Docker swarm - DNS service discovery, looking to internals

Question

First of all:
I have some poor experience with docker swarm (I mean I touch in production env). I read a lot of about it, and I know concepts like veth, overlay, labels-pinning, vtep, bridge and so on. I know also that docker swarm use some distributed key-value storage to deal with management of cluster.

However, there is something that I don't understand: service disorvery/DNS/resolving service name.

How does it work? Where is this DNS server placed? Who cares to resolve service names?
Is it possible to read the content of distributed key-value storage?

Answer 1

How does it work? Where is this DNS server placed? Who cares to resolve service names?

It's embedded in the docker daemon itself. Every container which is part of a user defined network, does is name resolving requests to 127.0.0.11.
https://docs.docker.com/v17.09/engine/userguide/networking/configure-dns/

Is it possible to read the content of distributed key-value storage?

Docker is using libkv. But I'm not sure if it's possible to bypass the docker daemon and access it.
https://github.com/docker/libkv

Answer 2

The DNS in Docker Swarm overlay networks works just like docker-compose bridge networks. Services resolve inside the same network by their service name. Other things like Swarm VIP and Routing Mesh make the whole solution slightly different but don't directly affect DNS resolution.

The Swarm raft log isn't meant to be easily read, but it's not more than just the service definitions of services and networks you create in Swarm. I've never needed to look at it directly in a production system.

Here's a 3.5 training video on all things Swarm (including lots of network details) from former Docker engineer Jerome Petazzoni and AJ Bowen

Also, Laura Frank has some details from last years DockerCon on how the raft log and consensus works and might point to some tools if you want to look under the hood.