Remote Procedure Call (RPC) errors troubleshooting guidance

Applies to:   Windows Client

You might encounter an "RPC server unavailable" error when you connect to Windows Management Instrumentation (WMI) or Microsoft SQL Server, during a Remote Procedure Call (RPC) session, or when you use various Microsoft Management Console (MMC) snap-ins. The following image shows an example of an RPC error.

Screenshot of an error message stating that the following error occurred: the RPC server is unavailable.

This is a common networking error that requires some basic familiarity with the process to successfully troubleshoot. To begin, there are several important terms to understand:

  • Endpoint mapper (EPM): A service that listens on the server and guides client apps to server apps by using port and UUID information.
  • Tower: Describes the RPC protocol to enable the client and server to negotiate a connection.
  • Floors: The layers of contents within a tower that contain specific data, such as ports, IP addresses, and identifiers.
  • UUID: A well-known GUID that identifies an RPC application. During troubleshooting, you can use the UUID to track the RPC conversations of a single type of application (among the many types that occur on a single computer at one time).
  • Opnum: Identifies a function that the client wants the server to perform. This is simply a hexadecimal number. However, a good network analyzer will translate the function for you. If the function can't be identified, contact your application vendor.
  • Port: The communication endpoint for client or server application. The EPM allocates dynamic ports (also known as high ports or ephemeral ports) for clients and servers to use.


    Typically the port number is the most important information that you'll use for troubleshooting.

  • Stub data: The data exchanged between the functions on the client and the functions on the server. This data is the payload, the important part of the communication.

How the connection works

The following diagram shows a client connecting to a server to run a remote operation. The client initially contacts TCP port 135 on the server, and then negotiates with EPM for a dynamic port number. After EPM assigns a port, the client disconnects, and then uses the dynamic port to connect to the server.

Diagram that shows how a client makes an RPC connection to a remote server.


If a firewall separates the client and the server, the firewall has to allow communication on port 135 and on the dynamic ports that EPM assigns. One approach to managing this scenario is to specify ports or ranges of ports for EPM to use. For more information, see Configure how RPC allocates dynamic ports.

Some firewalls also allow UUID filtering. In this scenario, if an RPC request uses port 135 to cross the firewall and contact EPM, the firewall notes the UUID that's associated with the request. When EPM responds and sends a dynamic port number for that UUID, the firewall also notes the port number. The firewall then allows RPC bind operations for that UUID and port.

Configure how RPC allocates dynamic ports

By default, EPM allocates dynamic ports randomly from the range that's configured for TCP and UDP (based on the implementation of the operating system that's used). However, this approach might not be practical, especially if the client and server must communicate through a firewall. An alternative method is to specify a port number or range of port numbers for EPM to use, and open those ports in the firewall.

Many Windows server applications that rely on RPC provide options (such as registry keys) to customize the allowed ports. Windows services use the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc\Internet subkey for this task.

When you specify a port or port range, use ports that are outside the range of commonly used ports. You can find a comprehensive list of server ports that are used in Windows and major Microsoft products in Service overview and network port requirements for Windows. The article also lists RPC server applications, and mentions which RPC server applications can be configured to use custom server ports beyond the capabilities of the RPC runtime.


This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For protection, back up the registry before you modify it so that you can restore it if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows.

By default, the Internet key doesn't exist. Therefore, you have to create it. For the Internet key, you can configure the following entries:

  • Ports REG_MULTI_SZ: Specifies a port or inclusive range of ports. The other entries that appear under Internet indicate whether these are the ports to use or the ports to exclude from use.

    • Value range: 0 - 65535
      For example, 5984 represents a single port, and 5000–5100 represents a set of ports. If any values are outside the range of 0 to 65535, or if any value can't be interpreted, the RPC runtime treats the entire configuration as invalid.
  • PortsInternetAvailable REG_SZ: Specifies whether the Ports value represents ports to include or ports to exclude.

    • Values: Y or N (not case-sensitive)
      • Y: The ports that are listed in the Ports entry represent all the ports on that computer that are available to EPM.
      • N: The ports that are listed in the Ports entry represent all ports that aren't available to EPM.
  • UseInternetPorts REG_SZ: Specifies the default system policy.

    • Values: Y or N (not case-sensitive)
      • Y: The processes that use the default system policy are assigned ports from the set of internet-available ports, as defined previously.
      • N: The processes that use the default system policy are assigned ports from the set of intranet-only ports.

You should open a range of ports that are greater than port 5000. Port numbers that are less than 5000 might already be in use by other applications, and they could cause conflicts with your DCOM applications. Furthermore, previous experience shows that a minimum of 100 ports should be opened. This is because several system services rely on these RPC ports to communicate with one another.


The minimum number of ports that are required may differ from computer to computer. Computers that support more traffic might encounter port exhaustion if the RPC dynamic ports are restricted. Take this into consideration if you restrict the port range.


If there's an error in the port configuration, or there aren't enough ports in the pool, EPM can't register RPC server applications (including Windows services such as Netlogon) that use dynamic endpoints. If a configuration error occurs, the error code is 87 (0x57) ERROR_INVALID_PARAMETER. For example, if there aren't enough ports, Netlogon logs event 5820:

Log Name: System
Event ID: 5820
Level: Error
Keywords: Classic
The Netlogon service could not add the AuthZ RPC interface. The service was terminated. The following error occurred: 'The parameter is incorrect.'

For more information about how RPC works, see RPC over IT/Pro.

