"very high" (probably?) MTU being set automatically on wireguard interface

"very high" (probably?) MTU being set automatically on wireguard interface

WireGuard interfaces often default to a high MTU, frequently inheriting from the underlying interface or using a standard 1500. While seemingly efficient, WireGuard itself adds a header (typically 20-32 bytes), meaning the effective payload MTU without fragmentation is lower. If this tunnel MTU exceeds the actual Path MTU (PMTU) to the peer, accounting for WireGuard's overhead, it triggers inefficient IP fragmentation or Path MTU Discovery (PMTUD) failures. PMTUD failures cause packet drops, performance degradation, or connectivity issues. Explicitly setting a conservative MTU (e.g., 1420 for a 1500-byte Ethernet link) on the WireGuard interface is vital for reliable operation, preventing fragmentation and PMTUD problems.

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