I cannot connect to external mongodb
server from my docker swarm
cluster.
As I understand this is because of cluster uses overlay network driver. Am I right?
If not, how does docker overlay driver works and how can I connect to external mongodb
server from cluster?
Docker uses embedded DNS to provide service discovery for containers running on a single Docker Engine and tasks running in a Docker Swarm. Docker Engine has an internal DNS server that provides name resolution to all of the containers on the host in user-defined bridge, overlay, and MACVLAN networks.
An overlay network is any virtual layer on top of physical network infrastructure. This may be as simple as a virtual local area network (VLAN) but typically refers to more complex virtual layers from software-defined networking (SDN) or a software-defined wide area network (SD-WAN).
The overlay network driver creates a distributed network among multiple Docker daemon hosts. This network sits on top of (overlays) the host-specific networks, allowing containers connected to it (including swarm service containers) to communicate securely when encryption is enabled.
Q. How does the docker overlay driver work?
I would recommend this good reference for understanding docker swarm network overlay, and more globally, Docker's architecture.
This states that:
Docker uses embedded DNS to provide service discovery for containers running on a single Docker Engine and tasks running in a Docker Swarm. Docker Engine has an internal DNS server that provides name resolution to all of the containers on the host in user-defined bridge, overlay, and MACVLAN networks.
Each Docker container ( or task in Swarm mode) has a DNS resolver that forwards DNS queries to Docker Engine, which acts as a DNS server.
So, in multi-host docker swarm mode
, with this example setup :
In this example there is a service of two containers called
myservice
. A second service (client
) exists on the same network. Theclient
executes twocurl
operations for docker.com andmyservice
.These are the resulting actions:
- DNS queries are initiated by
client
fordocker.com
andmyservice
.- The container's built-in resolver intercepts the DNS queries on
127.0.0.11:53
and sends them to Docker Engine's DNS server.myservice
resolves to the Virtual IP (VIP) of that service which is internally load balanced to the individual task IP addresses. Container names resolve as well, albeit directly to their IP addresses.docker.com
does not exist as a service name in the mynet network and so the request is forwarded to the configured default DNS server.
Back to your question:
How can I connect to an external mongodb server form cluster?
For your external mongodb
(let's say you have a DNS for that mongodb.mydomain.com
), you are in the same situation as the client
in above architecture, wanting to connect to docker.com
, except that you certainly don't wan't to expose that mongodb.mydomain.com
to the entire web, so you may have declared it in your internal cluster DNS server.
Then, how to tell docker engine to use this internal DNS server to resolve mongodb.mydomain.com
?
You have to indicate in your docker service
task that you want to use an internal DNS server, like so:
docker service create \ --name myservice \ --network my-overlay-network \ --dns=10.0.0.2 \ myservice:latest
The important thing here is --dns=10.0.0.2
. This will tell the Docker engine to use the DNS server at 10.0.0.2:53
as default if it can not resolve the DNS name in the VIP.
Finally, when you say :
I cannot connect to external mongodb server from my docker swarm cluster. As I understand this is because of cluster uses overlay network driver. Am I right?
I would say no, as there is a built in method in docker engine
to forward unknown DNS name coming from overlay network
to the DNS server you want.
Hope this helps!
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With