Всё разобрался.

proxy_pass $http_referer;
proxy_set_header x_ssl_client_m_serial $ssl_client_serial;
proxy_set_header x_ssl_client_s_dn $ssl_client_s_dn;
proxy_set_header x_ssl_client_i_dn $ssl_client_i_dn;
proxy_set_header x_ssl_cipher $ssl_cipher;