DOCUMENT:Q78209  30-JUL-2001  [lanman]
TITLE   :An Overview of the Netlogon Service and How It Works
PRODUCT :Microsoft LAN Manager
PROD/VER:
OPER/SYS:
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SUMMARY
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The following is a brief overview of the Netlogon service.

MORE INFORMATION
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The Netlogon service is executed to replicate the user accounts system (UAS)
database between a primary domain controller (PDC), a backup domain controller
(BDC), and member servers, and to validate logons to the logical domain the
servers are in. Changes to the UAS are allowed only from the primary domain
controller once the Netlogon service is started. Any attempt to modify the UAS
on a backup domain controller or member server will result in an error stating
that the UAS has been replicated and cannot be modified.

When Netlogon is started on a BDC or a member server, a request will be made to
do a full update of the UAS from the PDC. This can be avoided by using the
/update:no parameter at Netlogon start-up time. Other switches include:

   /PULSE        (60-3600) seconds         Defaults: 300
   /RANDOMIZE    (5-120) seconds           Defaults:  30
   /SCRIPTS      Path of Netlogon share

BDCs and member servers look at the UPDATE parameter and SCRIPTS path, but PDCs
ignore an /UPDATE switch.

After the PDC has been started successfully, it will maintain a list of changes
that have been made to the UAS in an internal list. At /PULSE time, the PDC will
issue a PULSE onto the network in the form of a second class mailslot. This
message will notify BDCs and members that changes have been made to the UAS.
This is a second class mailslot; therefore, a BDC or member server may miss this
call because its delivery is not guaranteed.

When a BDC or member server receives this message, it will request changes from
the PDC. BDCs and members use the /RANDOMIZE parameter to determine how long
they should wait before requesting updates from the PDC. This prevents the PDC
from receiving a flood of update requests from all the replicated systems. If a
BDC or member has missed a large number of these changes, it may request a
complete copy of the UAS from the PDC. This will ensure that the replicating
server has all the changes that have been made. BDCs and members never poll the
PDC for updates -- they always wait for update notices from the PDC.

Note: Members and backups assume the PDC has crashed if they do not receive a
pulse from the PDC within one minute of the time they expect a pulse. If this
failure is confirmed, a message is posted in the error log reporting the PDC
crash. After 60 minutes, the service will clear a flag and treat the next pulse
as a new failure.

Additional query words: 2.00 2.10 2.10a 2.20 netlogon

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