How can I let nginx log the used SSL/TLS protocol and ciphersuite?

My goal is to ensure proper security for clients connecting to my nginx. I'm following Mozilla's guide to configure TLS properly on my nginx installation, but I don't have an overview of the actual protocols/ciphersuites being used in practice.

What I have now:

server {
    listen 443;
    ssl on;
    ssl_certificate /path/to/signed_cert_plus_intermediates;
    ssl_certificate_key /path/to/private_key;
    ssl_dhparam /path/to/dhparam.pem;
    ssl_session_timeout 5m;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers 'the_long_ciphersuite_listed_there';
    ssl_prefer_server_ciphers on;
    ssl_session_cache shared:SSL:50m;
}

With this, I'd like to log which SSL protocol was used for a connection and what ciphersuite was chosen after the client/server negotiated. E.g.:

10.1.2.3 - - [13/Aug/2014:12:34:56 +0200] "GET / HTTP/1.1" 200 1234 "-" "User agent bla"

to

10.1.2.3 - - [13/Aug/2014:12:34:56 +0200] ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 "GET / HTTP/1.1" 200 1234 "-" "User agent bla"

This way I can quickly identify clients which are using outdated browsers or automated machines which do not support PFS or other relevant security enabling technologies.

How do I configure nginx to log this information?

Add $ssl_cipher to your log_format configuration.

Refer to http://nginx.org/en/docs/http/ngx_http_ssl_module.html#variables for all SSL-related variables.

Example

Define a custom log_format in the http context (e.g. /etc/nginx/nginx.conf):

log_format combined_ssl '$remote_addr - $remote_user [$time_local] '
                        '$ssl_protocol/$ssl_cipher '
                        '"$request" $status $body_bytes_sent '
                        '"$http_referer" "$http_user_agent"';

The above is based on the default combined format with an additional '$ssl_protocol/$ssl_cipher ' line.

Then add in a server context (with SSL enabled) the access_log directive with the custom log format:

server {
  listen 443;
  ssl on;
  access_log /var/log/nginx/access.log combined_ssl;
  [...]
}

After restarting nginx, logs appear like:

10.1.2.3 - - [13/Aug/2014:12:34:56 +0200] TLSv1.2/ECDHE-RSA-AES128-GCM-SHA256 "GET / HTTP/1.1" 200 1234 "-" "User agent bla"