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How to configure the Microsoft Distributed Transaction Coordinator (MSDTC) on Linux

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Applies to: SQL Server - Linux

This article describes how to configure the Microsoft Distributed Transaction Coordinator (MSDTC) on Linux.

MSDTC on Linux is supported on SQL Server 2017 (14.x) Cumulative Update 16 and later versions.

Overview

Distributed transactions are enabled on SQL Server on Linux by introducing MSDTC and remote procedure call (RPC) endpoint mapper functionality within SQL Server. By default, an RPC endpoint-mapping process listens on port 135 for incoming RPC requests and provides registered components information to remote requests. Remote requests can use the information returned by endpoint mapper to communicate with registered RPC components, such as MSDTC services. A process requires super user privileges to bind to well-known ports (port numbers less than 1024) on Linux. To avoid starting SQL Server with root privileges for the RPC endpoint mapper process, system administrators must use iptables to create Network Address Translation to route traffic on port 135 to SQL Server's RPC endpoint-mapping process.

MSDTC uses two configuration parameters for the mssql-conf utility:

mssql-conf setting Description
network.rpcport The TCP port that the RPC endpoint mapper process binds to.
distributedtransaction.servertcpport The port that the MSDTC server listens to. If not set, the MSDTC service uses a random ephemeral port on service restarts, and firewall exceptions need to be reconfigured to ensure that MSDTC service can continue communication.

For more information about these settings and other related MSDTC settings, see Configure SQL Server on Linux with the mssql-conf tool.

Supported transaction standards

The following MSDTC configurations are supported:

Transaction standard Data sources ODBC driver JDBC driver
OLE-TX transactions SQL Server on Linux Yes No
XA distributed transactions SQL Server, other ODBC, and JDBC data sources that support XA Yes (requires version 17.3 or higher) Yes
Distributed transactions on linked server SQL Server Yes No

For more information, see Understanding XA Transactions.

MSDTC configuration steps

There are three steps to configure MSDTC communication and functionality. If the necessary configuration steps aren't done, SQL Server doesn't enable MSDTC functionality.

  • Configure network.rpcport and distributedtransaction.servertcpport using mssql-conf.
  • Configure the firewall to allow communication on distributedtransaction.servertcpport and port 135.
  • Configure Linux server routing so that RPC communication on port 135 is redirected to SQL Server's network.rpcport.

The following sections provide detailed instructions for each step.

Configure RPC and MSDTC ports

First, configure network.rpcport and distributedtransaction.servertcpport using mssql-conf. This step if specific to SQL Server and common across all supported distributions.

  1. Use mssql-conf to set the network.rpcport value. The following example sets it to 13500.

    sudo /opt/mssql/bin/mssql-conf set network.rpcport 13500

  2. Set the distributedtransaction.servertcpport value. The following example sets it to 51999.

    sudo /opt/mssql/bin/mssql-conf set distributedtransaction.servertcpport 51999

  3. Restart SQL Server.

    sudo systemctl restart mssql-server

Configure the firewall

The second step is to configure the firewall to allow communication on servertcpport and port 135. This enables the RPC endpoint-mapping process and MSDTC process to communicate externally to other transaction managers and coordinators. The actual steps for this vary depending on your Linux distribution and firewall.

The following example shows how to create these rules on Ubuntu.

sudo ufw allow from any to any port 51999 proto tcp
sudo ufw allow from any to any port 135 proto tcp
sudo ufw allow from any to any port 13500 proto tcp

The following example shows how this could be done on Red Hat Enterprise Linux (RHEL):

sudo firewall-cmd --zone=public --add-port=51999/tcp --permanent
sudo firewall-cmd --zone=public --add-port=135/tcp --permanent
sudo firewall-cmd --reload

It's important to configure the firewall before configuring port routing in the next section. Refreshing the firewall can clear the port routing rules in some cases.

Configure port routing

Configure the Linux server routing table so that RPC communication on port 135 is redirected to SQL Server's network.rpcport. Configuration mechanism for port forwarding on different distribution might differ. The following sections provide guidance for Ubuntu, SUS Enterprise Linux (SLES), and Red Hat Enterprise Linux (RHEL).

Port routing in Ubuntu and SLES

Ubuntu and SLES don't use the firewalld service, so iptable rules are an efficient mechanism to achieve port routing. The iptable rules might not persist during restarts, so the following commands also provide instructions for restoring the rules after a restart.

