What is MSDTC?
MSDTC is the Microsoft Distributed Transaction Coordinator, which is a transaction manager program that permits client applications to include several different sources of data into one transaction. MSDTC then coordinates committing the transaction across all of the servers that are listed in the transaction. MSDTC runs on all Windows operating systems, and is also installed by a variety of Microsoft applications, including Personal Web Server and SQL Server.
What Network Ports are Used?
MSDTC uses a number of TCP network ports for sending and receiving messages. This fact must be considered when MSDTC is running in a network environment where the servers involved in the transactions. Say you are running a multi-tier application, and each tier is separated by a router or firewall for security purposes. An example would be an application server in one tier communicating with a SQL Server database You will need to know what port numbers need to be opened for MSDTC transaction information to be able to pass through successfully.
For sending out transaction messages, MSDTC always uses the same TCP port – 135. Dealing with the response message is a little more tricky. MSDTC response messages return on a dynamically assigned port anywhere in a range from 1024 – 5000.
Configuring the MSDTC Respone Port Range
As most of you can probably guess, network administrators are not very fond of opening a wide range of ports all at once. So in order for MSDTC communications to still work and keep the network administrator happy at the same time, you will need to reduce the port range used by the response messages. This change is configured in the registry of the servers involved in the MSDTC communications. You will need to add a couple of keys to the registry.
Note: Please make sure to always take a backup of the registry prior to making any changes!
The location of the change is:
HKEY_LOCAL_MACHINE\Software\Microsoft\Rpc\Internet
The following entries need to be added:
Ports : REG_MULTI-SZ : 1024-1054
PortsInternetAvailable : REG_SZ : Y
UseInternetPorts : REG_SZ : Y
(There is a space both before and after each colon)
In this particular example we are limiting the responses to a port range of only 1024-1054. The exact range of ports to use is basically up to each individual organization.
Testing MSDTC Communications
So how do you know your changes were successful? Microsoft provides a handy little tool called DTCPing.exe that you can use to test MSDTC communications between servers. DTCPing can be downloaded from:
http://www.microsoft.com/downloads/details.aspx?FamilyID=5e325025-4dcd-4658-a549-1d549ac17644&displaylang=en
This file is a self-extracting zip file. Confusingly, the zip file and actual executable file are both named “DTCPing.exe”, so you need to make sure to extract to a separate directory, otherwise you will receive a ‘Cannot create output file’ error message.
Once extracted, simply launch DTCPing.exe. The tool must be up and running on both the sending and receving servers, otherwise the test will fail. The initial screen of the tool will look like this:
From here just type in either the NetBIOS name or IP Address of the remote node and click Ping. Test messages will appear in the DTCPing windows of both the sending and responding servers, and a summary of the test will be presented at the end. The test scenario will also be written to a log file that can be found in the same directory as the DTCPing.exe file.
