How to force Kerberos to use TCP instead of UDP in Windows Server 2003, in Windows XP, and in Windows 2000 (244474)

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Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
Important If you use UDP for Kerberos, your client computer may stop responding (hang) when you receive the following message: Loading your personal settings. By default, the maximum size of datagram packets for which Windows Server 2003 uses UDP is 1,465 bytes. For Windows XP and for Windows 2000, this maximum is 2,000 bytes. Transmission Control Protocol (TCP) is used for any datagrampacket that is larger than this maximum. The maximum size of datagram packets for which UDP is used can be changed by modifying a registry key and value.

By default, Kerberos uses connectionless UDP datagram packets. Depending on a variety of factors including security identifier (SID) history and group membership, some accounts will have larger Kerberos authentication packet sizes. Depending on the virtual private network (VPN) hardware configuration, these larger packets have to be fragmented when going through a VPN. The problem is caused by fragmentation of these large UDP Kerberos packets. Because UDP is a connectionless protocol, fragmented UDP packets will be dropped if they arrive at the destination out of order.

If you change MaxPacketSize to a value of 1, you force the client to use TCP to send Kerberos traffic through the VPN tunnel. Because TCP is connection oriented, it is a more reliable means of transport across the VPN tunnel. Even if the packets are dropped, the server will re-request the missing data packet.


You can change MaxPacketSize to 1 to force the clients to use Kerberos traffic over TCP. To do this, follow these steps:

1. Start Registry Editor.
2. Locate and then click the following registry subkey:

   HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa\ Kerberos\Parameters

   Note If the Parameters key does not exist, create it now.
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type MaxPacketSize, and then press ENTER.
5. Double-click MaxPacketSize, type 1 in the Value data box, click to select the Decimal option, and then click OK.
6. Quit Registry Editor.
7. Restart your computer.

The following template is an administrative template that can be imported into Group Policy to let the MaxPacketSize value be set for all enterprise computers that are running Windows Server 2003, Windows XP, or Windows 2000. To view the MaxPacketSize settings in Group Policy Object Editor, click Show Policies Only on the View menu so that Show Policies Only is not selected. This template modifies registry keys outside the Policies section. By default, Group Policy Object Editor does not display these registry settings.

CLASS MACHINE
 
CATEGORY !!KRB_PARAMS
 
KEYNAME "SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters"
 
POLICY !!SET_MAXPACKETSIZE
EXPLAIN !!MAXPACKETSIZE_HELP
PART !!MAXPACKETSIZE NUMERIC REQUIRED
VALUENAME "MaxPacketSize"
MIN 1 MAX 2000 DEFAULT 2000
END PART
 
PART !!MAXPACKETSIZE_TIP TEXT
END PART
END POLICY
 
POLICY !!LOGLEVEL
EXPLAIN !!LOGLEVEL_HELP
VALUENAME "LogLevel"
END POLICY
END CATEGORY
 
[strings]
KRB_PARAMS="Kerberos Parameters"
SET_MAXPACKETSIZE="Set MaxPacketSize"
MAXPACKETSIZE_HELP="The Windows 2000 Kerberos Authentication package is 
the default in Windows 2000. It coexists with challenge/response (NTLM) 
and is used in instances in which both a client and server can negotiate 
Kerberos. Request for Comments (RFC) 1510 states that when a client 
contacts the Key Distribution Center (KDC), it should send a User 
Datagram Protocol (UDP) datagram to port 88 at the KDC's IP address. 
The KDC should  respond with a reply datagram to the sending port at the
sender's IP  address.\n\nWindows 2000, by default, uses UDP when the data
can be fit in  packets under 2,000 bytes. Any data above this value uses
TCP to carry the packets. The value of 2,000 bytes is configurable via 
this policy."
MAXPACKETSIZE="Bytes: "
MAXPACKETSIZE_TIP="Range is from 1 to 2000. Use 1 to force Kerberos to 
use TCP." 
LOGLEVEL="Kerberos Event Logging"
LOGLEVEL_HELP="Windows 2000
offers the capability of tracing detailed 
Kerberos events through the event log mechanism. You can use this 
information when you troubleshoot Kerberos. 
All Kerberos errors are logged to the System log."
				

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