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 ...

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 {
        error_log /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:

error_log /path/to/log debug;

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

Debugging log for selected clients

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

error_log /path/to/log;

events {
    debug_connection 192.168.1.1;
    debug_connection 192.168.10.0/24;
}

Logging to a cyclic memory buffer

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

error_log 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 = ngx_cycle->log

while $log->writer != ngx_log_memory_writer
    set $log = $log->next
end

set $buf = (ngx_log_memory_buf_t *) $log->wdata
dump binary memory debug_log.txt $buf->start $buf->end

Or using an lldb script as follows:

expr ngx_log_t *$log = ngx_cycle->log
expr while ($log->writer != ngx_log_memory_writer) { $log = $log->next; }
expr ngx_log_memory_buf_t *$buf = (ngx_log_memory_buf_t *) $log->wdata
memory read --force --outfile debug_log.txt --binary $buf->start $buf->end