Windows 95 (DUN 1.3) /98
With DUN 1.3 you can also trace PPP logs per Dialup connection. Enable "Record a log file for this connection" under the Server Types tab of the Dialup Networking connection.

The log file, named ppplog.txt, will be in the windows root directory.

Windows NT Server
Change the logging option under the following key :
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RASMAN\PPP\Parameters
Select the Logging value. From the Edit menu choose DWORD. Press 1 and choose OK

The log file will be in the %systemroot%\system32\RAS folder

Windows 2000 Professional
Open a command prompt window.
Type the follwing command, pressing ENTER after each command:
netsh
ras
set tracing PPP enable

The log file will be in the %systemroot%\tracing folder

To disable PPP logging type the following command:
set tracing PPP disabled

Windows 2000 Server
Start the Routing and Remote Access Services tool.
In the left pane, right-click the RRAS server for which you want to enable logging, then click Properties.
Click the Event Logging tab.
Click to select the Enable Point-to-Point Protocol (PPP) logging check box.

The log file will be in the %systemroot%\tracing folder

SCO Openserver 5

PPP debugger flag is set in the /etc/tcp file.

Look for this line:
   pppd ; echo "pppd \c"

Change it to:
   pppd -d 9 ; echo "pppd \c"

The log file will be appended to the /usr/adm/syslog file.

For exhaustive diagnostics you must additionally install Support Level Supplement (SLS) NET382E on your system and add the "debug"
option in /etc/ppphosts.

Linux

By default, pppd will log any warnings and error messages to syslog's daemon facility.
You have to add an entry to syslog.conf that redirects this to a file, or even the console, otherwise syslog simply discards these messages.
The following entry sends all messages to /var/log/ppp-log:

           daemon.*                /var/log/ppp-log