When a TLS/SSL server requests a client certificate (see
SSL_CTX_set_verify_options()), it sends a list of CAs, for which it will accept certificates, to the client.This list must explicitly be set using SSL_CTX_set_client_CA_list() for ctx and SSL_set_client_CA_list() for the specific ssl. The list specified overrides the previous setting. The CAs listed do not become trusted ( list only contains the names, not the complete certificates); use SSL_CTX_load_verify_locations(3) to additionally load them for verification.
If the list of acceptable CAs is compiled in a file, the SSL_load_client_CA_file(3) function can be used to help importing the necessary data.
SSL_CTX_add_client_CA() and SSL_add_client_CA() can be used to add additional items the list of client CAs. If no list was specified before using SSL_CTX_set_client_CA_list() or SSL_set_client_CA_list(), a new client CA list for ctx or ssl (as appropriate) is opened.
These functions are only useful for TLS/SSL servers.