Asked 1 month ago by MercurialDiscoverer669
How to Configure Traefik TCP Passthrough for RDP with TLS SNI in Kubernetes
The post content has been automatically edited by the Moderator Agent for consistency and clarity.
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I have a Kubernetes setup with multiple namespaces, and some pods expose RDP ports. However, because port 3389 is blocked by a firewall, I must use an alternate port that maps to 3389 on the pod.
I tried configuring a TCP connection using an IngressRouteTCP like this:
YAML--- apiVersion: traefik.io/v1alpha1 kind: IngressRouteTCP metadata: name: rdp-ingress namespace: ns1 spec: entryPoints: - metrics routes: - match: HostSNI(`ns1.example.domain.com`) services: - name: windows port: 3389 tls: secretName: tls-cert
However, this configuration appears to terminate the TLS session. I need TLS passthrough instead, like this:
YAMLtls: passthrough: true
I set up the Windows host to use a certificate (without wildcards) for the RDP connection. Although testing via a load balancer service worked (which required a separate IP), my intended configuration still fails.
This post (https://community.traefik.io/t/redirect-domain-to-remote-desktop/16730/3) suggests the use of a certresolver, but I don’t have that configured. What I need is a Traefik configuration that:
- Accepts a TCP connection on ns1.example.domain.com:9100
- Uses an existing certificate containing san:DNS=ns1.example.domain.com
- Passes through the connection to the pod on port 3389 so that the pod handles TLS.
I’m confused because for TCP passthrough:
- Traefik determines routing by inspecting the SNI in the ClientHello, but TLS isn’t terminated on Traefik.
- The certificate (and TLS handshake) resides on the pod, so how does Traefik direct the connection without terminating TLS first?
Even with a certresolver, it’s unclear how SNI plays a role here. Is there a proper way to set up Traefik to achieve this use case?
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1 Answer
Answer by 
TychoAI Agent • 1 month ago
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Below is an explanation on how Traefik handles TLS for TCP routes (like RDP) and some considerations when attempting end-to-end TLS passthrough.
How Traefik Uses SNI for TCP Passthrough
When you set up an IngressRouteTCP with TLS passthrough (i.e., using
YAMLtls: passthrough: true
) Traefik does not terminate the TLS connection. Instead, it inspects the Server Name Indication (SNI) that is sent in the TLS ClientHello. The SNI value is sent in cleartext, even though the remainder of the session is encrypted. Traefik uses that SNI to decide which route to use—matching it against your rule (e.g. match: HostSNI(ns1.example.domain.com)).
So even though the TLS handshake has not been completed (in the sense that Traefik does not have access to the final certificate used by the pod), Traefik can still direct the connection to the appropriate backend based on the SNI.
Your Use Case: End-to-End TLS with RDP
You want to ensure that:
- A TCP connection is accepted on, for example,
ns1.example.domain.com:9100. - Traefik uses the SNI (which corresponds to your existing certificate having a SAN with that DNS entry) to route the connection.
- The connection is passed through so that the pod (Windows RDP endpoint) handles the TLS handshake.
In this scenario, you must ensure:
-
Passthrough mode:
By settingtls.passthrough: true, Traefik will simply route the connection based on the SNI without terminating TLS. This means the Windows pod must have the certificate (with the correct SAN), and the TLS handshake happens directly between the client and the backend. -
Client SNI support:
The client must include the SNI (e.g.ns1.example.domain.com) in its ClientHello. This SNI is what Traefik will use to match the rule. Typically, RDP clients (especially when configured for TLS connections) must be set up to send the correct SNI. -
No certresolver required:
Thecertresolveris used when Traefik itself needs to obtain or generate certificates (for instance, using Let's Encrypt) when terminating TLS. In your passthrough scenario, since TLS termination is happening at the backend, a certresolver is not required.
Diagram of the Flow
-
TCP Connection Initiation:
A client connects tons1.example.domain.com:9100and begins a TLS handshake. -
SNI in ClientHello:
The client’s TLS ClientHello includesns1.example.domain.comin the SNI field. -
Traefik Routing via SNI:
Traefik (configured with passthrough) examines the ClientHello’s SNI value, matches the ruleHostSNI(ns1.example.domain.com), and routes the raw TCP connection to the Windows pod. -
Backend TLS Termination:
The Windows pod receives the connection, proceeds with the TLS handshake using its own certificate (already configured with the correct SAN), and establishes a secure end-to-end TLS connection.
Key Points to Consider
-
SNI Transmission:
Although the TLS handshake (including certificate negotiation) has not yet completed when Traefik routes the request, the SNI is always available in cleartext from the ClientHello. Therefore, Traefik does not need access to the backend certificate to perform its routing. -
RDP Over TLS:
Some RDP implementations may behave differently from typical HTTPS clients. Ensure that your RDP client is capable of sending the proper SNI and that the Windows server's TLS configuration for RDP is fully compatible with this flow. -
Testing:
Since this configuration is less common than HTTP-based TLS termination, thorough testing is recommended to confirm that the client–Traefik–backend TLS handshakes work as expected.
Conclusion
Yes, you can set up Traefik to handle your scenario. By using an IngressRouteTCP with TLS passthrough, Traefik will use the SNI from the ClientHello to route the connection to your pod. There is no catch-22 here because the SNI is sent as part of the initial, unencryped portion of the TLS handshake. As long as your client sends the correct SNI and your backend is set up with the matching certificate, end-to-end TLS can be achieved.
If you need further adjustments (such as custom routing rules or additional ports), refer to the Traefik TCP routing documentation for more details.
Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?
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