How to use the Docker host IP address within an application container (and allow communication across clusters)

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Published on - last updated on September 26th 2019 - Listed in Docker Rancher Containers Linux

When application containers talk to each other, they usually talk via the docker0 interface of the host. Either with other containers running on the same host or with containers running in the same cluster (Kubernetes or Swarm for example). In the latter case the cluster nodes use a trick/workaround using a third party communication channel, e.g. IPSec tunnels or CNI for a specific subnet to allow container communication across nodes.

Application containers cross-node communication using cni

This cross-node communication works (usually) very well and is (normally) hassle-free, as long as they run in the same LAN.

But my application is deployed across the world...

But what if application containers are physically separated into different locations and the hosts are not part of the same (Kubernetes) cluster?

In this particular scenario the same application is deployed into two clusters in two different locations. Because the application is cluster-aware it knows the other cluster peers from a central storage and tries to communicate with them. And because the application is started inside the container with its own internal address, the application now tries to connect to other containers using the internal address, e.g. And this fails, of course.

Internal container communication across locations and clusters

There are a couple of possibilities to achieve this. But depending on how the application inside the container works, not all of them might lead to a solution.

Use public DNS

The first obvious way would be to use public DNS records. Once the application is started, it announces its own public DNS record into the cluster storage. The public DNS obviously needs to be known to the container during startup. This could be a shell-script which determines the own public DNS record based on the public IP (using curl for example) or this information could come from an environment variable during startup of the container.

Container communication using public dns

The problem in this potential solution? If you run this application in multiple containers on each location, you might end up with a lot of DNS entries and even more Ingress rules. And you might even lose deployment flexibility and salability, depending on your cluster configuration.

This would have been a solution if the application is only deployed once on each location. In our case, this would not work out.

Use a cross-cluster communication method

Rancher recently announced a new project called Submariner. The goal of this project is to enable communications between Kubernetes clusters.

Communication across Kubernetes cluster using Submariner

Image Source:

However Submariner is not yet considered production ready, so one should be cautious. That's also the reason why this potential solution was not chosen.

VPN and announcing the Docker Host IP

The architecture drawing shows a VPN tunnel between the two locations. This means that the Docker hosts on each location are able to communicate with each other, however the container's can't. Sure, the hosts of each location could all be part of the same cluster, but to avoid cluster issues with a flaky VPN connection, separate location-based clusters were built (and this proved to be a good choice!).

So what if the application wouldn't announce its own container IP address, but use the primary IP address of the Docker host instead? This, in combination with exposing the application port to the host (and not using HTTP Ingress), allows communication between the containers across locations through the VPN tunnel. Because the Docker host addresses are used, all traffic happens through the VPN tunnel and is on each host translated to forward the traffic to the corresponding container.

This scenario even works when the application is deployed several times on each location, as long as it is not deployed more than once on the same host.

Communication between containers using host address and expose port

But how can the container dynamically know the main IP address of its host? Especially if it is not known which interface is the primary one (there may be eth0, ens160, eno16777984, bond1, etc)?

One way we came up with is to use the hostname of the host. The primary IP address can be found in /etc/hosts:

ckadm@dockerhost1:~$ grep $(hostname) /etc/hosts    dockerhost1

Note: Of course this requires a correctly setup hostname and /etc/hosts!

Using this method, the hosts's primary IP address can be read and saved into an environment variable during start of a container:

root@dockerhost1:~# docker run -it -e "DOCKER_HOST_IP=$(grep $(hostname) /etc/hosts | awk '{print $1}')" ubuntu /bin/bash

 Inside the container, the host's primary IP can be shown using the environment variable:

root@fcbb308c580c:/# echo $DOCKER_HOST_IP

Of course the application port for cluster communication needs to be exposed, too.

