I think that these details of the operation when the output= setting is not defined are not in the official documentation. I found the Log Window limit by looking at the source code. It is set in file ui_win_gui.c (#define LOG_LINES 1000). I have some familiarity with the 
code because I have been publishing 32bit binaries of stunnel for Windows for a few years on github (stunnel-win32).

Note that I think it is recommended to enable file logging, specially on Windows. This will let you investigate service failures, but currently you need to manually manage the the log rotation on Windows.

Regards,
Jose


On Sunday, May 31, 2026 at 03:21:40 PM GMT-5, Graham Jones <graham@lorien56.co.uk> wrote:


Hi Jose,

 

That’s a very clear explanation, thank you.  Is this actually explained in any documentation?  How did you find out these details?

 

Regards,

== Graham



 

From: Jose Alf. [mailto:josealf@rocketmail.com]
Sent: 31 May 2026 21:04
To: Graham Jones; stunnel-users@lists.stunnel.org
Subject: Re: [stunnel-users] Stunnel Show Log Window

 

Hi Graham,

 

Yes. You are absolutely right. I was a bit confused with your question at first. I apologize.

 

If we don't uncomment the output=filename.ext line, the log lines won't be written to any disk file. In this case, On Windows, the log lines go to the Log Window you mention. This is implemented as a circular buffer kept in RAM with hardcoded capacity for 1000 lines. That is, you will only see the last 1000 lines sent to the log and older lines are silently discarded. No crashes should happen. 

 

 

Best Regards,

Jose

 

 

--

On Sunday, May 31, 2026 at 10:57:37 AM GMT-5, Graham Jones <graham@lorien56.co.uk> wrote:

 

 

Hi Jose,

 

Thanks for the explanation about stunnel.log

 

My config file has the line:

 

;output = stunnel.log

 

I understand this to mean that the file stunnel.log is NOT created.  And there is no evidence of this file anywhere.

 

But the GUI allows me to see a "log window".  So my question is: what happens when this "log window" fills up?

 

Regards,

 

== Graham

 

 

 

> -----Original Message-----

> From: Josealf.rm [mailto:josealf@rocketmail.com]

> Sent: 31 May 2026 12:46

> Subject: Re: [stunnel-users] Stunnel Show Log Window

>

> Hi Graham,

>

> Stunnel does not implement automatic log rotation and it will not overwrite

> earlier log entries. It will always try to append new entries at the end. By

> default, the log file is named stunnel.log. If you run out of storage, stunnel

> will probably crash. In Unix-like OSes like Linux or FreeBSD, you can configure

> OS tasks to gracefully handle the log management. Periodically the stunnel

> server process is stopped, the oldest log file is deleted and/or compressed

> and the newest/current log is renamed. Then stunnel is started and a new

> stunnel log is created. In Windows you could do something similar by creating

> a suitable script or batch file and running it via Task Scheduler.

>

> Regards

> Jose

>

>

> > On 31/05/2026, at 1:26 AM, Graham Jones via stunnel-users <stunnel-

> >

> > I'm new to Stunnel.

> >

> > stunnel 5.78 on x64-pc-mingw32-gnu platform Windows 7 Pro.

> >

> > I see the option to "Show Log Window".  I have read the manual.  I see

> there

> > is an option to specify an output file; I am not using this option.

> >

> > My question:

> >

> > When the space allocated to the log information fills up, what happens?

> > Does stunnel overwrite the oldest log entries first?  Or does it crash?

> >

> > Regards,

> >

> > == Graham

> >

> >

> > _______________________________________________

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