Problem Scope
On a COSY 131, WAN-to-LAN port forwarding can provide customer-network access to a specific PLC or HMI service while preserving access separation for other services. The reported limitation is that enabling port forwarding prevents Talk2M Web (M2Web) from establishing browser-based access to the forwarded LAN device.
Marking the service Visible in M2Web in eCatcher does not resolve the conflict. M2Web displays Unable to build proxy when the user attempts to open the service.
Port Forwarding Compared with NAT 1:1
| Configuration | WAN exposure | Reported M2Web result |
|---|---|---|
| Port forwarding | Limits access to selected services, such as an HMI VNC service, without intentionally exposing a separate programming service | Fails with Unable to build proxy
|
| NAT 1:1 | Exposes all services of the destination device to the WAN side | M2Web remains available |
NAT 1:1 is therefore a functional workaround for M2Web access, but it changes the exposure boundary and may be unacceptable when only one destination service should be reachable from the customer WAN.
Confirm the Configuration Conflict
- Confirm that the COSY 131 uses port forwarding rather than NAT 1:1 for WAN access to the LAN device.
- In eCatcher, verify that the required service is marked Visible in M2Web.
- Open the service through Talk2M Web and record whether
Unable to build proxyappears.
If the error occurs under these conditions, the evidence supports a conflict between port forwarding and M2Web proxy creation. It does not identify an incorrect forwarding rule, service port, firmware version, or device-side configuration as the root cause.
Available Decision Paths
If browser-based M2Web access is mandatory, use NAT 1:1 only after reviewing the resulting WAN exposure. If restricted WAN exposure is mandatory, retain port forwarding and use a separate remote-access workflow instead of M2Web. The evidence identifies eCatcher with a separate client, such as a VNC client, as that workflow.
The requested ability to combine port forwarding with M2Web was recorded as a product wishlist item. No implementation commitment, firmware version, or delivery date was provided, so do not plan a deployment around a future firmware correction without confirmation through an official manufacturer channel.
FAQ
Why does COSY 131 M2Web show Unable to build proxy?
The reported error occurs when port forwarding is enabled and an operator opens a LAN-device service through M2Web, even when that service is marked Visible in M2Web.
Can COSY 131 use NAT 1:1 with M2Web?
Yes, the evidence reports that M2Web remains available with NAT 1:1. Review the security impact because NAT 1:1 exposes all services of the destination device to the WAN side.
Is there a firmware update date for M2Web with port forwarding?
No delivery date or firmware version is identified. The combined port-forwarding and M2Web capability was recorded only as a wishlist request.