Multiple Kubernetes Services Using Same Port Without SNI
I am running a bare metal Kubernetes cluster on k3s with Kube-VIP and Traefik. This works great for services that use SSL/TLS as Server Name Indication (SNI) can be used to reverse proxy multiple services listening on the same port. Consequently, getting Traefik to route multiple web servers receiving traffic on ports 80 or 443 is not a problem at all. However, I am stuck trying to accomplish the same thing for services that just use TCP or UDP without SSL/TLS since SNI is not included in TCP or UDP traffic.
I tried to setup Forgejo where clients will expect to use commands like git clone git@my.forgejo.instance.... which would ultimately use SSH on port 22. Since SSH uses TCP and Traefik supports TCPRoutes, I should be able to route traffic to Forgejo's SSH entry point using port 22, but I ran into an issue where the SSH service on the node would receive/process all traffic received by the node instead of allowing Traefik to receive the traffic and route it. I believe that I should be able to change the port that the node's SSH service is listening on or restrict the IP address that the node's SSH service is listening on. This should allow Traefik to receive the traffic on port 22 and route that traffic to Forgejo's SSH entry point while also allowing me to SSH directly into the node.
However, even if I get that to work correctly, I will run into another issue when other services that typically run on port 22 are stood up. For example, I would not be able to have Traefik reverse proxy both Forgejo's SSH entry point and an SFTP's entry point on port 22 since Traefik would only be able to route all traffic on port 22 to just one service due to the lack of SNI details.
The only viable solution that I can find is to only run one service's entry point on port 22 and run each of the other services' entry points on various ports. For instance, Forgejo's SSH entry point could be port 22 and the SFTP's entry point could be port 2222 (mapped to the pod's port 22). This would require multiple additional ports be opened on the firewall and each client would need its configuration and/or commands modified to connect to the service's a non-standard port.
Another solution that I have seen is to use other services like stunnel to wrap traffic in TLS (similar to how HTTPS works), but I believe this will likely lead to even more problems than using multiple ports as every client would likely need to be compatible with those wrapper services.
Is there some other solution that I am missing? Is there something that I could do with Virtual IP addresses, multiple load balancer IP addresses, etc.? Maybe I could route traffic on Load_Balancer#1_IP_Address:22 to Forgejo's SSH entry point and Load_Balancer#2_IP_Address:22 to SFTP's entry point?
tl;dr: Is it possible to host multiple services that do not use SSL/TLS (ie: cannot use SNI) on the same port in a single Kubernetes cluster without using non-standard ports and port mapping?
I guess I need to dig in a little deeper. I am currently only using Kube-VIP to provide a single IP address for the control plane. I think I may have it configured wrong though since that same IP address is the single load balancer IP used by Traefik.
I have struggled finding good documentation, hints, tutorials, etc. setting up Kube-VIP with Virtual IPs. Is there anything that you are aware of that might provide some assistant in setting that up correctly?
Okay, I'll try explaining it. Yes, there is especially for this very little documentation, so... Yeah.
You start by installing kube-vip into your cluster. Make sure to configure it correctly, so the uplink interface of your workers is being used for the vip, but not e.g. internal ones (see the env vars "vip_interface"). Make sure to enable service based functions and the respective election mechanism ("svc_enable", "vip_leaderelection"). I would also recommend the ARP usage, because others I've never tested.
Then you create a new "LoadBalancer"-service in k8s, on which you also set the "loadBalancerIP" field with the desired IPv4-VIP. Due to the previous kube-vip configuration, it should pick that up. You may take a look into its operators logs to learn more.
Theoretically that's it. Now one of your nodes will start serving the service-port under the vip. The service may target every TCP/UDP traffic, not only Traefik.
There is one more thing: The field "externalTrafficPolicy" on the LB-service allows you to disable any kind of internal routing via your CNI if set to "local", so you will even be able to see the real source IPv4 of your clients. Be careful with this on non kube-vip services, as nodes without the targeted pods will not be able to serve the traffic. Kube-vip only promotes nodes to serve the vip, if it also serves the pod targeted by it (see its docs/config).
I am unsure if I understood everything correctly, but I believe I am already doing everything that you mentioned. I followed the Kube-VIP's ARP daemonset's documentation. The leader election works. I am not using Kube-VIP for load balancing though. Instead, I am using Traefik, which is using the same IP address that was assigned to the control plane during both k3s's and Kube-VIP's setup. However, I am unable to get any additional VIP addresses to properly route to Traefik.
Even if I did get the additional VIP addresses working, I think I still have one last issue to overcome. I can control the local network's DNS so that service#1 is assigned VIP#1 and service#2 assigned VIP#2. However, how would this be handled for traffic received externally? If the external/public DNS has service#1 and service#2 assigned to the network's public IP address, both service's traffic would be received by the router/firewall on port 22. The router/firewall could forward traffic on port 22 to (presumably) a single IP address, which would only allow service#1 or service#2 (but not both) to receive traffic publicly, correct?
Ah yes, I see. Because TCP has no SNI built-in this is not really possible.
You could try IPv6, as within even a single /64 routable prefix you can choose the address section freely. Also take a look at overlay-vpn solutions like Netbird: They allow you to offer you multiple clients, which you could use to assign multiple IPv4 to your server and then routing them differently (you mentioned installing client software before)...
Finally, I'm not sure why you would inject Treafik into the networking chain. In the end is the direct, kernel-space connection always faster than having an user-space proxy in between.
I had not thought about using IPv6 for this. It's definitely something that I would need to research more as I know that this would expose my attack surface and may require an overhaul of the network (or at least a very thorough review).
I'm not sure I understand the concern about Traefik. I am using it as a reverse proxy and forcing HTTPS for all applicable services (which unfortunately does not apply to this particular situation). I am honestly a little confused about the control plane, tls-san, gateway, load balancer, ingress, etc. and how they all work together. I may not be using Traefik as the Load Balancer and instead have Kube-VIP as the LoadBalancer. I did not configure Kube-VIP any particular way for Load Balancing, but I did configure Traefik with a few Load Balancer specific options. When I tried to setup Kube-VIP with the additional IP addresses for load balancing, I was unable to get k3s to work correctly so I assumed that Traefik was my Load Balancer instead of Kube-VIP.