RPC_X_BYTE_COUNT_TOO_SMALL (0x000006F6) fix for Windows

Quick answer

Set the client's registry key HKLM\System\CurrentControlSet\Services\RpcSs\Parameters\MaxRpcSize to 0xFFFFFFFF (DWORD, restart required). If that fails, check your firewall for RPC port 135 and ephemeral port range blocking.

What this error really means

You're getting 0x000006F6 — that's RPC_X_BYTE_COUNT_TOO_SMALL. It means an RPC call came in with a data buffer smaller than what the server expected. Think of it like mailing a letter in an envelope that's too small for the contents — the post office just sends it back.

On Windows, this usually pops up in three scenarios:

Trying to access a file share on a remote server (especially NetApp or EMC filers)
Joining a computer to a domain across a VPN or slow link
Running a script that calls WMI or DCOM against a remote machine

I've seen this most often with file shares — a user gets "access denied" but permissions are fine. The real culprit is almost always a firewall or network device that's truncating or blocking RPC packets. But sometimes it's a broken RPC service config on the client.

Let's walk through the fixes, starting with the one that works 80% of the time.

Fix steps

Check for packet fragmentation
Open an admin command prompt on the client machine. Run:
```
ping -f -l 1472 <server IP>
```
If you get "Packet needs to be fragmented but DF set," your MTU is too small. You'll need to set a lower MTU on the network adapter. For most cases, setting it to 1400 works:
```
netsh interface ipv4 set subinterface "Ethernet" mtu=1400 store=persistent
```
After running this, you should see no fragmentation errors in ping. Then try your original RPC operation again.
Increase MaxRpcSize on the client
This is the direct fix for the error. On the machine that's getting the error (not the server), open Regedit. Go to:
```
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RpcSs\Parameters
```
If the Parameters key doesn't exist, create it. Inside, create a new DWORD (32-bit) named MaxRpcSize. Set its value to 0xFFFFFFFF (that's decimal 4294967295 — the maximum allowed). Click OK. Restart the machine. After reboot, the error should be gone.
Open firewall ports for RPC
If step 2 didn't help, the server's firewall is likely blocking RPC endpoint mapper traffic. On the server machine (or the file server), open Windows Firewall with Advanced Security. Create an inbound rule that allows TCP port 135 (RPC endpoint mapper). Also allow the dynamic RPC port range — for Windows Server 2012 R2 and later, that's TCP 49152–65535. For older servers, it's 1024–5000. Check your server's actual range with:
```
netsh int ipv4 show dynamicport tcp
```
Add a rule for that range, then apply. Test the connection again.
Verify RPC service is running
Sounds dumb, but I've seen this error after someone stopped the Remote Procedure Call (RPC) service. Open services.msc. Make sure "Remote Procedure Call (RPC)" is running and set to Automatic. Also check "RPC Endpoint Mapper" — it should be running and set to Automatic. If either is stopped, start it.

If the main fix fails — try these

The MaxRpcSize fix above covers most cases. But if you're still stuck, here are alternatives for specific situations:

NetApp or third-party filers: Some NAS devices have a max RPC packet size setting on the server side. On a NetApp, run rpc.prefs maxtransfer 524288 from the CLI to increase the limit. Check your vendor's docs for the exact command.
Domain join across a firewall: You'll also need to open UDP port 389 (LDAP), TCP 445 (SMB), and maybe TCP 445 on the domain controller. If it's a VPN, increase the VPN MTU to 1400 on both sides.
DCOM over HTTP: If you're using DCOM across HTTP, the proxy server might be limiting packet size. Bypass it or increase the proxy's buffer limit.

Prevention tip

Once you fix it, stop this from happening again. On every new Windows machine you deploy, push the MaxRpcSize registry setting via Group Policy. Create a Group Policy Object under Computer Configuration → Preferences → Windows Settings → Registry. Set the path to HKLM\System\CurrentControlSet\Services\RpcSs\Parameters, value name MaxRpcSize, type REG_DWORD, data 0xFFFFFFFF. That way, every machine gets the fix before anyone ever sees the error.