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

[Yousong Zhou]

On 28 July 2016 at 23:45, Leon George <leon at georgemail.eu> wrote:
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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 :-)

You may want to apply this kernel patch:
https://github.com/torvalds/linux/commit/018f8258582381bcce484312f0e9ec2970d0383e

                yousong

>
> cu,
> Leon
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> =2r70
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