[GH-ISSUE #2832] Windows: MTU from config is not applied to WinTUN adapter (defaults to 1280 instead of configured value) #1292

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opened 2026-10-13 20:39:35 +00:00 by bsakaguchi · 0 comments

bsakaguchi commented

2026-10-13 20:39:35 +00:00

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Originally created by @wsp-repo on GitHub (Jul 14, 2026).
Original GitHub issue: https://github.com/amnezia-vpn/amnezia-client/issues/2832

On Windows 11, the MTU value specified in the connection config (MTU = 1200 in the WireGuard/AmneziaWG interface section) is not applied to the created network adapter. The adapter ends up with MTU 1280 (likely a WinTUN default) instead of the configured 1200. This causes silent packet loss for traffic that doesn't fit the actual tunnel MTU: ICMP (ping) and traceroute work fine since packets are small, but TCP/TLS traffic (browser HTTPS requests) fails or hangs intermittently, since large packets get fragmented/dropped without a proper PMTU response back to the client.

Manually forcing the correct MTU on the adapter fixes the issue immediately:
netsh interface ipv4 set subinterface "AmneziaVPN" mtu=1200 store=persistent

Originally created by @wsp-repo on GitHub (Jul 14, 2026). Original GitHub issue: https://github.com/amnezia-vpn/amnezia-client/issues/2832 On Windows 11, the MTU value specified in the connection config (`MTU = 1200` in the WireGuard/AmneziaWG interface section) is not applied to the created network adapter. The adapter ends up with MTU 1280 (likely a WinTUN default) instead of the configured 1200. This causes silent packet loss for traffic that doesn't fit the actual tunnel MTU: ICMP (ping) and traceroute work fine since packets are small, but TCP/TLS traffic (browser HTTPS requests) fails or hangs intermittently, since large packets get fragmented/dropped without a proper PMTU response back to the client. Manually forcing the correct MTU on the adapter fixes the issue immediately: netsh interface ipv4 set subinterface "AmneziaVPN" mtu=1200 store=persistent

bsakaguchi referenced this issue

2026-10-13 21:20:41 +00:00

[PR #1292] [MERGED] Update README.md #2302