Question

How to set a custom log format for nginx, so that request got parsed and its parts are logged separate?

We serve a picture file to count mail opens. The url to the picture varies, but follows the rule:

http://www.server.com/counter/XXXXX/YYYYY/dailymail.gif

XXXXX – id of email campaign;
YYYYY – recipient id.

There’s a separate log for /counter location, which I’d like to give a format like

XXXXX   YYYYY    DATETIME

Location section looks like

    location    ~* ^counter/([0-9]+)/([^/]+)/dailymail\.gif$ {
        access_log /var/log/mailopened.log
        alias /var/www/site.com/1x1.gif?cid=$1&uid=$2&type=daily;
    }

So I have the values in variables $1 and $2. How can I use them in the log format?

Answer 1

log_format directive is allowed at http level only, so you have to define it in terms of other variables, e.g.

http {
  log_format tracking "$xxxx $yyyy $time_local";

Later, in your location, just set these variables and log in tracking format:

location ~* ^counter/([0-9]+)/([^/]+)/dailymail\.gif$ {
  set $xxxx $1;
  set $yyyy $2;
  access_log /var/log/mailopened.log tracking;
  alias /var/www/site.com/1x1.gif?cid=$1&uid=$2&type=daily;
}

With recent versions of Nginx and PCRE library it’s possible to omit set calls and name the captures explicitly in location (thanks to @kolbyjack):

location ~* ^counter/(?<xxxx>[0-9]+)/(?<yyyy>[^/]+)/dailymail\.gif$ {
  access_log /var/log/mailopened.log tracking;
  alias /var/www/site.com/1x1.gif?cid=$1&uid=$2&type=daily;
}