Example of a custom port configuration

In this example, ports 5000 through 6000 (inclusive) were arbitrarily selected to help illustrate how the new registry entries can be configured. This example isn't a recommendation of a minimum number of ports that any particular system requires. Such a configuration requires adding the Internet key under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc, and adding the following entries:

  • Ports MULTI_SZ
    • Data type: MULTI_SZ
    • Value: 5000-6000
  • PortsInternetAvailable REG_SZ
    • Data type: REG_SZ
    • Value: Y
  • UseInternetPorts REG_SZ
    • Data type: REG_SZ
    • Value: Y

The computer has to restart for this configuration to take effect. After that, all applications that use RPC are assigned dynamic ports in the range of 5000 through 6000 (inclusive).

Troubleshooting RPC errors


PortQry provides quick insight into how RPC is functioning before you delve into network trace data. You can quickly determine whether you can make a connection by running the following command on the client computer:

Portqry.exe -n <ServerIP> -e 135


In this command, <ServerIP> represents the IP address of the server that you're contacting.

For example, consider the following command:

Portqry.exe -n -e 135

This command produces output that resembles the following excerpt:

Querying target system called:
Attempting to resolve IP address to a name...
IP address resolved to
TCP port 135 (epmap service): LISTENING
Using ephemeral source port
Querying Endpoint Mapper Database...
Server's response:
UUID: d95afe70-a6d5-4259-822e-2c84da1ddb0d

By examining this output, you can determine the following information:

  • DNS is working correctly (it resolved the IP address to a fully qualified domain name (FQDN)).
  • PortQry contacted the RPC port (135) on the target computer.
  • EPM responded to PortQry, and assigned the dynamic port 49664 (enclosed in square brackets) for subsequent communication.
  • PortQry reconnected to port 49664.

If any of these steps fail, you can usually start collecting simultaneous network traces, as described in the next section.

For more information about PortQry, see Using the PortQry command-line tool.


You can use the Windows netsh tool to collect network trace data simultaneously on the client and the server.

To collect simultaneous network traces, open an elevated Command Prompt window on both the client and the server.

On the client, run the following command:

Netsh trace start scenario=netconnection capture=yes tracefile=c:\client_nettrace.etl maxsize=512 overwrite=yes report=yes

On the server, run the following command:

Netsh trace start scenario=netconnection capture=yes tracefile=c:\server_nettrace.etl maxsize=512 overwrite=yes report=yes

Now, try to reproduce your issue on the client computer. Then, run the following command at the command prompt in both windows to stop the traces:

Netsh trace stop

Open the trace files in Microsoft Network Monitor 3.4 or Message Analyzer, and filter the trace data for the IP address of the server or client computers and TCP port 135. For example, use filter strings such as the following:

  • Ipv4.address==<client-ip> and ipv4.address==<server-ip> and tcp.port==135

    In this filter string, <client-ip> represents the IP address of the client, and <server-ip> represents the IP address of the server.

  • tcp.port==135

In the filtered data, look for the EPM entry in the Protocol column.

Look for a response from EPM (on the server) that includes a dynamic port number. If the dynamic port number is present, note it for future reference.

Screenshot of Network Monitor that shows dynamic port highlighted.

Refilter the trace data for the dynamic port number and the server IP address. For example, use a filter string such as tcp.port==<dynamic-port-allocated> and ipv4.address==<server-ip>. In this filter string, <dynamic-port-allocated> represents the dynamic port number and <server-ip> represents the IP address of the server.

Screenshot of Network Monitor that has a filter applied.

In the filtered data, look for evidence that the client connected successfully to the dynamic port, or look for any network issues that might have occurred.

Port not reachable

The most common cause of "RPC server unavailable" errors is that the client can't connect to the dynamic port that was allocated. The client-side trace would then show TCP SYN retransmits for the dynamic port.

Screenshot of Network Monitor that shows TCP SYN retransmits.

This behavior indicates that one of the following conditions is blocking communication:

  • The dynamic port range is blocked on the firewall in the environment.
  • A middle device is dropping the packets.
  • The destination server is dropping the packets. This condition could be caused by such configurations as Windows Filtering Platform (WFP) packet drop, Network Interface Card (NIC) packet drop, or filter driver modifications.

Collecting data for deeper troubleshooting

Before you contact Microsoft support, we recommend that you gather information about your issue.


These procedures use the TroubleShootingScript (TSS) toolset. To use this toolset, you should be aware of the following prerequisites:

  • You must have Administrator-level permission on the local computer.

  • The first time that you run the toolset, you have to accept a EULA.

  • Make sure that the Windows PowerShell script execution policy for the computer is set to RemoteSigned. For more information about PowerShell execution policy, see about_Execution_Policies.


    If your environment prevents you from using RemoteSigned at the computer level, you can temporarily set it at the process level. To do this, run the following cmdlet in an elevated Powershell Command Prompt window before you start the tool:

    PS C:\> Set-ExecutionPolicy -scope Process -ExecutionPolicy RemoteSigned

    To verify that the change takes effect, run the PS C:\> Get-ExecutionPolicy -List cmdlet.

    The process-level permissions apply to only the current PowerShell session. After you close the PowerShell window, the execution policy reverts to the original setting.

Gather key information before contacting Microsoft support

  1. Download TSS on all nodes, and expand it to the C:\tss folder.

  2. Open the C:\tss folder in an elevated PowerShell Command Prompt window.

  3. Start traces on the problem computer by running the following cmdlet:

    TSS.ps1 -Scenario NET_RPC
  4. Respond to the EULA prompt.

  5. Reproduce the issue. You can use tools such as Event Viewer or wbemtest to monitor or test the issue.

  6. After you reproduce the issue, immediately stop collecting data.

  7. After the automated scripts finish collecting the required data, attach the data to your support request.