WNS sometimes not received

Question

WNS sometimes not received

Ole Bjørn Setnes 0

We are experiencing issues with Windows Push Notifications (WNS) not being delivered to our application endpoints. The push notification is sent successfully from our server, but the response indicates the message is dropped due to a blocked channel.

Here are the relevant response headers from the WNS server:

makefile
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X-WNS-MSG-ID: 2C9C70339C1E494  
X-WNS-STATUS: dropped  
X-WNS-ERROR-DESCRIPTION: Blocked channel URL  
X-WNS-DEBUG-TRACE: AM3PEPF0000624D  
MS-CV: j0Q59yxL0UmSaBRNeOmW8A.0  
MISE-CORRELATION-ID: a2cf4196-f380-41bf-be19-a50c82213157

We are using the WNS service directly via HTTPS POST requests. The push tokens were obtained using standard client-side registration, and the tokens are current as far as we know. However, messages to certain channels are being dropped with the above error.

Steps to Reproduce:

Send a push notification using WNS to a previously registered channel URI.

Observe the response headers showing the status as "dropped" and error description as "Blocked channel URL".

Expected Behavior:

The message should be delivered to the client device if the channel is valid and active.

Actual Behavior:

The message is dropped by WNS with error Blocked channel URL.

It works with most PCs and the above error is sometimes seen on a specific Windows 11 PC (23H2, build 22631.4460).

Bhargavi Naragani 7,445 Reputation points Microsoft External Staff Moderator

2025-06-10T15:34:12.5033333+00:00
Hi @Ole Bjørn Setnes,

If the channel URI becomes invalid or gets blocked, the Windows Notification Service (WNS) won’t be able to deliver notifications, and you’ll see a “Blocked channel URL” error. Based on the error details, it looks like this issue is only happening on certain devices, particularly a Windows 11 PC running version 23H2 (build 22631.4460). There could be a few reasons for this, like how the device is set up, network restrictions, or even problems with the WNS itself.

Make sure the device can connect to all the required WNS endpoints. These include:

*.notify.windows.com

*.wns.windows.com

sinwns1011421.wns.windows.com

sin.notify.windows.com

Firewalls or proxy servers could be blocking access, which would prevent the service from working. Allowing traffic to those domains might fix the problem.

Double-check that the device is properly configured to receive push notifications. Make sure the WNS service is turned on and that nothing on the device is preventing notifications from coming through.

Verify that the client-side registration process is correctly obtaining and using the push tokens. And look for the any specific settings or configurations on Windows 11 PC that might be causing the blocked channel URL error.

If the channel URI is no longer valid, try re-registering the device to get a new one. This usually means starting the registration process again on the device.

Occasionally, issues with WNS service itself can cause notifications to be dropped. Monitoring the health of the WNS service and checking for any reported outages can help determine if this is the cause.

Reference Documentations:
https://learn.microsoft.com/en-us/uwp/api/windows.networking.pushnotifications.pushnotificationchannel?view=winrt-26100
https://learn.microsoft.com/en-us/windows/apps/design/shell/tiles-and-notifications/troubleshoot-notifications#troubleshooting-specific-errors
https://learn.microsoft.com/en-us/windows/apps/windows-app-sdk/notifications/push-notifications/troubleshooting#wns-http-response-codes

Hope this information is helpful, if you have any further concerns or queries, please feel free to reach out to us.

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Ole Bjørn Setnes 0

Yes, this is observed on one test PC and because of this problem we cannot release the service we want to use Push Notifications for.

On the problematic PC, push Notifications works for a week or more and suddenly it stops working. Usually we see one Push Notification that's just not received and then all following notifications report this error. Perhaps the Blocked channel error means that there's already a notification under way and it cannot send another one?

Reregistering the service on the receiving endpoint solves the problem so it is not likely that it is because communication is blocked by firewalls/proxies and that the endpoint cannot connect to the mentioned services.

If the endpoint could know that something was wrong, it could restart the service, but it doesn't - everything seems fine. How can the endpoint check if everything is fine?

Is there a trace mechanish to get more information about why it fails and why the first message isn't delivered?

Shree Hima Bindu Maganti 5,480 Reputation points Microsoft External Staff Moderator

2025-06-11T00:45:47.2466667+00:00
Hi @Ole Bjørn Setnes,
Thanks for your Update.
One common cause is sending too many notifications in a short period, leading WNS to block the channel to prevent abuse.

Ensure you are not exceeding the notification frequency limits set by WNS. Sending too many notifications too quickly can result in the channel being blocked.

Channels can expire, and notifications sent to an expired channel will be dropped. Request a new channel URL periodically, especially if notifications stop being delivered.

If reregistering the service temporarily resolves the issue, the channel may be getting blocked or invalidated. Regularly refresh the channel URL to mitigate this problem.

Implement logging to capture the response headers from WNS. This can provide insights into the status of the notifications and identify patterns or conditions leading to the "Blocked channel URL" error.

Ensure your application has robust error handling to detect when notifications are not being delivered and take appropriate actions, such as reinitializing the notification service.
Unfortunately, there is no built-in trace mechanism in WNS to provide detailed information on why a notification fails. However, logging the response from WNS and monitoring the application behavior can help you diagnose the issue more effectively.
References:

Troubleshooting WNS push notifications

Troubleshooting push notifications
Sampath 3,865 Reputation points Microsoft External Staff Moderator

2025-06-11T17:36:38.26+00:00

Hi @Ole Bjørn Setnes, Could you please private message and reply back there.

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WNS sometimes not received

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