proposal: crypto/tls: dynamically reload root certificate authorities

[p0lyn0mial]

Proposal Details

The current state of the tls.Config.RootCAs presents a challenge when it comes to dynamically reloading the Root Certificate Authorities. The existing approaches involve either disabling automatic certificate and host verification or limiting functionality to non-proxied requests. This proposal seeks to address this limitation by introducing a mechanism for dynamic reloading of Root CAs.

The current workaround for dynamically updating Root CAs involves using the VerifyPeerCertificate, but it requires disabling automatic certificate and host verification by setting the InsecureSkipVerify field to true. This approach might introduce some security risks if the custom implementation is not correct.

Alternatively, a custom TLS Dialer could be used. However, this approach falls short when dealing with proxied requests, limiting its applicability in scenarios where proxies are used.

To enhance the flexibility of TLS configurations, this issue proposes modifying the behaviour of tls.Config.RootCAs to support dynamic reloading by introducing a function similar to GetClientCertificate.

Note that in the past, a similar request was rejected. Since commenting on that issue is disabled, I decided to create a new issue to see if something has changed. Having such a mechanism in the standard library would help the Kubernetes community address kubernetes/kubernetes#119483.

Update:

It largely depends on how the tls.Config.RootCAs is used internally. Another solution would be to accept an interface instead of *x509.CertPool and allow clients to inject a thread-safe implementation, enabling trust reloading.

[seankhliao]

cc @golang/security

[espadolini]

The discussion at #22836 mentions that it's possible to replicate the normal certificate verification behavior by using specifying VerifyConnection and setting InsecureSkipVerify, but from what I can tell there's no inherent way for VerifyConnection to have access to the ServerName in the tls.Config in use if it's an IP address (tls.ConnectionState.ServerName is set to the value sent by the client as SNI, but the spec prescribes that IP addresses should not be sent as SNI, and the Go TLS handshake follows that).

Obviously a VerifyConnection callback will have access to the ServerName as specified in the original tls.Config, but the general behavior with TLS clients in the ecosystem seems to be that the calling code is intended to pass a tls.Config with no ServerName set, and the callee will clone the config and set the server name right before initiating the handshake, which will obviously not work with a custom VerifyConnection that only has a reference to the original tls.Config.