OpenVPN MTU Calculator
OpenVPN adds about 69 bytes to every packet, so over a normal 1500-byte link the inner MTU is 1431.
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Encapsulation in the Path
Inner MTU
1500 − 69 bytes of headers
TCP MSS (IPv4)
TCP MSS (IPv6)
Largest Ping (IPv4)
Clamp MSS (nftables)
nft add rule inet filter forward tcp flags syn tcp option maxseg size set 1391
MTU & MSS Calculator
OpenVPN on a 1500-byte link
Overhead69 bytes
Inner MTU1431
TCP MSS (IPv4)1391
TCP MSS (IPv6)1371
Over PPPoE (1492)1423
Testping -M do -s 1403 <host>
If the inner interface keeps an MTU of 1500, packets larger than 1431 bytes are fragmented or dropped. The usual symptom: ping and small requests work, large downloads and TLS handshakes hang.
Set the OpenVPN interface's MTU to 1431, or clamp TCP MSS to 1391 on the router.
MTU by Tunnel
Questions
What MTU should I use for OpenVPN?
1431 over a standard 1500-byte link; 1423 over PPPoE.
How much overhead does OpenVPN add?
About 69 bytes per packet, varying with cipher and padding.
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