[OpenWrt-Devel] [L2TP] Bad UDP Checksum on TP-LINK hardware

[Leon George]

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Hi :-)

The UDP checksum for outgoing L2TP datagrams seems to be broken.
It has been tested with TP-LINK's WR842n, WDR3600, and WDR4300.
On two normal Linux computers it's working.

We've tried disabling UDP offloading using ethtool without success.

Is there any way we can make OpenWRT emit the datagrams with the
checksum in place?

This shows up in Wireshark. The first two datagrams are being received
on the other side. The ones after the l2tp aren't.

#this is received on the other end. normal udp
14.6944	UDP	575	52958 → 53201  Len=513
14.8159	UDP	152	53201 → 52958  Len=90
#ip l2tp add tunnel tunnel_id 52958 peer_tunnel_id 53201 encap udp
local '2001:920:18ae:3:20b:c0ff:fe46:42b0' udp_sport 52958 remote
'2a03:b0c0:3:d0::bb5:6001' udp_dport 53201
14.9130	UDP	184	52958 → 53201  Len=122 [ILLEGAL CHECKSUM (0)]
15.3230	UDP	152	52958 → 53201  Len=90 [ILLEGAL CHECKSUM (0)]
15.6929	UDP	184	52958 → 53201  Len=122 [ILLEGAL CHECKSUM (0)]
15.8198	UDP	164	53201 → 52958  Len=102

hoping somebody has an idea :-)

cu,
Leon
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