Fix 0x000006A8: Invalid RPC Protocol Sequence Error on Windows Server

What triggers 0x000006A8?

I've seen this error most often when a client tries to connect to a server using a protocol sequence the server doesn't have registered. Think of it like trying to call someone on a phone line they disconnected. The RPC endpoint mapper on the server only knows about certain transports—ncacn_ip_tcp (TCP/IP), ncalrpc (local), ncacn_np (named pipes), or ncacn_http (RPC over HTTP). If the client sends a request using, say, ncacn_spx (IPX/SPX) on a modern server that dropped SPX support, you get 0x000006A8.

The usual suspect? DNS. The client resolves the server's hostname to an IP, but that IP doesn't match the network interface the RPC service is listening on. Or the server's RPC service itself isn't running. I've also seen this with Exchange 2013/2016 Outlook Anywhere connections when the RPC over HTTP proxy is misconfigured.

Cause #1: DNS resolution points to the wrong interface

This is the #1 cause in my experience. The client looks up the server's FQDN and gets an IP that belongs to a different NIC—maybe a management interface or a VLAN that doesn't handle RPC traffic. The server's RPC endpoint mapper only registers protocol sequences on the NICs that have RPC enabled. If the client connects to the wrong IP, the server sees an invalid protocol sequence for that connection.

How to check DNS

1. On the client machine, open Command Prompt as admin.
2. Run nslookup servername.domain.com. Note the IP returned.
3. On the server, run ipconfig /all to see all IP addresses and their associated interfaces.
4. If the DNS record points to a management or backup IP, you need to update DNS to point to the primary application IP.

The real fix: Delete the stale A record and create a new one for the correct IP. Or, if the server has multiple NICs, bind RPC to a specific interface using the registry.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc\Internet\Ports
Value: Ports (REG_MULTI_SZ) – list allowed ports
Value: PortsInternetAvailable (REG_SZ) – set to Y

This forces RPC to listen only on the interfaces you specify. Reboot after this.

Cause #2: RPC service is stopped or the endpoint mapper isn't responding

If the Remote Procedure Call (RPC) service (RpcSs) isn't running, the endpoint mapper won't register any protocol sequences. Clients get 0x000006A8. This can happen after a security update or if someone killed the service manually.

Verify and restart the service

1. Press Win + R, type services.msc, press Enter.
2. Find Remote Procedure Call (RPC). Its status should be Running, startup type Automatic.
3. If it's stopped, right-click and select Start.
4. Also check RPC Endpoint Mapper (RpcEptMapper). It should be running too.

Pro tip: Check the System event log for Event ID 5719 or 5722. They often precede 0x000006A8 and point to the exact service failure.

Cause #3: Mismatched protocol sequence in client application or proxy

This one's sneaky. The client application is hardcoded to use a specific protocol sequence string—like ncacn_np (named pipes) while the server only accepts ncacn_ip_tcp. Or the RPC proxy (like in Exchange or SQL Server) expects ncacn_http but the client tries ncacn_ip_tcp.

How to diagnose

On the server, run this PowerShell command to list registered endpoints and their protocol sequences:

Get-WmiObject -Class Win32_Process -Filter "Name = 'svchost.exe'" | 
Select-Object -Property Name, ProcessId, @{Name='ProtocolSequences';Expression={
  $rpc = [System.Runtime.InteropServices.Marshal]::GetTypeFromCLSID('{...}')
  # Use rpcdump.exe instead for simplicity
}}

Actually, just use rpcdump.exe from the Windows SDK. Run it against the server IP:

rpcdump /i server_ip

Look for the line Registered endpoints: [ncacn_ip_tcp] or [ncacn_np]. If you see only ncalrpc, that means only local connections are allowed—remote clients will fail with 0x000006A8.

The fix: If you need remote RPC, add ncacn_ip_tcp as a protocol sequence. In the registry, go to:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Rpc\Internet

Create a new string value ProtocolSequence and set it to ncacn_ip_tcp. Restart the RPC service.

Quick-reference summary table

This error tripped me up the first time too. But once you check DNS first, then services, then protocol sequences, you'll nail it in under ten minutes. Good luck.