how to get common name from client cert in TLS connection instead of HTTPS

[Albert Zhang]

how to get common name from client cert in TLS connection instead of HTTPS. I am using TLS not https and want to get common name from client cert using nginx plus ami on was, 
I am using AWS elb(ssl)+nginx client certificate ssl I know use $ssl_client_s_dn but how to get/compare the value here is my config:
stream {
    upstream stream_backend {
         server 10.252.1.131:1983;
         server 10.252.1.131:2983;
    }
    server {
        listen                4443 ssl;
        proxy_pass            stream_backend;
 proxy_ssl on;
    proxy_ssl_certificate         /etc/ssl/certs/server.crt;
        proxy_ssl_certificate_key     /etc/ssl/certs/server.key;
        proxy_ssl_protocols           TLSv1 TLSv1.1 TLSv1.2;
        proxy_ssl_ciphers             HIGH:!aNULL:!MD5;
#  proxy_ssl_client_certificate /etc/ssl/certs/ca.pem;
proxy_ssl_trusted_certificate /etc/ssl/certs/ca.pem;
#proxy_ssl_session_reuse on;
    proxy_ssl_verify       on;
    proxy_ssl_verify_depth 4;
#    proxy_ssl_verify_client optional;
        ssl_certificate       /etc/ssl/certs/server.crt;
        ssl_certificate_key   /etc/ssl/certs/server.key;
        ssl_protocols         SSLv3 TLSv1 TLSv1.1 TLSv1.2;
        ssl_ciphers           HIGH:!aNULL:!MD5;
        ssl_session_cache     shared:SSL:20m;
        ssl_session_timeout   4h;
        ssl_handshake_timeout 30s;
     }

}

albert