nginx client authentication with 2 intermediate CAs

[hatlam]

I'm trying to get nginx to verify client certificate issued through the
following chain, with self-signed root: Root CA => Signing CA => Subordinate
CA => Client cert.

I installed root_CA.crt on the server, and on the client side, the certs are
concatenated with cat client.crt subordinate_CA.crt signing_CA.crt >
cert-chain.pem. My nginx setting looks like this:

ssl_client_certificate /path/to/root_CA.crt;
ssl_verify_client on;
ssl_verify_depth 3;

I tried to connect with curl -k server.url:443 --cert cert-chain.pem but it
gives me error curl: (35) error reading X.509 key or certificate file. If I
try that with --key client.key then it gives me 400 Bad Request. I also
tried to test with openssl s_client and the result is similar.

I've verified that the nginx setting works if I have no intermediate CA,
i.e., Root CA => Client cert. It also works if my intermediate CA certs are
installed on the server and only the leaf cert is on the client side.
However, in our case, the Signing CA and Subordinate CA certs cannot be
installed on server ahead of time. Any idea what to try next?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,264942,264942#msg-264942