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Published 20 hours ago • verified publisher icon OpenVPN

Missing packet fragmentation for packets larger than configured MTU

Open DevCybran opened this issue 7 years ago • 2 comments

It appears that UDP packets which are larger than the user-configured MTU are not properly fragmented before transmission.

For example, if attempting to transmit a UDP packet of size 1450 and the MTU is set to 1400, the packet is not fragmented via IP packet fragmentation but transmitted as-is. The driver based on NDIS 5 on the other hand does proper packet fragmentation. This can be seen by using for example Wireshark.

DevCybran avatar Apr 10 '17 17:04 DevCybran

More testing showed that all packets with a size up to 1514 bytes are being transmitted unfragmented (payload size in case of an UDP packet is 1472 bytes). All packets larger than that are automatically fragmented, regardless of the MTU setting. However, the configured MTU is very good at something else: Dropping incoming packets larger than it. Maybe the configured MTU is not correctly / only partially applied?

DevCybran avatar Apr 10 '17 18:04 DevCybran

Is this with OpenVPN, or with a 3rd-party software using the tap6-driver? I'm fairly sure I tested MTU setting for v4+v6 and subsequent large pings and it behaved properly.

cron2 avatar Oct 26 '22 12:10 cron2