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HTTP logging

Sometimes, while debugging your Web app (or client-side code using Necko), it can be useful to log HTTP traffic.  This saves a log of HTTP-related information from your browser run into a file that you can examine (or upload to Bugzilla if a developer has asked you for a log).

Note: The Web Console also offers the ability to peek at HTTP transactions within Firefox.  HTTP logging generally provides more detailed logging.

Using about:networking

This is available starting with Firefox 52.

This is the best and easiest way to do HTTP logging.  At any point during while your browser is running, you can turn logging on and off.  This allows you to capture only the "interesting" part of the browser's behavior (i.e. your bug), which makes the HTTP log much smaller and easier to analyze.

  1. Launch the browser and get it into whatever state you need to be in just before your bug occurs.
  2. Open a new tab and type in "about:networking" into the URL bar.
  3. Go to the "Logging section"
  4. Adjust the location of the log file if you don't like the default
  5. Adjust the list of modules that you want to log: this list has the exact same format as the MOZ_LOG environment variable (see below).  Generally the default list is OK, unless a Mozilla developer has told you to modify it.
  6. Click on Start Logging.
  7. Reproduce the bug (i.e. go to the web site that is broken for you and make the bug happen in the browser)
  8. Make a note of the value of "Current Log File".
  9. Click on Stop Logging.
  10. Go to the folder containing the specified log file, and gather all the log files. You will see several files that look like: log.txt-main.1806, log.txt-child.1954, log.txt-child.1970, etc.  This is because Firefox now uses multiple processes, and each process gets its own log file.
  11. For many bugs, the "log.txt-main" file is the only thing you need to upload as a file attachment to your Bugzilla bug (this is assuming you're logging to help a mozilla developer).  Other bugs may require all the logs to be uploaded--ask the developer if you're not sure.
  12. Pat yourself on the back--a job well done!  Thanks for helping us debug Firefox.

Logging HTTP activity by manually setting environment variables

Sometimes the about:networking approach won't work, for instance if your bug occurs during startup, or you're running on mobile, etc.  In that case you can set environment variables *before* you launch Firefox.  Note that this approach winds up logging the whole browser history, so files can get rather large (they compress well :)

Setting environment variables differs by operating system. Don't let the scary-looking command line stuff frighten you off; it's not hard at all!


  1. If Firefox is already running, exit out of it.
  2. Open a command prompt. On Windows XP, you can find the "Run..." command in the Start menu's "All Programs" submenu. On all newer versions of Windows, you can hold down the Windows key and press "R".
  3. Copy and paste the following lines one at a time into the "Run" command window. Press the enter key after each one.:
    For 32-bit Windows:
    cd c:\
    set MOZ_LOG=timestamp,rotate:200,nsHttp:5,cache2:5,nsSocketTransport:5,nsHostResolver:5 
    set MOZ_LOG_FILE=%TEMP%\log.txt 
    cd "Program Files\Mozilla Firefox"
    For 64-bit Windows:
    cd c:\
    set MOZ_LOG=timestamp,rotate:200,nsHttp:5,cache2:5,nsSocketTransport:5,nsHostResolver:5
    set MOZ_LOG_FILE=%TEMP%\log.txt
    cd "Program Files (x86)\Mozilla Firefox"
    (These instructions assume that you installed Firefox to the default location, and that drive C: is your Windows startup disk. Make the appropriate adjustments if those aren't the case.)
  4. Reproduce whatever problem it is that you're having.
  5. Once you've reproduced the problem, exit Firefox and look for the generated log files in your temporary directory. You can type "%TEMP%" directly into the Windows Explorer location bar to get there quickly.


This section offers information on how to capture HTTP logs for Firefox running on Linux.

  1. Quit out of Firefox if it's running.
  2. Open a new shell. The commands listed here assume a bash-compatible shell.
  3. Copy and paste the following commands into the shell one at a time. Make sure to hit enter after each line.
    export MOZ_LOG=timestamp,rotate:200,nsHttp:5,cache2:5,nsSocketTransport:5,nsHostResolver:5
    export MOZ_LOG_FILE=/tmp/log.txt
    cd /path/to/firefox
  4. Reproduce the problem you're debugging.
  5. When the problem has been reproduced, exit Firefox and look for the generated log files, which you can find at /tmp/log.txt.

Mac OS X

These instructions show how to log HTTP traffic in Firefox on Mac OS X.