  1. Create routing rules for port 135. In the following example, port 135 is directed to the RPC port, 13500, defined in the previous section. Replace <ipaddress> with the IP address of your server.

    sudo iptables -t nat -A PREROUTING -d <ip> -p tcp --dport 135 -m addrtype --dst-type LOCAL  \
   -j DNAT --to-destination <ip>:13500 -m comment --comment RpcEndPointMapper
sudo iptables -t nat -A OUTPUT -d <ip> -p tcp --dport 135 -m addrtype --dst-type LOCAL \
   -j DNAT --to-destination <ip>:13500 -m comment --comment RpcEndPointMapper

    The --comment RpcEndPointMapper parameter in the previous commands assists with managing these rules in later commands.

  2. View the routing rules you created with the following command:

    sudo iptables -S -t nat | grep "RpcEndPointMapper"

  3. Save the routing rules to a file on your machine.

    sudo iptables-save > /etc/iptables.conf

  4. To reload the rules after a restart, add the following command to /etc/rc.local (for Ubuntu) or to /etc/init.d/after.local (for SLES):

    iptables-restore < /etc/iptables.conf

    Note

    You must have super user (sudo) privileges to edit the rc.local or after.local files.

The iptables-save and iptables-restore commands, along with rc.local/after.local startup configuration, provide a basic mechanism to save and restore iptables entries. Depending on your Linux distribution, there might be more advanced or automated options available. For example, an Ubuntu alternative is the iptables-persistent package to make entries persistent.

The previous steps assume a fixed IP address. If the IP address for your SQL Server instance changes (due to manual intervention or DHCP), you must remove and recreate the routing rules if they were created with iptables. If you need to recreate or delete existing routing rules, you can use the following command to remove old RpcEndPointMapper rules:

sudo iptables -S -t nat | grep "RpcEndPointMapper" | sed 's/^-A //' | while read rule; do iptables -t nat -D $rule; done

Port routing in RHEL

On distributions that use firewalld service, such as Red Hat Enterprise Linux, the same service can be used for both opening the port on the server and internal port forwarding. For example, on Red Hat Enterprise Linux, you should use firewalld service (via firewall-cmd configuration utility with -add-forward-port or similar options) to create and manage persistent port forwarding rules instead of using iptables.

sudo firewall-cmd --permanent --add-forward-port=port=135:proto=tcp:toport=13500
sudo firewall-cmd --reload

Verify

At this point, SQL Server should be able to participate in distributed transactions. To verify that SQL Server is listening, run the netstat command (if you're using RHEL, you might have to first install the net-tools package):

sudo netstat -tulpn | grep sqlservr

You should see output similar to the following example:

tcp 0 0 0.0.0.0:1433 0.0.0.0:* LISTEN 13911/sqlservr
tcp 0 0 127.0.0.1:1434 0.0.0.0:* LISTEN 13911/sqlservr
tcp 0 0 0.0.0.0:13500 0.0.0.0:* LISTEN 13911/sqlservr
tcp 0 0 0.0.0.0:51999 0.0.0.0:* LISTEN 13911/sqlservr
tcp6 0 0 :::1433 :::* LISTEN 13911/sqlservr
tcp6 0 0 ::1:1434 :::* LISTEN 13911/sqlservr
tcp6 0 0 :::13500 :::* LISTEN 13911/sqlservr
tcp6 0 0 :::51999 :::* LISTEN 13911/sqlservr

However, after a restart, SQL Server doesn't start listening on the servertcpport until the first distributed transaction. In this case, you wouldn't see SQL Server listening on port 51999 in this example until the first distributed transaction.

Configure authentication on RPC communication for MSDTC

MSDTC for SQL Server on Linux doesn't use authentication on RPC communication by default. However, when the host machine is joined to an Active Directory domain, it's possible to configure MSDTC to use authenticated RPC communication using following mssql-conf settings:

Setting Description
distributedtransaction.allowonlysecurerpccalls Configure secure only RPC calls for distributed transactions. Default value is 0.
distributedtransaction.fallbacktounsecurerpcifnecessary Configure security only RPC calls for distributed transactions. Default value is 0.
distributedtransaction.turnoffrpcsecurity Enable or disable RPC security for distributed transactions. Default value is 0.

Additional guidance

Active directory

Microsoft recommends using MSDTC with RPC enabled if SQL Server is enrolled into an Active Directory configuration. If SQL Server is configured to use Active Directory authentication, MSDTC uses mutual authentication RPC security by default.

Windows and Linux

If a client on a Windows operating system needs to enlist into distributed transaction with SQL Server on Linux, it must have the following minimum version of Windows operating system:

Operating system Minimum version OS Build
Windows Server 1903 18362.30.190401-1528
Windows 10 1903 18362.267

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