Overview

iPhone devices experience severely limited upload speeds (typically below 5 Mbps) when connected to a network managed by Kerio Control, while Windows and Android devices maintain normal upload speeds on the same network. This issue specifically affects Apple devices including iPhones, iPads, and MacBooks.

Common Symptoms:

• iPhone upload speeds limited to below 5 Mbps (sometimes as low as 1 Mbps)
• Download speeds on iPhones remain normal (90-100 Mbps on a 100 Mbps connection)
• Windows and Android devices achieve full upload speeds without issues
• Problem persists across all WiFi bands (2.4 GHz and 5 GHz)
• Issue occurs on different WiFi access points
• Disabling traffic filters, rules, or IPS does not resolve the issue

Root Cause: The GRO (Generic Receive Offload) feature is enabled on Kerio Control network interfaces. GRO is a kernel-level optimization that aggregates received packets before passing them to the network stack. While this can improve performance for some devices, it creates a bottleneck specifically for upload traffic from Apple devices due to differences in how Apple devices handle network packet processing.

Solution

Step 1: Identify the Affected Network Interface

Determine which interface your WiFi access points are connected to. This is typically a LAN interface such as eth2. The GRO setting needs to be disabled at least on this specific interface.

To identify the correct interface:

• Check your Kerio Control network configuration
• Note which interface is configured for your local network where WiFi access points connect
• Common interface names include eth0, eth1, eth2, etc.

Step 2: Disable GRO Persistently

Kerio Control provides an official guide for disabling GRO persistently. Access this guide at: Configuring GRO Setting for Maximum Bandwidth in Kerio Control.

As long as GRO is disabled from the UI, per the above guide, it will persist reboots.

📌 Note that if you have disabled GRO using the script from the Fix performance after upgrading to 9.4.2 patch 1 by modifying the GRO article, then it will not persist a restart OR a version upgrade and you would need to use the GUI method above.

Step 3: Test iPhone Upload Speeds

Use the same iPhone devices that were experiencing slow upload speeds to verify the fix:

1. Connect an iPhone to the WiFi network
2. Open the Speedtest app (or visit speedtest.net in a browser)
3. Run a speed test and check the upload speed
4. Verify that upload speeds now match or closely approach download speeds
5. Test with multiple iPhone models and iOS versions to ensure consistency

Expected Result: On a 100 Mbps connection, iPhones should now achieve 90-100 Mbps upload speeds, matching the performance of Windows and Android devices.

Summary

Slow iPhone upload speeds on Kerio Control networks are caused by the GRO (Generic Receive Offload) feature being enabled on network interfaces. This issue specifically affects Apple devices while leaving Windows and Android devices unaffected. The solution requires persistently disabling GRO on the interface where WiFi access points are connected, following the official Kerio Control guide. After disabling GRO and rebooting, iPhone upload speeds should return to normal levels matching other devices on the network.

Key Points:

• GRO must be disabled persistently
• Apply the setting to at least the network interface where WiFi access points connect via the GUI

FAQ

Q1: Why does this issue only affect iPhones and not Android or Windows devices?

A1: The issue occurs due to differences in how Apple devices handle network packet processing at the kernel level. When GRO (Generic Receive Offload) is enabled, it aggregates received packets in a way that conflicts with Apple's network stack implementation, creating a bottleneck specifically for upload traffic. Android and Windows devices handle GRO differently and are not affected by this behavior.

Q2: Will disabling GRO affect the performance of other devices on my network?

A2: Disabling GRO typically has minimal impact on overall network performance. While GRO is designed to improve throughput by reducing CPU overhead, most networks will not see a noticeable difference in performance for non-Apple devices. The benefit of restoring normal upload speeds for Apple devices far outweighs any potential minor performance trade-offs for other devices.