  1. Quit Firefox is if it's currently running, by using the Quit option in the File menu. Keep in mind that simply closing all windows does not quit Firefox on Mac OS X (this is standard practice for Mac applications).
  2. Run the Terminal application, which is located in the Utilities subfolder in your startup disk's Applications folder.
  3. Copy and paste the following commands into the Terminal window, hitting the return key after each line.
    export MOZ_LOG=timestamp,rotate:200,nsHttp:5,cache2:5,nsSocketTransport:5,nsHostResolver:5
    export MOZ_LOG_FILE=~/Desktop/log.txt
    cd /Applications/Firefox.app/Contents/MacOS
    (The instructions assume that you've installed Firefox directly into your startup disk's Applications folder. If you've put it elsewhere, change the path used on the third line appropriately.)
  4. Reproduce whatever problem you're trying to debug.
  5. Quit Firefox and look for the generated log.txt log files on your desktop.

Note: The generated log file uses Unix-style line endings. Older editors may have problems with this, but if you're using an even reasonably modern Mac OS X application to view the log, you won't have any problems.

Advanced techniques

You can adjust some of the settings listed above to change what HTTP information get logged.

Limiting the size of the logged data

By default there is no limit to the size of log file(s), and they capture the logging throughout the time Firefox runs, from start to finish.  These files can get quite large (gigabytes)!  So we have added a 'rotate:SIZE_IN_MB' option to MOZ_LOG (we use it in the examples above).  If you are using Firefox >= 51, setting this option saves only the last N megabytes of logging data, which helps keep them manageable in size.  (Unknown modules are ignored, so it's OK to use 'rotate' in your environment even if you're running Firefox <= 50: it will do nothing).

This is accomplished by splitting the log into up to 4 separate files (their filenames have a numbered extension, .0, .1, .2, .3)  The logging back end cycles the files it writes to, while ensuring that the sum of these files’ sizes will never go over the specified limit. 

Note 1: the file with the largest number is not guarantied to be the last file written!  We don’t move the files, we only cycle.  Using the rotate module automatically adds timestamps to the log, so it’s always easy to recognize which file keeps the most recent data.

Note 2: rotate doesn’t support append.  When you specify rotate, on every start all the files (including any previous non-rotated log file) are deleted to avoid any mixture of information.  The append module specified is then ignored.

Use 'sync' if your browser crashes or hangs

By default, HTTP logging buffers messages and only periodically writes them to disk (this is more efficient and also makes logging less likely to interfere with race conditions, etc).  However, if you are seeing your browser crash (or hang) you should add ",sync" to the list of logging modules in your MOZ_LOG environment variable.  This will cause each log message to be immediately written (and fflush()'d), which is likely to give us more information about your crash.

Logging only HTTP request and response headers

There are two ways to do this:

  1. Replace MOZ_LOG=nsHttp:5 with MOZ_LOG=nsHttp:3 in the commands above.
  2. There's a handy extension for Firefox called Live HTTP Headers that you can use to capture just the HTTP request and response headers. This is a useful tool when you want to peek at HTTP traffic.

Turning off logging of socket-level transactions

If you're not interested in socket-level log information, either because it's not relevant to your bug or because you're debugging something that includes a lot of noise that's hard to parse through, you can do that. Simply remove the text nsSocketTransport:5 from the commands above.

Turning off DNS query logging

You can turn off logging of host resolving (that is, DNS queries) by removing the text nsHostResolver:5 from the commands above.

Enable Logging for try server runs

You can enable PR_LOGGING on try by hacking build/automation.py.in, in 'def environment'. For example:

def environment(...

  env['MOZ_LOG'] = 'nsHttp:5'
  return env

See also

  • There are similar options available to debug mailnews protocols. See this document for more info about mailnews troubleshooting.
  • On the Windows platform, nightly Firefox builds have FTP logging built-in (don't ask why this is only the case for Windows!). To enable FTP logging, just set MOZ_LOG=nsFtp:5 (in older versions of Mozilla, you need to use nsFTPProtocol instead of nsFtp).
  • When Mozilla's built-in logging capabilities aren't good enough, and you need a full-fledged packet tracing tool, two free products are Wireshark and ngrep. They are available for Windows and most flavors of UNIX (including Linux and Mac OS X), are rock solid, and offer enough features to help uncover any Mozilla networking problem.

Original Document Information

  • Author(s): Darin Fisher
  • Copyright Information: Portions of this content are © 1998–2007 by individual mozilla.org contributors; content available under a Creative Commons license | Details.