run/netns

find -L /var/run/netns -type l -delete

Also note that while the script above used modern ip command instead of old deprecated wrappers like ipconfig and route, these older commands would also have worked inside of our container. The ip addr command can be typed as ip a if you are in a hurry.

Finally, note the importance of the ip netns exec command, which let us reach inside and configure a network namespace as root. The same commands would not have worked if run inside of the container, because part of safe containerization is that Docker strips container processes of the right to configure their own networks. Using ip netns exec is what let us finish up the configuration without having to take the dangerous step of running the container itself with --privileged=true.

Tools and Examples

Before diving into the following sections on custom network topologies, you might be interested in glancing at a few external tools or examples of the same kinds of configuration. Here are two:

• Jérôme Petazzoni has created a pipework shell script to help you connect together containers in arbitrarily complex scenarios: https://github.com/jpetazzo/pipework

• Brandon Rhodes has created a whole network topology of Docker containers for the next edition of Foundations of Python Network Programming that includes routing, NAT'd firewalls, and servers that offer HTTP, SMTP, POP, IMAP, Telnet, SSH, and FTP: https://github.com/brandon-rhodes/fopnp/tree/m/playground

Both tools use networking commands very much like the ones you saw in the previous section, and will see in the following sections.

Building a point-to-point connection

By default, Docker attaches all containers to the virtual subnet implemented by docker0. You can create containers that are each connected to some different virtual subnet by creating your own bridge as shown in Building your own bridge, starting each container with docker run --net=none, and then attaching the containers to your bridge with the shell commands shown in How Docker networks a container.

But sometimes you want two particular containers to be able to communicate directly without the added complexity of both being bound to a host-wide Ethernet bridge.

The solution is simple: when you create your pair of peer interfaces, simply throw both of them into containers, and configure them as classic point-to-point links. The two containers will then be able to communicate directly (provided you manage to tell each container the other's IP address, of course). You might adjust the instructions of the previous section to go something like this:

# Start up two containers in two terminal windows

$ sudo docker run -i -t --rm --net=none base /bin/bash
root@1f1f4c1f931a:/#

$ sudo docker run -i -t --rm --net=none base /bin/bash
root@12e343489d2f:/#

# Learn the container process IDs
# and create their namespace entries

$ sudo docker inspect -f '{{.State.Pid}}' 1f1f4c1f931a
2989
$ sudo docker inspect -f '{{.State.Pid}}' 12e343489d2f
3004
$ sudo mkdir -p /var/run/netns
$ sudo ln -s /proc/2989/ns/net /var/run/netns/2989
$ sudo ln -s /proc/3004/ns/net /var/run/netns/3004

# Create the "peer" interfaces and hand them out

$ sudo ip link add A type veth peer name B

$ sudo ip link set A netns 2989
$ sudo ip netns exec 2989 ip addr add 10.1.1.1/32 dev A
$ sudo ip netns exec 2989 ip link set A up
$ sudo ip netns exec 2989 ip route add 10.1.1.2/32 dev A

$ sudo ip link set B netns 3004
$ sudo ip netns exec 3004 ip addr add 10.1.1.2/32 dev B
$ sudo ip netns exec 3004 ip link set B up
$ sudo ip netns exec 3004 ip route add 10.1.1.1/32 dev B

The two containers should now be able to ping each other and make connections successfully. Point-to-point links like this do not depend on a subnet nor a netmask, but on the bare assertion made by ip route that some other single IP address is connected to a particular network interface.

Note that point-to-point links can be safely combined with other kinds of network connectivity — there is no need to start the containers with --net=none if you want point-to-point links to be an addition to the container's normal networking instead of a replacement.

A final permutation of this pattern is to create the point-to-point link between the Docker host and one container, which would allow the host to communicate with that one container on some single IP address and thus communicate “out-of-band” of the bridge that connects the other, more usual containers. But unless you have very specific networking needs that drive you to such a solution, it is probably far preferable to use --icc=false to lock down inter-container communication, as we explored earlier.