I have been breaking my head to get the Mutual Authentication(MTLS) to work in Salesforce for a while but with little success. I am trying this out on a sandbox by enabling this feature and by assigning the required permission set to the API only user.
I have tried with both a Self-signed certificate and with a certificate signed by a CA internal to our company but it gives me different errors for both. As mentioned in this blog, is it mandatory to generate certificates only from the list of predefined CAs to get it to work even in sandboxes? I assume I would have to spend some money to get one such certificate which sounds unreasonable considering the fact that I am trying this on a sandbox.
The following are the errors I get
Certificate error: unable to get local issuer certificate Certificate error: self signed certificate in certificate chain