9

As far as I can see Nginx supports by default 2 log files: error_log (tracks issues related to the Nginx server itself) and access_log (tracks requests processed by Nginx). Whilst it is possible to control the format of access_log with the log_format directive, I have been unsuccessful at finding a way of logging only certain requests to a separate file, and therefore would like to ask the question on SF as a reference for future readers:

Is there a way to log certain requests to a different log file than the one defined by access_log?

FYI the reason behind this question is that I have a rule that denies access to unwanted crawlers with a 200 (because 403 would give them a hint that they're being blocked), and filtering those requests out of the access_log becomes more difficult.

Max Corbeau
  • 3,653

4 Answers4

10

cjc put me on the right track. Using access_log in an if statement by itself is not possible (You get a nginx: [emerg] "access_log" directive is not allowed here error). So the workaround is as follows:

if ($http_user_agent ~* (crawler) ) {
  set $crawler 'yes';
}
location ~ .* {
  if ($crawler = 'yes') {
    access_log /var/log/nginx/blockedbots.log;
    return 200;
    }
}
Max Corbeau
  • 3,653
7

access_log supports if:

(access_log path [format [buffer=size [flush=time]] [if=condition]];)

access_log /var/.... if $crawler;

Source:

http://nginx.org/en/docs/http/ngx_http_log_module.html

Thomas Decaux
  • 1,397
4

You should be able to put an access_log directive inside an if block, according to the documentation:

http://wiki.nginx.org/HttpLogModule

So, you should be able to do something like:

if ($http_user_agent ~* (crawler) ) {
   access_log /path/to/other/log/file ;
}
cjc
  • 25,492
2

When I came upon this post I was looking for a way to send AWS ELB healthcheck logs to a separate file as they were clogging up access.log. I was never able to get access_log to work with if inside a location block, and this does not work either:

map $http_user_agent $healthcheck {
    "ELB-HealthChecker/2.0" 1;
    default 0;
  }

access_log /var/log/nginx/elb-healthcheck.log main if=$healthcheck;
access_log /var/log/nginx/access.log main if=!$healthcheck;

In a sea of braces it is easy to forget that the NGINX configuration file is not a programming language.

What I ended up with, albeit clunky looking, is this:

map $http_user_agent $not_elb {
    "ELB-HealthChecker/2.0" 0;
    default 1;
}
map $http_user_agent $elb {
    "ELB-HealthChecker/2.0" 1;
    default 0;
}

access_log /var/log/nginx/access.log main if=$not_elb;
access_log /var/log/nginx/elb-healthcheck.log main if=$elb;

Regular traffic goes to access.log and ELB healthcheck logs go to elb-healthcheck.log.

Joe
  • 524