The containers are started on both locations using an expose port 8888 on the host, resolving to port 80 inside the container.

root@dockerhost1:~# docker run -it -p8888:80 -e "DOCKER_HOST_IP=$(grep $(hostname) /etc/hosts | awk '{print $1}')" nginx /bin/bash
root@dockerhost2:~# docker run -it -p8888:80 -e "DOCKER_HOST_IP=$(grep $(hostname) /etc/hosts | awk '{print $1}')" nginx /bin/bash

curl and ping were installed in both containers to make a test connection across the locations:

root@0e73605363b6:/# apt-get update && apt-get install curl iputils-ping net-tools
root@3d7551f8ed7f:/# apt-get update && apt-get install curl iputils-ping net-tools

The host IP address is verified:

root@0e73605363b6:/# echo $DOCKER_HOST_IP

root@3d7551f8ed7f:/# echo $DOCKER_HOST_IP

Note: The end application with adjusted settings would now announce this IP address and the exposed listener port to the central storage.

Now the communication using the host IP's can be tested:

root@0e73605363b6:/# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=61 time=10.4 ms
64 bytes from icmp_seq=2 ttl=61 time=9.47 ms
64 bytes from icmp_seq=3 ttl=61 time=9.34 ms

root@3d7551f8ed7f:/# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=61 time=10.4 ms
64 bytes from icmp_seq=2 ttl=61 time=9.45 ms
64 bytes from icmp_seq=3 ttl=61 time=9.38 ms

Checking nginx is running and listening:

root@0e73605363b6:/# nginx
root@0e73605363b6:/# netstat -lntup
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
tcp        0      0    *               LISTEN      2852/nginx: master 

root@3d7551f8ed7f:/# nginx
root@3d7551f8ed7f:/# netstat -lntup
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name   
tcp        0      0    *               LISTEN      2847/nginx: master 

And testing communication via the exposed port, through the VPN tunnel, using curl:

root@0e73605363b6:/# curl -I
HTTP/1.1 200 OK
Server: nginx/1.17.4
Date: Wed, 25 Sep 2019 10:22:23 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 24 Sep 2019 14:49:10 GMT
Connection: keep-alive
ETag: "5d8a2ce6-264"
Accept-Ranges: bytes

root@3d7551f8ed7f:/# curl -I
HTTP/1.1 200 OK
Server: nginx/1.17.4
Date: Wed, 25 Sep 2019 10:26:12 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 24 Sep 2019 14:49:10 GMT
Connection: keep-alive
ETag: "5d8a2ce6-264"
Accept-Ranges: bytes

What about Dockerfile?

Using docker run the environment variables can set with dynamic values, for example from a command output (as shown before). Unfortunately this does not work when images are built using a Dockerfile. A workaround (for the workaround) needs to be found.

By default the Docker container runs in a "managed" network in a subnet used by all the containers. But there's also the possibility to use the host's network directly. In our infrastructure we use Rancher and the service settings allow to use the Host network:

Rancher Service using network of Docker Host

Inside the container this will now look like this:

root@dockerhost2:/# ip a sh dev eth0
2: eth0: mtu 9001 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 02:6d:d2:e4:74:83 brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::6d:d2ff:fee4:7483/64 scope link
       valid_lft forever preferred_lft forever

root@dockerhost2:/# hostname

 So both IP address and hostname are taken from the host. This way this information can be used during the entrypoint script:

# Use Docker host hostname to find primary IP address of Docker host
# note: requires Docker container to use host network (Rancher -> Service -> Networking -> Network: Host)
DOCKER_HOST_IP=$(grep $(hostname) /etc/hosts | awk '{print $1}')

This variable $DOCKER_HOST_IP can then be used for further actions.

What did we learn?

Using the Docker host's primary IP address and expose ports can serve as a workaround to enable a cross-cluster-communication between containers. In this example a VPN tunnel was used, but this could also be replaced by a public IP (and/or DNAT) to the Docker host. However the most important point is the application itself: Although the application was built cluster-aware, it did not count the possibility to run a cluster across multiple networks, at least not inside an application container.

Either the application needs to allow such scenarios or on system side a workaround needs to be implemented. We chose the latter, ergo this article ;-).

If the whole Docker/Kubernetes infrastructure is too much hassle and you simply want to enjoy an available Kubernetes cluster to deploy your applications, check out the Private Kubernetes Cloud Infrastructure at Infiniroot!

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