A debugging log

Debugging log for selected clients
Logging to a cyclic memory buffer

To enable a debugging log, nginx needs to be configured to support debugging during the build:

./configure --with-debug ...

Then the debug level should be set with the error_log directive:

error_log /path/to/log debug;

To verify that nginx is configured to support debugging, run the nginx -V command:

configure arguments: --with-debug ...

Pre-built Linux packages provide out-of-the-box support for debugging log with the nginx-debug binary (1.9.8) which can be run using commands

service nginx stop
service nginx-debug start

and then set the debug level. The nginx binary version for Windows is always built with the debugging log support, so only setting the debug level will suffice.

Note that redefining the log without also specifying the debug level will disable the debugging log. In the example below, redefining the log on the server level disables the debugging log for this server:

error_log /path/to/log debug;

http {
    server {
        errorlog /path/to/log;
        ...

To avoid this, either the line redefining the log should be commented out, or the debug level specification should also be added:

errorlog /path/to/log debug;

http {
    server {
        error_log /path/to/log debug;
        ...

It is also possible to enable the debugging log for selected client addresses only:

error_log /path/to/log;

events {
    debugconnection 192.168.1.1;
    debugconnection 192.168.10.0/24;
}

The debugging log can be written to a cyclic memory buffer:

errorlog memory:32m debug;

Logging to the memory buffer on the debug level does not have significant impact on performance even under high load. In this case, the log can be extracted using a gdb script like the following one:

set $log = ngxcycle->log

while $log->writer != ngxlogmemory_writer
    set $log = $log->next
end
set $buf = (ngxlogmemorybuft *) $log->wdata
dump binary memory debug_log.txt $buf->start $buf->end