OpenSIPS: Enhancing TLS Client Domain With Certificate Flexibility

Hey guys! Let's dive into an interesting challenge in the world of OpenSIPS and TLS (Transport Layer Security). We're talking about a feature request that aims to make OpenSIPS more flexible when handling TLS client domains and certificates. As you know, ensuring secure communication is crucial in the world of VoIP and SIP. This proposed enhancement directly addresses upcoming changes in how TLS certificates are issued and used, particularly concerning client authentication. The current setup, while common, faces limitations that this feature request seeks to overcome. By allowing more flexible certificate handling, OpenSIPS can maintain robust security while adapting to evolving industry standards and practices. It's all about making sure your SIP infrastructure stays secure and compatible, without unnecessary headaches.

The Problem: Certificates, Client Auth, and the EKU Dilemma

Alright, let's get into the nitty-gritty. The core issue revolves around the usage of certificates in OpenSIPS for client authentication. Many of us are currently using the same certificate for both client and server authentication. This is a pretty standard practice. However, things are about to get a little tricky. Major changes are coming to TLS certificates, especially those issued by public Certificate Authorities (CAs). The clientAuth Extended Key Usage (EKU), which is super important for client authentication, is being phased out in many publicly trusted certificates. This shift means that certificates might soon lose their ability to handle client authentication effectively. When OpenSIPS is configured for a client_domain, it currently requires a certificate, even if mutual TLS (mTLS) isn't strictly necessary. This setup often leads to using the server certificate, which, after the aforementioned changes, might not include the crucial clientAuth EKU. Consequently, when the server asks for a client certificate, OpenSIPS might send the server certificate, and this could cause the session to be rejected.

This is where things get a bit complicated. As the industry evolves, the old ways of doing things might not always cut it. The feature request is designed to solve this issue and ensure that OpenSIPS keeps its cool in the face of these changes. In essence, it's about providing a more adaptable and secure solution for how OpenSIPS handles client certificates in TLS configurations. This helps maintain compatibility and security.

The Solution: Flexible Certificate Handling

So, what's the game plan? The proposed solution is pretty straightforward. It suggests allowing the certificate and private_key fields to be set to NULL in the tls_mgm module when the type is set to 2 (client). This allows OpenSIPS to send a certificate message containing no certificates when a client certificate isn't required. This aligns with RFC 8446 4.4.2.4 and RFC 5426 7.4.6, which specifies that if no suitable certificate is available, the client MUST send a certificate message containing no certificates. This is a pretty elegant solution because it allows for a more flexible and adaptive approach to certificate handling within OpenSIPS.

Implementation Details: Where Does the Magic Happen?

As for the implementation, the changes would likely focus on the proto_tls or tls_mgm components. The key is to allow certificate and private_key fields to accept NULL values when the client domain is configured. This small tweak can have a big impact, enhancing OpenSIPS's ability to navigate the evolving landscape of TLS certificates. The change is intended to integrate with existing behavior, providing a smoother transition and more robust security. This approach avoids any radical changes to the system's core functionality, which helps with backward compatibility.

Exploring Alternatives: What Other Options Are There?

Of course, we've considered other options. Let's explore them. One alternative is generating publicly trusted TLS certificates with the clientAuth EKU. However, this is rapidly becoming impossible, as public CAs are phasing out this option. Another option is using self-signed certificates or setting up a private CA. While this works in theory, it can lead to compatibility issues with some service providers. Therefore, the proposed solution is the most practical and future-proof approach.

The Challenges of Alternatives

Let's go deeper into the downsides of the alternatives. Generating certificates with clientAuth EKU is quickly becoming a non-starter because public CAs are no longer supporting it. Relying on self-signed certificates or private CAs introduces complexities. They require that the other party accepts your CA, which can cause issues with specific providers. You might run into trust problems and create extra setup steps, which you don't really want. These alternatives aren't always a good fit when you need things to