Static IPs Convention

[tandiljuan]

Hello everybody,

First of all, I want to thanks the team behind this project for all the work you did to make it. Great work!

In my projects, I'm used to work with host names. For this, and to make my life easier, I need to be sure that the VM's or container IPs are not going to change on every reboot... otherwise it would be tedious to update the hosts file every time.

After creating a dummy cluster (k3d cluster create --agents 3 --servers 5 --registry-create potato) and restarting it, this is the list of assigned IPs for each container:

Container           | IP 1st run  | IP 2nd run
--------------------+-------------+-------------
k3d-potato-server-0 | 172.20.0.2  | 172.20.0.2
k3d-potato-server-1 | 172.20.0.3  | 172.20.0.3
k3d-potato-server-2 | 172.20.0.4  | 172.20.0.4
k3d-potato-server-3 | 172.20.0.5  | 172.20.0.5
k3d-potato-server-4 | 172.20.0.6  | 172.20.0.6
k3d-potato-agent-0  | 172.20.0.7  | 172.20.0.9
k3d-potato-agent-1  | 172.20.0.8  | 172.20.0.8
k3d-potato-agent-2  | 172.20.0.9  | 172.20.0.7
k3d-potato-serverlb | 172.20.0.11 | 172.20.0.10
k3d-potato-registry | 172.20.0.10 | 172.20.0.11

Is it possible to define a convention where we could know/guess what's going to be the IP or at least that it won't change on any cluster restart? For example:

Container           | Proposed IPs
--------------------+-------------
k3d-potato-server-0 | 172.20.0.1
k3d-potato-server-1 | 172.20.0.2
k3d-potato-server-2 | 172.20.0.3
k3d-potato-server-3 | 172.20.0.4
k3d-potato-server-4 | 172.20.0.5
k3d-potato-agent-0  | 172.20.1.1
k3d-potato-agent-1  | 172.20.1.2
k3d-potato-agent-2  | 172.20.1.3
k3d-potato-serverlb | 172.20.2.1
k3d-potato-registry | 172.20.3.1

With this in mind, the IP could also be listed with the k3d node list command (or with the --output argument). Of course it also could be get with docker (docker inspect k3d-potato-server-1 --format '{{$network := index .NetworkSettings.Networks "k3d-potato"}}{{$network.IPAddress}}').

Thanks and regards!

[iwilltry42]

Hey @tandiljuan , thanks for starting this discussion.
Thanks for the nice feedback :)
This is actually a tough one, we're now following in this issue over here: #550
Docker does not provide an easy way to dynamically assign IPs that should then stick with a container forever, so we'll have to come up with something there.

[tandiljuan]

Hi @iwilltry42 !

Sorry, I though that this was possible because it can be done using the Docker CLI (https://docs.docker.com/config/containers/container-networking/#ip-address-and-hostname). But I guess it can't be done using the Docker API.

Thanks for the quick response!

[iwilltry42]

Sure thing 👍🏼
Well yes, it is possible to assign static IPs upfront. But that means, that k3d has to handle IP assignment, etc (IPAM).
Would've been awesome, if docker took care of assigning the IPs like it does now and then just keeps those static across container restarts 🤷‍♂️
K3d clusters are dynamic, so we have to take care of topology changes, etc.
Let's see how we can easily approach this 👍🏼